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CCS7-English

Information
Signaling
Common Channel Signaling System
No.7 (topic 8)
A30808-X2798-X6-1-7618


Common Channel Signaling System No.7 (topic 8)

Information Signaling

Copyright (C) Siemens AG 1995
Issued by the Public Communication Network Group
Hofmannstraße 51
D-81359 München
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 Signaling

Common Channel Signaling System No.7 (topic 8)

This document consists of a total of 43 pages. All pages are issue 1.

Contents
1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2

Signaling Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3
3.1
3.1.1
3.1.2
3.1.3
3.2
3.2.1
3.2.2
3.2.3

Structure of CCS7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Message Transfer Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Addressing of the Signal Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
User Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Integrated Services Digital Network User Part . . . . . . . . . . . . . . . . . . . . . .
Signaling Connection Control Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Transaction Capabilities Application Part . . . . . . . . . . . . . . . . . . . . . . . . . .

4


Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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3

11
12
13
16
16
26
27
31
36


Common Channel Signaling System No.7 (topic 8)

Information Signaling

Illustrations

4

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

2.1
2.2
2.3
3.1
3.2
3.3
3.4
3.5
3.6

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14

Signaling via a common channel signaling link. . . . . . . . . . . . . . . . . . . . . 8
Associated mode of signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Example of the quasi-associated mode of signaling. . . . . . . . . . . . . . . . . 9
Functional levels of CCS7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Message exchange between two signaling points with CCS7 . . . . . . . . 13
Format of the various signal units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Routing label of a message signal unit . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Distribution of functions in the message transfer part. . . . . . . . . . . . . . . 17
Acknowledgment cycle for the basic error correction procedure
in normal operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Exchange of signal units during initial alignment . . . . . . . . . . . . . . . . . . 25
Message transfer part users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
ISDN-UP message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Set-up of an ISDN call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Release of an ISDN call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
SCCP message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Protocol classes for message transfer via the SCCP . . . . . . . . . . . . . . . 35
TCAP message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Tables
Tab. 3.1

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Alignment status indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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Common Channel Signaling System No.7 (topic 8)

6

Information Signaling

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

1 Introduction
Communication networks generally connect two subscriber terminating equipment units
together via several line sections for message exchange (e.g. speech, data, text or
images). Control information has to be transferred between the exchanges for call
control and for the use of facilities. In analog communication networks, channel-associated signaling systems have so far been used to carry the control information. Faultfree
operation is guaranteed with the channel-associated signaling systems in analog
communication networks, but the systems do not meet the requirements in digital,
processor-controlled communication networks. Such networks offer a considerably
larger scope of performance as compared with the analog communication networks,
due, for instance, to a number of new services and facilities. The amount and variety of
the information to be transferred is accordingly larger. The information can no longer be
economically transported by the conventional channel-associated signaling systems.
For this reason, a new, efficient signaling system is required in digital, processorcontrolled communication networks.
The CCITT has therefore specified the common channel signaling system no. 7 (CCS7).
CCS7 is optimized for application in digital networks.
It is characterized by the following main features:
• internationally standardized (national variations possible)
• suitable for the national and international/intercontinental network level
• suitable for various communication services such as telephony, text services, data
services and other services
• suitable for service-specific communication networks and for the integrated services
digital network (ISDN)
• high performance and flexibility along with a future-oriented concept which will meet
new requirements
• high reliability for message transfer
• processor-friendly structure of the messages (signal units of multiples of 8 bits)
• signaling on separate signaling links; the bit rate of the circuits is therefore exclusively for communication
• signaling links always available, even during existing calls
• use of the signaling links for transferring user data also
• used on various transmission media
cable (copper, optical fiber)
radio relay
satellite (up to 2 satellite links)
• use of the transfer rate of 64 kbit/s typical in digital networks
• used also for lower bit rates and for analog signaling links if necessary
• automatic supervision and control of the signaling network.

