7/27/2019 C13_ISDN

1/55

ISDNIntegrated Services Digital Network

Chapter 13

7/27/2019 C13_ISDN

2/55

What is ISDN?

Can be used to transport voice, data, orvideo.

ISDN uses a system of digital phoneconnections for communication and datatransfer. Been available for over a decade

Allows data to be transmittedsimultaneously across the world using end-to-end digital connectivity

7/27/2019 C13_ISDN

3/55

Overview

ISDN uses different channels to sendand receive information. B-Channel(s) is used to send and receive

information. D-Channel is used for signaling purposes

only. Two types of service

Basic Rate Interface (BRI) Primary Rate Interface (PRI)

7/27/2019 C13_ISDN

4/55

Types of ISDN Service

Basic Rate Interface (BRI) Consists of 2 64 kbps B-Channels and one 16 kbps

D-Channel, for a total of 144 kbps

Intended to meet the needs of most customers Primary Rate Interface (PRI)

Consists of 23 B-Channels and one 64 kbps D-Channel, for a total of 1536 kbps

Intended for users with greater capacity needs

7/27/2019 C13_ISDN

5/55

Types of ISDN Service

7/27/2019 C13_ISDN

6/55

BRI

BRI service at an office or a home, ISDNline consists of the same twisted pair ofwires used for analog phones.

Up to 8 ISDN devices can connect to thissingle line.

Devices are distinguished through theuse of multiple subscriber numbers.

7/27/2019 C13_ISDN

7/55

BRI

7/27/2019 C13_ISDN

8/55

BRI

7/27/2019 C13_ISDN

9/55

BRI

7/27/2019 C13_ISDN

10/55

Whats Needed for BRI

ISDN? Subscribe to an ISDN phone line Must be within 5.5Km or about 3.4 miles

of the CO for BRI service. Must purchase ISDN Terminal Adapters

(Sometimes incorrectly called ISDNModems), and ISDN routers.

7/27/2019 C13_ISDN

11/55

PRI

Includes 23 B-channels in North America andJapan, 30 in most of the rest of the world. The number is limited due to the T-1.

Still only one D-channel, but its bandwidth isincreased to 64 kbps.

Only one device can be connected to a PRIline PBX, however, can reallocate the ISDN PRI

resources onto multiple BRI buses.

7/27/2019 C13_ISDN

12/55

PRI

A single PRI connection is usually muchless expensive than multiple BRIconnections.

Main benefit of PRI is that its bandwidthcan be dynamically allocated amongapplications.

Example, certain channels can be allocatedfor voice channels, can be reallocated forvideoconferencing.

7/27/2019 C13_ISDN

13/55

PRI

7/27/2019 C13_ISDN

14/55

PRI

7/27/2019 C13_ISDN

15/55

B-Channel

These are logical pipes in a singleISDN line.

Two can be combined to download dataat high speeds, or one can be freed upfor other devices such as a telephone orfax machine.

The B-Channels 64 kbps uses its entirebandwidth for data.

7/27/2019 C13_ISDN

16/55

B-Channel

Four types of connections can be set up. Circuit-Switched

Packet-Switched uses X.25

Frame Mode uses LAPF

Semipermanent Leased Line

Can be used to transport any layer 2 orhigher protocol.

7/27/2019 C13_ISDN

17/55

B-Channel

Connections

(Circuit-switched B-Channel Connections)

(Packet-switched B-Channel Connections)

7/27/2019 C13_ISDN

18/55

D-Channel

Used for administrative signaling Instruct carrier to setup or terminate a B-

Channel.

Insure that a B-Channel is available toreceive a call.

Provide signaling information for such

features as Caller ID.

7/27/2019 C13_ISDN

19/55

D-Channel

Uses packet-switched connections Associated with protocols at layer 2

(Q.921) and 3 (Q.931).

7/27/2019 C13_ISDN

20/55

D-Channel

7/27/2019 C13_ISDN

21/55

ISDN and the OSI Model

ISDN deals with the bottom 3 layers ofthe OSI model.

7/27/2019 C13_ISDN

22/55

Layer 1 Devices

Terminal Equipment 1 (TE1) Device thats compatible with ISDN.

Terminal Equipment 2 (TE2) Device thats not compatible with

ISDN.

