Institute of Technology,Sligo Dept of Computing

WANsWANs

Semester 4Semester 4

Chapter 2Chapter 2

Table of ContentsTable of Contents

2.1 WAN Service Providers2.1 WAN Service Providers 2.2 WAN Devices2.2 WAN Devices 2.3 How WANs relate to the OSI Model2.3 How WANs relate to the OSI Model 2.4 WAN Encapsulation Formats2.4 WAN Encapsulation Formats 2.5 WAN Link Options2.5 WAN Link Options

WAN ServicesWAN Services A WAN is a A WAN is a Data CommunicationsData Communications NetworkNetwork

operating beyond a LAN's geographic scope.operating beyond a LAN's geographic scope. You must subscribe to a WAN service You must subscribe to a WAN service

provider, such as a regional Bell operating provider, such as a regional Bell operating company (RBOC) to use WAN carrier company (RBOC) to use WAN carrier network services. network services.

A WAN connects the locations of an A WAN connects the locations of an organization to:organization to: each other each other locations of other organizationslocations of other organizations external services (such as databases)external services (such as databases) remote userremote user

WAN ServicesWAN Services A WAN uses A WAN uses data linksdata links, such as , such as

Integrated Services Digital Network Integrated Services Digital Network (ISDN) and Frame wide-area geographies(ISDN) and Frame wide-area geographies

WANs carry many types of WANs carry many types of traffictraffic, such as:, such as: voicevoice datadata videovideo

WAN technologies function at three layers of the OSI WAN technologies function at three layers of the OSI model: model: physicalphysical data linkdata link NetworkNetwork

WAN ServicesWAN Services TelephoneTelephone and and datadata servicesservices are the most are the most

commonly used WAN services. commonly used WAN services. Telephone and data services are connected from Telephone and data services are connected from

the building POP to the WAN provider's central the building POP to the WAN provider's central office (CO). office (CO).

The CO is the local telephone company office to The CO is the local telephone company office to which all local loops in that area connectwhich all local loops in that area connect

The services offered by the WAN provider are of 3 The services offered by the WAN provider are of 3 main types:main types: Call setup (also called Call setup (also called signallingsignalling)) Time Division Multiplexing (TDM)Time Division Multiplexing (TDM) Frame RelayFrame Relay

WAN ServicesWAN Services Call setup (also called Call setup (also called signallingsignalling): ):

Sets up and clears calls between telephone users. Sets up and clears calls between telephone users. Most commonly used call setup is Signaling System Most commonly used call setup is Signaling System 7 (SS7)7 (SS7)

Time Division Multiplexing (TDM): Time Division Multiplexing (TDM): Information from many sources has bandwidth Information from many sources has bandwidth

allocation on a single medium. Basic telephone allocation on a single medium. Basic telephone service and ISDN use TDM circuits.service and ISDN use TDM circuits.

Frame Relay: Frame Relay: Data contained in frames shares bandwidth with Data contained in frames shares bandwidth with

other WAN Frame Relay subscribers. Uses statistical other WAN Frame Relay subscribers. Uses statistical multiplexing (STDM)multiplexing (STDM)

CPE, demarc, "last mile", CO switch, toll CPE, demarc, "last mile", CO switch, toll networknetwork

The most commonly used terms associated with the main parts The most commonly used terms associated with the main parts of WAN services:of WAN services: Customer premises equipmentCustomer premises equipment ( (CPECPE) -- Devices physically located ) -- Devices physically located

on the subscriber's premises. on the subscriber's premises. Demarcation Demarcation (or (or demarcdemarc) -- The point at which the CPE ends and the ) -- The point at which the CPE ends and the

local loop portion of the service begins. Often occurs at the local loop portion of the service begins. Often occurs at the POPPOP of a of a building. building.

LocalLocal looploop (or "last-mile") -- Cabling (usually copper wiring) that (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office. extends from the demarc into the WAN service provider's central office.

COCO switchswitch -- A switching facility that provides the nearest point of -- A switching facility that provides the nearest point of presence for the provider's WAN service. presence for the provider's WAN service.

TollToll networknetwork -- The collective switches and facilities (called trunks) -- The collective switches and facilities (called trunks) inside the WAN provider's cloud. inside the WAN provider's cloud.