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Common Channel Signaling System No.7 (topic 8)

Information Signaling

2 Signaling Network
In contrast to channel-associated signaling, which has been standard practice until now,
in CCS7 the signaling messages are sent via separate signaling links (see Fig. 2.1).
One signaling link can convey the signaling messages for many circuits.
The CCS7 signaling links connect signaling points in a communication network. The
signaling points and the signaling links form an independent signaling network which is
overlaid over the circuit network.
Signaling points
A distinction is made between:
– signaling points (SP) and
– signaling transfer points (STP).
The signaling points are the sources (originating points) and the sinks (destination
points) of signaling traffic. In a communication network these are primarily the
exchanges.
The signaling transfer points switch signaling messages received to another signaling
transfer point or to a signaling point on the basis of the destination address. No call
processing of the signaling messages occurs in a signaling transfer point. A signaling
transfer point can be integrated in a signaling point (e.g. in an exchange) or can form a
node of its own in the signaling network. One or more levels of signaling transfer points
are possible in a signaling network, according to the size of the network.
All signaling points in the signaling network are identified by means of a code within the
framework of a corresponding numbering plan and can therefore be directly addressed
in a signaling message.
Switching
network

Switching
network

Circuits

Signaling link
terminal

Signaling link

Control

Fig. 2.1

8

Signaling link
terminal

Control

Signaling via a common channel signaling link

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Signaling links
A signaling link consists of a signaling data link (two data channels operating together
in opposite directions at the same data rate) and its transfer control functions. A channel
of an existing transmission link (e.g. a PCM30 link) is used as the signaling data link.
Generally, more than one signaling link exists between two signaling points in order to
provide redundancy. In the case of failure of a signaling link, functions of the CCS7
ensure that the signaling traffic is rerouted to fault-free alternative routes. The routing of
the signaling links between two signaling points can differ. All the signaling links
between two signaling points are combined in a signaling link set.
Signaling modes
Two different signaling modes can be used in the signaling network for CCS7.
In the associated mode of signaling, the signaling link is routed together with the
circuit group belonging to the link. In other words, the signaling link is directly connected
to signaling points which are also the terminal points of the circuit group (see Fig. 2.2).
This mode of signaling is recommended when the capacity of the traffic relation between
the signaling points A and B is heavily utilized.
Signaling point A

Signaling point B
Circuit group
Signaling link

Fig. 2.2

Associated mode of signaling
In the quasi-associated mode of signaling, the signaling link and the circuit group run
along different routes, the circuit group connecting the signaling point A directly with the
signaling point B. For this mode the signaling for the circuit group is carried out via one
or more defined signaling transfer points (see Fig. 2.3). This signaling mode is favorable for traffic relations with low capacity utilization, as the same signaling link can be
used for several destinations.
Signaling point A

Signaling point B
Circuit group (with quasi-associated signaling A-C-B)

Signaling links
Circuit group
(with associated
signaling)

Circuit group
(with associated
signaling)

Signaling point C/
Signaling transfer point

Fig. 2.3

Example of the quasi-associated mode of signaling

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Common Channel Signaling System No.7 (topic 8)

Information Signaling

Signaling routes
The route defined for the signaling between an originating point and a destination point
is called the signaling route. The signaling traffic between two signaling points can be
distributed over several different signaling routes. All signaling routes between two
signaling points are combined in a signaling route set.
Network structure
The signaling network can be designed in different ways because of the two signaling
modes. It can be constructed either with a uniform mode of signaling (associated or
quasi-associated) or with a mixed mode (associated and quasi-associated).
The worldwide signaling network is divided into two levels that are functionally independent of each other: an international level with an international network and a national
level with many national networks. Each network has its own numbering plans for the
signaling points.
Planning aspects
Economic, operational and organizational aspects must be considered in the planning
of the signaling network for CCS7.
An administration should also have discussions with the other administrations at an
early stage before CCS7 is introduced in order to make decisions, for example, on the
following points:
– signaling network
mode of signaling
selection of the signaling transfer points
signaling type (en bloc or overlap)
assignement of the addresses to signaling points
– signaling data links
e.g. 64 kbit/s digital or 4,8 kbit/s analog
– safety requirements
load sharing between signaling links
diverting the signaling traffic to alternative routes in the event of faults
error correction
– adjacent traffic relations.

