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13 avril 2011 3 13 /04 /avril /2011 16:38

An Ethernet hub, active hub, network hub, repeater hubor hubis a device for connecting multiple twisted pair or fiber optic Ethernetdevices together and making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater  HP 448007-001 battery.

Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or Attachment Unit Interface (AUI) connector to allow connection to legacy 10BASE2 or 10BASE5network segments  HP Pavilion DV7 battery.

The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications.

Technical information

A network hub is a fairly unsophisticated broadcast device. Hubs do not manage any of the traffic that comes through them, and any packet entering any port is regenerated and broadcast out on all other ports  HP DV6-1120SA battery.

Since every packet is being sent out through all other ports, packet collisions result—which greatly impedes the smooth flow of traffic.

100 Mbit/s hubs/repeater come in two different speed grades  HP DV6-1210SA battery:

Class I delay the signal for a maximum of 140 bit times (enabling translation between 100Base-TX, 100Base-FX and 100Base-T4) and Class II ones delay the signal for a maximum of 92 bit times (enabling installation of two hubs in a single collision domain)  Compaq CQ50 battery.

The need for hosts to be able to detect collisions limits the number of hubs and the total size of a network built using hubs (a network built using switches does not have these limitations). For 10 Mbit/s networks built using repeater hubs, the 5-4-3 rule must be followed: up to 5 segments (4 hubs) are allowed between any two end stations  HP Pavilion DV8 battery.

For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even that is only allowed if the hubs are of Class II. Some hubs have manufacturer specific stack ports allowing them to be combined in a way that allows more hubs than simple chaining through Ethernet cables  Sony VGP-BPS13 battery,

but even so, a large fast Ethernet network is likely to require switches to avoid the chaining limits of hubs.

Most hubs detect typical problems, such as excessive collisions and jabbering on individual ports, and partitionthe port, disconnecting it from the shared medium  HP DV9700 battery.

Thus, hub-based Ethernet is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire collision domain. Even if not partitioned automatically, a hub simplifies troubleshooting because they remove the need to troubleshoot faults on a long cable with multiple taps  Compaq CQ35-100 battery;

status lights on the hub can indicate the possible problem source or, as a last resort, devices can be disconnected from a hub one at a time much more easily than from a coaxial cable.

Hubs are classified as Layer 1 (physical layer) devices in the OSI model. At the physical layer, hubs support little in the way of sophisticated networking  Sony VGP-BPS11 battery.

Hubs do not read any of the data passing through them and are not aware of their source or destination. A hub simply receives incoming Ethernet frames, regenerates the electrical signal, and broadcasts these packets out to all other devices on the network  HP Pavilion DV6-1223EO Battery.

To pass data through the repeater in a usable fashion from one segment to the next, the packets and the Logical Link Control (LLC) protocols must be the same on the each segment. This means that a repeater will not enable communication, for example, between an 802.3 segment (Ethernet) and an 802.5 segment (Token Ring)  Sony Vaio VGN-CR190E/L Battery.

That is, they cannot translate an Ethernet packet into a Token Ring packet. In other words, repeaters do not translate anything.

Dual speed hubs

In the early days of fast Ethernet, Ethernet switches were relatively expensive devices. Hubs suffered from the problem that if there were any 10BASE-T devices connected then the whole network needed to run at 10 Mbit/s  Sony Vaio VGN-CR190E/P Battery.

Therefore a compromise between a hub and a switch was developed, known as a dual-speed hub. These devices consisted of an internal two-port switch, dividing the 10 Mbit/s and 100 Mbit/s segments. The device would typically consist of more than two physical ports  Sony Vaio VGN-CR190E/R Battery.

When a network device becomes active on any of the physical ports, the device attaches it to either the 10 Mbit/s segment or the 100 Mbit/s segment, as appropriate. This prevented the need for an all-or-nothing migration fast Ethernet networks. These devices are considered hubs because the traffic between devices connected at the same speed is not switchedSony Vaio VGN-CR190E/W Battery.

Uses

Historically, the main reason for purchasing hubs rather than switches was their price. This motivator has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances  Sony Vaio VGN-CR21/B Battery:

  • For inserting a protocol analyzer into a network connection, a hub is an alternative to a network tap or port mirroring.
  • When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteur) can bring down the network by connecting two ports together, causing a loopSony Vaio VGN-CR21E/L Battery.
  • This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. This hazard can also be avoided by using switches that can detect and deal with loops, for example by implementing the spanning tree protocol  Sony Vaio VGN-CR21E/P Battery.
  • A hub with a 10BASE2 port can be used to connect devices that only support 10BASE2 to a modern network. The same goes for linking in an old 10BASE5 network segment using an AUI port on a hub (individual devices that were intended for thicknet can be linked to modern Ethernet by using an AUI-10BASE-T transceiver)  Sony Vaio VGN-CR21E/W Battery.

A USB hubis a device that expands a single USB port into several so that there are more ports available to connect devices to a host system.

USB hubs are often built into equipment such as computers, keyboards, monitors, or printers  Sony Vaio VGN-CR21S/L Battery.

When such a device has many USB ports they all usually stem from one or two internal USB hubs rather than each port having independent USB circuitry.

Physically separate USB hubs come in a wide variety of form factors: from external boxes (looking similar to a network hub) connectible with a long cableSony Vaio VGN-CR21S/P Battery,

to small designs that can be directly plugged into a USB port (see the 'compact design' picture). In the middle case, there are "short cable" hubs which typically use an integral 6 inch cable to slightly distance a small hub away from physical port congestion and of course increase the number of available ports  Sony Vaio VGN-CR21S/W Battery.

Laptop computers may be equipped with many USB ports, but an external USB hub can consolidate several everyday devices (like a mouse and a printer) into a single hub to enable one-step attachment and removal of all the devices  Sony Vaio VGN-CR21Z/N Battery.

Inverse or Sharing Hubs

Also available are so-called "sharing hubs", which effectively are the reverse of a USB hub, allowing several PCs to access (usually) a single peripheral. They can either be manual, effectively a simple switch-box, or automatic, incorporating a mechanism that recognises which PC wishes to use the peripheral and switches accordingly  Sony Vaio VGN-CR21Z/R Battery.

They cannot grant both PCs access at once. Some models, however, have the ability to control multiple peripherals separately (e.g. 2 PCs and 4 peripherals, assigning access separately). Only the simpler switches tend to be automatic, and this feature generally places them at a higher price point too  Sony Vaio VGN-CR220E/R Battery.

Modern KVM switches can also often share USB devices between several computers.

Physical layout

A USB network is built from USB hubs connected downstream to USB ports, which themselves may stem from USB hubs. USB hubs can extend a USB network to a maximum of 127 ports  Sony Vaio VGN-CR23/B Battery.

The USB specification requires that bus-powered hubs may not be connected in series to other bus-powered hubs.

USB ports are often closely spaced. Consequentially plugging a device into one port may physically block an adjacent port, particularly when the plug is not part of a cable but is integral to a device such as a USB flash driveSony Vaio VGN-CR23/P Battery.

A horizontal array of horizontal sockets may be easy to fabricate, but may cause only two out of four ports to be usable (depending on plug width).

Port arrays in which the port orientation is perpendicular to the array orientation generally have fewer blockage problems  Sony Vaio VGN-CR23/R Battery.

"Octopus" or "Squid" hubs (with each socket at the end of a very short cable maybe 2 inches long), or "star" hubs (with each port pointing in a different direction, as pictured) avoid this problem completelySony Vaio VGN-CR23/L Battery.

Length limitations

USB cables are limited to 5m. A hub can be used as an active USB repeater to extend cable length for up to 5m lengths at a time  Sony Vaio VGN-CR23/N Battery.

Active Cables(specialized connector-embedded one-port hubs) perform the same function, but since they are strictly bus-powered, externally powered (non-bus-powered) USB hubs would likely be required for some of the segmentsSony Vaio VGN-CR23/W Battery.

Power

A bus-powered hubis a hub that draws all its power from the host computer's USB interface. It does not need a separate power connection. However, many devices require more power than this method can provide, and will not work in this type of hubHP Pavilion DM3 battery.

USB current (related to power) is allocated in units of 100 mA up to a maximum total of 500 mA per port. Therefore a compliant bus powered hub can have no more than four downstream ports and cannot offer more than four 100 mA units of current in total to downstream devices (since one unit is needed for the hub itself)  HP Pavilion DM4 battery.

If more units of current are required by a device than can be supplied by the port it is plugged into, the operating system usually reports this to the user.

In contrast a self-powered hubis one that takes its power from an external power supply unit and can therefore provide full power (up to 500 mA) to every port  HP Pavilion DV6-1210SA battery.

