WAN Tech CT

7/27/2019 WAN Tech CT

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A WAN is a data communications network thatcovers a relatively broad geographic area andoften uses transmission facilities provided bycommon carriers, such as telephone companies.

WAN technologies function at the lower threelayers of the OSI reference model: the physicallayer, the data link layer, and the network layer.



There are two prevailing definitions WAN

book definition

A network that

spans largegeographical

locations, usually to

interconnect

multiple Local AreaNetworks (LANs).

practical definition

A network that

traverses a publicnetwork or

commercial carrier,

using one of several

WAN technologies.



WANs are generally grouped into threeseparate connection types

Point-to-Point technologies

Circuit-switched technologies

Packet-switched technologies



Often called dedicated or leased lines Usually the most expensive form of WAN

technology.

Leased from a service provider, and provide

guaranteed bandwidth from location to another (hence point-to-point).

Cost is determined by the distance of theconnection, and the amount of bandwidth allocated.

Generally, point-to-point links require no call-setup,and the connection is usually always on. Examples ofpoint-to-point technologies include T1 lines

T3 lines



Circuit-Switched technologies require call-setupto occur before information can be transferred.

The session is usually torn down once datatransfer is complete (this is identified as an On-

Demand Circuit). Circuit-switched lines are generally low-speed

compared to point-to-point lines.

Examples of circuit-switched technologies

include: Dial-up

ISDN



Share a common infrastructure between allthe provider’s subscribers. Thus, bandwidth is not

guaranteed, but is instead allocated on a best

effort basis.

Packet-switched technologies are ill-suited

for applications that require consistent bandwidth,

but are considerably less expensive than

dedicated point-to-point lines. Examples of packet-switched technologies

include: Frame-Relay

X25



8.1

Main Ideas

Circuit-switched networks

use a direct, physicalconnection to transmit

data back and forth.

Message-switched

networks send entire

transmissions between

intermediaries. Packet-

switched networks route

packets through virtual

circuits.

Key Terms

carrier services

remote accessdial-up networking (DUN)

Virtual Private Network

(VPN)

circuit-switched network

message switching

packet switching

Packet Switching

Exchange (PSE)

How a WAN Works



8.1

After a network growsbeyond the confines of theLAN, a carrier service isrequired.

Carriers provide a varietyof services which areconnected throughremote access.

carrier services Datatransfer services provided bytelecommunicationscompanies.

remote access A directconnection to a networkusing regular dial-up lines. After the connection is made,the user can work on thenetwork as if in the office.

Carriers

How a WAN Works



8.1

Remote access

connections are

established through either

a dial-up network (DUN)

or virtual privatenetwork (VPN).

The advantages of a VPN

are that users benefit from

faster network

connections, privacy, and

security.

dial-up networking (DUN)

A client service using regular

telephone lines to connect to

a network.

Virtual Private Network

(VPN) An alternative to

using a dial-up network

connection. Is a network

connection between two

computers. With a VPN, a

remote user can securely

access the internal network

from a remote location.

Carriers

How a WAN Works



8.1

Data in the WAN istransported throughvarious technologies:

• Circuit switching• Message switching

circuit-switched network Anetwork that creates a directphysical connection betweenthe sender and receiver.

message switching Atechnology used to route anentire message from onesystem to another. Thatmessage is routed through

intermediate (go-between)station and does not involvea direct physical connectionbetween the sender andreceiver.

Switching Technologies

How a WAN Works



8.1


Message switching sends the entire message to

intermediary stations that decide which route to use.

How a WAN Works



8.1

Packet switching iscommonly found in LANs.

Packets are routed

through PacketSwitching Exchanges until they reach their destination.

packet switching Atransmission method inwhich all transmissions arebroken into small units andsent over the network.

Packets are reassembled atthe destination computer.

Packet SwitchingExchange (PSE) Go-betweens in the packetswitching process. The PSEinspects the packet’sdestination address, consultsa routing table, and forwardsthe packet at the highestpossible speed.


How a WAN Works



8.1


Packet-switching networks send packets along the best

route available.