CPE, demarc, "last mile", CO switch, toll CPE, demarc, "last mile", CO switch, toll networknetwork

CPE, demarc, "last mile", CO switch, toll CPE, demarc, "last mile", CO switch, toll networknetwork

A key interface is between the data terminal equipment (A key interface is between the data terminal equipment (DTEDTE) and ) and the data circuit-terminating equipment (the data circuit-terminating equipment (DCEDCE).). Typically, the Typically, the DTEDTE is the is the routerrouter, and the , and the DCEDCE is the device used to is the device used to

convert the user data from the DTE into a form acceptable to the convert the user data from the DTE into a form acceptable to the WAN service's facility (eg. WAN service's facility (eg. MODEM, CSU/DSU, TA/NT1)MODEM, CSU/DSU, TA/NT1). .

The WAN path between the DTEs is called the The WAN path between the DTEs is called the linklink circuitcircuit channel channel lineline

The DTE/DCE interface describes the physical layer standards. It The DTE/DCE interface describes the physical layer standards. It uses various protocols (such as uses various protocols (such as HSSIHSSI and and V.35V.35) that establish the ) that establish the codes that DTEs and DCEs use to communicate with each other. codes that DTEs and DCEs use to communicate with each other.

WAN Virtual CircuitsWAN Virtual Circuits A virtual circuit is a pathway through a packet-switched network A virtual circuit is a pathway through a packet-switched network

that that appearsappears to be a dedicated, physically-connected circuit (as to be a dedicated, physically-connected circuit (as opposed to an actual, physical, circuit-switched pathway opposed to an actual, physical, circuit-switched pathway through a network). through a network).

Virtual circuits are connection oriented Virtual circuits are connection oriented Two types of virtual circuits exist: Two types of virtual circuits exist:

switched virtual circuits (SVCs) switched virtual circuits (SVCs) permanent virtual circuits (PVCs). permanent virtual circuits (PVCs).

In a PVC, the customer and the carrier have negotiated the In a PVC, the customer and the carrier have negotiated the endpoints and characteristics of the virtual circuit ahead of endpoints and characteristics of the virtual circuit ahead of time, and they are constantly available. time, and they are constantly available. The The end pointsend points and a stated and a stated bandwidth bandwidth called a Committed called a Committed

Information Rate (CIR) constitute a PVC, which is defined to Information Rate (CIR) constitute a PVC, which is defined to the frame relay network devicesthe frame relay network devices

In a SVC, the virtual circuit is available only "on-demand".In a SVC, the virtual circuit is available only "on-demand". circuit establishment, data transfer, and circuit termination. circuit establishment, data transfer, and circuit termination.

PVC and SVCPVC and SVC SVCs are used in situations where data transmission between SVCs are used in situations where data transmission between

devices is sporadic. devices is sporadic. DisadvantageDisadvantage = increased bandwidth due to the circuit = increased bandwidth due to the circuit

establishment and termination phasesestablishment and termination phases AdvantageAdvantage = decreased costs compare with constant virtual- = decreased costs compare with constant virtual-

circuit availability.circuit availability. A PVC is a A PVC is a permanentlypermanently established virtual circuit that established virtual circuit that

consists of one mode: consists of one mode: data transfer. data transfer.

PVCs are used in situations where data transfer between PVCs are used in situations where data transfer between devices is constant.devices is constant. AdvantageAdvantage = PVCs decreased bandwidth use because there are = PVCs decreased bandwidth use because there are

no establishment and termination phasesno establishment and termination phases DisadvantageDisadvantage = increased costs due to constant availability. = increased costs due to constant availability.

WAN Line TypesWAN Line Types

OC-768 now exists running at 40Gbps

Fundamental WAN DevicesFundamental WAN Devices The router is The router is thethe essential WAN device, but essential WAN device, but

it is also considered a LAN deviceit is also considered a LAN device.. It offers many services, including LAN It offers many services, including LAN

and WAN interface ports and WAN interface ports WAN switches connect to WAN bandwidth WAN switches connect to WAN bandwidth

for voice, data, and video communication.for voice, data, and video communication. Modems interface voice-grade services Modems interface voice-grade services

(telephone lines). They include:(telephone lines). They include: CSUs/ DSUs devices that interface T1/E1 lines CSUs/ DSUs devices that interface T1/E1 lines TA/NT1 devices that interface ISDN services.TA/NT1 devices that interface ISDN services.