Many hubs can operate as either buspowered or self powered hubs.

However, there are many non-compliant hubs on the market which announce themselves to the host as self-powered despite really being bus-powered  Dell N3010 battery.

Equally there are plenty of non-compliant devices that use more than 100 mA without announcing this fact (or indeed sometimes without identifying themselves as USB devices at all). These hubs and devices do allow more flexibility in the use of power   Dell Inspiron N4010 battery

(in particular many devices use far less than 100 mA and many USB ports can supply more than 500 mA before going into overload shut-off) but they are likely to make power problems harder to diagnose  Dell INSPIRON 9100 battery.

Some powered hubs do not supply enough power to support a 500mA load on every port. For example, many 7 port hubs come with a 1A power adapter, when in fact seven ports could draw a maximum of 7 x 0.5 = 3.5A, plus power for the hub itself. The assumption is that the user will most likely connect many low power devices and only one or two requiring a full 500mADell XPS 1340 battery.

Speed

To allow high-speeddevices to operate in their fastest mode all hubs between the devices and the computer must be high speed. High-speed devices should fall back to full-speedwhen plugged in to a full-speed hub (or connected to an older full-speed computer port)  Dell Inspiron 1464 battery.

While high-speed hubs support all device speeds, low and full-speed traffic is combined and segregated from high-speed traffic through a transaction translator. Each transaction translator segregates lower speed traffic into its own pool, essentially creating a virtual full-speed bus  Dell Inspiron 1764 battery.

Some designs use a single transaction translator, while other designs have multiple translators. Having multiple translators is only a significant benefit when connecting multiple high-bandwidth full-speed devices.

It is an important consideration that in common language (and often product marketing) USB 2.0 is used as synonymous with high-speed  Dell Inspiron 1564 battery.

However, because the USB 2.0 specification, which introduced high-speed, incorporates and supersedes the USB 1.1 specification, any compliant full-speed or low-speed device is still a USB 2.0 device. Thus, not all USB 2.0 hubs operate at high-speed  Dell Inspiron 1720 battery.

Protocol

Each hub has exactly one upstream port and a number of downstream ports. The upstream port connects the hub (directly or through other hubs) to the host. Other hubs or devices can be attached to the downstream ports  Dell Inspiron Mini 12 battery.

During normal transmission, hubs are essentially transparent: data received from its upstream port is broadcast to all devices attached to its downstream ports; data received from a downstream port is generally forwarded to the upstream port only  Dell LATITUDE D800 battery.

This way, what is sent by the host is received by all hubs and devices, and what sent by a device is received by the host but not by the other devices (an exception is resume signaling).

Hubs are not transparent when dealing with changes in the status of downstream ports, such as insertion or removal of devices  Dell Inspiron 6400 AC Adapter.

In particular, if a downstream port of a hub changes status, this change is dealt with an interaction between the host and this hub; the hubs between them act as transparent in this case.

To this aim, each hub has a single interrupt endpoint "1 IN" (endpoint address 1, hub-to-host direction) used to signal changes in the status of the downstream ports  SONY VAIO VGN-FWZJ/H Battery.

When a device is attached, the hub detects the device pull-up resistor on either D+ or D- and signals the insertion to the host via this interrupt endpoint. When the host polls this interrupt endpoint, it is informed of the presence of the new device. It then instructs the hub (via the default control pipe) to reset the port where the new device is connected  SONY VAIO VGN-FW41M/H Battery.

This reset makes the new device assume address 0, and the host can then interact with it directly; this interaction will result in the assignment of a new (non-zero) address to the device.

Electronic design

Most USB hubs use one or more integrated controller ICs, of which several designs are available from various manufacturersSONY VAIO VGN-FW37J Battery.

Most support a four-port hub system, but hubs using seven-port hub controllers are also available. (It is worth noting, though, that a seven-port hub may actually consist of two cascaded four-port controllers.) Additional features on some hub controllers include control of port LEDsSONY VAIO VGN-FW90NS Battery

(sometimes automatic, sometimes under control of the host PC) and PS/2 to USB conversion for mice and keyboards.

A network switchor switching hubis a computer networking device that connects network segmentsSONY VAIO VGN-FW83XS Battery.

The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the network layer (Layer 3) and above are often referred to as Layer 3 switches or multilayer switches  SONY VAIO VGN-FW83JS Battery.

The term network switchdoes not generally encompass unintelligent or passive network devices such as hubs and repeaters.

The first Ethernet switch was introduced by Kalpana in 1990.

Function

The network switch plays an integral part in most modern Ethernet local area networks (LANs)  SONY VAIO VGN-FW30B Battery.

Mid-to-large sized LANs contain a number of linked managed switches. Small office/home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as a gateway to access small office/home broadband services such as DSL or cable internet  SONY VAIO VGN-AW41MF/H Battery.

In most of these cases, the end-user device contains a router and components that interface to the particular physical broadband technology. User devices may also include a telephone interface for VoIP.

An Ethernet switch operates at the data link layer of the OSI model to create a separate collision domain for each switch portSONY VAIO VGN-AW41MF Battery.

With 4 computers (e.g., A, B, C, and D) on 4 switch ports, A and B can transfer data back and forth, while C and D also do so simultaneously, and the two conversations will not interfere with one another. In the case of a hub, they would all share the bandwidth and run inhalf duplex, resulting in collisions, which would then necessitate retransmissions  SONY VAIO VGN-AW41JF/H Battery.

Using a switch is called microsegmentation. This allows computers to have dedicated bandwidth on a point-to-point connections to the network and to therefore run in Full duplex without collisions.

Role of switches in networks

Switches may operate at one or more layers of the OSI model, including data link, network, or transport (i.e., end-to-end)SONY VAIO VGN-AW41JF Battery.

A device that operates simultaneously at more than one of these layers is known as a multilayer switch.

In switches intended for commercial use, built-in or modular interfaces make it possible to connect different types of networks, including Ethernet, Fibre Channel, ATM, ITU-T G.hn and 802.11  SONY VAIO VGN-FW38M Battery.

This connectivity can be at any of the layers mentioned . While Layer 2 functionality is adequate for bandwidth-shifting within one technology, interconnecting technologies such as Ethernet and token ringare easier at Layer 3.

Interconnection of different Layer 3 networks is done by routers  SONY VAIO VGN-FW31Z Battery.

If there are any features that characterize "Layer-3 switches" as opposed to general-purpose routers, it tends to be that they are optimized, in larger switches, for high-density Ethernet connectivity.

In some service provider and other environments where there is a need for a great deal of analysis of network performance and security, switches may be connected between WAN routers as places for analytic modules  SONY VAIO VGN-FW31M Battery.

Some vendors provide firewall, network intrusion detection, and performance analysis modules that can plug into switch ports. Some of these functions may be on combined modules.

In other cases, the switch is used to create a mirror image of data that can go to an external device  SONY VAIO VGN-FW31B Battery.

Since most switch port mirroring provides only one mirrored stream, network hubs can be useful for fanning out data to several read-only analyzers, such as intrusion detection systems and packet sniffers.

Layer-specific functionality

While switches may learn about topologies at many layers, and forward at one or more layers, they do tend to have common features  SONY VAIO VGN-FW31E Battery.

Other than for high-performance applications, modern commercial switches use primarily Ethernet interfaces, which can have different input and output bandwidths of 10, 100, 1000 or 10,000 megabits per second.

At any layer, a modern switch may implement power over Ethernet (PoE), which avoids the need for attached devices, such as an VoIP phone orwireless access point, to have a separate power supplySONY VAIO VGN-FW31J Battery.

Since switches can have redundant power circuits connected to uninterruptible power supplies, the connected device can continue operating even when regular office power fails.

Layer 1 hubs versus higher-layer switches

A network hub, or repeater, is a simple network device. Hubs do not manage any of the traffic that comes through themSony VGP-BPS21 Battery.

Any packet entering a port is broadcast out or "repeated" on every other port, except for the port of entry. Since every packet is repeated on every other port, packet collisions result, which slows down the network.

There are specialized applications where a hub can be useful, such as copying traffic to multiple network sensors  Sony VGP-BPS21A Battery.

High end switches have a feature which does the same thing called port mirroring.

There is no longer any significant price difference between a hub and a low-end switch.

Layer 2

A network bridge, operating at the data link layer, may interconnect a small number of devices in a home or the officeSony VGP-BPS21/S Battery.

This is a trivial case of bridging, in which the bridge learns the MAC address of each connected device.

Single bridges also can provide extremely high performance in specialized applications such as storage area networks.

Classic bridges may also interconnect using a spanning tree protocol that disables links so that the resulting local area network is a tree without loops  Sony VGP-BPS21A/B Battery.