How a WAN Works



Setup, data transfer, and teardown phases as in a circuit-switched network (CSN)

Resource allocated during setup phase, as in a CSN, or ondemand as in a datagram network (DN)

As in DN, data are packetized and each packet carries an

address in the header. The address has local jurisdiction,not end-to-end jurisdiction.

As in CSN, all packets follow the same path establishedduring the connection

VCN is normally implemented in the data link layer, while

CSN is in physical layer and DN in the network layer



Two types of addressing in a virtual-circuit network: globaland local (virtual-circuit identifier: VCI)

Global address is used only to create a VCI

Virtual Circuit Identifier



Setup phase, Data transfer phase, Teardown phase Switch and tables in a virtual-circuit network



Source-to-destination data transfer in a virtual-circuitnetwork



In virtual-circuit switching, all packets belonging to thesame source and destination travel the same path; but thepackets may arrive at the destination with different delaysif resource allocation is on demand.



8.2

Main Ideas

X.25 is a packet-switchingprotocol originally designed

for analog telephone lines.

Frame relay usespermanent, virtual circuitsand reliable, digital lines totransmit data.

ATM transmits data, voice,and video over many typesof lines.

Key Terms

X.25

Data Terminal Equipment

(DTE)packet assembler/disassembler (PAD)

Data Circuit-terminatingEquipment (DCE)

frame relay

Asynchronous Transfer Mode (ATM)

cell relay

broadband ISDN (B-ISDN)

Sending Data across a WAN



8.2

Computers on an X.25

network can receive and

transmit data at the same

time. This is also called

full-duplexcommunication.

X.25 A connection-oriented,

packet-switching protocol

designed for use on analog

telephone lines. Computers

on an X.25 network canreceive and transmit data at

the same time.

X.25 Packet-Switching Protocol




8.2

The X.52 protocolconcentrates on theinterface through whichtransmission flows. The

interface is made up of the following devices:

• Data Terminal Equipment(DTE)

• Packet

assembler/dissembler (PAD)

Data Terminal Equipment(DTE) X.25 deviceterminology. There are nocomputers, hosts, or nodes.Instead, sending and

receiving computers arereferred to simply as this.

packetassembler/disassembler

(PAD) A device in X.25transmissions that preparespackets for transmission anddisassembles the packetsthat come in.





8.2

• Data Circuit-terminating

Equipment (DCE)

• Packet switching

exchanges (PSEs)

Data Circuit-Terminating

Equipment (DCE) Packets

are sent here from the DTE

or PAD. A DCE might be a

modem or packet switch. ADCE is located at each end

of the connection.





X.25 and Frame Relay



Designed to provide a low cost alternative for data communication over public networks Pay only for bandwidth actually used

Ideal for “bursty” communication over low

quality circuits Standard provides error detection and

correction for reliable data transfer X.25 standard approved in 1976 by CCITT (now

known as ITU) Can support speeds of 9.6 Kbps to 2 Mbps Can provide multiplexing of up to 4095 virtual

circuits over on DTE-DCE link



Data Terminal Equipment (DTE) Terminals, personal computers, and network hosts

Located on premises of subscriber

Data Circuit-terminating Equipment (DCE) Modems and packet switches

Usually located at carrier facility

Packet Switching Exchange (PSE) Switches that make up the carrier network



X.25

WAN

Personal Computer

DTE

Terminal

DTE

Server

DTE

Modem

DCE

Modem

DCE

Modem

DCE

PSE

PSE

PSE

PSE



Used for DTE devices that are too simple toimplement X.25 (such as character-mode terminals)

Acts as intermediary device between DTE and DCE

Performs three functions Buffering to store data until a device is ready to process

it

Packet Assembly

Packet Disassembly



Terminal

DTEModem

DCE

PAD

Buffer

Data

Assembly/

Disassembly

Data

X.25 Packet

PSE

PSE



Application

Presentation

Session

Transport

Network

Data Link

Physical

PLP

LAPB

x.21 bis, EIA/TIA-232, EIA/TIA-449,

EIA-530, G.703

Other Services

X.25

ProtocolSuite



Several well-known standards are used for X.25 networks X.21bis – supports up to 2 Mbps

