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Information

Subscriber Administration

Digital Subscriber Signaling SystemNo.1 (topic 9)

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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration

Copyright (C) Siemens AG 1995

Issued by the Public Communication Network Group

Hofmannstrae 51

D-81359 Mnchen

Technical modifications possible.

Technical specifications and features are binding only insofar as

they are specifically and expressly agreed upon in a written contract.

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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)

This document consists of a total of 40 pages. All pages are issue 1.

Contents

1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 User-Network Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Reference Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Basic Access and Primary Rate Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 Functional Groups. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 OSI Reference Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 The Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Communication Between the Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 Protocol Architecture in the B and D Channels. . . . . . . . . . . . . . . . . . . . . . 13

4 Physical Layer (Layer 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1 Basic Access. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.1.1 Reference Point S/T Between Terminal Equipment and the Network Termina-tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.1.2 Reference Point U Between the Network Termination and the Exchange . 17

4.2 Primary Rate Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2.1 2048-kbit/s Primary Rate Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2.2 1544-kbit/s Primary Rate Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5 Data Link Layer (Layer 2 of DSS1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1 Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.2 Layer 2 Addressing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.3 Commands and Responses and their Functions . . . . . . . . . . . . . . . . . . . . 26

5.4 Assignment of the Terminal Endpoint Identifier . . . . . . . . . . . . . . . . . . . . . 28

6 Network Layer (Layer 3 of DSS1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6.1 Message Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6.2 Use of Layer 3 Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7 Example of a Complete DSS1 Message. . . . . . . . . . . . . . . . . . . . . . . . . . . 37

8 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

IllustrationsFig. 1.1 Signaling in ISDN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Fig. 1.2 ISDN subscriber line types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Fig. 2.1 Reference configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Fig. 2.2 ISDN basic access with more than one terminal device (in this case eight)

8

Fig. 2.3 Examples of connections between ISDN subscribers and an exchange via

basic access (2 B+D) and primary rate access (30 B+D or 23 B+D). . . . 9

Fig. 3.1 The seven layers of the OSI reference model . . . . . . . . . . . . . . . . . . . . 11

Fig. 3.2 Communication between entities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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Fig. 3.3 Forwarding signaling information via the D channel (in this case from a

terminal to the exchange) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Fig. 3.4 Protocol architecture for the transfer of user information on the B channel

14Fig. 3.5 Protocol architecture for the transfer of signaling information on the D

channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Fig. 3.6 Example of a protocol architecture in ISDN terminals . . . . . . . . . . . . . . 15

Fig. 4.1 Basic access pulse frame structure between TE and NT with possible

pseudo-ternary values for the individual bits. . . . . . . . . . . . . . . . . . . . . . 17

Fig. 4.2 Pulse frame structure of the 2048-kbit/s primary rate access. . . . . . . . . 18

Fig. 4.3 Pulse frame structure of the 1544-kbit/s primary rate access. . . . . . . . . 19

Fig. 5.1 Layer 2 frame structure with and without an information field. . . . . . . . . 21

Fig. 5.2 Address field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Fig. 5.3 Control field formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Fig. 5.4 Example of accessing terminals with a layer 2 address (SAPI+TEI) from

the exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Fig. 5.5 Handling the sequence numbers for acknowledged information transfer27

Fig. 5.6 Assignment of a TEI or denial of assignment . . . . . . . . . . . . . . . . . . . . . 28

Fig. 6.1 Structure of a DSS1 message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Fig. 6.2 Call reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Fig. 6.3 Information elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Fig. 6.4 Example of an information element with the called party number . . . . . 35

Fig. 6.5 Connection setup (principle for digit selection) . . . . . . . . . . . . . . . . . . . . 36

Fig. 7.1 A complete message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Tables

Tab. 5.1 Meaning of the command/response bit. . . . . . . . . . . . . . . . . . . . . . . . . . 22

Tab. 5.2 Defined information classes of the service access point identifiers . . . . 22

Tab. 5.3 Defined applications of the terminal endpoint identifiers. . . . . . . . . . . . . 23

Tab. 5.4 Commands and responses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Tab. 5.5 Tasks of the commands and responses of the S frames . . . . . . . . . . . . 27

Tab. 5.6 Tasks of the SABME and DISC commands and of the UA and DM

responses of the U frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Tab. 6.1 Protocol discriminator codes and their meanings . . . . . . . . . . . . . . . . . . 31

Tab. 6.2 Codes for the message types for call setup, call cleardown and miscella-

neous messages as defined in CCITT Recommendation Q.391.. . . . . . 32

Tab. 6.3 Codes for the message types for supplementary services as defined in

CCITT Recommendation Q.932 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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1 IntroductionIn view of the wide range of services and features offered in the integrated services

digital network (ISDN), a very high-capacity signaling system, more so than those used

for conventional communication systems, is needed to handle signaling functions

between the exchanges and

between the exchanges and the terminal equipment/telecommunication systems

The International Telegraph and Telephone Consultative Committee (CCITT) has spec-

ified two systems (see Fig. 1.1) for transmitting the signaling information (control infor-

mation) in the ISDN:

common channel signaling system no. 7 (CCS7) for use between exchanges (see

topic 8)

digital subscriber signaling system no.1 (DSS1, previously known as the D channel

protocol) for use between exchanges and the terminal equipment; this system is the

subject of this document.

Fig. 1.1 Signaling in ISDN

For certain supplementary services these two systems also permit end-to-end signaling

between subscriber terminal equipment. This topic document is based on the relevant

CCITT Recommendations (Blue Book).

In order to ensure that, say, speech and signaling information or text and signaling infor-

mation can be transmitted simultaneously in ISDN with no mutual interference, separate

channels are provided, known as the B channels and the D channel. The B channels

carry speech, text, data and images whereas the D channel is used exclusively for

signaling.

CCITT has specified two types of ISDN access line (Fig. 1.2):

the basic access

a basic access has two B channels and one D channel

the primary rate access for PABXs;

a primary rate access has 30 or 23 B channels and one D channel

PABXExchangeExchange

Exchange

DSS1 CCS7 DSS1

Terminal equipment

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Fig. 1.2 ISDN subscriber line types

The separate D channel has a very high capacity and is permanently available. Its trans-

mission capacity is such that it can handle not only the signaling information but also low

transfer rate data such as packet data, telemetry data and user-user information. Priority

is always given to signaling information.

The D channel transfers signaling information and data irrespective of the busy/idle

status of the B channels, which means, for example, that a subscriber can be simulta-

neously sending a fax on one B channel and making a telephone call on the other B

channel while the directory number of a calling subscriber is displayed on his terminal.

This directory number is transmitted via the D channel. Access to the D channel is

assured at all times from all terminal equipment and from the exchange.

The principal features of DSS1 are as follows: internationally standardized

very high integrity and flexibility

suitable for all communication services (telephony, facsimile, teletex, data transfer

etc.)

short reaction times

future-proof to accommodate new requirements.