In contrast to routers, spanning tree bridges must have topologies with only one active path between two points. The older IEEE 802.1D spanning tree protocol could be quite slow, with forwarding stopping for 30 seconds while the spanning tree would reconverge      SONY VGP-BPS21 Battery.

A Rapid Spanning Tree Protocol was introduced as IEEE 802.1w, but the newest edition of IEEE 802.1D adopts the 802.1w extensions as the base standard.

The IETF is specifying the TRILL protocol, which is the application of link-state routing technology to the layer-2 bridging problemSONY VGP-BPS21A Battery.

Devices which implement TRILL, called RBridges, combine the best features of both routers and bridges.

While "layer 2 switch" remains more of a marketing term than a technical term, the products that were introduced as "switches" tended to use microsegmentation and Full duplex to prevent collisions among devices connected to EthernetSONY VGP-BPS21B Battery.

By using an internal forwarding plane much faster than any interface, they give the impression of simultaneous paths among multiple devices.

Once a bridge learns the topology through a spanning tree protocol, it forwards data link layer frames using a layer 2 forwarding method  SONY VGP-BPS21/S Battery.

There are four forwarding methods a bridge can use, of which the second through fourth method were performance-increasing methods when used on "switch" products with the same input and output port bandwidths:

  1. Store and forward: The switch buffers and verifies each frame before forwarding it  SONY VGP-BPS21A/B Battery.
  2. Cut through: The switch reads only up to the frame's hardware address before starting to forward it. Cut-through switches have to fall back to store and forward if the outgoing port is busy at the time the packet arrives. There is no error checking with this method  SONY VGP-BPS13/Q Battery.
  3. Fragment free: A method that attempts to retain the benefits of both store and forward and cut through. Fragment free checks the first 64 bytes of the frame, where addressing information is stored. According to Ethernet specifications, collisions should be detected during the first 64 bytes of the frame  SONY VGP-BPS13/Q Battery,
  4. so frames that are in error because of a collision will not be forwarded. This way the frame will always reach its intended destination. Error checking of the actual data in the packet is left for the end device.
  5. Adaptive switching: A method of automatically selecting between the other three modesSONY VGP-BPS13B/Q Battery .

While there are specialized applications, such as storage area networks, where the input and output interfaces are the same bandwidth, this is rarely the case in general LAN applications. In LANs, a switch used for end user access typically concentrates lower bandwidth (e.g., 10/100 Mbit/s) into a higher bandwidth (at least 1 Gbit/s)  SONY VGP-BPS13A/Q Battery.

Alternatively, a switch that provides access to server ports usually connects to them at a much higher bandwidth than is used by end user devices.

Layer 3

Within the confines of the Ethernet physical layer, a layer 3 switch can perform some or all of the functions normally performed by a router  SONY VGP-BPS13A/Q Battery.

The most common layer-3 capability is awareness of IP multicast through IGMP snooping. With this awareness, a layer-3 switch can increase efficiency by delivering the traffic of a multicast group only to ports where the attached device has signaled that it wants to listen to that group  Toshiba Satellite M300 Battery.

Layer 4

While the exact meaning of the term Layer-4 switch is vendor-dependent, it almost always starts with a capability for network address translation, but then adds some type of load distribution based onTCP sessions.

The device may include a stateful firewall, a VPN concentrator, or be an IPSec security gateway  Toshiba Satellite 1200 Battery.

Layer 7

Layer 7 switches may distribute loads based on URL or by some installation-specific technique to recognize application-level transactions. A Layer-7 switch may include a web cache and participate in a content delivery network  Toshiba Satellite A200 Battery.

Form factor

  • Desktop, not mounted in an enclosure, typically intended to be used in a home or office environment outside of a wiring closet
  • Rack mounted
  • Chassis — with swappable "switch module" cards. e.g. Alcatel's OmniSwitch 9000; Cisco Catalyst switch 4500 and 6500; 3Com 7700, 7900E, 8800 Sony VGP-BPS21 battery.
  • DIN rail mounted, normally seen in industrial environments or panels

Configuration options

  • Unmanagedswitches — These switches have no configuration interface or options. They are plug and play. They are typically the least expensive switches, found in home, SOHO, or small businesses. They can be desktop or rack mounted  Sony VGP-BPS14 battery .
  • Managedswitches — These switches have one or more methods to modify the operation of the switch. Common management methods include: a command-line interface (CLI) accessed via serial console, telnet or Secure Shell  Sony VGP-BPL8 battery,
  • an embedded Simple Network Management Protocol (SNMP) agent allowing management from a remote console or management station, or a web interface for management from a web browser. Examples of configuration changes that one can do from a managed switch include  Sony VGP-BPS22 battery :
  • enable features such as Spanning Tree Protocol, set port bandwidth, create or modify Virtual LANs (VLANs), etc. Two sub-classes of managed switches are marketed today:
    • Smart(or intelligent) switches — These are managed switches with a limited set of management features  Sony VGP-BPS11 battery .
    • Likewise "web-managed" switches are switches which fall in a market niche between unmanaged and managed. For a price much lower than a fully managed switch they provide a web interface (and usually no CLI access) and allow configuration of basic settings, such as VLANs, port-bandwidth and duplex  Sony VGP-BPS9 battery .
    • Enterprise Managed(or fully managed) switches — These have a full set of management features, including CLI, SNMP agent, and web interface. They may have additional features to manipulate configurations, such as the ability to display, modify, backup and restore configurations  Sony VGP-BPS10 battery.
    • Compared with smart switches, enterprise switches have more features that can be customized or optimized, and are generally more expensive than smart switches. Enterprise switches are typically found in networks with larger number of switches and connections  Toshiba PA3399U-2BRS battery,
    • where centralized management is a significant savings in administrative time and effort. A stackable switch is a version of enterprise-managed switch.

Traffic monitoring on a switched network

Unless port mirroring or other methods such as RMON or SMON are implemented in a switch, it is difficult to monitor traffic that is bridged using a switch because only the sending and receiving ports can see the traffic. These monitoring features are rarely present on consumer-grade switches  Toshiba Satellite T4900 Battery.

Two popular methods that are specifically designed to allow a network analyst to monitor traffic are:

  • Port mirroring — the switch sends a copy of network packets to a monitoring network connection.
  • SMON — "Switch Monitoring" is described by RFC 2613 and is a protocol for controlling facilities such as port mirroringToshiba Satellite L305 Battery.

Another method to monitor may be to connect a Layer-1 hub between the monitored device and its switch port. This will induce minor delay, but will provide multiple interfaces that can be used to monitor the individual switch port  SONY VGN -FZ210CE Battery.

Typical switch management features

  • Turn particular port range on or off
  • Link bandwidth and duplex settings
  • Priority settings for ports  Dell Precision M70 Battery
  • MAC filtering and other types of "port security" features which prevent MAC flooding
  • Use of Spanning Tree Protocol
  • SNMP monitoring of device and link health
  • Port mirroring (also known as: port monitoring, spanning port, SPAN port, roving analysis port or link mode port)
  • Link aggregation (also known as bonding, trunkingor teaming)  Acer Aspire One battery 10400mAh
  • VLAN settings
  • 802.1X network access control
  • IGMP snooping

Link aggregation allows the use of multiple ports for the same connection achieving higher data transfer rates. Creating VLANs can serve security and performance goals by reducing the size of the broadcast domain  Toshiba Satellite P10 Battery .

A console server(console access server, console management server, serial concentrator, or serial console server) is a device or service that provides access to the system console of a computing device via networking technologies  Dell RM791 battery.

Most commonly, a console server provides a number of serial ports, which are then connected to the serial ports of other equipment, such as servers, routers or switches. The consoles of the connected devices can then be accessed by connecting to the console server over a serial link such as a modem  Dell Inspiron E1505 battery,

or over a network with terminal emulator software such as telnet or ssh, maintaining survivable connectivity that allows remote users to log in the various consoles without being physically nearby.

Description

Dedicated console server appliances are available from a number of manufacturers in many configurationsDell Studio 1737 battery,

with the number of serial ports ranging from one to 48. These Console Servers are primarily used for secure remote access to Unix Servers, Linux Servers, Windows Servers and any device on the network with a console port  Dell Inspiron 300M Battery.

The purpose is to allow network operations center (NOC) personnel to perform secure remote data center management and out-of-band management of IT assets from anywhere in the world. Products marketed as Console Servers usually have highly advanced security functionality to ensure that only qualified personnel  Dell Latitude E4200 Battery

can access various servers and that any data that is transmitted across the LAN, or over the Internet, is encrypted. Marketing a product as a console server is very application specific because it really refers to what the user wants to do - remotely control, monitor, diagnose and troubleshoot equipment over a network or the Internet  Dell Latitude E5400 Battery.

