BugFix

EWSD
Digital Subscriber Signaling System No.1 (DSS1)

Contents
DSS1 SS7 DSS1
User-network interface
OSI Reference model
Physical layer (layer 1) Network Network PBX
node node
Data link layer (layer 2 of DSS1)
Network layer (layer 3 of DSS1)
Network
Terminal
node
equipment

In view of the wide range of services and features of-

fered in the integrated services digital network (ISDN), For certain supplementary services these two systems
a very high-capacity signaling system, more so than also permit end-to-end signaling between subscriber
those used for conventional communication systems, is terminal equipment. This topic document is based on
needed to handle signaling functions the relevant ITU-T Recommendations.
– between the network nodes and
In order to ensure that, say, speech and signaling infor-
– between the network nodes and the terminal equip-
mation or text and signaling information can be trans-
ment/telecommunication systems
mitted simultaneously in ISDN with no mutual
The International Telecommunication union - Telecom- interference, separate channels are provided, known
munication standardization sector (ITU-T) has speci- as the B channels and the D channel. The B channels
fied two systems for transmitting the signaling carry speech, text, data and images whereas the D
information (control information) in the ISDN: channel is used exclusively for signaling.
– signaling system no. 7 (SS7) for use between net-
work nodes
(see Signaling System No.7 (SS7))
– digital subscriber signaling system no.1 (DSS1) for
use between network nodes and the terminal
equipment.

1
ITU-T has specified two types of ISDN access line: For communication between terminal equipment and
– the basic access (BA) for single subscriber lines between terminal equipment and the network nodes,
and for telecommunication systems: DSS1 is based on
a basic access has two B channels and one D – defined characteristics of the user-network inter-
channel face (see User-network interface)
– the primary rate access (PA) for PBXs: – the Open System Interconnection (OSI) reference
a primary rate access has 30 or 23 B channels and model (see OSI Reference model).
one D channel

User-network interface
Basic access (BA) Reference configuration
One of the principal requirements that have to be met
B 64 kbit/s: speech, text data, images before ISDN can be introduced is that the system must
B 64 kbit/s: speech, text data, images be digital up to and including the terminal equipment.
D 16 kbit/s: signaling, low transfer rate data
For the subscriber access line the ITU-T has defined
functional groups with intermediate reference points.
This arrangement defines the tasks that the individual
Primary rate access (PA) subscriber and network node functional groups have to
perform so that compatibility is assured between differ-
ent network nodes and terminal equipment.
B 64 kbit/s: speech, text data, images
ITU-T defines the user-network interface both for the
B 64 kbit/s: speech, text data, images basic access and for the primary rate access. These
D 16 kbit/s: signaling, low transfer rate data
B 64 kbit/s: speech, text data, images specifications are based on the OSI reference model
(see OSI Reference model). They relate not only to the
B 64 kbit/s: speech, text data, images 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 ser-
vices
The separate D channel has a very high capacity and – portability of terminal equipment
is permanently available. Its transmission capacity is – independent development of the technologies, con-
such that it can handle not only the signaling informa- figurations and installations for terminal equipment
tion but also low transfer rate data such as packet data, and networks
telemetry data and user-user information. Priority is al- – cost-effective connection to specialized storage
ways given to signaling information. and data processing media and to other networks.
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 si- Terminal Network termina- Network node
multaneously sending a fax on one B channel and mak- equipment tion equipment equipment
ing a telephone call on the other B channel while the
directory number of a calling subscriber is displayed on Refer- R S T U V
his terminal. This directory number is transmitted via ence
points TE1
the D channel. Access to the D channel is assured at
all times from all terminal equipment and from the net-
or NT2 NT1 LT ET
work node.
The principal features of DSS1 are as follows: Access
TE2 TA
line
– internationally standardized
– very high integrity and flexibility
– suitable for all communication services (telephony, Functional groups
facsimile, teletex, data transfer etc.)
– short reaction times
– future-proof to accommodate new requirements

2
The user-network interface can cover reference points The primary rate access with the channel structure 30
S and T, as follows: or 23 B+D is used for point-to-point connections be-
– if a type 2 network termination (NT2, e.g. a PBX) is tween medium- to large-size PBXs and the network
installed, the user-network interface is at reference node. The transmission path via the U interface is pro-
point T vided by two balanced wire pairs of a low-frequency ca-
– if there is no NT2 installed, reference points S and T ble, optical waveguides or radio relay routes.
coincide (reference point S/T) and the user-network If a PBX is used the interfaces at reference points S and
interface is then effectively at reference point S. T may be different. The subscribers, for example, may
This means that the presence or absence of an NT2 be connected to the PBX via basic access (reference
determines the location of the user-network interface; point S) and the PBX may be linked to the network node
generally speaking this will be the same user-network via primary rate access (reference point T).
interface in both cases. Depending on their size, PBXs can be connected to a
ITU-T does not specify the interface at reference point network node
U but does represent the connection between network – via basic and primary rate access (see the following
termination 1 (NT1) and the network node as a digital figure)
transmission system (ITU-T Recommendation G.961). – via basic access only
– via primary rate access only
Basic access and primary rate access
The basic access with the 2 B+D channel structure is Network termination Access lines Network node
used for linking one or more terminal devices to a net- equipment equipment
work node. A single device port requires a point-to-
point connection, whereas a multiple device port re- S/T U
quires a point-to-multipoint connection. If more than
2 B+D
one terminal device is connected (up to eight devices a) NT1 LT ET

are normal) the passive bus at the subscriber equip-

2 B+D
ment is used (reference point S/T). The passive bus b) NT1 LT ET
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 2 B+D 1)
Remote LT ET
the bus as required and can be accessed directly with c) both digital
a) and b) concen-
a multiple subscriber number. As far as the basic ac- 2 B+D 1)
trator LT ET
cess is concerned, conventional two-wire copper sub-
scriber lines can be used at reference point U for the
connection between NT1 and the network node. S/T T

2 B+D
Reference points PBX NT1 LT ET
S/T U via
Network PBXs 30 B+D,
2-wire (23 B+D
node NT1 LT ET
subscriber also possible)
line
ISDN terminal NT LT ET
equipment (TE1) 1