Communication servers concentrate dial-in Communication servers concentrate dial-in and dial-out user communication. and dial-out user communication.

Routers and WAN SwitchesRouters and WAN Switches Routers provide interfaces for a wide range of links and subnetworks at a Routers provide interfaces for a wide range of links and subnetworks at a

wide range of speeds. wide range of speeds. Routers are active and intelligent network devices and can participate in Routers are active and intelligent network devices and can participate in

managing the network by:managing the network by: providing dynamic control over resourcesproviding dynamic control over resources supporting the tasks and goals for networks such as:supporting the tasks and goals for networks such as:

ConnectivityConnectivity Reliable performanceReliable performance Management controlManagement control FlexibilityFlexibility SecuritySecurity

A WAN switch is a multiport networking device that operates at A WAN switch is a multiport networking device that operates at the data link layer of the OSI reference model. A WAN switch the data link layer of the OSI reference model. A WAN switch typically switches traffic such as:typically switches traffic such as:

Frame RelayFrame Relay X.25X.25 Switched Multimegabit Data Service (SMDS)Switched Multimegabit Data Service (SMDS)

Routers and WAN SwitchesRouters and WAN Switches

Describe modems on a WANDescribe modems on a WAN A device that interprets digital and analog signals by A device that interprets digital and analog signals by

modulating and demodulating the signal, which modulating and demodulating the signal, which enables enables datadata to be transmitted over to be transmitted over voice-gradevoice-grade telephone lines.telephone lines.

At the source, digital signals are converted to a form At the source, digital signals are converted to a form suitable for transmission over analog communication suitable for transmission over analog communication facilities. facilities. That is digital signals are converted into That is digital signals are converted into analog signalsanalog signals. At the destination, these analog . At the destination, these analog signals are returned to their digital form. signals are returned to their digital form.

CSU/DSUs on a WANCSU/DSUs on a WAN A CSU/DSU is :A CSU/DSU is :

A digital-interface device that connects a A digital-interface device that connects a T1/E1T1/E1

Sometimes two separate digital devicesSometimes two separate digital devices Sometimes its integrated into the routerSometimes its integrated into the router

Adapts the physical interface on a DTE Adapts the physical interface on a DTE device (such as a terminal or router) to device (such as a terminal or router) to the interface of a DCE device (such as a the interface of a DCE device (such as a switch) in a switched-carrier networkswitch) in a switched-carrier network

ISDN Terminal Adapters on a ISDN Terminal Adapters on a WANWAN

An ISDN Terminal Adapter (TA) is a device used to An ISDN Terminal Adapter (TA) is a device used to connect ISDN Basic Rate Interface (BRI) connect ISDN Basic Rate Interface (BRI) connections to other interfacesconnections to other interfaces

Its is an adapter that allows non ISDN devices Its is an adapter that allows non ISDN devices (terminals or routers) to communicate on an ISDN (terminals or routers) to communicate on an ISDN networknetwork

The ISDN TA can be either a standalone device or The ISDN TA can be either a standalone device or a board inside the TE2. a board inside the TE2.

Organizations that deal with WAN Organizations that deal with WAN StandardsStandards

WANs use the OSI model layered approach to WANs use the OSI model layered approach to encapsulation, just as LANs doencapsulation, just as LANs do

WANs mainly focus on the physical and data-link WANs mainly focus on the physical and data-link layerslayers Physical layer includes delivery of dataPhysical layer includes delivery of data Data-link requirements include addressing, flow control, Data-link requirements include addressing, flow control,

encapsulationsencapsulations WAN standards are defined and managed by:WAN standards are defined and managed by:

International Telecommunication Union-International Telecommunication Union-Telecommunication Standardization Sector (ITU-T)Telecommunication Standardization Sector (ITU-T)

International Organization for Standardization (ISO)International Organization for Standardization (ISO) Internet Engineering Task Force (IETF) Internet Engineering Task Force (IETF) Electronic Industries Association (EIA)/ Electronic Industries Association (EIA)/

Telecommunications Industries Association (TIA)Telecommunications Industries Association (TIA)

WAN Physical Layer WAN Physical Layer StandardsStandards

WAN physical-layer protocols describe how to WAN physical-layer protocols describe how to provide electrical, mechanical, operational, and provide electrical, mechanical, operational, and functional connections for WAN services.functional connections for WAN services.