Some users have created their own console servers using off-the-shelf commodity computer hardware, usually with multiport serial cards typically running a slimmed-down Unix-like operating system such as Linux. Such "home-grown" console servers can be less expensive  Sony VGN-FW139E/H battery,

especially if built from components that have been retired in upgrades, and allow greater flexibility by putting full control of the software driving the device in the hands of the administrator. This includes full access to and configurability of a wide array of security protocols and encryption standards, making it possible to create a console server that is more secureSony VGN-FW11M Battery.

However, this solution may have a higher TCO, less reliability and higher rack-space requirements, since most industrial console servers have the physical dimension of one rack unit (1U), whereas a desktop computer with full-size PCI cards requires at least 3Us, making the home-grown solution more costly in the case of a co-located infrastructure  Sony VGN-FW11S Battery.

An alternative approach to a console server used in some cluster setups is to null-modem wire and daisy-chain consoles to otherwise unused serial ports on nodes with some other primary function.

An Ethernet network is usually composed of routers, hubs, and workstations  Dell Inspiron Mini 10 Battery.

These workstations share the Ethernet connection with each other. In a fully switched networkall of the hubs that would usually reside on an Ethernet network are replaced with one or more switches. These switches allow for a dedicated connection to each workstation. A switch allows for many conversations to occur simultaneouslyDell Latitude D620 Battery.

Before switches existed data could only be transmitted in one direction at a time, this was called half-duplex. By using a switch the network is able to maintain full-duplex Ethernet. This means that data can now be transmitted in both directions at the same time. A good analogy for this would be traveling on a highway with traffic flowing in both directions  Dell Latitude D830 Battery .

The core function of a switch is to allow each workstation to communicate only with the switch instead of with each other. This in turn means that data can be sent from workstation to switch and from switch to workstation simultaneously. The above describes a fully switched network  Toshiba Satellite M60 battery.

The core purpose of a switch is to decongest network flow to the workstations so that the connections can transmit more effectively; receiving transmissions that were only specific to their network address. With the network decongested and transmitting data in both directions simultaneously this can in fact double network speed and capacity when two workstations are trading informationToshiba Satellite M65 battery .

For example if your network speed is 5 Mbit/s, than each workstation is able to simultaneously transfer data at 5 Mbit/s.

Fully switched networks employ either twisted-pair or fiber-optic cabling, both of which use separate conductors for sending and receiving data  Dell Latitude D610 Battery .

In this type of environment, Ethernet nodes can forego the collision detection process and transmit at will, since they are the only potential devices that can access the medium. Simply stated a fully switched network is a collision-free environment.

Network performancerefers to the service quality of a telecommunications product as seen by the customerToshiba NB100 Battery .

It should not be seen merely as an attempt to get "more through" the network.

The following list gives examples of Network Performance measures for a circuit-switched network and one type of packet-switched network, viz. ATM  Dell Studio 1457 Battery:

  • Circuit-switched networks: In circuit switched networks, network performance is synonymous with the grade of service. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads. Other types of performance measures can include noise, echo and so on  Dell Studio 1450 Battery.
  • ATM: In an Asynchronous Transfer Mode (ATM) network, performance can be measured by line rate, quality of service (QoS), data throughput, connect time, stability, technology, modulation technique and modem enhancements.

There are many different ways to measure the performance of a network, as each network is different in nature and designDell Inspiron 1320n Battery.

Performance can also be modelled instead of measured; one example of this is using state transition diagrams to model queuing performance in a circuit-switched network. These diagrams allow the network planner to analyze how the network will perform in each state, ensuring that the network will be optimally designed  SONY VGP-BPS13AS Battery.

8-second rule

A June 2001 Zona Research report entitled "The Need for Speed II" found that the average web user will wait about eight seconds for a page to download, but that current average download time across backbone connection on most web sites is almost ten seconds  SONY VGP-BPS13A/S Battery.

The 8-second ruleis an old (by Internet standards) way of measuring the adequate response time of a webserver through different bandwidth connections. It specified that if the load-time of a web page exceeds eight seconds, users are unlikely to wait, or "stick around", for its completion  SONY VGP-BPS13B/B Battery.

In order to increase the "stickiness" of a website, faster ways to deliver the content to the user needed to be devised. These included stripping away unnecessary HTML code and using fewer images.

It is generally believed that this rule no longer applies  SONY VGP-BPS13A/B Battery,

since a much higher percentage of Internet users now have broadband available, making almost every website load up much faster, in some cases in less than a second. However, the rule has remained as a rough unit to measure the performance of a webserverSONY VGP-BPS13/S Battery.

Relationship between latency and throughput

A common concern in the development or procurement of a telecommunications system is a simple question: "will my data arrive fast enough?". This question in fact contains many subtle parts, based on the interplay of several factors  SONY VGP-BPS8 Battery.

The perceived 'fastness' (speed being a scientific quantity related to propagation and so is not used in this context) is highly dependent on user requirements and measurement technique. A common misunderstanding is that having greater throughput means a "faster" connection  SONY VAIO VGN-FZ38M Battery.

However, throughput, latency, the type of information transmitted, and the way that information is applied all affect the perceived speedof a connection.

Terms

Throughputis the number of messages successfully delivered per unit time  SONY VAIO VGN-FZ31Z Battery.

Throughput is controlled by available bandwidth, as well as the available signal-to-noise ratio and hardware limitations. Throughput for the purpose of this article will be understood to be measured from the arrival of the first bit of data at the receiver, to decouple the concept of throughput from the concept of latency  SONY VAIO VGN-FZ31M Battery.

For discussions of this type the terms 'throughput' and 'bandwidth' are often used interchangeably.

The Time Windowis the period over which the throughput is measured. Choice of an appropriate time window will often dominate calculations of throughput, and whether latency is taken into account or not will determine whether the latency affects the throughput or not  SONY VAIO VGN-FZ31J Battery.

Interplay of factors

All of the factors above, coupled with user requirements and user perceptions, play a role in determining the perceived 'fastness' or utility, of a network connection. The relationship between throughput, latency, and user experience is most aptly understood in the context of a shared network medium, and as a scheduling problem  SONY VAIO VGN-FZ31B Battery.

For systems that are heavily dominated by either latency or throughput considerations.

Physical limitations

  • The speed of light imposes a minimum propagation time on all electromagnetic signals. It is not possible to reduce the latency below t=(distance)/(speed of light)  SONY VAIO VGN-FZ31E Battery.
  • The available channel bandwidth and achievable signal-to-noise ratio dominate the throughput. It is not generally possible to send more data than dictated by the Shannon-Hartley Theorem.

Algorithms and protocols

For some systems, latency and throughput are coupled entities  SONY VAIO VGN-FZ4000 Battery.

In TCP/IP, latency can also directly affect throughput. In TCP connections, the large bandwidth-delay product of high latency connections, combined with relatively small TCP window sizes on many devices, effectively causes the throughput of a high latency connection to drop sharply with latency  SONY VAIO VGN-FZ480E Battery.

This can be remedied with various techniques, such as increasing the TCP congestion window size, or more drastic solutions, such as packet coalescing, TCP acceleration, and forward error correction, all of which are commonly used for high latency satellite links  SONY VAIO VGN-FZ460E Battery.

TCP acceleration converts the TCP packets into a stream that is similar to UDP. Because of this, the TCP acceleration software must provide its own mechanisms to ensure the reliability of the link, taking the latency and bandwidth of the link into account, and both ends of the high latency link must support the method used  SONY VAIO VGN-FZ340E Battery.

Examples of latency or throughput dominated systems

Many systems can be characterized as dominated either by throughput limitations or by latency limitations in terms of end-user utility or experience  SONY VAIO VGN-FZ220E Battery.

In some cases, hard limits such as the speed of light present unique problems to such systems and nothing can be done to correct this. Other systems allow for significant balancing and optimization for best user experience.

Satellite telephony

A telecom satellite in geosynchronous orbit imposes a path length of at least 71000 km between transmitter and receiver  SONY VAIO VGN-FZ180E Battery.

which means a minimum delay between message request and message receipt, or latency of 473 ms. This delay can be very noticeable and affects satellite phone service regardless of available throughput capacity  Sony VAIO VGN-NR11Z/T Battery.

Deep space communication

These long path length considerations are exacerbated when communicating with space probes and other long-range targets beyond Earth's atmosphere. The Deep Space Network implemented by NASA is one such system that must cope with these problems. Largely latency driven, the GAO has criticized the current architecture  Sony VAIO VGN-NR11M/S Battery.