1) digital transmission line

Non-ISDN
terminal Terminal
Direct connection to the network node:
equip- adapters
a) with a single terminal (single device port)
ment
b) with more than one terminal via a passive bus
(TE2)
R (multiple device port)
c) via remote concentrators (in most cases connection to the
network node is implemented with digital transmission
a/b TA a/b links)

X.21 passive bus

TA X.21
bis (two 2-wire
bis
lines)
X.21 TA X.21

X.25 TA X.25

3
Functional groups OSI Reference model
The functional groups of an ISDN access are described The Open Systems Interconnection (OSI) reference
in brief below: model developed by the International Standards Orga-
– ISDN terminal equipment (TE1) nization (ISO) provides a structure for the logical oper-
A TE1 is equipped with the protocols relevant to the ations in a communication network. Consistent
interface at reference point S and can be connected application of the OSI reference model permits terminal
directly to the passive bus. equipment from different manufacturers to communi-
– Non-ISDN terminal equipment (TE2) cate in a network (open system). Suitable network and
In contrast to TE1, a TE2 has a conventional inter- service gateways allow open communication among all
face (e.g. a/b, X.21, X.25, V.24) and can be con- subscribers in these networks.
nected to the passive bus only via an appropriate
The OSI reference model provides the necessary
terminal adapter.
framework for arranging and developing protocols and
– Terminal adapter (TA)
interfaces for communication in open systems but it
The terminal adapter (TA) enables conventional
does not offer any technical solutions. It merely speci-
non-ISDN terminal equipment to be used in the
fies how the technology should behave outwardly. The
ISDN by adapting conventional interfaces to proto-
following brief description of the OSI reference model is
cols of the interface at reference point S.
intended as a basic introduction to permit better under-
– Network termination (NT)
standing of the sections to follow.
A network termination (NT) may consist of two com-
ponents (NT1 and NT2). Network termination 1 The layers
(NT1) ensures that the terminal equipment is phys-
ically matched to the network node access line. It The OSI reference model assigns the necessary com-
also enables the access line to be shared by more munication functions to seven layers.
than one terminal. In addition, an NT1 can support Layers 1 to 7 of an open system are arranged in a ver-
centralized maintenance irrespective of the operat- tical hierarchical structure. A lower layer, possibly sup-
ing state of the subscriber equipment (test loop) ported by a layer or layers beneath it, provides certain
and report transmission quality criteria to the net- defined services for the layer above it (if it exists). The
work node. The NT2 option contains switching functions of the lower layers are fundamental to the
functions, in other words it can be a PBX. If no such functions of the higher levels. The functions of layers 1
NT2 functions are needed then NT2 is not installed to 7 are defined in ITU-T Recommendation X.200. The
(zero NT2). principal tasks of the seven layers are as follows:
– Line termination (LT) – Layer 1: Controls the physical transmission medium
A terminates an access line in the network node as – Layer 2: Ensures data transfer via the links
far as transmission is concerned. Depending on – Layer 3: Establishes and switches the entire net-
whether it is used for a basic access or a primary work connection
rate access it can perform functions such as feed- – Layer 4: Provides an end-to-end transport service
ing the NT or the intermediate regenerators, provid- – Layer 5: Controls the end-to-end communication
ing test loops, signal regeneration and code link
conversion. – Layer 6: Creates a form of presentation for data
– Exchange termination (ET) communication which is not user- and device-de-
An ET terminates an access line in the network pendent
node as far as control is concerned; user and sig- – Layer 7: Controls user-specific communication
naling information pass via the exchange termina-
tion. In the network node it handles the protocol of
the data link layer (layer 2 of the OSI reference
model) of DSS1. If necessary, the signaling infor- Communi- 7 Application layer
mation received by the terminal equipment is con- cation
functions 6 Presentation layer
verted into a different format before being further
processed outside the ET. 5 Session layer
OSI
LTs and ETs may be integrated in a single function- reference 4 Transport layer
al unit. model
3 Network layer

2 Data link layer

1 Physical layer

4
The design of the physical connection paths (copper
wires, coaxial cable, optical waveguides, radio relay Terminal Network
equipment node
links or satellite links) needed to transfer user and sig-
Layer
naling information is not described in layer 1 of the OSI Layer 3 protocol
reference model. Entity of the (logical Entity of the
network network 3
connection)
Although the stipulations of the OSI reference model layer layer
are valid for all seven layers, only layers 1, 2 and 3 are Service Service
described in detail below since only these three layers primitives primitives
are relevant to DSS1. References to the higher layers Layer 2 protocol
Entity of the Entity of the
are included for the sake of completeness only. (logical 2
data link data link
connection)
layer layer
Communication between the layers
Service Service
In open systems the individual layers consist of func- primitives primitives
tions performed by various equipment (such as termi- Layer 1 protocol
Entity of the Entity of the
nal equipments and text and databases). The OSI (Setting up and
physical clearing down the physical 1
reference model defines the assignment of functions layer layer
physical
(entities) to the individual layers but not the way in connection)
which these functions are implemented. In keeping with
the structure of the OSI reference model, the entities of D channel
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 com-
munication between adjacent entities of a terminal and Example:
communication with entities of other terminals. (Forwarding of signaling information from a
Within a terminal two vertically adjacent entities com- terminal equipment to the ISDN network node)
municate, in an abstract sense, by means of ”service With the aid of service primitives as the means of
primitives” in order to make use of the services of the communication, the signaling information in a termi-
layer below (if there is such a layer) so that it can per- nal equipment, for example, passes step by step
form its own services and make its services available to from the layer 3 entity to the layer 2 entity and then
the layer above. to the layer1 entity.
Entities in one terminal can only communicate with en-
Terminal equipment Network node
tities of the same layer in other terminals (horizontal
Layer
communication). These communicating entities in dif-
ferent terminals are called peer entities. Communica- Entity of the Entity of the
tion between peer entities is regulated by a protocol. network layer network layer 3
Peer entities may be in the same open system or in dif-
ferent open systems. The figure below shows an exam-
P3 Signaling P3 Signaling
ple of peer entities of layers 1 to 3 for DSS1 between a
terminal equipment and an ISDN network node.
The peer entities involved exchange information in the Entity of the data Entity of the data
link layer link layer 2
form of protocols with protocol elements (dotted lines).
Logical connections are set up for communication in
the horizontal direction. The physical connections,
P2 P3 Signaling P2 P2 P3 Signaling P2
however, all pass via the D channel (layer 1). The ser-
vice primitives are used for communication in the verti-
cal direction via the entities.
Entity of the Entity of the
physical layer physical layer 1