15-pin connector

RS-232 (EIA/TIA-232) – supports up to 19.2

Kbps 25-pin connector

RS-449 (EIA/TIA-449) – supports up to 64Kbps 37-pin connector

V.35 – supports up to 2 Mbps 34-pin connector

Uses serial communications in either asynchronous or synchronous modes



Link Access Procedure, Balanced (LAPB) is theprotocol used for this layer

LAPB is a version of HDLC

HDLC in Asynchronous Balanced Mode (ABM)

DTE and DCE are peers and can both perform allfunctions

LAPB manages communication and packetframing between DTE and DCE devices

Makes sure that frames are delivered in sequenceand error-free

Uses sliding window of 8 or 128 frames



Packet Layer Protocol (PLP) is the X.25 networklayer protocol

PLP manages calls between a pair DTE devicesusing a Permanent Virtual Circuit (PVC) or aSwitched Virtual Circuit (SVC)

PLP handles segmentation, reassembly, bitpadding and error and flow control

PLP uses X.121 Addressing Scheme to setup avirtual circuit



Call Setup Used to setup virtual circuit for SVC

Data Transfer Used for transferring data with both SVC and PVC

Idle Used when SVC call has been established but no data

is currently being transferred

Call Clearing Used to end communication between DTEs for a SVC

Restarting Used to synchronize DTE and DCE for all virtual circuits

that exist between them



Call

Setup

Phase

Data

Transfer

Phase

Call

Clearing

Phase

DTE to DCE

Interface

DCE to DTE

Interface

Call Request

Call Connected

Data Packet

Incoming Data

Clear Request

Clear Confirm

Incoming Call

Call Accepted

Incoming Data

Data Packet

Clear Indication

Clear Response



Frame Relay was originally designed for use onIntegrated Services Digital Network (ISDN)

Usually considered a replacement for X.25 usingmore advanced digital and fiber optic connections

Does not perform error correction at intermediatenodes making it faster than X.25

When an error is detected (FCS) the frame is discardedand correction is left up to higher layer protocols

Original standard proposed in 1984 but widespread

acceptance did not occur until the late 1980’s



Data Terminal Equipment (DTE) Terminals, Personal Computers, routers, and bridges

typically at the customer location

Data Circuit-terminating Equipment (DCE)

Typically packet switches owned by the carrier thattransmit data through the WAN



Frame Relay

WAN

Personal Computer

DTETerminal

DTE

Network Host

DTE

Packet Switch

DCE

Packet Switch

DCE

Packet Switch

DCE

Packet Switch

DCE



To handle frames from other protocols a FRAD isused to provide conversion to Frame Relaypackets

A FRAD can either be a separate device or part ofa router/switch

Frame RelayFRAD FRAD



Application

Presentation

Session

Transport

Network

Data Link

Physical

LAPF

Other Services

FrameRelay

ProtocolAny Standard



No specific protocol is defined Any protocol recognized by ANSI can be

implemented



Link Access Protocol for Frame Modes Services(LAPF) is the protocol defined for Frame RelayLayer 2 services

LAPF is a version of HDLC

Does not provide flow or error control Uses Address field for DLCI (addressing) as well as for

congestion control



Flag FlagAddress Information FCS

DLCI: (10 bits) Data Link Connection Identifier is used to identify the Virtual

Circuit number

C/R: (1 bit) Provided for up layers to determine commands and responses

EA: (1 bit) Determines if this byte is last byte of address (0=more, 1=last)FECN: (1 bit) Forward Explicit Congestion Notification indicates congestion in

the direction the frame is traveling

BECN: (1 bit) Backward Explicit Congestion Notification indicates congestion

in the opposite direction the frame is traveling

DE: (1 bit) Discard Eligibility indicates that a frame is low priority when set

C/R EA DLCIDLCI EADEBECNFECN



To increase the number of virtual circuits theDLCI can be expanded from 10 bits to 16 bits and23 bits

The EA field is set to 0 to indicate that additional

address bytes are present. The last address bytewill have a 1 in the EA field



C/R 0

DLCI

DLCI

1DEBECNFECN

C/R 0

DLCI

DLCI

0DEBECNFECN

DLCI 10

C/R 0

DLCI

DLCI

0DEBECNFECN

DLCI 0

DLCI 10

Two-byte Address

(10 bit DLCI)