Several different methods have been proposed to handle the intermittent connectivity and long delays between packets, such as delay-tolerant networking  .

Even deeper space communication

At interstellar distances, the difficulties in designing radio systems that can achieve any throughput at all are massive  Sony VAIO VGN-NR11Z/S Battery.

In these cases, maintaining communication is a bigger issue than how long that communication takes.

Offline data transport

Transportation is concerned almost entirely with throughput, which is why physical deliveries of backup tape archives are still largely done by vehicle  Sony VAIO VGN-NR11S/S Battery.

Optimized systems

Web surfing

Users browsing the Internet feel that responses are "instant" when delays are less than 100 ms from click to response . Latency and throughput together affect the perceived speed of a connection  Sony VAIO VGN-FW21E Battery .

However, the perceived performance of a connection can still vary widely, depending in part on the type of information transmitted and how it is used.

In a 2001 study, it was found that a typical web page was 53,400 bytes in sizeSONY VAIO VGN-NR11Z/T Battery .

Round-trip packet latency over the Internet is fairly low – typically less than a tenth of a second across North America – and an average web page of 30-100 kilobytes would normally transfer fully in 10–30 seconds, over a 56-kbit/s modem, which yields a 3 kbit/s transfer rate  SONY VAIO VGN-NR11Z/S Battery.

If a user had to wait 10–30 seconds to see anything, after every web-page click, it would be intolerable.

Because latency is so important, the HTTP protocol and HTML markup language were invented to reduce the rendering time of hypertext over the internet  SONY VAIO VGN-NR11M/S Battery.

These protocols allow incremental rendering, meaning that page text can begin display after the first packet arrives. HTTP and nearly all browsers support gzip (compressed) transfer encoding, which can typically compress text by 2x. Moreover, HTTP 1.0 and later protocols support a rich set of caching primitives  SONY VAIO VGN-NR11S/S Battery,

allowing content to be stored closer to the user, in both browser-caches and ISP proxy-caches, all to reduce latency. And finally, in the early days of HTTP, interlaced photos were transmitted via GIF, which allowed a rough version of an embedded picture to appear when only half the scan lines had arrived  SONY VAIO VGN-FW21E Battery.

A few years later JPEGwas invented, allowing an almost arbitrary tradeoff between latency and image quality. These optimizations of HTTP and HTML, GIF, and JPEG were crucial to reducing latency and improving the perceived performance of the World Wide WebSONY VAIO VGN-FW31J Battery 

a>.

Hence, when a user clicks on a web page, there is a delay of 500-550 milliseconds to transfer a 1500-byte packet over a 56 kbit/s modem, before the user can begin to see up to 3,000 bytes (uncompressed) of text. A DSL line with a throughput of 256kbit/s would produce a delay of roughly 60-110 ms, which would be perceived as an "instant" response  SONY VAIO VGN-FW31J Battery.

By comparison, to transfer the contents of a DVD over a modem could take a week or more at a 56 kbit/s modem rate. Simply packing the DVD into an envelope and mailing it could be faster.

Online gaming

Some online games utilize the Internet and/or a Local Area Network to coordinate a multiplayer game experience among two or more players  SONY VAIO VGN-FZ21E Battery,

each of whom is running a copy of the game on a local game system (typically a video game console or gaming computer), with messages sent among the multiple game systems (either directly or through a game server reporting the actions of each player so that all the game systems stay synchronized  SONY VAIO VGN-FZ18 Battery.

If the nature of the game is such that the game's local action cannot proceed until it synchronizes with one or more remote game systems, then the latency of the Internet and/or LAN will accordingly delay the responsiveness of a game system. Although such systems may only require very low throughputHP Pavilion DV8000 battery

(e.g. messages of game controller actions may be only a few kilobits per second), the latency of the Internet and/or LAN must be low enough to meet the requirements of the game.

The maximum acceptable latency is game-type dependent  HP Pavilion DV6000 battery.

For example, generally, twitch gameplay games such as a first-person shooter like Quake 3 require lower latency for the best experience, while generally, a turn-based game such as chess can tolerate higher latency. But, it entirely depends on the specifics of each game  HP Pavilion DV8000 battery.

For example, fast chess is a turn-based game that may have low latency requirements. And, in the case of twitch games, some games can be designed such that only events that impact the outcome of the game are subject to synchronization latency, allowing for fast local response time most of the time  HP Pavilion DV6000 battery .

Cloud gaming is a type of online gaming where the entire game is hosted on a game server in a data center, and the user is only running a thin client locally that forwards game controller upstream to the game server. The game server then renders the next frame of the game video which is compressed using low-latency video compression and is sent downstream and decompressed by the thin client  SONY VAIO VGN-FZ Battery.

For the cloud gaming experience to be acceptable, the round-trip latency of all elements of the cloud gaming system (the thin client, the Internet and/or LAN connection the game server, the game execution on the game server, the video and audio compression and decompression  HP PAVILION DV8000 Battery,

and the display of the video on a display device) must be low enough that the user perception is that the game is running locally.  Because of such tight latency requirements, distance considerations of the speed of light through optical fiber come into play  HP PAVILION DV6000 Battery,

currently limiting the distance between a user and a cloud gaming game server to approximately 1000 miles, according to OnLive, the only company thus far operating a cloud gaming service.

Online game systems utilizing a wireless network may be subject to significant latency, depending on the architecture of the wireless network and local electromagnetic interference impacting that network  HP Pavilion dv8000 battery.

Although radio propagation through air is faster than light through optical fiber, wireless systems are often shared among many users and may suffer from latency incurred due to network congestion which can trigger bufferbloat related issues, or due to network protocols that introduce latency  HP Pavilion dv6000 Battery.

And, in the event of electromagnetic interference, transmitted packets may be lost, requiring a retransmission which also incurs latency.

A local area network (LAN)is a computer network that connects computers and devices in a limited geographical area such as home, school, computer laboratory or office building  Dell Inspiron E1505 battery .

The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

ARCNET, Token Ring and other technology standards have been used in the past, but Ethernet over twisted pair cabling, and Wi-Fi are the two most common technologies currently in use  Sony VGP-BPL15 battery.

History

As larger universities and research labs obtained more computers during the late 1960s, there was an increasing pressure to provide high-speed interconnections. A report in 1970 from the Lawrence Radiation Laboratory detailing the growth of their "Octopus" network  gives a good indication of the situation  Sony VGP-BPS13B/B battery.

Cambridge Ring was developed at Cambridge University in 1974 but was never developed into a successful commercial product.

Ethernet was developed at Xerox PARC in 1973–1975, and filed as U.S. Patent 4,063,220. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper, "Ethernet: Distributed Packet-Switching For Local Computer Networks.Sony VGP-BPL9 battery"

ARCNET was developed by Datapoint Corporation in 1976 and announced in 1977. It had the first commercial installation in December 1977 at Chase Manhattan Bank in New York.

Standards evolution

The development and proliferation of CP/M-based personal computers from the late 1970s and then DOS-based personal computers from 1981 meant that a single site began to have dozens or even hundreds of computers  Sony VGP-BPS13B/B battery.

The initial attraction of networking these was generally to share disk space and laser printers, which were both very expensive at the time. There was much enthusiasm for the concept and for several years, from about 1983 onward, computer industry pundits would regularly declare the coming year to be “the year of the LAN”  Sony VGP-BPS13A/B battery.

In practice, the concept was marred by proliferation of incompatible physical Layer and network protocol implementations, and a plethora of methods of sharing resources. Typically, each vendor would have its own type of network card, cabling, protocol, and network operating system  Sony VGP-BPS13/S battery.

A solution appeared with the advent of Novell NetWare which provided even-handed support for dozens of competing card/cable types, and a much more sophisticated operating system than most of its competitors. Netware dominated  the personal computer LAN business from early after its introduction in 1983 until the mid 1990s when Microsoft introduced Windows NT Advanced Server and Windows for Workgroups  Sony VGP-BPS13/B battery .

Of the competitors to NetWare, only Banyan Vines had comparable technical strengths, but Banyan never gained a secure base. Microsoft and 3Com worked together to create a simple network operating system which formed the base of 3Com's 3+Share, Microsoft's LAN Manager and IBM's LAN Server - but none of these were particularly successful  Sony VGP-BPS13 battery.

During the same period, Unix computer workstations from vendors such as Sun Microsystems, Hewlett-Packard, Silicon Graphics, Intergraph, NeXT and Apollo were using TCP/IP based networking. Although this market segment is now much reducedDell Studio 1555 battery,

the technologies developed in this area continue to be influential on the Internet and in both Linux and Apple Mac OS X networking—and the TCP/IP protocol has now almost completely replaced IPX, AppleTalk, NBF, and other protocols used by the early PC LANsDell KM958 battery.