D channel

P2 P3 Signaling P2

P = Protocol element

5
The layer 3 and 2 entities add appropriate protocol ele- ITU-T has defined layers 1 to 3 for ISDN to provide se-
ments to the signaling information which are needed for cure transfer of signaling information and low
performing the tasks in question. The layer 1 entity transfer rate data on the D channel. The figure below
does not add any protocol elements as such but does shows how the layers are distributed for the exchange
have recourse to a simple protocol. It sends a bit of signaling information between two terminal equip-
stream containing the signaling information and the ments. For the transfer of low transfer rate data on the
protocol elements to the layer 1 entity in the network D channel the application-oriented layers (4 to 7) have,
node. Layers 1 to 3 in the network node check the in- at most, only an end-to-end function between the termi-
formation received for formal correctness with the aid of nals.
their respective protocols. When it has performed its
check and removed its own protocol elements each lay-
ISDN
er passes the information to the next higher layer or to
Terminal Terminal
the signaling application. equipment Layer Layer equipment

Protocol architecture in the B and D channels

Circuit-switched communication networks provide only
a layer 1 connection between the two terminal equip- Network Network
ments for exchanging user information via the B chan- node node
nel. The network nodes through-connect transparent 3 3
layer 1 connections in their switching networks accord- 2
2
ing to the signaling information received. The functions
1 1
of the other layers (2 to 7) are used in the terminal
equipments according to the particular service and are
D channel D channel
implemented only in these terminal equipments. Con-
sequently, they are significant only as end-to-end func- ITU-T recommendations
tions. Q.930/I.450 and Q.931/I.451
Q.920/I.440 and Q.921/I.441
I.430 for BA or I.431 for PA

ISDN
Terminal Terminal
equipment Layer Layer equipment The complete protocol architecture in an ISDN terminal
7 7 for communication via the B and D channels is shown
6 6 in the figure below, using a multiservice terminal con-
nected to a basic access as an example. The relevant
5 5
ITU-T Recommendations are included in the diagram.
4 4

3 3
Network Network
ISDN multiservice terminal
2 node node 2

1 1 Signal- Low transfer Service 1 Service 2

ing rate data
Layer
B channel B channel
High-level 7
protocols
for low 6
ITU-T recommendations transfer rate High-level proto- High-level proto- 5
I.430 for BA or I.431 for PA data cols appropri- cols appropri-
ate to service 1 ate to service 2 4
I.450 and I.451 *) 3
I.440 and I.441 *) 2

I.430 *) 1

B channel
B channel
D channel

*) ITU-T Recommendations for basic access Basic

access

6
Physical layer (layer 1) A pseudo-ternary code is used for the transmission of
pulse frames between the TE and the NT. In this code
Layer 1 provides the higher layers with the digital trans- the binary values of "1" are transmitted at zero voltage
mission paths for both directions of transmission, i.e. and binary values of "0" alternately at positive and neg-
with the B channels for user information and the D ative voltage. Two intentional code violations are used
channel for signaling information. The transmission ca- for pulse frame detection.
pacity of the D channel and the number of B channels – 1st code violation:
depend on whether the connection is a basic access (2 The L bit (bit number 2) and the first zero bit after
B+D), a 2048-kbit/s primary rate access (30 B+D) or a the L bit (but no later than the FA bit (bit numbers 3
1544-kbit/s primary rate access (23 B+D). Additional to 14)) are both transmitted at negative voltage.
services for layer 2 include setting up and clearing – 2nd code violation:
down the physical connection, D channel access for the The last zero bit of a pulse frame and the following
basic access, maintenance functions and a layer 1 sta- F bit (bit number 1) of the next pulse frame are both
tus indication. The characteristics of layer 1 are de- transmitted at positive voltage.
scribed in detail below with reference to the D channel.
To ensure satisfactory transmission of information
Basic access (from the TE to the NT) via a passive bus, ordered ac-
cess to the D channel is assured for each TE in a multi-
Reference point S/T between terminal equipment
device configuration. Defined priorities ensure that
and the network termination
transmission of the signaling information takes prefer-
ITU-T Recommendation I.430 provides for one 16- ence over all other forms of information (packet data,
kbit/s D channel and two 64-kbit/s B channels for both telemetry data, user-user information). Before informa-
directions of transmission between the terminal equip- tion is transmitted on the D channel a TE must check for
ment (TE) and the network termination (NT) of a basic the idle state (permanent binary "1" on the D echo
access. Transmissions between TE and NT take place channel). If information is transmitted simultaneously
in full duplex mode at a bit rate of 192 kbit/s. The pulse from two or more TEs there is a mechanism to ensure
frames used contain 48 bits each and have a total that only one TE can complete transmission (D channel
transmission time of 250 µs. In one second 4000 such contention resolution). For this purpose, the NT loops
pulse frames are transmitted (48 bits x 4000/s = 192 back the D channel bits received from the TE (D bits) to
kbit/s). Four of the 48 bits of each pulse frame (D bits) the TEs on the D echo channel (E bits). The TEs com-
constitute the D channel (4 bits x 4000/s = 16 kbit/s). pare the bit received on the D echo channel with the
Within the pulse frame the D bits occupy bit numbers last D bit sent. If a TE ascertains that the bits sent and
12, 25, 36 and 47. received are the same it continues sending information;
In the direction of transmission from the NT to the TE if, however, it finds that they are not the same (binary
there are also four E bits which form a D echo channel "0" = positive or negative voltage instead of binary "1"
(4 bits x 4000/s = 16 kbit/s). The bit numbers of the E = zero voltage) it immediately stops sending informa-
bits in such a pulse frame are 11, 24, 35 and 46. The D tion (collision detection). The other TE continues to
echo channel is used to control TE access to the D transmit. TEs which interrupt transmission have to wait
channel (collision detection). for the next opportunity to transmit via the D channel.
The decisive factor for setting the time difference be-
tween 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 direction of the
NT with an offset of 2 bits.