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

3 Structure of CCS7
The signaling functions in CCS7 are distributed among the following parts:
– message transfer part (MTP, see Section 3.1)
– function-specific user parts (UP, see Section 3.2).
The message transfer part represents a user-neutral means of transport for messages
between the users. The term user is applied here for all functional units which use the
transport capability of the message transfer part.
Each user part encompasses the functions, protocols and coding for the signaling via
CCS7 for a specific user type (e.g. telephone service, data service, ISDN). In this way,
the user parts control the set-up and release of circuit connections, the processing of
facilities as well as administration and maintenance functions for the circuits.
The functions of the message transfer part and the user parts of CCS7 are divided into
4 levels. Levels 1 to 3 are allotted to the message transfer part while the user parts form
level 4 (see Fig. 3.1).
ISDN-UP,
level 4

SCCP,
level 4

TUP,
level 4

Examples of the user parts

Signaling network functions, level 3

Signaling link functions, level 2

Message transfer part

Signaling data link functions, level1

Fig. 3.1

Functional levels of CCS7

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Common Channel Signaling System No.7 (topic 8)

3.1

Information Signaling

Message Transfer Part
(CCITT Blue Book, Recommendations Q.701 to Q.707)
The message transfer part (MTP) is used in CCS7 by all user parts as a transport
system for message exchange. Messages to be transferred from one user part to
another are given to the message transfer part (see Fig. 3.2). The message transfer
part ensures that the messages reach the addressed user part in the correct order
without information loss, duplication or sequence alteration and without any bit errors.
Functional levels
Level 1 (signaling data link) defines the physical, electrical and functional characteristics of a signaling data link and the access units. Level 1 represents the bearer for a
signaling link. In a digital network, 64-kbit/s channels are generally used as signaling
data links. In addition, analog channels (preferably with a bit rate of 4.8 kbit/s) can also
be used via modems as a signaling data link.
Level 2 (signaling link) defines the functions and procedures for a correct exchange of
user messages via a signaling link. The following functions must be carried out in level 2:
– delimitation of the signal units by flags
– elimination of superfluous flags
– error detection using check bits
– error correction by retransmitting signal units
– error rate monitoring on the signaling data link
– restoration of fault-free operation, for example, after disruption of the signaling data
link.
Level 3 (signaling network) defines the interworking of the individual signaling links. A
distinction is made between the two following functional areas:
– message handling, i.e. directing the messages to the desired signaling link, or to the
correct user part
– signaling network management, i.e. control of the message traffic, for example, by
means of changeover of signaling links if a fault is detected and changeback to
normal operation after the fault is corrected.
The various functions of level 3 operate with one another, with functions of other levels
and with corresponding functions of other signaling points.

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Signaling point A

Signaling point B

User part
(e.g. ISDN-UP)

User part
(e.g. ISDN-UP)

Circuits

User
message

User
message

Signaling data link
Message
transfer part

Message
transfer part

Signal unit

MTP
component

User information

Address

MTP
component

User message

Fig. 3.2

Message exchange between two signaling points with CCS7

3.1.1

Signal Units
The message transfer part transports messages in signal units of varying length. A
signal unit is formed by the functions of level 2. In addition to the message it also
contains control information for the message exchange. There are three different types
of signal units:
– message signal units (MSU)
– link status signal units (LSSU)
– fill-in signal units (FISU).
Using message signal units the message transfer part transfers user messages, i.e.
messages from user parts (level 4) and messages from the signaling network management (level 3). The link status signal units contain information for the operation of the
signaling link (e.g. for the alignment) and the fill-in signal units are used to maintain the
acknowledgment cycle when no user messages are to be sent in one of the two directions of the signaling link. Fig. 3.3 illustrates the structure of the signal units.

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Common Channel Signaling System No.7 (topic 8)

F

CK

Information Signaling

SIF

LI

SIO*

F
I
B

FSN

B
I
B

BSN

F

Message signal unit

F

SF

CK

LI

F
I
B

FSN

B
I
B

BSN

F

Link status signal unit

F

CK

LI

F
I
B

FSN

B
I
B

Direction of transfer
BSN

F
* Double assignement of abbreviation
(see Section 4)

Fill-in signal unit

Fig. 3.3

Format of the various signal units

Flag (F)
The signal units are of varying length. In order to clearly separate them from one
another, each signal unit begins and ends with a flag. The closing flag of one signal unit
is usually also the opening flag of the next signal unit. However, in the event of overloading of the signaling link, several consecutive flags can be sent. The flag is also used
for the purpose of alignment. The bit pattern of a flag (F) is 01111110.
Backward sequence number (BSN)
The backward sequence number is used as an acknowledgment carrier within the
context of error control. It contains the forward sequence number of a signal unit in the
opposite direction whose reception is being acknowledged. A series of signal units can
also be acknowledged with one backward sequence number.
Backward indicator bit (BIB)
The backward indicator bit is needed during general error correction (see Section 3.1.3,
subheading ”Correction of transmission errors”). With this bit, faulty signal units are
requested to be retransmitted for error correction.
Forward sequence number (FSN)
A forward sequence number (FSN) is assigned consecutively to each signal unit to be
transmitted. On the receive side it is used for supervision of the correct order for the
signal units and for safeguarding against transmission errors. The numbers 0 to 127 are
available for the forward sequence number.

