4.CCS7 Signalling

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4.2 ADVANTAGES OF COMMON CHANNEL SIGNALLINGCommon Channel Signaling is being adopted throughout the world in national and

international networks for numerous reasons. The main reasons are:

The rapidly changing control techniques of exchanges.

The limitations of CAS systems.

The evolutionary potential of CCS systems.

Fig : 1 CAS Signaling with Separate Control and Switch Blocks

Further advantages result from the evolutionary process of CCS and call control. The drive to

provide an unrestricted communication capability between exchange processors eliminates

per-circuit signaling termination costs. These costs are inevitable in per-circuit CAS systems,

but for funneling all signaling information into a single common-channel, only one signaling

termination cost is incurred for each transmission link. There are costs penalties for CCS

systems, e.g. the message received by an exchange have to be analyzed, resulting in a

processing overhead. However, these cost penalties are more than covered by the advantages

of increased scope of inter-processor communication and more efficient processor activity.

The separation of CCS from traffic circuits, and the direct inter-connection of exchange

processors, is the early steps in establishing a cohesive CCS network to allow unimpeded

signaling transfer between customers and nodes and between nodes in the network. The

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Exchange A Exchange B

Control

X

X

Switch Block

X

X

Switch Block

Control

Speech

Signaling

LegendSpeech

Signaling


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concept of a cohesive CCS network opens up the opportunity for the implementation of a

wide range of network management administrative, operation and maintenance function. A

major example of such function is the quasi-associated mode of operation. This mode of

operation provides great deals of flexibility in network security, reduces the cost of CCS on

small traffic routes and extends the data-transfer capabilities for non-circuit related signaling.

Fig : 2 Common channel signaling

CAS systems posses limited information-transfer capability due to:

The restricted number of conditions that can be applied (e.g. the limit variation that

can be applied to a D.C. loop or the limited number frequency combinations that can

be implemented in a voice frequency system)

The limited number of opportunities to transfer signals (e.g. it is not possible to

transmit voice-frequency signals during the conversation phase of a call without

inconveniencing the customers or taking special measures.)

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Exchange A Exchange B

X X

X X

XX

Switch Block Switch Block

Control Control

Speech

Signaling


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Neither of these restrictions applies to CCS. The flexible message-based approach allows a

vast range of information to be defined and the information can be during any stage of a call.

Hence, the repertoire of CCS is far greater than channel-associated versions and messages

can be transferred at any stage of a call without affecting the calling and called subscribers.

CCS systems transfer signals very quickly, i.e. at 64 Kbps. This speedy signaling also permitsthe inclusion of far more information without an increase in post dialing delay.

Techniques used in modern CCS system can further improve the flexibility provided to

customers. 'User-to-user' signaling and end-to-end signaling techniques are used whereby

messages can be transferred from one customer to another without undergoing a full analysis

at each exchange in the network. Whilst forms of end-to-end signaling are possible using

CAS systems, the technique can be more efficiently implemented with CCS systems.

One of the problems that prompted the development of CCS systems was 'speech clipping' in

the international network. In some case CAS systems, it is necessary to split the speech path

during call set-up to avoid tones being heard by the calling customer. This results in a slowreturn of the answer signal and, if the called customer starts speaking immediately after

answer, then the first part of the statement by the called customer is lost. As the first

statement is usually the identity of the called customer, this causes a great deal of confusion

and inconvenience. CCS systems avoid the problem by transferring the answer signal

quickly.

As a result of the processing ability of CCS systems, a high degree of reliability can be

designed into the signaling network. Error detection and correction techniques can be applied

which ensure reliable transfer of uncorrupted information. In the case of an intermediate

exchange failure, re-routing can take place within the signaling network, enabling signaling

transfer to be continued. While these features introduce extra requirements, the common

channel approach to signaling allows a high degree of reliability to be implemented

economically.

A major restriction of CAS is the lack of flexibility, e.g. the ability to add new features is

limited. One factor that led to the development of CCS was the increasing need to add new

features and respond to new network requirements. Responses to new requirements in CCS

can be far more rapid and comprehensive than for channel associated versions.

CCS systems are not just designed to meet current needs. They are designed to the as flexible

as possible in meeting future requirements. One way of achieving the objective is to definemodern CCS systems in a structured way, specifying the signaling system in a number of

tiers. The result is flexibility signaling system that reacts quickly to evolving requirements

and future services can be incorporated in a flexible and comprehensive manner. Changes to

existing services can be implemented more quickly and at lower cost than with CAS systems.