7
Reference point U between the network termination 2048-kbit/s primary rate access
and the network node The 2048-kbit/s primary rate access is specified in ITU-
The NT encodes the B and D channel information re- T Recommendation I.431. It uses a pulse frame as de-
ceived from the TEs in a suitable form for transmission fined in ITU-T Recommendation G.704. The pulse
via the two-wire copper subscriber line to the network frame contains 32 channel time slots of 8 bits each:
node (non-ITU-T-standardized interface at reference – one channel time slot for frame alignment, service
point U, see User-network interface). In the opposite di- signals etc.
rection of transmission the NT receives the information – 30 channel time slots for the 30 B channels
sent from the network node, decodes this information – one channel time slot for the D channel
and forwards it to the TEs. In addition to two B channels 2048-kbit/s transmission systems use the same pulse
(64+64 kbit/s) and one D channel (16 kbit/s) synchroni- frame structure.
zation and control information is transmitted in both di-
rections. Transmission on the subscriber line involves
8 7 6 5 4 3 2 1 Bit number
signals with no dc components in full duplex mode. The
common-frequency procedure used separates the di- Channel for frame alignment Channel time slot 0
rections of transmission by means of echo compensa- word/service word
tion (see also Signaling System No.7 (SS7)). This B channel 1 Channel time slot 1

32 channels x 8 bits = 256 bits

procedure enables a transmission range to be Channel time slot 2
B channel 2
achieved which is adequate for public systems. Be-
cause of echo compensation those block codes which ......
lead to a reduction in the transmission speed on the
B channel 15 Channel time slot 15
subscriber line at the same bit rate are particularly use-
ful for coding. Examples of these are the 4B/3T and D channel Channel time slot 16
2B/1Q codes. With the 4B/3T code four bits of a binary B channel 16 Channel time slot 17
signal can be represented by three ternary signal ele-
......
ments, and with the 2B/1Q code two bits of a binary sig-
nal by one quaternary signal element. The transmission Channel time slot 31
B channel 30
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 infor-
mation) is transferred on the subscriber line at 120 1544-kbit/s primary rate access
kbaud (4B/3T code) or 80 kbaud (2B/1Q code). The 1544-kbit/s primary rate access also conforms to
ITU-T Recommendation I.431 and also uses a pulse
Primary rate access
frame as defined in ITU-T Recommendation G.704.
In this case all the channels, in other words the B chan- The pulse frame contains one time slot of 1 bit and 24
nels (user information) and the D channels (signaling channel time slots of 8 bits each:
information), have a bit rate of 64 kbit/s. Channels with – one 1-bit time slot (F bit) for frame alignment, per-
the same number in the pulse frame are used for trans- formance monitoring etc.
mitting user information in both directions. There are – 23 channel time slots for the 23 B channels
two types of primary rate access (see also Signaling – one channel time slot for the D channel
System No.7 (SS7)).
1544-kbit/s transmission systems use the same pulse
frame structure.

8 7 6 5 4 3 2 1 Bit number

F-
Bit time slot
1 bit + 24 channels x 8 bits

Bit
B channel 1 Channel time slot 1
= 193 bits

B channel 2 Channel time slot 2

......

B channel 23 Channel time slot 23

D channel Channel time slot 24

8
Data link layer (layer 2 of DSS1)
8 7 6 5 4 3 2 1 Bit numbering
The data link layer (layer 2, ITU-T Recommendations
Flag Octet 1
Q.920/I.440 and Q.921/I.441) ensures reliable error-
free transfer of layer 3 information (signaling informa- Address field Octet 2
tion and low transfer rate data) via the D channel. For Octet 3

actual transfer, layer 2 makes use of the services of the Control field Octet 4
physical layer (layer 1). *)
Information field
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 balanced
(LAPB, ITU-T Recommendation X.25) and the HDLC Frame check Octet m-2
sequence field Octet m-1
(high-level data link control) standards defined by the
International Standardization Organization (ISO 3309 Flag Octet m
and ISO 4355). LAPD offers the following:
– establishment of one or more layer 2 connections 8 7 6 5 4 3 2 1 Bit numbering
on the D channel for several terminals connected to
Flag Octet 1
a basic access and several layer 3 entities
– frame formation with transparent transfer for layer 3 Address field Octet 2
information Octet 3
– frame sequence control Control field Octet 4
– error detection and automatic frame repetition *)
– protocol error recording Frame check Octet n-2
– flow control sequence field
Octet n-1
– administration functions for layer 2 Flag Octet n

Frame *) The second octet of the control field is not used in the U frame
without a sequence number for the transfer of unnumbered
The layer 2 frame structure with and without the infor- unacknowledged information and control functions.
mation field is shown in the figure below. The frames
are used for activating and deactivating layer 2, for
transferring layer 3 information and for performing inter- – Flag
nal layer 2 control and supervision functions. Each frame starts and ends with a flag. The flags
The layer 2 frames are divided into two categories: always have the same bit pattern: 01111110. Be-
commands (C) and responses (R). Whether com- tween the opening and closing flags of a frame the
mands have to be acknowledged (i.e. require respons- transmitter automatically inserts a "0" after five con-
es) or not depends on the particular functions being secutive "1"s. The receiver then masks out these in-
performed. ITU-T has defined the "multiple frame oper- serted "0" bits. This makes flag detection
ation" procedure for information transfer with acknowl- unambiguous (in the idle state – between frames –
edgments. In this procedure a number of frames sent the terminals send a continuous sequence of "1"
one after the other can be acknowledged as a group, signal elements). The closing flag of a frame can
which means that there is no need for each frame to be also be the opening flag of the next frame.
acknowledged immediately (see control field). – Address field
The layer 2 frames with their changing addresses and The address field consists of two octets and
frame lengths differ considerably from the repetitive uniquely identifies a layer 2 connection. It contains
pulse frames with permanently assigned channel time two address field extension bits (EA), a com-
slots of digital transmission systems or of the primary mand/response bit (C/R bit), a service access point
rate access (see Signaling System No.7 (SS7)). identifier (SAPI) and a terminal endpoint identifier
(TEI).