Also describes the interface between the DTE and Also describes the interface between the DTE and the DCEthe DCE

WAN Physical Layer StandardsWAN Physical Layer Standards Physical layer standards define the rules for the Physical layer standards define the rules for the

DTE/DCE interface:DTE/DCE interface: EIA/TIA-232: developed by TIA/EIA, supports signal speeds of EIA/TIA-232: developed by TIA/EIA, supports signal speeds of

up to 64kbps, formerly called RS-232up to 64kbps, formerly called RS-232 EIA/TIA-449: a faster version of EIA/TIA-232 (up to 2 Mbps), EIA/TIA-449: a faster version of EIA/TIA-232 (up to 2 Mbps),

capable of longer cable runs.capable of longer cable runs. EIA/TIA-612/613: A standard describing HSSI, which provides EIA/TIA-612/613: A standard describing HSSI, which provides

access to services at T3 (45 Mbps), E3 (34 Mbps), SONET access to services at T3 (45 Mbps), E3 (34 Mbps), SONET (51.84 Mbps) rates.(51.84 Mbps) rates.

V.35 : An ITU-T standard describing a synchronous, physical-V.35 : An ITU-T standard describing a synchronous, physical-layer protocol used for communications between a network layer protocol used for communications between a network access device and a packet network. For speeds up to 48kbpsaccess device and a packet network. For speeds up to 48kbps

X.21 -- An ITU-T standard for serial communications over X.21 -- An ITU-T standard for serial communications over synchronous digital lines.synchronous digital lines.

Name and describe 6 data-link Name and describe 6 data-link encapsulationsencapsulations

The WAN data link layer defines how data is encapsulated for The WAN data link layer defines how data is encapsulated for transmission to remote sitestransmission to remote sites Frame Relay: uses Frame Relay: uses simplifiedsimplified encapsulation with no error correction over encapsulation with no error correction over

high-quality high-quality digital digital facilities. A very fast protocol compared to the other facilities. A very fast protocol compared to the other WAN protocols.WAN protocols.

Point-to-Point Protocol (PPP): developed by the IETF. PPP contains a Point-to-Point Protocol (PPP): developed by the IETF. PPP contains a protocol fieldprotocol field to identify the network-layer protocol. to identify the network-layer protocol.

ISDN: a set of digital services that transmits ISDN: a set of digital services that transmits voice voice andand data data over existing over existing phone lines.phone lines.

Link Access Procedure, Balanced (LAPB): For packet-switched networks Link Access Procedure, Balanced (LAPB): For packet-switched networks used to encapsulate packets at Layer 2 of the X.25 stack. Provides used to encapsulate packets at Layer 2 of the X.25 stack. Provides reliability reliability and and flow controlflow control on a point-to-point basis. on a point-to-point basis.

Cisco/IETF: Used to encapsulate Frame Relay traffic. The Cisco option is Cisco/IETF: Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers.proprietary and can be used only between Cisco routers.

High-Level Data Link Control (HDLC): an ISO standard, HDLC not High-Level Data Link Control (HDLC): an ISO standard, HDLC not compatible between different vendors because of the way each vendor compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both has chosen to implement it. HDLC supports both point-to-pointpoint-to-point and and multipoint multipoint configurations. configurations.

Serial line frame fieldsSerial line frame fields The two most common point-to-point WAN The two most common point-to-point WAN

encapsulations are HDLC and PPPencapsulations are HDLC and PPP All the serial line encapsulations share a All the serial line encapsulations share a

common frame format, which has the common frame format, which has the following fieldsfollowing fields

The choice of encapsulation protocol The choice of encapsulation protocol depends on the WAN technology and the depends on the WAN technology and the communicating equipment. communicating equipment.

PPP and HDLCPPP and HDLC PPP is a standard serial-line encapsulation methodPPP is a standard serial-line encapsulation method

This protocol can check for link quality during connection This protocol can check for link quality during connection establishment.establishment.

Provides authentication through Password Authentication Protocol Provides authentication through Password Authentication Protocol (PAP)(PAP) and Challenge Handshake Authentication Protocol and Challenge Handshake Authentication Protocol (CHAP).(CHAP).