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Forward indicator bit (FIB)
The forward indicator bit (FIB) is needed during general error correction (see Section
3.1.3, subheading ”Correction of transmission errors”). It indicates whether a signal unit
is being sent for the first time or whether it is being retransmitted.
Length indicator (LI)
The length indicator (LI) is used to differentiate between the three signal units. It gives
the number of octets between the check-bit field and the length indicator field. The
length indicator field contains different values according to the type of signal unit:
– 0 = fill-in signal unit
– 1 oder 2 = link status signal unit
– greater than 2 = message signal unit.
The maximum value in the length indicator field is 63 even if the signaling information
field contains more than 62 octets.
Service information octet (SIO)
The service information octet (SIO) only exists in message signal units. It contains the
service indicator and the network indicator. A service indicator is assigned to each user
of the message transfer part. It informs the message transfer part which user part has
sent the message and which user part is to receive it. The network indicator indicates
whether the traffic is national or international. The message transfer part evaluates both
items of information.
Signaling information field (SIF)
The signaling information field (SIF) only exists in message signal units. It contains the
actual user message. The user message also includes the address of the destination to
which the message is to be transferred. The maximum length of the user message is
272 octets (one octet = 8 bits). The format and the coding of the user message are separately defined for each user part.
Check bits (CK)
The check bits (CK) are formed on the transmission side from the contents of the signal
unit and are added to the signal unit as redundancy. On the receive side, the message
transfer part can determine with the check bits whether the signal unit was transferred
without any errors. The signal unit is acknowledged as either positive or faulty on the
basis of the check.
Status field (SF)
The status field (SF) only exists in link status signal units. It contains status indications
for the alignment of the transmit and receive directions.

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Common Channel Signaling System No.7 (topic 8)

3.1.2

Information Signaling

Addressing of the Signal Units
The destination of a signal unit is specified in a routing label. The routing label is a
component of every user message and is transported in the signaling information field
(SIF). The routing label in a message signal unit consists of the following (see Fig. 3.4):
– destination point code (DPC)
– originating point code (OPC)
– signaling link selection (SLS) field
A code is assigned to each signaling point in the signaling network according to a
numbering plan. The message transfer part uses the code for message routing. The
destination point code in a message signal unit identifies the signaling point to which this
message is to be transferred. The originating point code specifies the signaling point
from which the message originates. The contents of the signaling link selection field
determine the signaling route along which the message is to be transmitted. In this way,
the signaling link selection field is used for load sharing on the signaling links between
two signaling points.
The service information octet (SIO) contains additional address information. Using the
service indicator, the destination message transfer part identifies the user part for which
the message is intended. The network indicator, for example, enables a message to be
identified as being for national or international traffic.
Link status signal units and fill-in signal units require no routing label as they are only
exchanged between level 2 of adjacent message transfer parts.

User information

SLS

OPC

DPC

Signaling information field (SIF)

Fig. 3.4

Routing label of a message signal unit

3.1.3

Functions
The message transfer part is responsible for transmitting and receiving signal units, for
correcting transmission errors, for the signaling network management and for the alignment. Its functions are spread over the functional levels 1, 2 and 3.

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

TUP
UP

ISDN-UP

Other UPs

Level 4

Message
distribution

Signaling
traffic
management

Signaling
route
management

Signaling
link
management

Level 3
Message
routing

Message
discrimination

Signaling message handling

Signaling network management

Other signaling links
Signaling link status
control
MTP
Error rate monitoring

Level 2

Transmission
control,
transmission buffer,
retransmission
buffer

Receive
control

Control for the
initial alignment

Check bit
and flag
generator

Level 1

Fig. 3.5

Flag, alignment and
error detection

Signaling data link

Distribution of functions in the message transfer part

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Common Channel Signaling System No.7 (topic 8)