8 7 6 5 4 3 2 1 Bit numbering

SAPI C/R EA (=0) Octet 2

TEI EA Octet 3

9
– Address field extension bits – Terminal endpoint identifier (TEI)
With the EA bits the address field length is extend- The TEI in the address field denotes a terminal for
ed to or defined at two octets. The EA bit of the first explicit transfer of a message. Functional groups of
address field octet is assigned the binary value "0" multiservice terminals can have their own TEI in the
and the EA bit of the second address field octet the same way as individual terminals. TEIs enable ter-
binary value "1". Binary "1" of the second EA bit in- minals within an information class (same SAPI) to
dicates the last octet of the address field. be differentiated, and with a particular common TEI
– Command/response bit it is possible to access a number of terminals at the
The C/R bit indicates whether a frame contains a same time (broadcasting). Depending on the de-
command or a response. sign of the terminal, a TEI can be assigned to a ter-
minal either by the user or automatically by the
Frame Transmission direction Binary value of network. The available 7 bits of the address field
contents the C/R bit give a possible 128 different TEI values, numbered
Command Network –> Terminal 1 from 0 to 127. Bit 2 of octet 3 is the least significant
bit (LSB) and bit 8 the most significant bit (MSB).
Terminal –> Network 0
The applications of the TEI are shown in the table
Response Network –> Terminal 0 below.
Terminal –> Network 1
TEI Applications
– Service access point identifier (SAPI)
0 through 63 Assigned by the user
The SAPI in the address field denotes the class of
information to be transferred. These information 64 through 126 Automatically assigned by the network node
classes are used to differentiate between signaling,
127 Broadcasting and for assigning TEIs 64
layer 2 administrative information and packet data through 126
including user-user information. With the six bits of
the address field a total of 64 information classes, – Control field
numbered from 0 to 63, can be identified. Bit 3 of The control field contains the code for identifying
octet 2 is the least significant bit (LSB) and bit 8 the the type of frame. There are three formats for con-
most significant bit (MSB). The meanings of the de- trol fields:
fined SAPIs are shown in the table below. I format for serially numbered acknowledged infor-
mation transfer (I frame)
SAPI Information class S format for supervisory control functions (S
0 Signaling frame)
U format for unnumbered unacknowledged infor-
1 Packet data (Q.931 signaling procedures)
mation transfer and control functions (U frame).
16 Packet data (X.25 layer 3 procedures) The control fields with the I format of all the frames
to be transmitted are each given a send sequence
63 Layer 2 administration function number N(S). The receive sequence numbers N(R)
(TEI administration)
of I and S frames acknowledge the error-free recep-
2 through 15 For future applications tion of all I frames up to send sequence number
and 17 through N(S) = N(R) –1. Sequence numbers N(S) and N(R)
62
are used to supervise the continuous and error-free
exchange of frames.
In multiple frame operation with modulus 128,N(S)
and N(R) can be assigned the value 0 to 127 (128
different values). With this operating mode the max-
imum difference (window size) between N(S) and
N(R) can be 127. The window size indicates the
number of I frames that a layer 2 instance can
transmit without receiving an acknowledgment.
ITU-T has defined the following window sizes:
for signaling:
- basic access: 1 I frame
- primary rate access: 7 I frame

10
 for packet data Layer 2 addressing
- basic access: 3 I frame Information transfer via the D channel of a subscriber
- primary rate access: 7 I frame line takes place in the same way whether from the ter-
The value of the P (poll) bit and the F (final) bit may minals to the network node or from the network node to
be binary ”0” or ”1”: the terminals. The layer 2 addressing procedure via the
A command with P bit = ”1” requests a response D channel can best be described with reference to an
from the layer 2 entity of the receiver. In the result- example. To simplify matters this example only deals
ant receive response the F bit has the binary value with the addressing procedure for information transfer
”1”. from the network node to the terminal equipment. For
A command with P bit = ”0” does not require any the purposes of information transfer the relevant net-
particular response. In a non-requested response work node provides appropriate frames and inserts an
the F bit has the binary value ”0”. appropriate service access point identifier (SAPI) in the
The S and M bits determine the function of the address field. The value of this SAPI depends on
frame. whether, for example, signaling information (SAPI = 0)
or packet data (SAPI = 16) is being transferred. The
I format network node also includes the relevant terminal end-
8 7 6 5 4 3 2 1 Bit numbering
point identifier (TEI) in the address field. In our example
TEI = 64 or 71 for a particular user terminal (the TEIs
N(S) 0 Octet 4 have been automatically assigned by the network
N(R) P Octet 5 node) or TEI = 127 for addressing all the terminals si-
multaneously (broadcasting).
S format

8 7 6 5 4 3 2 1 Bit numbering Terminal equipment Subscriber Network node

line
0 0 0 0 S S S 1 Octet 4
Multiservice terminal 1
N(R) P/F Octet 5
B channels
Voice text (64 kbit/s) Signal-
U format
data images ing
8 7 6 5 4 3 2 1 Bit numbering D
channel SAPI
M M M M M TEI = 64
P/F 1 1 Octet 4 (16 kbit/s) =0
Signal- SAPI = 0 TEI = 71
ing TEI = 64 TEI =
TEI = 127 127

– Information field
The information field consists of an integer number SAPI = 16
of octets and may contain as many as 260 octets. TEI = 64
Packet
TEI = 127
The contents of the information field form part of data
layer 3.
– Frame check sequence field
The frame check sequence field consists of two oc- Multiservice terminal 2
tets. A frame check sequence (FCS) is used to de-
tect transmission errors on the D channel. The Voice text
transmitter uses an algorithm to form a 16-bit FCS data images
from the contents of the address, control and infor-
mation fields of a frame. The receiver uses the Signal- SAPI = 0
same algorithm to calculate the FCS and compares ing TEI = 71 Packet
it with the FCS received from the transmitter. If the TEI = 127 data
two FCSs are identical then transmission is error- TEI = 64
free. SAPI = 16 SAPI TEI = 71
TEI = 71 =16 TEI =
Packet
TEI = 127 127
data

