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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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
B 64 kbit/s speech, text, data, images
D 16 kbit/s signaling, low transfer rate data
a) Basic access
B 64 kbit/s speech, text, data, images
B 64 kbit/s speech, text, data, images
D 64 kbit/s signaling, low transfer rate data
B 64 kbit/s speech, text, data, images
B 64 kbit/s speech, text, data, images
b) Primary rate access
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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Terminal equipment Exchange
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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 17
Channel time slot 16
Channel time slot 15
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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 24
Channel time slot 23
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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Bit numbering8 7 6 5 4 3 2 1
Address field
Control field
*)
Frame check sequence 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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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
Bit numbering8 7 6 5 4 3 2 1
EA(=0)
TEI
Octet 2C/R
EA(=1)
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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Bit numbering8 7 6 5 4 3 2 1
0
N(R)
Octet 4
P
a) I format
Octet 5
Bit numbering8 7 6 5 4 3 2 1
1 Octet 4
P/F
b) S format
S
MM M M 1
Bit numbering8 7 6 5 4 3 2 1
1 Octet 4
c) U format
P/F
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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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.
Frame check sequence field
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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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)
Terminal equipment Exchange
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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration
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]
Terminal equipment Exchange
In all three cases: address field SAPI=63, TEI=127ID= identityRi= reference numberAi= action indicator
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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Bit numbering8 7 6 5 4 3 2 1
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
Bit numbering8 7 6 5 4 3 2 1
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 Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Bit numbering8 7 6 5 4 3 2 1
1
a) Type 1
1
Bit numbering8 7 6 5 4 3 2 1
Single-octet informationelements
Bit numbering8 7 6 5 4 3 2 1
Identifier
Contents
0Multiple-octet informationelements
Length of contents
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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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
Bit numbering8 7 6 5 4 3 2 1
1st digit
nth digit
1
0
1 1 1 0 0 0 0
Directory number type
Length of contents
0
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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Information Subscriber Administration Digital Subscriber Signaling System No.1 (topic 9)
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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Digital Subscriber Signaling System No.1 (topic 9) Information Subscriber Administration