Information Signaling

Transmission of a signal unit
As an example, the transmission of a message signal unit is described. The message
sent from a user to the message transfer part for transmission contains: the user information, the routing label, the service indicator, the network indicator and a length indicator. The processing of a user message to be transmitted in the message transfer part
begins in level 3 (see Fig. 3.5).
The message routing (level 3) determines the signaling link on which the user
message is to be transmitted. To do this, it analyzes the destination point code and the
signaling link selection field in the routing label of the user message, and then transfers
the message to the appropriate signaling link (level 2).
The transmission control (level 2) assigns the next forward sequence number and the
forward indicator bit to the user message. In addition, it includes the backward sequence
number and the backward indicator bit as an acknowledgment for the last received
message signal unit. The transmission control simultaneously enters the part of the
message signal unit formed so far in the transmission and retransmission buffers. All
message signal units to be transmitted are stored in the retransmission buffer until their
fault-free reception is acknowledged by the receive side. Only then are they deleted.
The check bit and flag generator (level 2) generates check bits for safeguarding
against transmission errors for the message signal unit and sets the flag for separating
the signal units. In order that any section of code identical to the flag (01111110) occurring by chance is not mistaken for the flag, the user messages are monitored before the
flag is added to see if five consecutive ones (1) appear in the message. A zero (0) is
automatically inserted after five consecutive 1s. On the receive side, the zero following
the five 1s is then automatically removed and the user message thereby regains its original coding.
The check-bit and flag generator transfers a complete message signal unit to level 1. In
level 1, the message signal unit is sent on the signaling data link.

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Reception of a signal unit
The bit stream along a signaling data link is received in level 1 and transferred to level 2.
Flag detection (level 2) examines the received bit stream for flags. The bit sequence
between two flags corresponds to one signal unit.
The alignment detection (level 2) monitors the synchronism of the transmit and receive
sides with the bit pattern of the flags.
Using the check bits also transmitted, error detection (level 2) checks whether the
signal unit was correctly received. A fault-free signal unit is transferred to the receive
control, while a faulty signal unit is discarded. The reception of a faulty signal unit is
reported to error rate monitoring, in order to keep a continuous check on the error rate
on the receive side of the signaling link. If a specified error rate is exceeded, this is
reported to the signaling link status control by error rate monitoring. The signaling link
status control then takes the signaling link out of service and sends a report to level 3.
The receive control (level 2) checks whether the transferred signal unit contains the
expected forward sequence number and the expected forward indicator bit. If this is the
case and if it is a message signal unit, the receive control transfers the user message to
level 3 and causes the reception of the message signal unit to be positively acknowledged. If the forward sequence number of the transferred message signal unit does not
agree with that expected, the receive control detects a transmission error and causes
this and all subsequent message signal units to be retransmitted (see subheading
”Correction of transmission errors”).
The message discrimination (level 3) accepts the correctly received user message. It
first determines whether the user message is to be delivered to one of the immediately
connected user parts or to be transferred to another signaling link (quasi-associated
message). This preselection is achieved in the message discrimination by evaluation of
the destination point code. A user message which only passes through a signaling point
(signaling transfer point) is transferred by the message discrimination to the message
routing, where it is treated as a user message to be transmitted.
If a received user message is intended for one of the connected user parts (signaling
point), it is transferred to the message distribution (level 3). The message distribution
evaluates the service information octet (SIO), thereby determining the user part
concerned, and delivers the user message there.

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Common Channel Signaling System No.7 (topic 8)

Information Signaling

Correction of transmission errors
As signal mutilations in the signaling can bring about faulty reactions particularly in call
processing, transmission errors must be reduced to a minimum. In CCS7 two methods
exist for this purpose:
– basic error correction
– error correction with preventive cyclic retransmission.
Both methods are based on retransmitting received faulty message signal units. For the
basic error correction method, all message signal units from the received faulty
message signal unit onwards are specifically requested to be retransmitted. For the
preventive cyclic retransmission method, on the other hand, all the message signal units
in the retransmission buffer are cyclically retransmitted as a preventive measure. The
error correction takes place in level 2.
The basic error correction method is applied for signaling links with small propagation
delays (< 15 ms, e.g. terrestrial links). It functions with both positive and negative
acknowledgements. For this method, an acknowledgement comprises the backward
sequence number (BSN) and the backward indicator bit (BIB). For a positive acknowledgement, the backward indicator bit has the same value as the backward indicator bit
in the previous acknowledgement (see Fig. 3.6). For a negative acknowledgement, the
backward indicator bit is the inverse of the backward indicator bit in the previous
acknowledgement.
When a positive acknowledgment is received, the receive control on the originating side
causes the corresponding message signal unit (or a sequence of message signal units)
in the retransmission buffer to be deleted. In the case of a negative acknowledgment,
the receive control on the originating side requests the transmission control to stop the
transmission of new signal units and to retransmit the negatively acknowledged
message signal unit. Then all further message signal units still in the retransmission
buffer are retransmitted in the order they were first transmitted. After detecting a transmission error the receive control on the destination side discards all incoming message
signal units until it receives the negatively acknowledged message signal unit correctly.
Only then does it resume evaluation of the message signal units. This ensures that user
messages do not overtake one another. The receive control at the destination recognizes the retransmitted message signal units by the forward indicator bit (FIB). The
forward indicator bit is inverted for the retransmitted message signal units and for all
later newly transmitted message signal units with regard to the message signal units
transmitted before the error.