11
The individual frames arrive at the terminals via the D – I frame:
channel. All the terminals examine the address field The numbered I frames transfer the layer 3 informa-
(TEI and SAPI) and ascertain whether the frame is in- tion to be acknowledged via the layer 2 connection.
tended for them. If this is the case, the layer 2 message The figure shows how the send and receive se-
is evaluated and, if there are I and U frames, the infor- quence numbers N(S) and N(R) are incremented.
mation field of the frame is forwarded in transparent
form to the relevant layer 3.
The information field may, for example, contain signal- Terminal Network node
equipment
ing, packet data or layer 2 administration data. In the
case of signaling, the actual voice, text, data or image Send 0 I frame (Control field: Receive 0
information is sent via an assigned B channel or, if the N(S)=0, N(R)=0)
video telephony service is being used, currently via two Receive 0, I frame (Control field: Send 0,
0 acknowledged N(S)=0, N(R)=1) acknowledge 0
B channels.
Send 1, I frame (Control field: Receive 1,
If TEI = 127 all the terminals equipped with an appropri-
acknowledge 0 N(S)=1, N(R)=1) 0 acknowledged
ate SAPI receive the messages. What happens then
1 acknowledged S frame (Control field: Acknowledge 1
depends on the type of message. Layer 2 administra- N(R)=2)
tion messages with SAPI = 63 (administration function) Send 2, Receive 2,
I frame (Control field:
can be used, for example, for assigning TEIs to the ter- (0 acknowledged) N(S)=2, N(R)=1) (0 acknowledged
minals. In all terminals with appropriate SAPIs the infor- 2 acknowledged Acknowledge 2
S frame (Control field:
mation fields are forwarded to layer 3. N(R)=3)
Send 3, I frame (Control field: Receive 3,
Commands and responses and their functions (0 acknowledged (0 acknowledged
N(S)=3, N(R)=1)
The commands and responses of layer 2 are listed in Receive 1, I frame (Control field: Send 1,
the table below. The information as to which command 3 acknowledged N(S)=1, N(R)=4) acknowledge 3
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 ITU-T Recommendation
Q.921.
– S frame:
The RR, RNR and REJ control functions are used
Applica- Control field Commands Responses
for controlling layer 2 transfers (see table). If there
tions formats
is no layer 3 information to be sent, the control func-
Unacknowl- Serially-num- Information (I) tions may also acknowledge received I frames.
edged and bered informa-
multiple tion transfer (I)
frame ac- Command/response Tasks
Supervisory Receive ready Receive ready
knowledged
control func- (RR) (RR) Receive ready (RR) Indicate ”ready to receive” (I
information
tions (S) frame)
transfer Receive not Receive not
ready (RNR) ready (RNR) Acknowledge received I frames
Cancel a temporary busy state
Reject (REJ) Reject (REJ) previously indicated by RNR
Unnumbered in- Set asynchro- Receive not ready (RNR) Indicate a temporary busy state
formation trans- nous balanced Interrogate the status of a peer
fer and control mode extend- entity (if P bit = 1)
functions (U) ed (SABME)
Reject (REJ) Request retransmission of an I
Disconnected frame, possibly in connection
mode (DM) with the
– cancelation of a temporary
Unnumbered
busy state previously indicated
information
by RNR
(UI)
– implicit indication of the
Disconnect receive ready status
(DISC) – status interrogation of a peer
entity (if P bit = 1)
Unnumbered
acknowledg-
ment (UA)

Frame reject
(FRMR)

Connection Exchange Exchange

manage- identification identification
ment (XID) (XID)

12
– U frames: Assignment of the terminal endpoint identifier
The SABME, DISC, UA and DM control functions For a terminal to be able to communicate with the net-
(see table) are used to set up and clear down ac- work node it must be assigned a unique TEI value. As
knowledged layer 2 connections for multiple frame far as TEI assignment is concerned, there are two cat-
operation. The UI commands of the U frame (UI egories of terminal:
frame) are used for transferring information which – terminals without automatic TEI assignment
does not have to be acknowledged. These com- (TEI values from 0 to 63)
mands relate to the assignment, checking, removal, – terminals with automatic TEI assignment
identification and confirmation of TEIs (for SAPI = (TEI values from 64 to 126)
63, assignment of a TEI, shown in figure) and also
In the case of terminals without automatic TEI assign-
to the broadcasting of information to all the termi-
ment, the user must ensure that unique TEI values are
nals of the called party. The FRMR control function
assigned to the terminals (e.g. by setting the TEI value
acknowledges received frames which do not con-
on the terminal).
form to the protocol in use, indicates the protocol
error and calls for the layer 2 connection to be reset. The TEI assignment procedure for terminals with auto-
Frames with the XID control function can be ex- matic TEI assignment is described below.
changed between the layer 2 entities so that proto- If a terminal has automatic TEI assignment it is easier
col parameters can be changed as required. for the user to use this terminal on different access
lines.
Command/ Tasks – Each time the terminal is plugged in it uses a UI
response
frame to request a TEI from the layer 2 administra-
Set asynchronous Setup request for an acknowledged tion entity in the network node. In addition to the ad-
balanced mode ex- layer 2 connection dress field with SAPI = 63 and TEI = 127, such a UI
tended (SABME)
frame contains a randomly generated reference
Disconnect (DISC) Disconnection request for an number Ri, the "identity request" message type and
acknowledged layer 2 connection
an action indicator Ai = 127. The reference number
Unnumbered ac- Positive response to SABME or DISC, ranges from 0 to 65,535 and is used to discriminate
knowledgment (UA) possibly in connection with
– cancellation of a temporary busy
between different simultaneous operations. If there
state previously indicated by RNR are TEI values in the 64 to 126 range free then the
Disconnected Indicate lack of readiness to accept an
layer 2 administration entity in the network node will
mode (DM) acknowledged layer 2 connection assign a free TEI to the terminal. This TEI is for-
warded to the terminal in the Ai field of an "identity
assigned" UI frame. The terminal checks the refer-
Terminal Network node ence number to make sure that the TEI is intended
equipment
for it and stores this TEI. All subsequent messages
UI frame (SAPI, TEI)
to or from this particular terminal will contain this
(information field: ID request,
Ri, Ai) TEI in the address field. The TEI remains valid for
UI frame (SAPI, TEI) the terminal until the terminal is disconnected from
(information field: the network, either intentionally or as a result of a
ID assigned, Ri, Ai) fault/error, or the TEI is withdrawn by the layer 2 ad-
OR ministration entity in the network node.
UI frame (SAPI, TEI) If there are no free TEI values available when the
(informationfield:
ID denied, Ri, Ai)
request is made, the layer 2 administration entity in
the network node cannot assign a TEI value to the
terminal. In this case, the terminal receives an
In all three cases: address field SAPI=63, TEI=127
ID = identity
"identity denied" UI frame.
Ri = reference number The layer 2 administration entity in the network
Ai = action indicator node can also verify the assigned TEIs of the vari-
ous 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.