7/27/2019 C13_ISDN

23/55

Layer 1 Devices

Network Termination 1 (NT1) Connects 4 wire ISDN wiring to the

conventional 2 wire local loop facility. Network Termination 2 (NT2)

Intelligent device that performs

switching among the attached devicesto the NT1.

7/27/2019 C13_ISDN

24/55

Layer 1 Devices

7/27/2019 C13_ISDN

25/55

Layer 1

The ISDN Physical Layer is specified bythe ITU I-series and G-seriesdocuments.

The U interface provided by the Telco forBRI is a 2-wire, 160 kbps digitalconnection.

7/27/2019 C13_ISDN

26/55

Layer 1

U Interface Each U interface frame is 240 bits long. At the prescribed data rate of 160 kbps,

each frame is therefore 1.5 msec long. Each frame consists of:

Frame overhead - 16 kbps

D channel - 16 kbps

2 B channels at 64 kbps - 128 kbps

7/27/2019 C13_ISDN

27/55

Layer 1

U Interface

Sync field 9 Quaternaries (2 bits each)

pattern +3 +3 -3 -3 -3 +3 -3 +3 -3.

(B1

+ B2

+ D)

18 bits of data consisting 8 bits first B channel 8 bits second B channel 2 bits D channel data.

Sync 12 * (B1 + B2 + D) Maintenance

7/27/2019 C13_ISDN

28/55

Layer 1

U Interface Maintenance CRC information

block error detection flags

embedded operator commands

loop back testing without disrupting userdata.

Data is transmitted in a superframe 8 240-bit frames (1920 bits = 240 octets)

Sync field of first frame is inverted.

7/27/2019 C13_ISDN

29/55

Layer 1

S/T Interface T interface: Serial interface

S interface Four-wire bus that ISDN consumer devices

plug into

S & T reference points are commonlyimplemented as a single interface la 'S/T'on an NT1

7/27/2019 C13_ISDN

30/55

Layer 1

R Interface Defines the point between a non-ISDN

device (TE2) and a terminal adapter (TA)

which provides translation to and fromsuch a device.

7/27/2019 C13_ISDN

31/55

Layer 1

Diagram

7/27/2019 C13_ISDN

32/55

Layer 2 - Q.921

Use only the D-Channel. Establishes and maintains

communication between the ISDNterminal and the ISDN switch. Uses LAPD.

7/27/2019 C13_ISDN

33/55

LAPD and ISDN

LAPD gives low-level signaling to ISDNdevices.

Frame's information ensures thatincoming calls are routed to theappropriate ISDN device.

7/27/2019 C13_ISDN

34/55

LAPD and ISDN

Routing information: Terminal Endpoint Identifier (TEI)

Service Access Point Identifier (SAPI)

7/27/2019 C13_ISDN

35/55

TEI - Terminal Endpoint Identifiers

Unique identifier of device (TE) on anISDN S/T bus.

Given by the switch Assignation

statically when the TE is installed

dynamically when activated.

7/27/2019 C13_ISDN

36/55

TEI - Terminal Endpoint Identifiers

TEI Description

0-63 Fixed TEI assignments

64-126 Dynamic TEI assignment (assigned by the switch)

127 Broadcast to all devices

7/27/2019 C13_ISDN

37/55

SAPI - Service Access Point Identifier

Identifies the point where Layer 2provides a service to Layer 3.

SAPI Description

0 Call control procedures

1 Packet Mode using Q.931 call procedures

16 Packet Mode communications procedures

32-47 Reserved for national use

63 Management Procedures

Others Reserved for Future Use

7/27/2019 C13_ISDN

38/55

LAPD Frame

FLAG7E

ADDRESS CONTROL INFORMATON CRCFLAG

7E

7/27/2019 C13_ISDN

39/55

LAPD Frame

SAPI (Service access pointidentifier),6-bits

C/R (Command/Response)bitindicates if the frame is acommand or a response

EA0 (Address Extension) bitindicates whether this is thefinal octet of the address or not

TEI (Terminal EndpointIdentifier) 7-bit device identifier

EA1 (Address Extension) bit,same as EA0

Address (2 octets)

1 2 3 4 5 6 7 8

SAPI (6 bits) /E

0

TEI (7 bits)

E

1

7/27/2019 C13_ISDN

40/55

LAPD Frame

Control (2 octets) The frame level control

Frame type

Information Supervisory Unnumbered

Sequence numbers N(r) N(s)

7/27/2019 C13_ISDN

41/55

LAPD Frame

Information Layer 3 protocol information

User data

CRC (2 octets) Cyclic Redundancy Check

Low-level test for bit errors on the userdata.