Three-byte Address

(16 bit DLCI)

Four-byte Address

(23 bit DLCI)



Two Byte Address Format0 In-channel signaling

1-15 Reserved

16-991 Assigned using Frame Relay connection procedures

992-1001 Layer 2 management of Frame Relay service1002-1022 Reserved

1023 In channel layer management

Three Byte Address Format0 In-channel signaling

1-1023 Reserved1024-63,487 Assigned using Frame Relay connection procedures

63,488-64,511 Layer 2 management of Frame Relay service

64,512-65,534 Reserved

65,535 In channel layer management

Four Byte Address Format0 In-channel signaling

1-131,071 Reserved

131,072-8,126,463 Assigned using Frame Relay connection procedures

8,126,464-8,257,535 Layer 2 management of Frame Relay service

8,257,536-8,388,606 Reserved

8,288,607 In channel layer management



Original Frame Relay standard only covered PVC

SVC support was added but does not have widespreadimplementation

PVC States Data Transfer – data is being transmitted between DTE devices

Idle – connection is still active but no data is being transferred

SVC required the addition of two additional states Call Setup – virtual circuit between DTE devices is established

Call Termination – virtual circuit between DTE devices isterminated



Because of the shared resources of a virtual circuit,congestion can cause the loss of packets as buffersbecome full

Frame Relay defines a congestion control mechanismusing the FECN and BECN bits in the address field

When a switch determines that congestion has occurredit will set the FECN bit on packets traveling in thedirection of the congestion to alert the receiver to slowdown requests for data. The BECN bit will be set for packets going in the opposite direction of thecongestion to let the sender know to send data more

slowly The FECN and BECN bits will allow higher layer protocols

to manage flow. Discard Eligible bit is used to identify frames that are low

priority and can be discarded in the event of congestion



LMI is a set of extensions to Frame Relaydeveloped in 1990 by Cisco Systems, StrataCom,Northern Telecom, and Digital EquipmentCorporation

LMI provides global addressing which allowsadditional management capability such asstandard address resolution and discovery

LMI allows status messages to be passedbetween DCE and DTE devices to providecommunication and synchronization (uses DLCI1023 on a 2-byte address)

LMI specifies multicast capability to allowcreation of multicast groups to limit bandwidth

use



X.25 and Frame Relay

X.25 Frame Relay

Layer 1 Specification Yes None

Layer 2 Protocol Family HDLC HDLC

Layer 3 Support PLP None

Error Correction Node to Node None

Propagation Delay High Low

Ease of Implementation Difficult Easy

Good for Interactive Applications Too Slow Yes

Good for Voice No Yes

Good for LAN File Transfer Slow Yes



Many X.25 networks have been replaced byFrame Relay or X.25 over Frame Relay Networks

X.25 still in use for low bandwidth applicationssuch as credit card verification

It is likely that ATM Networks will ultimatelyreplace Frame Relay and X.25 Networks



8.2

Frame relay is faster thanthe X.52 protocol. Framerelay networks connectingLANs to a WAN rely onrouters and switchingequipment.

frame relay A newer form of

packet switching that relies

on digital technologies, such

as fiber optic and ISDN.

Frame Relay




Two approaches for the VC setup Permanent virtual circuit (PVC):

Switched virtual circuit (SVC): setup, data transfer,teardown



Frame Relay is a virtual circuit wide area network VCIs in Frame Relay are called DLCIs(Data Link Connection

Identifier)



Frame relay operates at a higher speed. It can easily be usedinstead of a mesh of T-1 or T-3 lines (1.544 Mbps or 44.376 Mbps)

Frame relay operates just the physical and data link layers. It isgood as a backbone to provide services to protocols that alreadyhave a network layer protocol, such as Internet

It allows bursty data

It allows a frame size of 9000 bytes accommodating all LAN framesizes

It is less expensive than other traditional WANs

It has error detection at the data link layer only. There is no flowcontrol pr error control

X.25 Leased Lines Frame Relay



Frame relay operates only at the physical anddata link layers



Frame relay requires congestion control, because Frame Relay does not have a network layer