Cabling

Early LAN cabling had always been based on various grades of coaxial cable. However shielded twisted pair was used in IBM's Token Ring implementation, and in 1984 StarLAN showed the potential of simple unshieldedtwisted pair by using Cat3—the same simple cable used for telephone systems  Dell XPS M1210 Battery.

This led to the development of 10Base-T (and its successors) and structured cabling which is still the basis of most commercial LANs today. In addition, fiber-optic cabling is increasingly used in commercial applications.

As cabling is not always possible, wireless Wi-Fi is now the most common technology in residential premises, as the cabling required is minimal and it is well suited to mobile laptops and smartphones  SONY VAIO VGN-FW33GW Battery.

Technical aspects

Switched Ethernet is the most common Data Link Layer and Physical Layer implementation for local area networks. At the higher layers, the Internet Protocol (TCP/IP) has become the standard    SONY VAIO VGN-FW33G/E1 Battery.

Smaller LANs generally consist of one or more switches linked to each other, often at least one is connected to a router, cable modem, or ADSL modem for Internet access  .

Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loopsSONY VAIO VGN-FW290JTH Battery,

their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic with VLANs. Larger LANs also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, and sensors.

LANs may have connections with other LANs via leased lines, leased services, or by tunneling across the Internet using virtual private network technologies  SONY VAIO VGN-FW290JTB Battery.

Depending on how the connections are established and secured in a LAN, and the distance involved, a LAN may also be classified as metropolitan area network (MAN) or wide area networks (WAN)

Network topology describes the layout pattern of interconnections between devices and network segments  SONY VAIO VGN-FW29/B Battery.

The most common topology types are bus, ring, star, bus-star composite and mesh.

In data communications, bandwidth-delay productrefers to the product of a data link's capacity (in bits per second) and its end-to-end delay (in seconds)  SONY VAIO VGN-FW27T/H Battery .

The result, an amount of data measured in bits (or bytes), is equivalent to the maximum amount of data on the network circuit at any given time, i.e. data that has been transmitted but not yet received. Sometimes it is calculated as the data link's capacity times its round trip time    SONY VAIO VGN-FW31ZJ Battery.

A network with a large bandwidth-delay product is commonly known as a long fat network(shortened to LFNand often pronounced "elephant"). As defined in RFC 1072, a network is considered an LFN if its bandwidth-delay product is significantly larger than 105bits (12500 bytes)    SONY VAIO VGN-FW27/W Battery.

A high bandwidth-delay product is an important problem case in the design of protocols such as TCP in respect of performance tuning, because the protocol can only achieve optimum throughput if a sender sends a sufficiently large quantity of data before being required to stop and wait until a confirming message is received from the receiver  SONY VAIO VGN-FW27/B Battery,

acknowledging successful receipt of that data. If the quantity of data sent is insufficient compared with the bandwidth-delay product, then the link is not being kept busy and the protocol is operating below peak efficiency for the link. Protocols that hope to succeed in this respect need carefully designed self-monitoring, self-tuning algorithms  SONY VAIO VGN-FW26T/B Battery.

Ultra-high speed LANs may fall into this category, where protocol tuning is critical for achieving peak throughput, on account of their extremely high bandwidth, even though their delay is not great.

An important example of a system where the bandwidth-delay product is large is that of GEO satellite connectionsSONY VAIO VGN-FW25T/B Battery,

where end-to-end delivery time is very high and link throughput may also be high. The high end-to-end delivery time makes life difficult for stop-and-wait protocols and applications that assume rapid end-to-end response.

In telecommunications and computing  SONY VAIO VGN-FW21Z Battery,

bitrate(sometimes written bit rate, data rateor as a variable Ror fb) is the number of bits that are conveyed or processed per unit of time.

The bit rate is quantified using the bits per second (bit/sor bps) unit, often in conjunction with an SI prefix such as kilo- (kbit/s or kbps), mega- (Mbit/s or Mbps), giga- (Gbit/s or Gbps) or tera- (Tbit/s or Tbps)  SONY VAIO VGN-FW21M Battery.

Note that, unlike many other computer-related units, 1 kbit/s is traditionally defined as 1,000 bit/s, not 1,024 bit/s, etc., also before 1999 when SI prefixes were introduced for units of information in the standard IEC 60027-2.

The formal abbreviation for "bits per second" is "bit/s" (not "bits/s", see writing style for SI units)  SONY VAIO VGN-FW21L Battery.

In less formal contexts the abbreviations "b/s" or "bps" are often used, though this risks confusion with "bytes per second" ("B/s", "Bps"). 1 Byte/s (Bps or B/s) corresponds to 8 bit/s (bps or b/s).

Physical layer gross bit rate

In digital communication systems, the gross bitrate, raw bitrate  SONY VAIO VGN-FW21J Battery ,

data signaling rate or uncoded transmission rate is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead. The gross bit rate is related to, but should not be confused with, the symbol rate in baud, symbols/s or pulses/s  Dell Latitude E6400 ATG Battery.

Gross bit rate can be used interchangeably with "baud" onlywhen each modulation transition of a data transmission system carries exactly one bit of data; something not true for modern modem modulation systems and modern LANs, for example.

For most line codes and modulation methods  Asus M52N Battery:

Symbol rate ? Gross bit rate

More specifically, a line code representing the data using pulse-amplitude modulation with 2Ndifferent voltage levels, or a digital modulation method using 2Ndifferent symbols, for example 2Namplitudes, phases or frequencies, can transfer Nbit/symbol, or Nbit/pulse  SONY VAIO VGN-FW21E Battery.

This results in:

Gross bit rate = Symbol rate · N

The exception from the above is some self-synchronizing line codes, for example Manchester coding and return-to-zero (RTZ) coding, where each bit is represented by two pulses (signal states), resulting in  Ibm THINKPAD X200 Battery:

Gross bit rate = Symbol rate/2

A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certain analog bandwidth in hertz is given by the Nyquist law:  Ibm THINKPAD X200 Battery

Symbol rate ? Nyquist rate = 2 · bandwidth

In practice this upper bound can only be approached for line coding schemes (or baseband transmission) and for so-called vestigal sideband digital modulation  Ibm THINKPAD X200 Battery .

Most other digital carrier-modulated schemes (or passband transmission schemes), for example ASK, PSK and QAM, can be characterized as double sideband modulation, resulting in the following approximative relation:

Symbol rate ? Bandwidth  HP Pavilion DV7 battery

Physical layer net bit rate

The physical layer net bitrate, peak bitrate, useful bit rate, information rate, payload rate, coded transmission rate, effective data rate or wire speed (informal language) of a digital communication link is the capacity excluding the physical layer protocol overhead  Dell Latitude E6500 battery,

for example time division multiplex (TDM) framing bits, redundant forward error correction (FEC) codes, equalizer training symbols and other channel coding. Error-correcting codes are common especially in wireless communication systems and broadband modem standards  Dell Latitude E6500 battery.

The relationship between the gross bit rate and net bit rate is affected by the FEC code rate according to the following.

Net bit rate ? Gross bit rate · code rate

Some operational systems indicate the "connection speed" (informal language) of a network access technology or communication device  Dell Latitude E6500 battery.

The connection speed of a technology that involves forward error correction typically refers to the physical layer net bit ratein accordance with the above definition.

For example, the net bitrate (and thus the "connection speed") of a IEEE 802.11a wireless network is the net bit rate of between 6 and 54 Mbit/s, while the gross bit rate is between 12 and 72 Mbit/s inclusive of error-correcting codesDell Latitude E6500 battery.

The net bit rate of ISDN Basic Rate Interface (2 B-channels + 1 D-channel) of 64+64+16 = 144 kbit/s also refers to the payload data rates, while the signalling rate is 160 kbit/s.

The net bit rate of the Ethernet 100Base-TX physical layer standard is 100 Mbit/s, while the gross bitrate is 125 Mbit/second, due to the 4B5B (four bit over five bit) encoding  Dell Latitude E6400 Battery.

In this case, the gross bit rate is equal to the symbol rate or pulse rate of 125 Mbaud, due to the NRZI line code.

In communications technologies without forward error correction and other physical layer protocol overhead, there is no distinction between gross bit rate and physical layer net bit rate  Dell Latitude E6400 Battery.

For example, the net as well as gross bit rate of Ethernet 10Base-T is 10 Mbit/s. Due to the Manchester line code, each bit is represented by two pulses, resulting in a pulse rate of 20 Mbaud.