For communication between terminal equipment and between terminal equipment and

the exchanges, DSS1 is based on

defined characteristics of the user-network interface (Section 2)

the Open System Interconnection (OSI) reference model (Section 3).

B 64 kbit/s speech, text, data, images


D 16 kbit/s signaling, low transfer rate data

a) Basic access



D 64 kbit/s signaling, low transfer rate data



b) Primary rate access

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2 User-Network Interface

2.1 Reference ConfigurationOne of the principal requirements that have to be met before ISDN can be introduced is

that the system must be digital up to and including the terminal equipment. For the

subscriber access line the CCITT has defined functional groups with intermediate refer-

ence points (see Fig. 2.1). This arrangement defines the tasks that the individual

subscriber and exchange functional groups have to perform so that compatibility is

assured between different exchanges and terminal equipment

Fig. 2.1 Reference configuration

CCITT defines the user-network interface both for the basic access and for the primary

rate access. These specifications are based on the OSI reference model (Section 3).

They relate not only to the physical characteristics of such interfaces but also, for

example, to access options and protocols. In particular, the defined interfaces guarantee

the following:

use of different terminal equipment for different services

portability of terminal equipment

independent development of the technologies, configurations and installations for

terminal equipment and networks

cost-effective connection to specialized storage and data processing media and to

other networks.The user-network interface can cover reference points S and T, as follows:

if a type 2 network termination (NT2, e.g. a PABX) is installed, the user-network

interface is at reference point T

if there is no NT2 installed, reference points S and T coincide (reference point S/T)

and the user-network interface is then effectively at reference point S.

This means that the presence or absence of an NT2 determines the location of the user-

network interface; generally speaking this will be the same user-network interface in

both cases.

CCITT does not specify the interface at reference point U but does represent the

connection between network termination 1 (NT1) and the exchange as a digital trans-

mission system (CCITT Recommendation G.961).

Referencepoints

Terminal equipment

TE2

Functional groups

NT2 ET

TA

or

TE1

NT1 LT

R S T U V

Network termination equipmentExchangeequipment

Accessline

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2.2 Basic Access and Primary Rate Access

The basic accesswith the 2 B+D channel structure is used for linking one or more

terminal devices to an exchange. A single device port requires a point-to-point connec-

tion, whereas a multiple device port requires a point-to-multipoint connection. If morethan one terminal device is connected (up to eight devices are normal) the passive bus

at the subscriber equipment is used (Fig. 2.2, reference point S/T). The passive bus

consists of two 2-wire lines, one for each direction of transmission, and is equipped with

sockets (e.g. 16). The terminal devices can be plugged and unplugged on the bus as

required and can be accessed directly with a multiple subscriber number. As far as the

basic access is concerned, conventional two-wire copper subscriber lines can be used

at reference point U for the connection between NT1 and the exchange.

Fig. 2.2 ISDN basic access with more than one terminal device (in this case eight)

The primary rate access with the channel structure 30 or 23 B+D is used for point-to-point connections between medium- to large-size PABXs and the exchange. The trans-

2-wiresubscriberline

ETNT1 LT

U

ISDN terminal equipment

Reference points S/T

R

Terminaladapters

TA a/b

Non-ISDN ter-minal equipment

a/b

Passive bus(two 2-wirelines)

Exchange

TA X.21 bisX.21 bis

TA X.21X.21

TA X.25X.25

Digital telephone

Multiserviceterminal

Fax terminal

Teletex terminal

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mission path via the U interface is provided by two balanced wire pairs of a low-

frequency cable, optical waveguides or radio relay routes.

If a PABX is used the interfaces at reference points S and T may be different. The

subscribers, for example, may be connected to the PABX via basic access (referencepoint S) and the PABX may be linked to the exchange via primary rate access (reference

point T).

Depending on their size, PABXs can be connected to an exchange

via basic and primary rate access (Fig. 2.3)

via basic access only

via primary rate access only

Fig. 2.3 Examples of connections between ISDN subscribers and an exchange via basic access (2 B+D) and

primary rate access (30 B+D or 23 B+D)

2.3 Functional Groups

The functional groups (Fig. 2.1) of an ISDN access are described in brief below:

Direct connection to the exchangewith a single terminal (single device port, a)with more than one terminal via a passive bus (multiple device port, b).

Via remote concentrators (in most cases connection to the exchange is implemented with digital transmission links, c).Via PABXs (d).

a)

Remotedigital

concen-trator

ETNT1 LT

2 B+D

U

Network termination equipmentExchangeequipment

Accesslines

2 B+D

b) ETNT1 LT2 B+D

c)

2 B+D

1)

1)

ETLT

ETLT

d) ETNT1 LT2 B+D

ETNT1 LT30 B+D 2)

TS

PABX

1) Digital transmission link2) 23 B+D also possible

Botha) and

b)

S/T

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ISDN terminal equipment (TE1, terminal equipment type 1)

A TE1 is equipped with the protocols relevant to the interface at reference point S and

can be connected directly to the passive bus.

Non-ISDN terminal equipment (TE2, terminal equipment type 2)In contrast to TE1, a TE2 has a conventional interface (e.g. a/b, X.21, X.25, V.24) and

can be connected to the passive bus only via an appropriate terminal adapter.

Terminal adapter (TA)

The terminal adapter enables conventional non-ISDN terminal equipment to be used in

the ISDN by adapting conventional interfaces to protocols of the interface at reference

point S.

Network termination (NT)

A network termination may consist of two components (NT1 and NT2). Network termi-

nation 1 (NT1) ensures that the terminal equipment is physically matched to the

exchange access line. It also enables the access line to be shared by more than one

terminal. In addition, an NT1 can support centralized maintenance irrespective of theoperating state of the subscriber equipment (test loop) and report transmission quality

criteria to the exchange. The NT2 option contains switching functions, in other words it

can be a PABX. If no such NT2 functions are needed then NT2 is not installed (zero

NT2).

Line termination (LT)

An LT terminates an access line in the exchange as far as transmission is concerned.

Depending on whether it is used for a basic access or a primary rate access it can

perform functions such as feeding the NT or the intermediate regenerators, providing

test loops, signal regeneration and code conversion.

Exchange termination (ET)

An ET terminates an access line in the exchange as far as control is concerned; userand signaling information pass via the exchange termination. In the exchange it handles

the protocol of the data link layer (layer 2 of the OSI reference model, see Sections 3

and 5) of DSS1. If necessary, the signaling information received by the terminal equip-

ment is converted into a different format before being further processed outside the ET.

LTs and ETs may be integrated in a single functional unit.

3 OSI Reference ModelThe Open System Interconnection (OSI) reference model developed by the Interna-

tional Standardization Organization provides a structure for the logical operations in acommunication network. Consistent application of the OSI reference model permits

terminal equipment from different manufacturers to communicate in a network (open

system). Suitable network and service gateways allow open communication among all

subscribers in these networks.