No flow control at the data link layer

Frame Relay allows the user to transmit bursty data

Congestion avoidance

Two bits in the frame are used BECN(Backward Explicit Congestion Notification)

FECN(Forward Explicit Congestion Notification)

Discard eligibility(DE): Priority level of the frame for traffic control

Discarding frame to avoid the congestion or collapsing



FRAD



Asynchronous Transfer Mode

ATM is the cell relay protocol designed by ATM forum andadopted by ITU-T

ATM uses asynchronous TDM

Cells are transmitted along virtual circuits

Design Goals Large bandwidth and less susceptible to noise degradation

Interface with existing systems without lowering theireffectiveness

Inexpensive implementation

Support the existing telecommunications hierarchies

Connection-oriented to ensure accurate and predictabledelivery

Many functions are hardware implementable



Multi-speed network environment that providesa variety of complex network services

Can carry voice, data, video separately or simultaneously

Can be used in LANs, MANs, or WANs Fixed-lenth packets (cells)

Allows multiple logical connections to bemultiplexed

Minimal error and flow control capabilities Connection-oriented virtual channel



Similar to frame relay Difference?

Frame relay switches variable length frames withinframe relay cloud from source to destination

ATM switches fixed-length cells (48 byte information

field, 5 byte header) Based on packet switching (connection-oriented)

Cell sequence integrity preserved via virtual channel

VCC – virtual channel connection – is set up betweenend users, variable rate, full duplex

VCC also used for control

Information field is carried transparently through thenetwork, with minimal error control



So far, ATM has been implemented in : PC, workstation, and server network interface cards

Switched-Ethernet and token-ring workgroup hubs

ATM enterprise network switches

ATM multiplexers

ATM-edge switches

ATM-backbone switches

72



Main features of ATM

Service is connection oriented, with data transferred over aVC

A cell-switched network (architecture).

Fixed-size cell (53-Bytes) Uses Asynchronous time-division multiplexing

(Asynchronous TDM)

The Quantity of Service (QofS) enable carriers to transmitvoice, data, and video.

ATM is independent of the transmission medium. ATM cellscan be sent on a wire or fiber, and can also be packagedinside the payload of other carrier system.

73



Advantage:

Transmitted with predictability and uniformity.

Easy to be multiplexed with other cells, and routed through thecell network.

With high speed of the links, small and fixed-size cells seem toarrive their respective destinations in an approximation of

continuous stream, despite interleaving. E.g. phone call.

Simpler buffer hardware, avoiding memory fragmentationproblem

Simpler cells scheduling:

- Easier to allocate different bandwidths and delays to different

VCs.- Easier to implement priority

- Fixed sized can be switched in parallel in synchronous fashion.

It’s suitable for time-critical information such as voice or video

Quicker recovery in case of circuit failure.74



Disadvantage: Processing overhead as messages are segmented

into cells

Segmentation mismatch, as the last cell in afragmented message may not be fully used. Thiseffect will decrease as the message lengthincreases.

75



The variety of packet sizes makes trafficunpredictable A cell network uses the cell as thebasic unit of data exchange A cell is defined as a small, fixed sized block of

information

Cells are interleaved so that non suffers a long delay

A cell network can handle real-time transmissions

Network operation is more efficient and cheaper



UNI: user-to-network interface

NNI: network-to-network interface



Connection between two endpoints isaccomplished through Transmission path (TP)

Virtual path (VP)

Virtual circuit (VC)

A virtual connection is defined by a pair ofnumbers: VPI and VCI



84

ATM Layers

ATM Technology

Reference Model



85

Reference Model

Control

responsible for generating and managing signaling request (connection

management).

User

deals with data transport, flow control, error correction, and other user

functions.

Layer Management :

manages layer-specific functions (detection of failures and protocol problems)

Plane Management:

manages and coordinates functions related to the complete system.

ATM Technology

Reference Model



86

Physical Medium-Dependent (PMD) – having two functions:

Synchronizes transmission and reception by sending and

receiving a continuous flow of bits with associated timing info.

Specifies the physical media for the physical medium used,

including connector type and cable.