HDLC is Cisco's default encapsulation for serial HDLC is Cisco's default encapsulation for serial lineslines No windowing or flow controlNo windowing or flow control Address field is 11111111, just like Address field is 11111111, just like PPPPPP A proprietary type code is inserted in the frame which means that A proprietary type code is inserted in the frame which means that

HDLC framing is not interoperable with other vendors' equipment.HDLC framing is not interoperable with other vendors' equipment. Used when both ends of a Used when both ends of a dedicated-linededicated-line connection are routers connection are routers

running Cisco IOSrunning Cisco IOS

Two basic WAN link optionsTwo basic WAN link options Dedicated lines are physical circuits that involve no Dedicated lines are physical circuits that involve no

switchingswitching Switched lines are broken up into 2 categories: Switched lines are broken up into 2 categories:

Circuit-switched: Circuit-switched: A physical circuit is established for the A physical circuit is established for the duration of the data transferduration of the data transfer

Packet-switched: Packet-switched: Packets may take different physical routes Packets may take different physical routes through the cloud on the way to the destinationthrough the cloud on the way to the destination

Dedicated LinesDedicated Lines Dedicated lines, also called leased lines, provide Dedicated lines, also called leased lines, provide fullfull--time time

service. Typically are used to carry:service. Typically are used to carry: DataData VoiceVoice VideoVideo

Generally provide core or backbone connectivity between Generally provide core or backbone connectivity between major sites or campuses, as well as LAN-to-LAN connectivity. major sites or campuses, as well as LAN-to-LAN connectivity.

The following are required for dedicated line connections:The following are required for dedicated line connections: A A routerrouter port port CSUCSU / / DSUDSU An An actual actual circuitcircuit from the service provider. from the service provider.

Most important dedicated lines are T1, E1, and J1 depending Most important dedicated lines are T1, E1, and J1 depending on where in the world you are locatedon where in the world you are located

Leased LinesLeased Lines Leased lines are a form of dedicated lines.Leased lines are a form of dedicated lines. Dedicated lines also are referred to as Dedicated lines also are referred to as point-to-point linkspoint-to-point links

because their established path is permanent and fixed for because their established path is permanent and fixed for each remote network reached through the carrier facilities. each remote network reached through the carrier facilities.

Connections are made using the router's synchronous serial Connections are made using the router's synchronous serial ports with typical bandwidth use of up to 2 Mbps (E1) ports with typical bandwidth use of up to 2 Mbps (E1) available through the use of a available through the use of a CSU/DSUCSU/DSU..

Point-to-point links provide a single, pre-established WAN Point-to-point links provide a single, pre-established WAN communications path from the subscriber through a carrier communications path from the subscriber through a carrier network (such as a telephone company), to a remote network (such as a telephone company), to a remote networknetwork

The service provider The service provider reserves reserves point-to-point links for the point-to-point links for the private use of the customer.private use of the customer.

Point-to-point is used for Point-to-point is used for direct physicaldirect physical links or for links or for virtualvirtual links consisting of links consisting of multiple physicalmultiple physical links. links.

Packet-Switched ConnectionsPacket-Switched Connections Network devices Network devices share a (PVC) share a (PVC) permanent virtual circuit to transport permanent virtual circuit to transport

packets from a source to a destination across a carrier network,packets from a source to a destination across a carrier network, Switched networks can carry variable-size frames or fixed-size cells. Switched networks can carry variable-size frames or fixed-size cells. The most common packet-switched network type is Frame Relay. The most common packet-switched network type is Frame Relay.

Others include X.25 and SMDSOthers include X.25 and SMDS

Frame RelayFrame Relay Designed to be used over high-speed, high quality digital facilitiesDesigned to be used over high-speed, high quality digital facilities Does not offer much error checking or reliability, but expects upper-Does not offer much error checking or reliability, but expects upper-

layer protocols to attend to these issueslayer protocols to attend to these issues Connect multiple network devices on a multipoint WANConnect multiple network devices on a multipoint WAN Frame Relay access is typically at 56 kbps, 64 kbps, or 1.544 MbpsFrame Relay access is typically at 56 kbps, 64 kbps, or 1.544 Mbps Each site can be connected to every other by a virtual circuit. Each site can be connected to every other by a virtual circuit. Frame Relay service is offered through a PVC. Frame Relay service is offered through a PVC. A data-link connection identifier (DLCI) identifies a PVCA data-link connection identifier (DLCI) identifies a PVC The DLCI number is a local identifier between the DTE and the DCEThe DLCI number is a local identifier between the DTE and the DCE The DLCI identifies the logical circuit between the source and The DLCI identifies the logical circuit between the source and

destination devices. destination devices.