The OSI reference model provides the necessary framework for arranging and devel-

oping protocols and interfaces for communication in open systems but it does not offer

any technical solutions. It merely specifies how the technology should behave

outwardly. The following brief description of the OSI reference model is intended as a

basic introduction to permit better understanding of the sections to follow.

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3.1 The Layers

The OSI reference model assigns the necessary communication functions to seven

layers (Fig. 3.1).

Layers 1 to 7 of an open system are arranged in a vertical hierarchical structure. A lower

layer, possibly supported by a layer or layers beneath it, provides certain defined

services for the layer above it (if it exists). This applies to all layers from 1 to 7. The func-

tions of the lower layers are fundamental to the functions of the higher levels. The func-

tions of layers 1 to 7 are defined in CCITT Recommendation X.200. The principal tasks

of the seven layers are as follows:

Layer 1:Controls the physical transmission medium

Layer 2:Ensures data transfer via the links

Layer 3:Establishes and switches the entire network connection

Layer 4:Provides an end-to-end transport service

Layer 5:Controls the end-to-end communication link

Layer 6:Creates a form of presentation for data communication which is not user-and device-dependent

Layer 7:Controls user-specific communication

Fig. 3.1 The seven layers of the OSI reference model

The design of the physical connection paths (copper wires, coaxial cable, optical

waveguides, radio relay links or satellite links) needed to transfer user and signaling

information is not described in layer 1 of the OSI reference model.

Although the stipulations of the OSI reference model are valid for all seven layers, only

layers 1, 2 and 3 are described in detail below since only these three layers are relevant

to DSS1. References to the higher layers are included for the sake of completeness

only.

3.2 Communication Between the Layers

In open systems the individual layers consist of functions performed by various equip-

ment (such as terminal equipments and text and databases). The OSI reference model

defines the assignment of functions (entities) to the individual layers but not the way in

which these functions are implemented. In keeping with the structure of the OSI refer-

ence model, the entities of a terminal are shown in a vertical hierarchy. In order for

terminal equipment to perform its principal task (that of providing the necessary

services) there must be communication between adjacent entities of a terminal and

communication with entities of other terminals.

Application layer 7

Presentation layer 6

Session layer 5

Transport layer 4

Network layer 3

Data link layer 2Physical layer 1

OSI reference model

Communica-tion functions

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Within a terminal two vertically adjacent entities communicate, in an abstract sense, by

means of service primitives in order to

make use of the services of the layer below (if there is such a layer) so that it can

perform its own services and

make its services available to the layer above.

Entities in one terminal can only communicate with entities of the same layer in other

terminals (horizontal communication). These communicating entities in different termi-

nals are called peer entities. Communication between peer entities is regulated by a

protocol. Peer entities may be in the same open system or in different open systems.

Fig. 3.2 shows an example of peer entities of layers 1 to 3 for DSS1 between a terminal

equipment and an ISDN exchange. The peer entities involved exchange information in

the form of protocols with protocol elements (dotted lines). Logical connections are set

up for communication in the horizontal direction. The physical connections, however, all

pass via the D channel (layer 1). The service primitives are used for communication in

the vertical direction via the entities.

Fig. 3.2 Communication between entities

Example: Forwarding of signaling information from a terminal equipment to the ISDN

exchange.

With the aid of service primitives as the means of communication, the signaling informa-

tion in a terminal equipment, for example, passes step by step from the layer 3 entity to

the layer 2 entity and then to the layer1 entity (Fig. 3.3). The layer 3 and 2 entities add

appropriate protocol elements to the signaling information which are needed for

performing the tasks in question (see Sections 5 and 6). The layer 1 entity does not add

any protocol elements as such but does have recourse to a simple protocol (see Section

4). It sends a bit stream containing the signaling information and the protocol elements

to the layer 1 entity in the exchange. Layers 1 to 3 in the exchange check the information

received for formal correctness with the aid of their respective protocols. When it has

Serviceprimitives

Entity of the network layer

LayerLayer

3 Entity of the network layer 3Layer 3 protocol

(logical connection)

Serviceprimitives

Entity of the data link layer Entity of the data link layer

(logical connection)

Layer 2 protocol

Serviceprimitives

2 2

Serviceprimitives

Entity of the physical layer Entity of the physical layer

(Setting up and clearing down thephysical connection)

Layer 1 protocol1 1

D channel

Terminal equipment Exchange

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performed its check and removed its own protocol elements each layer passes the infor-

mation to the next higher layer or to the signaling application.

Fig. 3.3 Forwarding signaling information via the D channel (in this case from a terminal to the exchange)

3.3 Protocol Architecture in the B and D Channels

Circuit-switched communication networks provide only a layer 1 connection between the

two terminal equipments for exchanging user information via the B channel (Fig. 3.4).

The exchanges through-connect transparent layer 1 connections in their switching

networks according to the signaling information received. The functions of the other

layers (2 to 7) are used in the terminal equipments according to the particular service

and are implemented only in these terminal equipments. Consequently, they are signif-icant only as end-to-end functions.

Entity of the network layer

LayerLayer

3 Entity of the network layer 3

Entity of the data link layer Entity of the data link layer2 2

Entity of the physical layer Entity of the physical layer1 1

D channel


P3 P3

SignalingP2 P3 P2 SignalingP2 P3 P2

SignalingP2 P3 P2P1

P=Protocol elementSignaling Signaling

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Fig. 3.4 Protocol architecture for the transfer of user information on the B channel

CCITT has defined layers 1 to 3 for ISDN to provide secure transfer of signaling infor-

mation and low transfer rate data on the D channel (see Sections 4 to 6). Fig. 3.5 shows

how the layers are distributed for the exchange of signaling information between two

terminal equipments. For the transfer of low transfer rate data on the D channel the

application-oriented layers (4 to 7) have, at most, only an end-to-end function between

the terminals.

Fig. 3.5 Protocol architecture for the transfer of signaling information on the D channel

The complete protocol architecture in an ISDN terminal for communication via the B and

Terminalequipment

Terminalequipment

Exchange Exchange

Layer

7

6

5

4

3

2

1

CCITTRecommenda-tion I.430(basic access)or I.431(primary rateaccess)

B channel

7

6

5

4

3

2

1

Layer

B channel

1

LayerLayer

1

ISDN

Exchange ExchangeLayer Layer

Terminalequipment

3

2

1

Q.930/I.450 andQ.931/I.451

Q.920/I.440 andQ.921/I.441

I.430(basic access)or I.431(primary rateaccess)

CCITT Recom-mendations

D channel

3

2

1

D channel

3

LayerLayer

3

Terminalequipment

ISDN

1

2 2

1

DSS1

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D channels is shown in Fig. 3.6, using a multiservice terminal connected to a basic

access as an example. The relevant CCITT Recommendations are included in the

diagram.