13
Network layer (layer 3 of DSS1)
For basic access or primary rate access
The network layer comprises functions for establishing, (see call reference)
maintaining and releasing connections (ITU-T Recom-
mendations Q.930/I.450 and Q.931/I.451). It is also 8 7 6 5 4 3 2 1 Bit numbering

used for controlling supplementary services (ITU-T Protocol discriminator Octet 1

Recommendation Q.932). For all its functions layer 3
Call reference Octet 2
uses the services of layers 1 and 2 to ensure reliable
Octet 3
transfer of the necessary messages.
Message type Octet 4
Message structure Information elements Octet 5
The layer 3 entities supply the complete messages for ......
transfer in layer 2 information fields. The number of oc-
tets in the message may vary but is never more than For primary rate access
260. The DSS1 messages, internationally standardized (see call reference)
by ITU-T, have a uniform structure and each contains
8 7 6 5 4 3 2 1 Bit numbering
the prescribed protocol discriminator, a call reference,
the message type and a number of information ele- Protocol discriminator Octet 1
ments.
Call reference Octet 2
Octet 3
Protocol discriminator octet Meaning
(Bit numbering) Octet 4
Message type Octet 5

8 7 6 5 4 3 2 1 Information elements Octet 6

......
0 0 0 0 0 0 0 0 Protocol discriminators in user-
to user information elements. Not
0 0 0 0 0 0 0 0 available for messages for
user-network call control

0 0 0 0 1 0 0 0 Messages for user-network – Protocol discriminator

call control The protocol discriminator occupies the first octet in
ITU-T Recommendation Q.931) each layer 3 message. It identifies the layer 3 pro-
0 0 0 1 0 0 0 0 Reserved for other network tocol used to define the meaning and use (mes-
to layer or layer 3 protocols sage type) of the message. For user-network call
0 0 1 1 1 1 1 1 (including X.25 protocol)
control in DSS1 ITU-T has defined the bit pattern
0 1 0 0 0 0 0 0 National applications outlined in gray in the table above. This table shows
to
0 1 0 0 1 1 1 1
the meanings of the various protocol discriminator
values according to ITU-T. Within this given frame-
0 1 0 1 0 0 0 0 Reserved for other layer 3
to protocols (including X.25
work individual operating companies can freely as-
1 1 1 1 1 1 1 0 protocol) sign meanings to the protocol discriminators and
thus define specific protocols.
All other values yet to be defined. – Call reference
The call reference is the second part of each layer
3 message. Each call reference contains a call ref-
erence value. In layer 3 a call reference value es-
tablishes the unique relationship between the
message and a particular call or a particular sup-
plementary service control operation. These rela-
tionships apply only to the relevant layer 2 path
between a terminal equipment and a network node;
in other words they have no end-to-end signifi-
cance. 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 as-
signed 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.

14
 The call reference may consist of:
two octets in the case of basic access and Message type octet Meaning
three octets for primary rate access or (Bit numbering)
two octets as a network option.
Bits 1 to 4 of the first call reference octet indicate 8 7 6 5 4 3 2 1
the length of the subsequent call reference value
0 0 0 0 0 0 0 0 National application: message type
(i.e. one or two octets). Call references consisting defined in subsequent octet
of one octet may be assigned values from 0 to 127,
0 0 0 - - - - - Messages for call setup
those consisting of two octets may be assigned val-
ues from 0 to 32,767. The originating side of the 0 0 0 0 1 ALERTING
calls defines the call reference values relevant to it. 0 0 0 1 0 CALL PROCEEDING
0 0 1 1 1 CONNECT
The full range of call reference values is available to
0 1 1 1 1 CONNECT ACKNOWLEDGE
each originating side. A marker bit (bit eight in the 0 0 0 1 1 PROGRESS
second call reference octet) identifies the origin 0 0 0 0 1 SETUP
(subscriber terminal or network node equipment) of 0 1 1 0 1 SETUP ACKNOWLEDGE

a call reference. The originating side sets the mark- 0 0 1 - - - - - Messages during the active call
er bit to binary "0". In call-related messages from phases
0 0 1 1 0 RESUME
the remote end the marker bit is always inverted (bi- 0 1 1 1 0 RESUME ACKNOWLEDGE
nary "1"). 0 0 0 1 0 RESUME REJECT
0 0 1 0 1 SUSPEND
0 1 1 0 1 SUSPEND ACKNOWLEDGE
8 7 6 5 4 3 2 1 Bit numbering 0 0 0 0 1 SUSPEND REJECT
0 0 0 0 0 USER INFORMATION
Length of the call
Octet 2 0 1 0 - - - - - Messages for call cleardown
0 0 0 0 reference value
0 0 0 0 0 0 1 0 1 DISCONNECT
0 1 1 0 1 RELEASE
Marker Call reference Octet 3 1 1 0 1 0 RELEASE COMPLETE
bit value 0 0 1 1 0 RESTART
0 1 1 1 0 RESTART ACKNOWLEDGE
a) Call reference value contained in one octet
0 1 1 - - - - - Miscellaneous messages

0 0 0 0 0 SEGMENT
8 7 6 5 4 3 2 1 Bit numbering 1 1 0 0 1 CONGESTION CONTROL
1 1 0 1 1 INFORMATION
Length of the call 0 0 0 1 0 FACILITY
0 0 0 0 reference value Octet 2 0 1 1 1 0 NOTIFY
0 0 0 0 1 1 1 0 1 STATUS
1 0 1 0 1 STATUS ENQUIRY
Marker Call Octet 3
bit reference
value
Octet 4 Message type octet Meaning
(Bit numbering)
b) Call reference value contained in two octets
8 7 6 5 4 3 2 1