20

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Signaling equipment at the origin

Signaling data link

Signaling equipment at the destination

Transmission control

Receive control

MSU number 36
BSN

BSN

BIB

BIB

FSN

FSN

FIB

FIB

CK

1

36

1

11

F

CK

1

35

1

10

F

CK

1

34

Retransmission buffer

1

9

F

Retransmission buffer

36

1

1

9

35

1

1

10

34

1

1

11

33

1

1

12

32

1

1

13

F

32

1

11

1

CK

F

33

1

12

1

CK

F

34

1

13

1

FIB

FIB

FSN

FSN

BIB
BSN

CK

BIB
BSN
MSU number 13

Receive control

Fig. 3.6

Transmission control

Acknowledgment cycle for the basic error correction procedure
in normal operation

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21


Common Channel Signaling System No.7 (topic 8)

Information Signaling

The preventive cyclic retransmission error correction method is applied for
signaling links with large propagation delays (>=15 ms, e.g. long terrestrial links, satellite
links). Unlike the basic error correction method, the PCR method works only with positive acknowledgments. The retransmission of the message signal units does not first
occur on request after a transmission error; instead, all the message signal units in the
retransmission buffer are cyclically retransmitted as a preventive measure whenever no
new message signal units are available for transmission. The acknowledgment for
correctly received message signal units consists of the backward sequence number
only. The backward indicator bit and the forward indicator bit are not required for this
method. To keep the format of the signal units almost identical for both error correction
methods, the backward and forward indicator bits receive a fixed value and are transmitted with every signal unit without alteration.
As in the case of basic error correction, when the receive control on the originating side
receives an acknowledgement it causes the corresponding message signal unit (or a
sequence of message signal units) in the retransmission buffer to be deleted. If, with the
PCR method, the receive control at the destination detects a transmission error, it
continues to acknowledge the last correctly received message signal unit and waits until
it receives the faulty message signal unit correctly by cyclic retransmission. The retransmitted message signal units are then accepted, processed and acknowledged on the
destination side until the retransmission has reached the first transmission of new signal
units. Thus, with the PCR method, automatic error correction occurs. It is dependent on
the usage of the signaling link (on average approx. 20%). The smaller the load, the
greater the free capacity for retransmitting the message signal units.

22

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Signaling network management
The signaling network management is a function of level 3. It controls the operation and
the interworking of the individual signaling links in the signaling network. To this end, the
signaling network management exchanges messages and control instructions with the
signaling links of level 2, sends messages to the user parts and works together with the
signaling network management in adjacent signaling points. For the interworking with
other signaling points the signaling network management uses the transport function of
the message transfer part. Management messages are transferred in message signal
units like user messages. For discrimination, the management messages have their
own service indicator. The signaling network management contains 3 function blocks:
The signaling link management controls and monitors the individual signaling links. It
receives the messages concerning the alignment and status of the individual signaling
links, or concerning operating irregularities and effects any change in status which may
be necessary. In addition, the signaling link management controls the putting into
service of signaling links, including initial alignment and automatic realignment of
signaling links after failures or alignment losses due to persistent faults. If necessary,
the signaling link management transfers messages to the signaling traffic management
or receives instructions from there.
The signaling route management controls and monitors the operability of signaling
routes. It exchanges messages with the signaling route management in the adjacent
signaling transfer points for this purpose. The signaling route management receives, for
example, messages concerning the failure or reavailability of signaling routes or the
overloading of signaling transfer points. In cooperation with the signaling traffic managment, it initiates the appropriate actions in order to maintain the signaling operation to
the signaling destinations involved.
The signaling traffic management controls the diversion of the signaling traffic from
faulty signaling links or routes to fault-free signaling links or routes. It also controls the
load distribution on the signaling links and routes. To achieve this it can initiate the
following actions:
– changeover; on failure of a signaling link the signaling traffic management switches
the signaling traffic from the failed signaling link to a fault-free signaling link
– changeback; when a signaling link becomes available again after a fault has been
corrected, the signaling traffic management reverses the effect of the changeover
– rerouting; when a signaling point can no longer be reached on a normal route, the
signaling traffic management diverts the signaling traffic to a predefined alternative
route.
When overloading occurs, the signaling traffic management sends messages to the
users in its own signaling point in order that they reduce the load. The management also
informs the adjacent signaling points of the overloading in its own signaling point and
requests them to also reduce the load.