Fig. 3.6 Example of a protocol architecture in ISDN terminals

4 Physical Layer (Layer 1)Layer 1 provides the higher layers with the digital transmission paths for both directions

of transmission, i.e. with the B channels for user information and the D channel for

signaling information. The transmission capacity of the D channel and the number of B

channels depend on whether the connection is a basic access (2 B+D), a 2048-kbit/s

primary rate access (30 B+D) or a 1544-kbit/s primary rate access (23 B+D). Additional

services for layer 2 include setting up and clearing down the physical connection, D

channel access for the basic access, maintenance functions and a layer 1 status indi-

cation. The characteristics of layer 1 are described in detail below with reference to the

D channel.

Basic access

Service 2 Layer

7

6

5

4

3

2

1

High-level protocolsappropriate to service 2

B channel

B channel

D channel

I.430 *)

I.440 and I.441 *)

High-levelprotocolsfor lowtransfer ratedata

I.450 and I.451 *)

High-level protocolsappropriate to service 1

Service 1

Signal- anding

Low transferrate data

ISDN multiservice terminal

This signalingapplication is controlledby services 1 and 2 andlow transfer rate services

*) CCITTRecommendations forbasic access

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4.1 Basic Access

4.1.1 Reference Point S/T Between Terminal Equipment and the Network

Termination

CCITT Recommendation I.430 provides for one 16-kbit/s D channel and two 64-kbit/s B

channels for both directions of transmission between the terminal equipment (TE) and

the network termination (NT) of a basic access. Transmissions between TE and NT take

place in full duplex mode at a bit rate of 192 kbit/s. The pulse frames used contain 48

bits each and have a total transmission time of 250 ms. In one second 4000 such pulse

frames are transmitted (48 bits 4000/s = 192 kbit/s). Four of the 48 bits of each pulse

frame (D bits) constitute the D channel (4 bits 4000/s = 16 kbit/s). Within the pulse

frame the D bits occupy bit numbers 12, 25, 36 and 47 (Fig. 4.1).

In the direction of transmission from the NT to the TE there are also four E bits which

form a D echo channel (4 bits 4000/s = 16 kbit/s). The bit numbers of the E bits in sucha pulse frame are 11, 24, 35 and 46. The D echo channel is used to control TE access

to the D channel (collision detection, see below).

The decisive factor for setting the time difference between the receive pulse frame and

the transmit pulse frame is the reception of the first bit of each pulse frame (F bit) at the

TEs. On this basis the terminal equipment involved sends the pulse frames in the direc-

tion of the NT with an offset of 2 bits.

A pseudo-ternary code is used for the transmission of pulse frames between the TE and

the NT. In this code the binary values of "1" are transmitted at zero voltage and binary

values of "0" alternately at positive and negative voltage. Two intentional code violations

are used for pulse frame detection.

1st code violation:The L bit (bit number 2) and the first zero bit after the L bit (but no later than the FA bit

(bit numbers 3 to 14)) are both transmitted at negative voltage.

2nd code violation:

The last zero bit of a pulse frame and the following F bit (bit number 1) of the next pulse

frame are both transmitted at positive voltage.

To ensure satisfactory transmission of information (from the TE to the NT) via a passive

bus, ordered access to the D channel is assured for each TE in a multi-device configu-

ration. Defined priorities ensure that transmission of the signaling information takes pref-

erence over all other forms of information (packet data, telemetry data, user-user

information). Before information is transmitted on the D channel a TE must check for the

idle state (permanent binary "1" on the D echo channel). If information is transmitted

simultaneously from two or more TEs there is a mechanism to ensure that only one TE

can complete transmission (D channel contention resolution). For this purpose, the NT

loops back the D channel bits received from the TE (D bits) to the TEs on the D echo

channel (E bits). The TEs compare the bit received on the D echo channel with the last

D bit sent (see arrows in Fig. 4.1). If a TE ascertains that the bits sent and received are

the same it continues sending information; if, however, it finds that they are not the same

(binary "0" = positive or negative voltage instead of binary "1" = zero voltage) it immedi-

ately stops sending information (collision detection). The other TE continues to transmit.

TEs which interrupt transmission have to wait for the next opportunity to transmit via the

D channel.

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bits of a binary signal can be represented by three ternary signal elements, and with the

2B/1Q code two bits of a binary signal by one quaternary signal element. The transmis-

sion speed on the subscriber line is reduced by 25% and 50% respectively compared

with the equivalent binary signal: an information signal with a total bit rate of, say, 160

kbit/s (2 B+D + synchronization and control information) is transferred on the subscriber

line at 120 kbaud (4B/3T code) or 80 kbaud (2B/1Q code).

4.2 Primary Rate Access

In this case all the channels, in other words the B channels (user information) and the D

channels (signaling information), have a bit rate of 64 kbit/s. Channels with the same

number in the pulse frame are used for transmitting user information in both directions.

There are two types of primary rate access (see also topic 7).

4.2.1 2048-kbit/s Primary Rate AccessThe 2048-kbit/s primary rate access is specified in CCITT Recommendation I.431. It

uses a pulse frame as defined in CCITT Recommendation G.704. The pulse frame (Fig.

4.2) contains 32 channel time slots of 8 bits each:

one channel time slot for frame alignment, service signals etc.

30 channel time slots for the 30 B channels

one channel time slot for the D channel.

2048-kbit/s transmission systems use the same pulse frame structure.

Fig. 4.2 Pulse frame structure of the 2048-kbit/s primary rate access

Channel time slot 31




Channel time slot 2

Channel time slot 1

Bit number8 7 6 5 4 3 2 1

Channel for frame alignment word/service word

B channel 1

B channel 2

B channel 15

B channel 16

B channel 30

Channel time slot 0

32 channels

8 bits=

256 bits D channel

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4.2.2 1544-kbit/s Primary Rate Access

The 1544-kbit/s primary rate access also conforms to CCITT Recommendation I.431

and also uses a pulse frame as defined in CCITT Recommendation G.704. The pulse

frame (Fig. 4.3) contains one time slot of 1 bit and 24 channel time slots of 8 bits each: one 1-bit time slot (F bit) for frame alignment, performance monitoring etc.

23 channel time slots for the 23 B channels

one channel time slot for the D channel.

1544-kbit/s transmission systems use the same pulse frame structure.

Fig. 4.3 Pulse frame structure of the 1544-kbit/s primary rate access

5 Data Link Layer (Layer 2 of DSS1)The data link layer (layer 2, CCITT Recommendations Q.920/I.440 and Q.921/I.441)

ensures reliable error-free transfer of layer 3 information (signaling information and low

transfer rate data) via the D channel. For actual transfer, layer 2 makes use of the

services of the physical layer (layer 1).

The protocol used for layer 2 of the D channel is called the link access procedure on the

D channel (LAPD). LAPD is based on link access procedure B (LAPB, CCITT Recom-

mendation X.25) and the HDLC (high-level data link control) standards defined by the

International Standardization Organization (ISO 3309 and ISO 4355). LAPD offers the

following:

establishment of one or more layer 2 connections on the D channel for several termi-

nals connected to a basic access and several layer 3 entities

frame formation with transparent transfer for layer 3 information

frame sequence control

error detection and automatic frame repetition

protocol error recording

flow control

administration functions for layer 2

5.1 Frame

The layer 2 frame structure with and without the information field is shown in Fig. 5.1.