0 0 1 - - - - - Messages during the active call

phases (Q.931, see Table above)
– Message type HOLD
0 0 1 0 0 HOLD ACKNOWLEDGE
The message type constitutes the third part of each 0 1 0 0 0 HOLD REJECT
layer 3 message. It indicates the function of the 1 0 0 0 0 RETRIEVE
messages just sent. For the codes defined here bit 1 0 0 0 0 RETRIEVE ACKNOWLEDGE
1 0 0 1 1 RETRIEVE REJECT
8 of the message type octet is always set to "0" and 1 0 1 1 1
is provided as a possible extension bit for the future.
0 1 1 - - - - - Miscellaneous messages (Q.931,
The codes for the individual message type octets see Table above)
are listed in the two tables below. 0 0 0 1 0 FACILITY
0 1 0 0 REGISTER

15
– Information elements In a message the multiple-octet information ele-
The fourth and last part of a message consists of ments within a codeset appear in ascending order
the information elements assigned to the message in accordance with the binary values of the informa-
type. The information elements contain the actual tion elements. This makes it easier for the receiving
information to be transferred which is needed, for equipment (entities) to detect the information ele-
example, for setting up a call or for controlling a ser- ments. Single-octet information elements, on the
vice. A layer 3 message may contain one or more other hand, may appear at any point within a mes-
information elements, or none at all. There are two sage.
categories of information element:
single-octet information elements Type 1 (Single-octet information elements)
multiple-octet information elements
There are two types of single-octet information 8 7 6 5 4 3 2 1 Bit numbering

element: 1 Identifier Contents

Type 1 consists of an information element identifier
(bits 5 to 7) and a contents part (bits 1 to 4) with var- Type 2 (Single-octet information elements)
ious parameters. These parameters may, for exam-
ple, be a code set changeover (shift, see below), an 8 7 6 5 4 3 2 1 Bit numbering
overload level or a repeat indicator. 1 0 1 0 Identifier
Type 2 transfers only an information element iden-
tifier. This corresponds to a message such as Multiple-octet information elements
"Sending complete" or "More data". Bits 5 to 7 are
permanently set to "010". 8 7 6 5 4 3 2 1 Bit numbering
The multiple-octet information elements vary in 0 Identifier
length (three or more octets). The first octet of such
Length of contents
an element contains the information element iden-
tifier. This indicates whether the information ele- Contents
......
ment contains, say, the called party number, user-
user information or a call status (see ITU-T Recom-
mendations Q.931 and Q.932 for complete lists).
The second octet of the multiple-octet information 8 7 6 5 4 3 2 1 Bit
element specifies the number (length) of the subse- numbering
quent octets (a binary value between 0 and 255). 0 1 1 1 0 0 0 0
Transfer of some of the octets of an information el-
ement is optional, which means that information el- Length of contents

ements with the same identifier may nevertheless

consist of different numbers of octets. 1 Directory Numbering plan
number type identification
With the bits available it is possible to encode the
following numbers of information element identifi- 0 1st digit

ers:
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 vari- 0 nth digit

able);
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 oc-
tets. The shift octets enable several codesets with
different meanings 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 information elements until
the next shift (locking shift).

16
Use of layer 3 messages The network node on the called side of the connection
The layer 3 messages are identified by their message transfers the connection request with a SETUP mes-
type. Layer 3 messages are generally transferred via sage to all the terminal equipment (TE1 and TE2) con-
acknowledged layer 2 connections. Only those layer 3 nected to the called subscriber line. The SETUP
messages which are sent from the network node to a message contains information on the service and may
group of terminals (e.g. a SETUP message for an in- also contain a multiple subscriber number for access-
coming call) are transferred via unacknowledged layer ing a particular terminal. The SETUP message is trans-
2 connections (UI frame) with TEI = 127. ferred without acknowledgment. If no message (e.g.
ALERTING or CONNECT) is received from the called
The following description of how a telephone call is set
side the network node sends the SETUP message
up with digit selection illustrates the use of layer 3 mes-
again after a period of time T to the called subscriber
sages: the calling party initiates connection setup (lifts
line.
his handset) by sending a SETUP message to the net-
work node. In this example the network node acknowl- In this example a global SETUP message is used
edges this with a SETUP ACKNOWLEDGE message, which is targeted at all the called party's service-com-
with which the terminal of the calling party is assigned patible terminals. The two service-compatible terminals
a B channel. If there are no digits in the SETUP mes- of the called party each respond with an ALERTING
sage the subscriber can receive the dialing tone. The message. On receipt of the ALERTING messages the
outstanding digits are sent to the network node in one network node sets up separate, individually address-
or more INFORMATION messages. able signaling connections via the D channel. By send-
ing these messages the terminals indicate that they are
ready to accept the call. Receipt of the first ALERTING
message is reported to the terminal of the called party
likewise by an ALERTING message. The calling party
receives ringing tone.

Calling party Exchange Called party

TE ET ET TE1 TE2

SETUP

SETUP ACK

INFORMA- Network
TION signaling

INFORMA-
TION SETUP SETUP

ALERT-ING
ALERT-ING

ALERT-ING

CON-NECT CON-NECT

CON-NECT CON-NECT
ACK ACK

RELEASE

RELEASE
COMP

17
If the called party answers (e.g. by lifting his handset) On the called side all the other terminals which have
then his terminal will send its network node a CON- sent an ALERTING or CONNECT message but which
NECT message. The first CONNECT message to ar- have not been given the call receive a RELEASE mes-
rive determines which terminal is responsible for the sage. These terminals each acknowledge the RE-
call. The network node of the called party informs this LEASE message with a RELEASE-COMPLETE
terminal with the aid of a CONNECT-ACKNOWLEDGE message and switch over to the idle state.
message containing the individual layer 2 address that
it has been selected for the telephone call. The terminal
connects itself to the B channel assigned to the call. A
CONNECT message informs the terminal of the calling
party that the call has been accepted. Ringing tone is
disconnected and call charge metering is started for the
telephone call now completely through-connected via
the B channel. The calling terminal may optionally ac-
knowledge receipt of a CONNECT message with a
CONNECT-ACKNOWLEDGE message.

Copyright (C) Siemens AG 2002

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