ATM Technology

Reference Model



87

Transmission Convergence (TC) – having four functions:

Cell delineation, generating cell boundaries.

Header error control (HEC) sequence generation and verification

Cell-rate decoupling, maintaining synchronization and inserting or suppressing idle ATM cells to rate of valid ATM cells to the payload

capacity of transmission system.

Transmission frame adaptation, packaging cells into frame acceptable

to the particular physical layer implementation.

ATM Technology

Reference Model



88

ATM Layer

Provides

Defining cells layout

Defining header

Routing

Est and release VC.

Switching

Multiplexing

Congestion control.

0



ATM Technology

Reference Model



90



ATM Adaptation Layer (AAL) Enables ATM to accept any type of payload, both data

frames and streams of bits

Fragments data from upper-layer into 48-byte dataunits for the ATM cells

Reassembles Cells Convergence sub layer (CS): Before the payload is

segmented CS prepares data to ensure their integrity, providing standard interface.

Segmentation and Reassembly (SAR): Segmentsthe payload into 48-byte cells, and at thedestination, reassemble them to recreate theoriginal payload.



ATM defines four versions of the AAL:

AAL1: Support Constant-bit-rate data (CBR) fromupper layer; video and voice.

AAL2: Used for low-bit-rate and short-frame trafficsuch as audio (compressed or uncompressed), video,

or fax. AAL2 allows the multiplexing of short framesinto one cell.

AAL3/4: support connection-oriented and connectionless data services

AAL5: –SEAL (Simple and Efficient Adaptation Layer)No sequencing and error control mechanismsAssumes that all cells belonging to a single messagetravel sequentially and that control functions areincluded in the layers of the sending application.



Frame relay uses variable length frames ATM fixed length cells

ATM has higher overhead, but faster speed andtraffic management (better suited for video and

voice)



SONET is a transport mechanism, transportingdata over fiber.

Can act as a transport carrier for ATM (or FDDI, or ISDN, etc.)

ATM is a technology and protocol designed touse SONET as its carrier service



Replace “network” (e.g., LAN segment) with

ATM network

ATM addresses, IP addresses



AALATMphyphy

Eth

IP

ATMphy

ATM

phy

app

transportIPAALATMphy

apptransportIPEthphy



at Source Host: IP layer maps between IP, ATM dest address

(using ARP)

passes datagram to AAL5

AAL5 encapsulates data, segments cells, passes toATM layer

ATM network: moves cell along VC todestination

at Destination Host: AAL5 reassembles cells into original datagram

if CRC OK, datagram is passed to IP



IP datagrams into ATMAAL5 PDUs

from IP addresses toATM addresses

just like IPaddresses to 802.3MAC addresses!

ATMnetwork

Ethernet

LANs



Minimal error and flow control Reduces overhead of processing ATM cells

Reduces number of required overhead bits

Fixed size simplified processing at each ATMnode (can be switched more efficiently – moreefficient use of router)

Small cells reduce queuing delay

Minimal addressing info on each cell

Efficient traffic management



ATM is mainly a wide-area network (WAN ATM);however, the technology can be adapted to local-area networks (ATM LANs). The high data rate ofthe technology has attracted the attention ofdesigners who are looking for greater and

greater speeds in LANs.



Connectionless versus connection-oriented

Physical addresses versus virtual-circuit identifiers

Multicasting and broadcasting delivery

Interoperability

Client/Server model in a LANE LANE Configuration Server (LECS), LANE Server (LES), LANE

Client (LEC)

Broadcast/Unknown Server (BUS)



Anticipating user demand for end-to-end digitalservices the world’s telephone companiesagreed in 1984 under CC I TT to build a new,fully digital, circuit -switched telephone system.

This system was called I S DN ( IntegratedServices Digital Net work)

and it’s primary goal was to integrate the voiceand non-voice services .



ISDN incorporates all communication connections

in a home or building into a single interface

With ISDN all customer services will become digital

rather than analog.



Buttons for instant call set up t o arbitrarytelephones anywhere in the world.

Displaying the caller’s telephone number, nameand address while ringing.

Connecting the telephone to a computer enablingthe caller ’s database record to be displayed onthe screen as the call comes in.