7/27/2019 C13_ISDN

42/55

Establishing the Link Layer The TE (Terminal Endpoint) and the Network exchange

Receive Ready (RR) frames, listening for someone toinitiate a connection

The TE sends an Unnumbered Information (UI) framewith a SAPI of 63 (management procedure, querynetwork) and TEI of 127 (broadcast)

The Network assigns an available TEI (in the range 64-126)

7/27/2019 C13_ISDN

43/55

Establishing the Link Layer

Now the Layer 2 establishment process is very similarto the LAP-B setup (X.25).

The TE sends a Set Asynchronous BalancedMode (SABME) frame with a SAPI of 0 (call

control, used to initiate a SETUP) and a TEIof the value assigned by the network

The network responds with an UnnumberedAcknowledgement (UA), SAPI=0,TEI=assigned.

At this point, the connection is ready for a Layer 3setup.

7/27/2019 C13_ISDN

44/55

Layer 3

Layer 3 signaling is based on the DigitalSignaling System (Q.931)

D-channel uses link layer protocol to

transmit messages (Q.931) Responsible for establishing,

maintaining, and releasing connections

in an ISDN network

7/27/2019 C13_ISDN

45/55

Layer 3

User data is mapped onto B-channels Q.931 signaling is mapped onto D-

channel

7/27/2019 C13_ISDN

46/55

Layer 3

Service Profiles Identifies and characterizes the servicethat is offered

Is carried in a layer 3 Q931 message Service Profile ID (SPID)

User service ID (USID)

7/27/2019 C13_ISDN

47/55

Layer 3Service Profile ID

Identifies services and features of an ISDNcircuit.

Optional Accessed only at call setup time. Format:

10-digit phone number of ISDN line

Prefix and Suffix: Features on the line. Assigned by a phone company (provider) to a

terminal on B-channel.

7/27/2019 C13_ISDN

48/55

Layer 3Service Profile ID

If an ISDN line requires a SPID, but it isnot correctly supplied:

Layer 2 initialization take place

Layer 3 do not initialize

Device unable to place or accept calls.

7/27/2019 C13_ISDN

49/55

Layer 3

Q.931 SpecificationInformation Field1 2 3 4 5 6 7 8

Protocol Discriminator

0 0 0 0 Length of CRV

Call ReferenceValue (1 or 2 octets)

0 Message Type

Mandatory & OptionalInformation Elements (variable)

7/27/2019 C13_ISDN

50/55

Layer 3

Q.931 Specification Messages all contain 3 common fields:

Protocol discriminator Used to distinguish

messages for user-network call control fromother message types. Call reference Identifies the user-channel

call to which this message refers.

Message type Identifies which I-451message is being sent.

7/27/2019 C13_ISDN

51/55

Layer 3

Call Setup Points where messages are sent and

received:

The Caller The ISDN Switch

The Receiver.

7/27/2019 C13_ISDN

52/55

Layer 3

Call SetupCaller sends a SETUP to the Switch.

If the SETUP is OK, the switch sends a CALLPROCEEDING to the Caller, and then a SETUP tothe Receiver.

The Receiver gets the SETUP. If it is OK, then it ringsthe phone and sends an ALERTING message to theSwitch.

The Switch forwards the ALERTING message to theCaller.

7/27/2019 C13_ISDN

53/55

Layer 3

Call SetupWhen the receiver answers the call, is sends aCONNECT message to the Switch

The Switch forwards the CONNECT message to the

Caller.The Caller sends a CONNECT ACK message to theSwitch

The Switch forwards the CONNECT ACK message to

the Receiver.Done. The connection is now up.

7/27/2019 C13_ISDN

54/55

Layer 3

Call Setup

7/27/2019 C13_ISDN

55/55

Summary

Types of ISDN Service BRI

PRI

Types of Channels B-Channel

D-Channel

ISDN works on Layers 1, 2 and 3 Layer 2 and Layer 3 setup