Frame RelayFrame Relay Two common topologies can be used in a Two common topologies can be used in a

Frame Relay solution:Frame Relay solution: Fully meshed topology: Fully meshed topology: Every Frame Relay Every Frame Relay

network device has a PVC to network device has a PVC to everyevery other other device on the multipoint WAN.device on the multipoint WAN.

Partially meshed topology: Partially meshed topology: also often called also often called a a star star topology or hub-and-spokes topology. In topology or hub-and-spokes topology. In a partially meshed topology, a partially meshed topology, not everynot every device device on the Frame Relay cloud has a PVC to every on the Frame Relay cloud has a PVC to every other device. other device.

Circuit-switched connectionsCircuit-switched connections A dedicated physical circuit is A dedicated physical circuit is established, maintained, established, maintained,

andand terminated terminated through a carrier network for each through a carrier network for each communication session.communication session.

Operates much like a normal telephone callOperates much like a normal telephone call Connections from one site to another are brought up Connections from one site to another are brought up

when needed and generally require low bandwidthwhen needed and generally require low bandwidth ISDN connections are limited to 64 or 128 kbpsISDN connections are limited to 64 or 128 kbps Used primarily:Used primarily:

to connect remote users and mobile users to corporate LANsto connect remote users and mobile users to corporate LANs as backup lines for higher-speed circuits like Frame Relay and as backup lines for higher-speed circuits like Frame Relay and

T1 lines.T1 lines.

DDRDDR Dial on Demand Routing is a technique that routers use to initiate Dial on Demand Routing is a technique that routers use to initiate

and close circuit-switched sessions when transmitting end stations and close circuit-switched sessions when transmitting end stations need them.need them.

DDR means that the connection is brought up only when a specific DDR means that the connection is brought up only when a specific type of traffic type of traffic (interesting traffic)(interesting traffic) initiates the call or when you need a initiates the call or when you need a backup linkbackup link..

DDR is a substitute for dedicated lines when full-time circuit DDR is a substitute for dedicated lines when full-time circuit availability is not required.availability is not required.

DDR can be used to provide DDR can be used to provide backup load sharingbackup load sharing and interface and interface backupbackup

Describe ISDNDescribe ISDN Terminal Equipment 1 (TE1): Designates a device that is Terminal Equipment 1 (TE1): Designates a device that is compatiblecompatible with the with the

ISDN network. A TE1 connects to an NT of either Type 1 or Type 2. ISDN network. A TE1 connects to an NT of either Type 1 or Type 2. Terminal Equipment 2 (TE2): Designates a device that is Terminal Equipment 2 (TE2): Designates a device that is not compatiblenot compatible with with

ISDN and requires a TA. ISDN and requires a TA. TA: TA: ConvertsConverts standard electrical signals into the form used by ISDN so that standard electrical signals into the form used by ISDN so that

non-ISDN devicesnon-ISDN devices can connect to the ISDN network. can connect to the ISDN network. NT Type 1 (NT1): Connects NT Type 1 (NT1): Connects four-wirefour-wire ISDN subscriber wiring to the ISDN subscriber wiring to the

conventional conventional two-wire local looptwo-wire local loop facility. facility. NT Type 2 (NT2): Directs traffic to and from different subscriber devices and the NT Type 2 (NT2): Directs traffic to and from different subscriber devices and the

NT1. The NT2 is an intelligent device that performs NT1. The NT2 is an intelligent device that performs switching and switching and concentrating. concentrating.

ISDN BRI operates mostly over the copper twisted-pair ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. telephone wiring in place today.

ISDN BRI delivers a total bandwidth of a ISDN BRI delivers a total bandwidth of a 144 kbps144 kbps line into line into threethree separate channels. separate channels.

Two of the channels, called Two of the channels, called B (bearer) channelsB (bearer) channels, operate at , operate at 6464 kbpskbps and are used to carry and are used to carry voicevoice or or datadata traffic. traffic.

The third channel, the The third channel, the D (delta) channelD (delta) channel, is a , is a 1616--kbpskbps signalling channel used to carry instructions that tell the signalling channel used to carry instructions that tell the telephone network how to handle each of the B channels. telephone network how to handle each of the B channels.

ISDN BRI often is referred to as ISDN BRI often is referred to as 2B+D2B+D..

ISDN