Call forwarding.

Conference calls worldwide. On line medical, burglar, and smoke alarms giving

the address to speed up response.



A Digital bit pipe between the customer and thecarrier through which bits flow in both directions.

The digital bit pipe can support multipleindependent channels by time division multiplexingof the bit stream.

Two principal standards for the bit pipe havebeen developed: A low bandwidth standard for home use.

A higher bandwidth standard for business use that

support s multiple channels identical to the home usechannels (multiple bit- pipes ).



U = connection between t he I S DN exchangeand NT1

T = connector on NT1 to the customer ,

S = interface between the I S DN PBX and theISDN terminal,

R = the connection between the terminaladapter and non-I S DN terminal.



The ISDN bit pipe supports multiple channels

interleaved by time division multiplexing.Several channel types have beenstandardized:

A - 4 kHz analog telephone channel

B - 64 kbps digital PCM channel for voice or data

C - 8 or 16 kbps digital channel for out -of -bandsignalling

D - 16 kbps digital channel for out -of -bandsignalling

E - 64 kbps digital channel for internal I S DNsignalling

H - 384, 1536, or 1920 kbps digital channel.



It is not allowed to make arbitrary combination ofchannels on the digital pipe.

Three combinations have been standardized so far: Basic rate: 2B + 1D. I t should be viewed as a

replacement f or P OTS ( Plain Old Telephone Service) .

Each of the 64 kbps B channels can handle a singlePCM voice channel with 8 bits samples made 8000times per second. D channel is for signaling ( i.e. , t oinform t he local I S DN exchange of the address of thedestination) . The separate channel for signaling results

in a significantly faster set up time. N-ISDN Primary rate: 23B + 1D ( US and Japan) or 30B + 1D

(Europe) . It is intended for use at the T referencepoint for businesses with a PBX.

Hybrid: 1A + 1C



Basic

B

B

D

Primary

B

B

D

B

Basic Service:Management rate: 192 kbps

Standard throughput: 144 kbps

Composition: B + B + D

channels, + Synch & framing

Information:Voice, Data

Signaling:Overhead or

telemetry, etc.

PCM voice

channels

Signaling



Basic Rate Interface (BRI): Used for homeand small office connectivity.

BRI services include two B channels and asingle D channel.

A B channel offers 64 Kbps and carriesuser data. A BRI D channel operates at 16Kbps and carries control and signalinginformation.

Through these two channels, a homeconnection can reach 128 Kbps of datathroughput.



Primary Rate Interface (PRI): Used for WANs andruns across leased lines.

The PRI service is composed of 23 B channels at64 Kbps each for user data, along with a single Dchannel, also operating at 64 Kbps to handle

control information.

Overall, the PRI service provides a throughputrate of 1.544 Mbps.



A computer with an ISDN line is able to connectto any other computer that also uses ISDN simplyby dialing its ISDN number.

Terminal adapter (TA): Also called an ISDNmodem, this is either an internal or external

adapter to connect equipment to an ISDN line.



Digital

reliable connection

Speed 128 kb/s (160 kb/s) for BRI

1920 kb/s (2048 kb/s) for PRI

Fast call setup 2 seconds

Bandwidth on Demand adding new channels to the bundle of channels

Multiple devices phone, fax, PC, videoconferencing system, router,

terminal adapter,.. each with its own sub-address



N-ISDN was an attempt to replace the analogtelephone system with a digital one.

N-ISDN basic rate is too low so for home as for business today.

N-ISDN may be partly saved, but by an unexpectedapplication: Internet access .

Various companies now sell ISDN adapters thatcombine the 2B+D channels into a single 144 kbpsdigital channel.

Many Internet providers also support theseadapters . So the people can access Internet over a144 kbps digital link, instead of a 28.8 kbps



When CCITT found that the N-ISDN was not goingto solve the actual communication problems , ittried to think of a new service.

The result was broadband ISDN ( B-ISDN) , basicallya digital virtual circuit for moving fixed-sized

packets (cells ) at 155 Mbps.

Broadband ISDN is based on ATM technology thatis fundamentally a packet -switching technology.

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