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Common Channel Signaling System No.7 (topic 8)

Information Signaling

The signaling traffic management accomplishes its functions by
– receiving messages from the signaling link and signaling route managements
– sending control instructions to the signaling link and signaling route managements
– directly accessing the signaling links, e.g. during emergency alignment
– modifying the message routing on failure of signaling routes
– exchanging management messages with the signaling traffic management in adjacent signaling points.
Alignment
For fault-free transmission of user messages via a signaling link it is necessary for the
transmit and receive directions to operate at the same rate. For this purpose, an initial
alignment is carried out when putting a signaling link into service.
The initiation of the initial alignment is possible from either end of the signaling link.
The alignment is done by exchanging status indications. The alignment itself takes place
in level 2, while the initiation comes from level 3. Several stages are provided for the
initial alignment. Each stage is assigned a special type of status indication (see Table
3.1). The status indications are transmitted in link status signal units.
Status indication

Explanation

SIO*

out of alignment

SIN

normal alignment

SIE

emergency alignment

SIOS

out of service

* Double assignment of abbreviation (see Section 4)

Tab. 3.1

Alignment status indications

At the start of the initial alignment the control for the alignment on the initiation side
causes the transmission control to send SIO status indications (see Fig. 3.7). The
opposite side receives these signals and replies to them with SIN status indications.
After receiving the first SIN status indication, the initiation side also sends SIN indications. This is the start of the proving period for the signaling link. The initial alignment
control records any signal errors occurring during the proving period (8.2 s for a
signaling link with 64 kbit/s) and decides whether the link can be released for normal
service. It reports the result of the proving period to level 3. With the transition to normal
service both ends of the signaling link first send fill-in signal units (FISU) until the first
message signal unit (MSU) is to be transmitted. If the proving period yields a negative
result, the initial alignment can be repeated.
The initial alignment can also be carried out in emergency form if necessary. Emergency
alignment can be applied, for example, when a non-aligned standby signaling link has
to be put into service. SIE status indications are then sent in place of SIN status indications. The proving period is drastically reduced (to 0.5 s for a signaling link with 64 kbit/s)
and the requirements for the error rate are lessened.

24

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Information Signaling

Common Channel Signaling System No.7 (topic 8)

Signaling point A

Signaling point B

Transmission
control
Status:

Receive
control

Transmission
control

Receive
control

Signaling data link

Status:

out of service
out of service

SIO

not aligned

aligned

proving period

SIO

SIO

SIO

SIN

SIN

SIN

SIN

SIN

SIN

SIN

SIN

SIN

SIN

SIN

FISU

SIN

MSU

SIN

not aligned

aligned

proving period

FISU

service

MSU
service

Fig. 3.7

Exchange of signal units during initial alignment
In the event that, following successful initial alignment the signaling link is able to send
or receive fill-in signal units but, for other reasons, e.g. errors in level 3, no message
signal units can be processed, link status signal units containing SIOS are sent instead
of the fill-in signal units. In this way the signaling link is taken out of service again and
the alignment procedure is restarted. Level 3 detects errors by testing the aligned
signaling link before message signal units are transmitted (signaling link test).
The maintenance of the alignment is ensured by the flags between the signal units on
the basis of their bit patterns. The alignment can be lost for a few signal units at most,
but it is recovered immediately with the next correct signal unit received and its corresponding flag.
If a signaling link is taken out of service due to an overly large signal error rate, an
attempt is first made to put the signaling link into service again with the initial alignment.

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25


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