The "connection speed" of a V.92 voiceband modem typically refers to the gross bit rate, since there is no additional error-correction code      Dell Latitude E6400 battery.

It can be up to 56,000 bit/s downstreams and 48,000 bit/supstreams. A lower bit rate may be chosen during the connection establishment phase due to adaptive modulation - slower but more robust modulation schemes are chosen in case of poor signal-to-noise ratio  Dell Vostro 1310 battery .

Due to data compression, the actual data transmission rate or throughput (see below) may be higher.

The channel capacity, also known as the Shannon capacity, is a theoretical upper bound for the maximum net bitrate, exclusive of forward error correction coding, that is possible without bit errors for a certain physical analog node-to-node communication linkDell Studio 1735 battery .

Net bit rate ? Channel capacity

The channel capacity is proportional to the analog bandwidth in hertz. This proportionality is called Hartley's law. Consequently the net bit rate is sometimes called digital bandwidth capacity in bit/s  Dell Studio 1535 battery .

Note that the term line ratein some textbooks is defined as gross bit rate, in others as net bit rate.

Network throughput

The term throughput, essentially the same thing as digital bandwidth consumption, denotes the achieved average useful bit rate in a computer network over a logical or physical communication link or through a network nodeDell Vostro A860 battery ,

typically measured at a reference point above the datalink layer. This implies that the throughput often excludes data link layer protocol overhead. The throughput is affected by the traffic load from the data source in question, as well as from other sources sharing the same network resources. See also Measuring network throughputDell Vostro A840 battery.

Goodput (data transfer rate)

Goodputor data transfer raterefers to the achieved average net bit rate that is delivered to the application layer, exclusive of all protocol overhead, data packets retransmissions, etc      Dell Studio XPS 1640 battery .

For example, in the case of file transfer, the goodput corresponds to the achieved file transfer rate. The file transfer rate in bit/s can be calculated as the file size (in bytes), divided by the file transfer time (in seconds), and multiplied by eight  Dell Vostro 1720 battery .

As an example, the goodput or data transfer rate of a V.92 voiceband modem is affected by the modem physical layer and data link layer protocols. It is sometimes higher than the physical layer data rate due to V.44 data compression, and sometimes lower due to bit-errors and automatic repeat request retransmissions    Dell Vostro 1710 battery.

If no data compression is provided by the network equipment or protocols, we have the following relation:

Goodput ? Throughput ? Maximum throughput ? Net bit rate

for a certain communication path.      Dell Inspiron 1320n battery

Multimedia encoding bit rate

In digital multimedia, bit rateoften refers to the number of bits used per unit of playback time to represent a continuous medium such as audio or video after source coding (data compression). The encoding bit rate of a multimedia file is the size of a multimedia file in bytes divided by the playback time of the recording (in seconds), multiplied by eightDell Inspiron 1320 battery .

In case of realtime streaming multimedia, the encoding bit rate is the goodput that is required to avoid interrupts. For streaming multimedia without interrupts, we have the following relationship:

Encoding bit rate = Required goodputDell Inspiron 1764 battery

The term average bitrate is used in case of variable bitrate multimedia source coding schemes.

A theoretical lower bound for the encoding bit rate for lossless data compression is the source information rate, also known as the entropy rate.

Entropy rate ? Multimedia bit rate  Dell Inspiron 1564 battery

Prefixes

When quantifying large bit rates, SI prefixes (also known as Metric prefixes or Decimal prefixes) are used, thus:

1,000 bit/s rate = 1 kbit/s (one kilobit or one thousand bits per second)
1,000,000 bit/s rate = 1 Mbit/s (one megabit or one million bits per second)
1,000,000,000 bit/s rate = 1 Gbit/s (one gigabit or one billion bits per second)

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Binary prefixes have almost never been used for bitrates, although they may occasionally be seen when data rates are expressed in bytes per second (e.g. 1 kByte/s or kBps is sometimes interpreted as 1000 Byte/s, sometimes as 1024 Byte/s)  Dell Inspiron N7010 battery .

A 1999 IEC standard (IEC 60027-2) specifies different abbreviations for Binary and Decimal (SI) prefixes (e.g. 1 kiB/s = 1024 Byte/s = 8192 bit/s, and 1MiB/s = 1024 kiB/s), but these are still not very common in the literature, and therefore sometimes it is necessary to seek clarification of the units used in a particular context  Dell Inspiron N5010 battery.

In computer networks, goodputis the application level throughput, i.e. the number of useful information bits, delivered by the network to a certain destination, per unit of time. The amount of data considered excludes protocol overhead bits as well as retransmitted data packets  Dell Inspiron N3610 battery.

This is related to the amount of time from the first bit of the first packet is sent (or delivered) until the last bit of the last packet is delivered, see below.

For example, if a file is transferred, the goodput that the user experiences corresponds to the file size in bits divided by the file transfer time  Dell Inspiron N4010 battery .

The goodput is always lower than the throughput (the gross bit rate that is transferred physically), which generally is lower than network access connection speed (the channel capacity or bandwidth).

Examples of factors that cause lower goodput than throughput are:Dell Inspiron 9100 battery

  • Protocol overhead;Typically, transport layer, network layer and sometimes datalink layer protocol overhead is included in the throughput, but is excluded from the goodput.
  • Transport layer flow control and congestion avoidance, for example TCP slow start, may cause a lower goodput than the maximum throughput.  Dell Studio XPS 1340 battery
  • Retransmission of lost or corrupt packets due to transport layer automatic repeat request (ARQ), caused by bit errors or packet dropping in congested switches and routers, is included in the datalink layer or network layer throughput but not in the goodput  Dell XPS M2010 battery.

Example

Imagine that a file is being transferred using HTTP over a switched Ethernet connection with a total channel capacity of 100 megabits per second. The file cannot be transferred over Ethernet as a single contiguous stream; instead, it must be broken down into individual chunks  Dell XPS M1730 battery.

These chunks must be no larger than the maximum transmission unit of Ethernet, which is 1500 bytes. Each packet requires 20 bytes of IP header information and 20 bytes of TCP header information, so only 1460 bytes are available per packet for the file transfer data itself (Unix systems, Linux, and Mac OS X are further limited to 1448 bytes as they also carry a 12 bytes time stamp )Dell Inspiron Mini 12 battery .

Furthermore, the data are transmitted over Ethernet in a frame, which imposes a 26 byte overhead per packet. Given these overheads, the maximum goodputis 1460/1526 × 100 Mbit/s which is 95.67 megabits per second or 11.959 megabytes per secondHp 2230 Battery.

Note that this example doesn't consider some additional Ethernet overhead, such as the interframe gap (a minimum of 96 bit times), nor collisions (which have a variable impact, depending on the network load). TCP itself also adds the overhead of acknowledgementsHp cq20 Battery

(which along with the round-trip delay time and the TCP window size in effect will rate-limit each individual TCP connection, seebandwidth-delay product). This example also does not consider the overhead of the HTTP protocol itself, which becomes relevant when transferring small files  Hp Pavilion cq72 Battery .

Data delivery time

The goodput is a ratio between delivered amount of information, and the total delivery time. This delivery time includes:

  • inter-packet time gaps caused by packet generation processing time (a source that does not use the full network capacity), or by protocol timing (for example collision avoidance)  Hp Pavilion cq42 Battery
  • data and overhead transmission delay (amount of data divided by bit rate)
  • propagation delay (distance divided by wave propagation speed),
  • packet queuing delay  Hp Pavilion cq62 Battery
  • intermediate node store-and-forward processing delay,
  • packet retransmission time (in case of deleted pakcets in congested routers, or detected bit errors),
  • delayed acknowledge due to flow control, congestion avoidance and processing delay  Hp Pavilion dm4 Battery.

In telecommunication engineering, and in particular teletraffic engineering, the quality of voice service is specified by two measures: the grade of service(GoS) and the quality of service(QoS)  Hp Pavilion dm3 Battery.

Grade of serviceis the probability of a call in a circuit groupbeing blocked or delayed for more than a specified interval, expressed as a vulgar fraction or decimal fraction. This is always with reference to the busy hour when the traffic intensity is the greatest  Samsung P467 battery.

Grade of service may be viewed independently from the perspective of incoming versus outgoing calls, and is not necessarily equal in each direction or between different source-destination pairs.

On the other hand, the quality of servicewhich a singlecircuit is designed or conditioned to provide, e.g. voice grade or program grade is called the quality of service  Samsung R522 battery .

Quality criteria for such circuits may include equalization for amplitude over a specified band of frequencies, or in the case of digital data transported via analogue circuits, may include equalization for phase. Criteria for mobile quality of service in cellular telephone circuits include the probability of abnormal termination of the call  Apple A1281 battery.