F bit



Channel time slot 2

Channel time slot 1

Bit number8 7 6 5 4 3 2 1

B channel 1

B channel 2

B channel 23

D channel

Bit time slot

1 bit

+24 channels

8 bits=

193 bits

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The frames are used for activating and deactivating layer 2, for transferring layer 3 infor-

mation and for performing internal layer 2 control and supervision functions.

The layer 2 frames are divided into two categories: commands (C) and responses (R).

Whether commands have to be acknowledged (i.e. require responses) or not dependson the particular functions being performed. CCITT has defined the "multiple frame

operation" procedure for information transfer with acknowledgments. In this procedure

a number of frames sent one after the other can be acknowledged as a group, which

means that there is no need for each frame to be acknowledged immediately (see

control field).

The layer 2 frames with their changing addresses and frame lengths differ considerably

from the repetitive pulse frames with permanently assigned channel time slots of digital

transmission systems or of the primary rate access (see Section 4.2 and also topic 7).

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Fig. 5.1 Layer 2 frame structure with and without an information field

Flag

Each frame starts and ends with a flag. The flags always have the same bit pattern:

01111110. Between the opening and closing flags of a frame the transmitter automati-

cally inserts a "0" after five consecutive "1"s. The receiver then masks out these inserted

"0" bits. This makes flag detection unambiguous (in the idle state between frames

the terminals send a continuous sequence of "1" signal elements). The closing flag of a

Address field

Octet m-2

Octet 4

Octet 3

Octet 2

Bit numbering8 7 6 5 4 3 2 1

Flag

Control field

*)

Information field

Octet 1

Frame check sequence field

Flag

Octet m-1

Octet m

Octet n-2

Octet 4

Octet 3

Octet 2


Address field

Control field

*)


Octet 1

Flag

Octet n-1

Octet n

*) The second octet of the control field is not used in the U frame without asequence number for the transfer of unnumbered unacknowledgedinformation and control functions.

Flag

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frame can also be the opening flag of the next frame.

Address field

The address field consists of two octets (Fig. 5.2) and uniquely identifies a layer 2

connection. It contains two address field extension bits (EA), a command/response bit

(C/R bit), a service access point identifier (SAPI) and a terminal endpoint identifier (TEI).

Fig. 5.2 Address field

Address field extension bits

With the EA bits the address field length is extended to or defined at two octets. The EA

bit of the first address field octet is assigned the binary value "0" and the EA bit of the

second address field octet the binary value "1". Binary "1" of the second EA bit indicates

the last octet of the address field.

Command/response bit

The C/R bit indicates whether a frame contains a command or a response (Table 5.1).

Service access point identifier

The SAPI in the address field denotes the class of information to be transferred. These

information classes are used to differentiate between signaling, layer 2 administrative

information and packet data including user-user information. With the six bits of the

address field a total of 64 information classes, numbered from 0 to 63, can be identified.Bit 3 of octet 2 is the least significant bit (LSB) and bit 8 the most significant bit (MSB).

The meanings of the defined SAPIs are shown in Table 5.2.

Frame contents Transmission direction Binary value of the

C/R bit

Command Network >Terminal 1

Terminal >Network 0

Response Network >Terminal 0

Terminal >Network 1

Tab. 5.1 Meaning of the command/response bit

SAPI Information class

0

1

16

63

2 through 15

and 17 through

62

Signaling

Packet data (Q.931 signaling procedures)

Packet data (X.25 layer 3 procedures)

Layer 2 administration function (TEI administration)

For future applications

Tab. 5.2 Defined information classes of the service access point identifiers

SAPI

Octet 3


EA(=0)

TEI

Octet 2C/R

EA(=1)

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Terminal endpoint identifier

The TEI in the address field denotes a terminal for explicit transfer of a message. Func-

tional groups of multiservice terminals can have their own TEI in the same way as indi-

vidual terminals. TEIs enable terminals within an information class (same SAPI) to be

differentiated, and with a particular common TEI it is possible to access a number of

terminals at the same time (broadcasting). Depending on the design of the terminal, a

TEI can be assigned to a terminal either by the user or automatically by the network (see

Section 5.4). The available 7 bits of the address field give a possible 128 different TEI

values, numbered from 0 to 127. Bit 2 of octet 3 is the least significant bit (LSB) and bit

8 the most significant bit (MSB). The applications of the TEI are shown in Table 5.3.

Control field

The control field contains the code for identifying the type of frame. There are three

formats for control fields (Fig. 5.3):

I format for serially numbered acknowledged information transfer (I frame)

S format for supervisory control functions (S frame)

U format for unnumbered unacknowledged information transfer and control func-

tions (U frame).

Fig. 5.3 Control field formats

TEI Applications

0 through 63

64 through 126

127

Assigned by the user

Automatically assigned by the exchange

Broadcasting and for assigning TEIs 64 through 126

Tab. 5.3 Defined applications of the terminal endpoint identifiers

N(R)

M

000 0 S 0

N(S)

Octet 5


0

N(R)

Octet 4

P

a) I format

Octet 5


1 Octet 4

P/F

b) S format

S

MM M M 1


1 Octet 4

c) U format

P/F

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The control fields with the I format of all the frames to be transmitted are each given a

send sequence number N(S). The receive sequence numbers N(R) of I and S frames

acknowledge the error-free reception of all I frames up to send sequence number

N(S) = N(R) 1. Sequence numbers N(S) and N(R) are used to supervise the contin-

uous and error-free exchange of frames (see also Section 5.3).

a) for signaling

basic access 1 I frame

primary rate access 7 I frame

b) for packet data

basic access 3 I frame

primary rate access 7 I frame

The value of the P (poll) bit and the F (final) bit may be binary 0 or 1:

A command with P bit = 1 requests a response from the layer 2 entity of the

receiver. In the resultant receive response the F bit has the binary value 1.

A command with P bit = 0 does not require any particular response. In a non-

requested response the F bit has the binary value 0.

The S and M bits determine the function of the frame.

Information field

The information field consists of an integer number of octets and may contain as many

as 260 octets. The contents of the information field form part of layer 3 and are described

in Section 6.1.


The frame check sequence field consists of two octets. A frame check sequence (FCS)

is used to detect transmission errors on the D channel. The transmitter uses an algo-rithm to form a 16-bit FCS from the contents of the address, control and information

fields of a frame. The receiver uses the same algorithm to calculate the FCS and

compares it with the FCS received from the transmitter. If the two FCSs are identical

then transmission is error-free.

5.2 Layer 2 Addressing

Information transfer via the D channel of a subscriber line takes place in the same way

whether from the terminals to the exchange or from the exchange to the terminals. The

layer 2 addressing procedure via the D channel can best be described with reference to

an example (Fig. 5.4). To simplify matters this example only deals with the addressingprocedure for information transfer from the exchange to the terminal equipment. For the

purposes of information transfer the relevant exchange provides appropriate frames and

inserts an appropriate service access point identifier (SAPI) in the address field. The

value of this SAPI depends on whether, for example, signaling information (SAPI = 0) or

packet data (SAPI = 16) is being transferred. The exchange also includes the relevant

terminal endpoint identifier (TEI) in the address field. In our example TEI = 64 or 71 for

a particular user terminal (the TEIs have been automatically assigned by the exchange,

see also Section 5.4) or TEI = 127 for addressing all the terminals simultaneously

(broadcasting).

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5.3 Commands and Responses and their Functions

The commands and responses of layer 2 are listed below in Table 5.4. The information

as to which command and which response is being used is contained in the control field

of the frame. For the bit patterns used in the individual control fields see CCITT Recom-mendation Q.921.

I frame:

The numbered I frames transfer the layer 3 information to be acknowledged via the layer

2 connection. Figure 5.5 shows how the send and receive sequence numbers N(S) and

N(R) are incremented.

Applications Control field formats Commands Responses

Unacknowledged and multiple

frame acknowledged information

transfer

Serially-numbered information

transfer (I)

Information (I)

Supervisory control functions (S) Receive ready (RR) Receive ready (RR)

Receive not ready (RNR)Receive not ready (RNR)

Reject (REJ) Reject (REJ)

Unnumbered information

transfer and control functions (U)

Set asynchronous balanced

mode extended (SABME)

Disconnected mode (DM

Unnumbered information (UI)

Disconnect (DISC)

Unnumbered acknowledgment

(UA)

Frame reject (FRMR)

Connection management Exchange identification (XID) Exchange identification (XID)

Tab. 5.4 Commands and responses

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Fig. 5.5 Handling the sequence numbers for acknowledged information transfer

S frame:

The RR, RNR and REJ control functions are used for controlling layer 2 transfers (Table

5.5). If there is no layer 3 information to be sent, the control functions may also acknowl-

edge received I frames.

U frames:The SABME, DISC, UA and DM control functions (Table 5.6) are used to set up and

clear down acknowledged layer 2 connections for multiple frame operation. The UI

commands of the U frame (UI frame) are used for transferring information which does

not have to be acknowledged. These commands relate to the assignment, checking,

removal, identification and confirmation of TEIs (for SAPI = 63, see example in Fig. 5.6,

assignment of a TEI) and also to the broadcasting of information to all the terminals of

the called party (Section 6.2). The FRMR control function acknowledges received

frames which do not conform to the protocol in use, indicates the protocol error and calls

for the layer 2 connection to be reset. Frames with the XID control function can be

exchanged between the layer 2 entities so that protocol parameters can be changed as

required.

Command/response Tasks

Receive ready (RR) Indicate ready to receive (I frame)

Acknowledge received I frames

Cancel a temporary busy state previously indicated by RNR

Receive not ready (RNR) Indicate a temporary busy state

Interrogate the status of a peer entity (if P bit = 1)

Reject (REJ) Request retransmission of an I frame, possibly in connection with thecancelation of a temporary busy state previously indicated by RNR

implicit indication of the receive ready status

status interrogation of a peer entity (if P bit = 1)

Tab. 5.5 Tasks of the commands and responses of the S frames

Send 0 I frame (Control field: N(S)=0, N(R)=0) Receive 0

Receive 0, 0 acknowledged Send 0, acknowledge 0I frame (Control field: N(S)=0, N(R)=1)

Send 1, acknowledge 0 I frame (Control field: N(S)=1, N(R)=1) Receive 1, 0 acknowledged

1 acknowledged Acknowledge 1S frame (Control field: N(R)=2)

Send 2, (0 acknowledged) Receive 2, (0 acknowledged)I frame (Control field: N(S)=2, N(R)=1)

2 acknowledged Acknowledge 2S frame (Control field: N(R)=3)

Send 3, (0 acknowledged) Receive 3, (0 acknowledged)I frame (Control field: N(S)=3, N(R)=1)

Receive 1, 3 acknowledged Send 1, acknowledge 3I frame (Control field: N(S)=1, N(R)=4)


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Fig. 5.6 Assignment of a TEI or denial of assignment

5.4 Assignment of the Terminal Endpoint Identifier

For a terminal to be able to communicate with the exchange it must be assigned a

unique TEI value. As far as TEI assignment is concerned, there are two categories of

terminal:

terminals without automatic TEI assignment (TEI values from 0 to 63)

terminals with automatic TEI assignment (TEI values from 64 to 126).

In the case of terminals without automatic TEI assignment, the user must ensure that

unique TEI values are assigned to the terminals (e.g. by setting the TEI value on the

terminal).

If a terminal has automatic TEI assignment it is easier for the user to use this terminal

on different access lines.

Automatic TEI assignment:

Each time the terminal is plugged in it uses a UI frame to request a TEI from the layer 2

administration entity in the exchange (Fig. 5.6). In addition to the address field with SAPI

= 63 and TEI = 127, such a UI frame contains a randomly generated reference number

Ri, the "identity request" message type and an action indicator Ai = 127. The reference

number ranges from 0 to 65,535 and is used to discriminate between different simulta-

neous operations. If there are TEI values in the 64 to 126 range free then the layer 2administration entity in the exchange will assign a free TEI to the terminal. This TEI is

Command/response Tasks

Set asynchronous balanced mode extended

(SABME)

Setup request for an acknowledged layer 2 connection

Disconnect (DISC) Disconnection request for an acknowledged layer 2 connection

Unnumbered acknowledgment (UA) Positive response to SABME or DISC, possibly in connection with

cancelation of a temporary busy state previously indicated by RNR

Disconnected mode (DM) Indicate lack of readiness to accept an acknowledged layer 2

connection

Tab. 5.6 Tasks of the SABME and DISC commands and of the UA and DM responses of the U frame

OR

UI frame (SAPI, TEI) [information field: ID request, Ri, Ai]

UI frame (SAPI, TEI) [information field: ID assigned, Ri, Ai]

UI frame (SAPI, TEI) [information field: ID denied, Ri, Ai]


In all three cases: address field SAPI=63, TEI=127ID= identityRi= reference numberAi= action indicator

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forwarded to the terminal in the Ai field of an "identity assigned" UI frame. The terminal

checks the reference number to make sure that the TEI is intended for it and stores this

TEI. All subsequent messages to or from this particular terminal will contain this TEI in

the address field. The TEI remains valid for the terminal until the terminal is discon-

nected from the network, either intentionally or as a result of a fault/error, or the TEI is

withdrawn by the layer 2 administration entity in the exchange.

If there are no free TEI values available when the request is made, the layer 2 adminis-

tration entity in the exchange cannot assign a TEI value to the terminal. In this case, the

terminal receives an "identity denied" UI frame.