What is Grade of Service and how is it measured?

When a user attempts to make a telephone call, the routing equipment handling the call has to determine whether to accept the call, reroute the call to alternative equipment, or reject the call entirely  Apple A1280 battery.

Rejected calls occur as a result of heavy traffic loads (congestion) on the system and can result in the call either being delayed or lost. If a call is delayed, the user simply has to wait for the traffic to decrease, however if a call is lost then it is removed from the systemIbm ThinkPad X200T Battery.

The Grade of Service is one aspect of the quality a customer can expect to experience when making a telephone call.  In a Loss System, the Grade of Service is described as that proportion of calls that are lost due to congestion in the busy hour. For a Lost Call system, the Grade of Service can be measured using Equation 1  Ibm ThinkPad G40 Battery .

\mbox{Grade of Service}=\frac{\mbox{number of lost calls}}{\mbox{number of offered calls}}\qquad(1)

For a delayed call system, the Grade of Service is measured using three separate terms  HP Pavilion DV6 Battery :

  • The mean delay td– Describes the average time a user spends waiting for a connection if their call is delayed.
  • The mean delay to– Describes the average time a user spends waiting for a connection whether or not their call is delayedHP Pavilion DV6-1000ET Battery.
  • The probability that a user may be delayed longer than time twhile waiting for a connection. Time tis chosen by the telecommunications service provider so that they can measure whether their services conform to a set Grade of Service  HP Pavilion DV6-1001TX Battery.

Where and when is Grade of Service measured?

The Grade of Service can be measured using different sections of a network. When a call is routed from one end to another, it will pass through several exchanges. If the Grade of Service is calculated based on the number of calls rejected by the final circuit group, then the Grade of Service is determined by the final circuit group blocking criteriaHP Pavilion DV6-1001XX Battery.

If the Grade of Service is calculated based on the number of rejected calls between exchanges, then the Grade of Service is determined by the exchange-to-exchange blocking criteriaHP Pavilion DV6-1002TX Battery.

The Grade of Service should be calculated using both the access networks and the core networks as it is these networks that allow a user to complete an end-to-end connection. Furthermore, the Grade of Service should be calculated from the average of the busy hour traffic intensities of the 30 busiest traffic days of the year  HP Pavilion DV6-1003TX Battery.

This will cater for most scenarios as the traffic intensity will seldom exceed the reference level.

Class of Service

Different telecommunications applications require different Qualities of Service  …  HP Pavilion DV6-1004TX Battery.

For example, if a telecommunications service provider decides to offer different qualities of voice connection, then a premium voice connection will require a better connection quality compared to an ordinary voice connection. Thus different Qualities of Service are appropriate, depending on the intended use  HP Pavilion DV6-1005EA Battery.

To help telecommunications service providers to market their different services, each service is placed into a specific class. Each Class of Service determines the level of service required.

To identify the Class of Service for a specific service, the network’s switches and routers examine the call based on several factorsHP Pavilion DV6-1005EZ Battery.

Such factors can include:

  • The type of service and priority due to precedence
  • The identity of the initiating party
  • The identity of the recipient party  HP Pavilion DV6-1005TX Battery

Quality of Service in broadband networks

In broadband networks, the Quality of Service is measured using two criteria. The first criterion is the probability of packet losses or delays in already accepted calls. The second criterion refers to the probability that a new incoming call will be rejected or blockedHP Pavilion DV6-1006TX Battery.

To avoid the former, broadband networks limit the number of active calls so that packets from established calls will not be lost due to new calls arriving. As in circuit-switched networks, the Grade of Service can be calculated for individual switches or for the whole network  HP Pavilion DV6-1007TX Battery.

Maintaining a Grade of Service

The telecommunications provider is usually aware of the required Grade of Service for a particular product. To achieve and maintain a given Grade of Service, the operator must ensure that sufficient telecommunications circuits or routes are available to meet a specific level of demand  HP Pavilion DV6-1008TX Battery.

It should also be kept in mind that too many circuits will create a situation where the operator is providing excess capacity which may never be used, or at the very least may be severely underutilized. This adds costs which must be borne by other parts of the network  HP Pavilion DV6-1009EL Battery.

To determine the correct number of circuits that are required, telecommunications service providers make use of Traffic Tables. An example of a Traffic Table can be viewed in Figure 1. It follows that in order for a telecommunications network to continue to offer a given Grade of Service  HP Pavilion DV6-1009TX Battery,

the number of circuits provided in a circuit group must increase (non-linearly) if the traffic intensity increases.

Erlang's lost call assumptions

To calculate the Grade of Service of a specified group of circuits or routes, A.K. Erlang used a set of assumptions that relied on the network losing calls when all circuits in a group were busy  HP Pavilion DV6-1010EA Battery.

These assumptions are:

  • All traffic through the network is pure-chance traffic, i.e. all call arrivals and terminations are independent random events
  • There is statistical equilibrium, i.e., the average number of calls does not change  HP Pavilion DV6-1010ED Battery
  • Full availability of the network, i.e., every outlet from a switch is accessible from every inlet
  • Any call that encounters congestion is immediately lost.

From these assumptions Erlang developed the Erlang-B formula which describes the probability of congestion in a circuit group. The probability of congestion gives the Grade of Service experienced  HP Pavilion DV6-1010ET Battery.

Calculating the Grade of Service

To determine the Grade of Service of a network when the traffic load and number of circuits are known, telecommunications network operators make use of Equation 2, which is the Erlang-B equation.

\mbox{Grade of Service}=\frac{\left(\frac{A^N}{N!}\right)}{\left(\sum_{k=0}^N\frac{A^k}{k!}\right)}\qquad(2)

A= Expected traffic intensity in Erlangs, N= Number of circuits in group  HP Pavilion DV6-1010TX Battery.

This equation allows operators to determine whether each of their circuit groups meet the required Grade of Service, simply by monitoring the reference traffic intensity.

(For delay networks, the Erlang-C formula allows network operators to determine the probability of delay depending on peak traffic and the number of circuits. )  HP Pavilion DV6-1011TX Battery

Other settings of interactivity include radio and television talk shows, letters to the editor, listener participation in such programs, and computer and technological programming. Interactive new media has become a true benefit to every one because people can express their artwork in more than HP Pavilion DV6-1012TX Battery

one way with the technology that we have today and there is no longer a limit to what we can do with our creativity.

Interactivity can be considered as a central concept in understanding new media, but different media forms possess different degrees of interactivity,even some forms of digitized and converged media are not in fact interactive at all HP Pavilion DV6-1013EA Battery.

Tony Feldman considers digital satellite television as an example of a new media technology that uses digital compression to dramatically increase the number of television channels that can be delivered, and which changes the nature of what can be offered through the service  HP Pavilion DV6-1013TX Battery,

but does not transform the experience of television from the user’s point of view, as it lacks a more fully interactive dimension. It remains the case that interactivity is not an inherent characteristic of all new media technologies, unlike digitization and convergence   HP Pavilion DV6-1016EZ Battery.

Terry Flew (2005) argues that "the global interactive games industry is large and growing, and is at the forefront of many of the most significant innovations in new media" (Flew 2005: 101). Interactivity is prominent in these online computer games such as World of Warcraft, The Sims Onlineand Second Life HP Pavilion DV6-1018EL Battery .

These games, developments of "new media", allow for users to establish relationships and experience a sense of belonging, despite temporal and spatial boundaries. These games can be used as an escape or to act out a desired life. Will Wright, creator of The Sims, "is fascinated by the way gamers have become so attached to his invention-with some even living their lives through it".HP Pavilion DV6-1020EC Battery

New media have created virtual realities that are becoming mere extensions of the world we live in. With the creation of Second Lifeand Active Worlds before it, people have even more control over this virtual world where anything that a participant can think of in their mind can become a reality inSecond Life.HP Pavilion DV6-1020ED Battery

New Media changes continuously because it is constantly modified and redefined by the interaction between the creative use of the masses, emerging technology, cultural changes, etc.

Industry

The new media industry shares an open association with many market segments in areas such as software/video game design, television  HP Pavilion DV6-1018EL Battery,

radio, and particularly movies, advertising and marketing, through which industry seeks to gain from the advantages of two-way dialogue with consumers primarily through the Internet. The advertising industry has capitalized on the proliferation of new media with large agencies running multi-million dollar interactive advertising subsidiaries  HP Pavilion DV6-1020EC Battery.

Interactive websites and kiosks have become popular. In a number of cases advertising agencies have also set up new divisions to study new media. Public relations firms are also taking advantage of the opportunities in new media through interactive PR practicesHP Pavilion DV6-1020ED Battery.

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