The layer 2 administration entity in the exchange can also verify the assigned TEIs of

the various terminals. Withdrawal or verification of the TEIs takes place with the aid of

UI frames in much the same way as assignment of the TEIs.

6 Network Layer (Layer 3 of DSS1)The network layer comprises functions for establishing, maintaining and releasing

connections (CCITT Recommendations Q.930/I.450 and Q.931/I.451). It is also used for

controlling supplementary services (CCITT Recommendation Q.932). For all its func-

tions layer 3 uses the services of layers 1 and 2 to ensure reliable transfer of the neces-

sary messages.

6.1 Message Structure

The layer 3 entities supply the complete messages for transfer in layer 2 information

fields (one per field, see Section 5.1). The number of octets in the message may vary

but is never more than 260. The DSS1 messages, internationally standardized byCCITT, have a uniform structure (Fig. 6.1) and each contains the prescribed protocol

discriminator (Table 6.1), a call reference, the message type and a number of informa-

tion elements.

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Call reference

The call reference is the second part of each layer 3 message. Each call reference

contains a call reference value. In layer 3 a call reference value establishes the unique

relationship between the message and a particular call or a particular supplementary

service control operation. These relationships apply only to the relevant layer 2 pathbetween a terminal equipment and an exchange; in other words they have no end-to-

end significance. For layer 3 the use of different call reference values allows multiple use

of a layer 2 connection. A particular call reference value is permanently assigned to a

call from the start of setup to the end of cleardown. Only when the call has been cleared

down can this call reference value be assigned to another call.

The call reference (Fig. 6.2) may consist of

two octets in the case of basic access and

three octets for primary rate access or

two octets as a network option.

Bits 1 to 4 of the first call reference octet indicate the length of the subsequent call refer-

ence value (i.e. one or two octets). Call references consisting of one octet may beassigned values from 0 to 127, those consisting of two octets may be assigned values

from 0 to 32,767. The originating side of the calls defines the call reference values rele-

vant to it. The full range of call reference values is available to each originating side. A

marker bit (bit eight in the second call reference octet) identifies the origin (subscriber

terminal or exchange equipment) of a call reference. The originating side sets the

marker bit to binary "0". In call-related messages from the remote end the marker bit is

always inverted (binary "1").

Protocol discriminator octet

(Bit numbering)

Meaning

8 7 6 5 4 3 2 1

0

0

0

0

0

0

0

0

0

to

0

0

0

0

0

0

0

Protocol discriminators in user-user information elements. Not avail-

able for messages for user-network call control

0 0 0 0 1 0 0 0 Messages for user-network call control

(CCITT Recommendation Q.931)

0

0

0

0

0

1

1

1

0

to

1

0

1

0

1

0

1

Reserved for other network layer or layer 3 protocols

(including X.25 protocol)

0

0

1

1

0

0

0

0

0

to

1

0

1

0

1

0

1

National applications

0

1

1

1

0

1

1

1

0

to

1

0

1

0

1

0

0

Reserved for other layer 3 protocols

(including X.25 protocol)

All other values yet to be defined.

Tab. 6.1 Protocol discriminator codes and their meanings

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Fig. 6.2 Call reference

Message type

The message type constitutes the third part of each layer 3 message. It indicates the

function of the messages just sent. For the codes defined here bit 8 of the message type

octet is always set to "0" and is provided as a possible extension bit for the future. The

codes for the individual message type octets are listed in Tables 6.2 and 6.3.

Message type octet

(Bit numbering

Meaning

8 7 6 5 4 3 2 1

0 0 0 0 0 0 0 0 National application: message type defined in subsequent octet

0 0 0 -

0

0

0

0

0

00

-

0

0

0

1

0

01

-

0

0

1

1

0

01

-

0

1

1

1

1

00

-

1

0

1

1

1

11

Messages for call setup

ALERTING

CALL PROCEEDING

CONNECT

CONNECT ACKNOWLEDGE

PROGRESS

SETUPSETUP ACKNOWLEDGE

0 0 1 -

0

0

0

0

0

0

0

-

0

1

0

0

1

0

0

-

1

1

0

1

1

0

0

-

1

1

1

0

0

0

0

-

0

0

0

1

1

1

0

Messages during the active call phases

RESUME

RESUME ACKNOWLEDGE

RESUME REJECT

SUSPEND

SUSPEND ACKNOWLEDGE

SUSPEND REJECT

USER INFORMATION

Tab. 6.2 Codes for the message types for call setup, call cleardown and miscellaneous messages as definedin CCITT Recommendation Q.391.

Call reference value

00000

Octet 3


Length of the call reference value

Octet 2

Mar-

ker bit

0 0 0

a) Call reference value

contained in one octet

00000

Octet 3


Length of the call reference valueOctet 2

Mar-

ker bit

0 0 0

b) Call reference valuecontained in two octets

Octet 4

Call reference value

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Information elements

The fourth and last part of a message consists of the information elements assigned to

the message type. The information elements contain the actual information to be trans-

ferred which is needed, for example, for setting up a call or for controlling a service. A

layer 3 message may contain one or more information elements, or none at all. There

are two categories of information element (Fig. 6.3):

single-octet information elements

multiple-octet information elements.

There are two types of single-octet information element:

Type 1 consists of an information element identifier (bits 5 to 7) and a contents part (bits

1 to 4) with various parameters. These parameters may, for example, be a code setchangeover (shift, see below), an overload level or a repeat indicator.

0 1 0 -

0

0

1

0

0

-

0

1

1

0

1

-

1

1

0

1

1

-

0

0

1

1

1

-

1

1

0

0

0

Messages for call cleardown

DISCONNECT

RELEASE

RELEASE COMPLETE

RESTART

RESTART ACKNOWLEDGE

0 1 1 -

0

1

1

0

01

1

-

0

1

1

0

11

0

-

0

0

0

0

11

1

-

0

0

1

1

10

0

-

0

1

1

0

01

1

Miscellaneous messages

SEGMENT

CONGESTION CONTROL

INFORMATION

FACILITY

NOTIFYSTATUS

STATUS ENQUIRY

Message type octet

(Bit numbering)

Meaning

8 7 6 5 4 3 2 1

0 0 1 -

00

1

1

1

1

-

01

0

0

0

0

-

10

0

0

0

1

-

00

0

0

1

1

-

00

0

0

1

1

Messages during the active call phases (Q.931, see Table 6.2)

HOLDHOLD ACKNOWLEDGE

HOLD REJECT

RETRIEVE

RETRIEVE ACKNOWLEDGE

RETRIEVE REJECT

0 1 1 -

0

0

-

00

-

0

1

-

1

0

-

0

0

Miscellaneous messages (Q.931, see Table 6.2)

FACILITY

REGISTER

Tab. 6.3 Codes for the message types for supplementary services as defined in CCITT RecommendationQ.932

Message type octet

(Bit numbering

Meaning

8 7 6 5 4 3 2 1

Tab. 6.2 Codes for the message types for call setup, call cleardown and miscellaneous messages as definedin CCITT Recommendation Q.391.

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Type 2 transfers only an information element identifier. This corresponds to a message

such as "Sending complete" or "More data". Bits 5 to 7 are permanently set to "010".

The multiple-octet information elements vary in length (three or more octets). The

first octet of such an element contains the information element identifier. This indicateswhether the information element contains, say, the called party number (Fig. 6.4), user-

user information or a call status (see CCITT Recommendations Q.931 and Q.932 for

complete lists). The second octet of the multiple-octet information element specifies the

number (length) of the subsequent octets (a binary value between 0 and 255). Transfer

of some of the octets of an information element is optional, which means that information

elements with the same identifier may nevertheless consist of different numbers of

octets.

Fig. 6.3 Information elements

b) Type 2

1 0 Identifier0

ContentsIdentifier


1

a) Type 1

1


Single-octet informationelements


Identifier

Contents

0Multiple-octet informationelements

Length of contents

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Fig. 6.4 Example of an information element with the called party number

With the bits available it is possible to encode the following numbers of information

element identifiers:

Single-octet information elements

Type 1: up to eight (3-bit identifier)

Type 2: up to 16 (4 bits of the 7-bit identifier are variable);

Multiple-octet information elements

up to 128 (7-bit identifier) per codeset.

These numbers of identifiers can be increased by using single-octet information

elements as shift octets. The shift octets enable several codesets with different mean-

ings to be accessed. Up to eight codesets are possible. A shift can either relate only to

the subsequent information element (non-locking shift) or to all subsequent informationelements until the next shift (locking shift).

In a message the multiple-octet information elements within a codeset appear in

ascending order in accordance with the binary values of the information elements. This

makes it easier for the receiving equipment (entities) to detect the information elements.

Single-octet information elements, on the other hand, may appear at any point within a

message.

6.2 Use of Layer 3 Messages

The layer 3 messages are identified by their message type. Layer 3 messages are

generally transferred via acknowledged layer 2 connections. Only those layer 3

messages which are sent from the exchange to a group of terminals (e.g. a SETUP

message for an incoming call) are transferred via unacknowledged layer 2 connections

(UI frame) with TEI = 127 (Section 5).

The following description of how a telephone call is set up with digit selection illustrates

the use of layer 3 messages: the calling party initiates connection setup (lifts his

handset) by sending a SETUP message to the exchange (Fig. 6.5). In this example the

exchange acknowledges this with a SETUP ACKNOWLEDGE message, with which the

terminal of the calling party is assigned a B channel. If there are no digits in the SETUP

message the subscriber can receive the dialing tone. The outstanding digits are sent to

the exchange in one or more INFORMATION messages.

The exchange on the called side of the connection transfers the connection request with

Numbering planidentification

0


1st digit

nth digit

1

0

1 1 1 0 0 0 0

Directory number type

Length of contents

0

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receipt of a CONNECT message with a CONNECT-ACKNOWLEDGE message.

On the called side all the other terminals which have sent an ALERTING or CONNECT

message but which have not been given the call receive a RELEASE message. These

terminals each acknowledge the RELEASE message with a RELEASE-COMPLETEmessage and switch over to the idle state.

7 Example of a Complete DSS1 MessageFig.7.1 shows an example of a complete DSS1 message (layers 2 and 3) for a basic

access. The message chosen for this example is a SETUP message. For an outgoing

connection setup a terminal can send this as the first message to the exchange. The

message conforms to CCITT Recommendation Q.931 and contains three multiple-octet

information elements with the following meanings:

Bearer capabilityA CCITT-coded transparent circuit-switched 64-kbit/s transmission path is

requested.

Channel identification

The B1 channel is preferred for transfer but use of the B2 channel cannot be

excluded.

Called party number

The number of the called party is 6 54 32.

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Fig. 7.1 A complete message

Prefer-redchan-nel

0

8

StandardCCITT coding

P Receive sequence numberN(R)=0

Transfermode(= circuit-switched)

0

Call reference

Send sequence numberN(S)=0

I format

3rd information element

Numberingplan identification(unknown)

Digit 2Digit 3Digit 4

765431 2

Protocol discriminatorControl field

Address flagOpening flag

0 1Layer 2

1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1

1 2 3 4 5 6 7 8 EA C/R SAPI= 0 EA TEI= 64

0 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 00 00 0

Closing flagFrame check sequence

1 1 1 0 0 0 1 0 1 1 0 0 0 1 1 1 1 1 1 01 01 0 Layer 2

0 1 0 1 1 0 0 0 0 1 1 0 0 1 0 0 1 1 0 01 00 1

Digit 5Digit 6Directorynumber type(unknown)

0 0 0 0 0 0 1 1 0 1 1 0 0 0 1 0 1 1 0 01 10 0

Channelselec-tion(B1)

Length of contents(=6 octets)

Called party number

0 0 0 0 0 1 0 0 1 1 1 0 1 1 0 0 0 0 00 01 0

2nd information element

0

Information transfer rate(=64 kbit/s)

Length of contents(=1 octet)

Channel identification

0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 0 00 10 0

Message type = SETUP

0

Length of callreference value

Mar-ker

Call reference valuee.g. 22

0 0 0 0 0 1 0 1 0 0 0 0 1 0 0 0 0 01 11 0

1st information element

1

Bearer capability Unrestricted digitalinformation

Length of contents(=2 octets)

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 11 00 0

Layer 3(Layer 2information field)

0

Bit numbering

Bit numbering

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8 AbbreviationsC command

CCITT International Telegraph and Telephone Consultative Committee

CCS7 common channel signaling system no. 7

DISC disconnect (layer 2 command)

DM disconnected mode (layer 2 response)

DSS1 digital subscriber signaling system no. 1

EA address field extension bit

ET exchange termination

FCS frame check sequence

FRMR frame reject (layer 2 response)

HDLC high-level data link control

I information (layer 2 command)

I sequentially numbered information transfer (format and frameISDN integrated services digital network

ISO International Organization for Standardization

LAPB link access procedure balanced

LSB least significant bit

LT line termination

MSB most significant bit

N(R) receive sequence number

N(S) send sequence number

NT network termination

OSI open system interconnection

PABX private automatic branch exchangePCM pulse code modulation

R response

REJ reject (layer 2 command or response

RNR receive not ready (layer 2 command or response)

RR receive ready (layer 2 command or response)

S supervisory control functions (format and frame)

SABME set asynchronous balanced mode extended (layer 2 command)

SAPI service access point identifier

TA terminal adapter

TEI terminal endpoint identifier

TE1 terminal equipment type 1

TE2 terminal equipment type 2

U unnumbered information transfer and control functions (format and

frame)

UA unnumbered acknowledgment (layer 2 response)

UI unnumbered information (layer 2 command)

XID exchange identification (layer 2 command or response)

2B/1Q code in which 2 bits of a binary signal are represented by 1 quaternary

signal element

4B/3T

code

code in which 4 bits of a binary signal are represented by 3 ternary

signal elements

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