4.3 OVERVIEW OF SIGNALLING SYSTEM NO. 7Signaling System No. 7 (CCS7) is a message based signaling system between Stored

Program Controlled (SPC) switches. Where the intermediate nodes may be used a Signal

Transfer Points (STPs) CCS7 network can be used for transmitting call related messages, as

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well as show speed data packets between ISDN users. The Signaling Connection Control Part

(SCCP) enables it to act like a packet network. Thus it is an important pre-requisite to

Integrated Service Digital Network (ISDN) and Intelligent Network (IN) features. Enhanced

service for the public telephone network can also be provided using this message based

signaling system.

Some of the salient features of CCS7 are:

Fast, reliable and economical.

Bit- oriented protocol.

Labeled messages

Associated and quasi-associated mode of working

Error correction is supported at link level (level 2) by transmission and sequence

control.

Signaling message handler at level 3 supports message routing.

Redundancy and load sharing is possible on signaling links. Change back on link

restoration is possible. Redundancy and load sharing is possible on signaling routes, along with diversion on

route failure.

4.4 CCS7 PROTOCOL STACKThe CCS7 protocol stack comprises of four layers. With reference to the OSI 7 layer model,

the correspondence between the layers is depicted in Fig. 4.3. The functions defined for each

layer or level are briefly described in the following paragraphs.

4.4.1 LEVEL 1

Any node with the capability of handling CCS7 is termed a 'Signaling point'. The direct

interconnection of two signaling points with CCS7 uses one or more 'signaling link (s)'. Level

1 of the 4-level structure (shown in Fig. 22) defines the physical, electrical and functional

characteristics of the signaling link. Defining such characteristics within level 1 means that

the rest of the signaling system (level 2 to 4) can be independent of the transmission medium

adopted. By keeping the interface between levels 1 and 2 constant, any changes within level 1

do not affect the higher levels. In a digital environment, usually the physical link is a 64 Kbps

channel. This is typically within a digital transmission system using pulse-code modulation

(PCM). However, other types of link (including analogue) can be used without affecting

levels 2 to 4.

4.4.2 LEVEL 2

Level 2 defines the functions that are relevant to an individual signaling link, including error

control and link monitoring. Thus, level 2 is responsible for the reliable transfer of signaling

information between two directly connected signaling points. If errors occur during

transmission of the signaling information, it is the responsibility of level 2 to invoke

procedures to correct the errors. Such characteristics can be optimized without affecting the

rest of the signaling system, provided that the interfaces to level 1 and 3 remain constant.

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4.4.3 LEVEL 3

The functions that are common to more than one signaling link, i.e. signaling network

functions, are defined in level 3 : these include 'message handling' functions and 'signaling

network management' functions. When a message is transferred between two exchanges,

there are usually several routes that the message can take including via a signal-transfer point.

The message-handling functions are responsible for the routing of the messages through the

signaling network to the correct exchange. Signaling network management functions control

the configuration of the signaling network. These functions include network reconfiguration

is response to status changes in the network. For example, if an exchange within the signaling

network fails, the level 3 of CCS7 can re-route message and avoid the exchange that has

failed.

Fig : 3

ASE = APPLICATION SERVICE ELEMENT

ISUP = ISDN USER PART

MTP = MESSAGE TRANSFER PART

OMAP = OPERATIONS MAINTENANCE AND ANDINISTRATION PART

TCAP = IRANSACTION CAP ABILITIES APPLICATION PART

TUP = TELEPHONE USER PART

SCCP = Signalling connection control Part

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CCS#7 PROTOCOL

MODEL

OSI

MODEL

APPLICATIO

N

PRESENTATIO

N

TRANSPORT

SESSION

NETWOR

K

DATA

LINKPHYSICA

LMTP LEVEL

1

MTP LEVEL

2

MTP LEVEL

3

TCA

P

OMA

P

ASE

s

NULL

SCCP

ISUP/TU

P


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4.4.4 MESSAGE TRANSFER PART (MTP)

Level 1 to 3 constitutes a transfer mechanism that is responsible for transferring information

in messages from one signaling point to another. The combination of level 1 to 3 is known as

the message transfer part (MTP). The MTP controls a number of signaling message links and

network management functions to ensure correct delivery of messages. This means that the

messages are delivered to the appropriate exchange in an uncorrupted form and in the

sequence that they were sent, even under failure conditions in the network.

4.4.5 LEVEL 4

Level 4 comprises the 'user parts'. The meaning of the messages transferred by the MTP and

the sequence of actions for a particular application (e.g. telephony) is defined by the 'user

parts'. A key feature is that many different user parts may use the standardized MTP. Hence,

if new requirements arise, that had not been foreseen previously, the relevant user part can be

enhanced (or a new user part derived) without modifying the transfer mechanism or affecting

other user parts. Three user parts have been defined, the Telephone User Part (TUP), the

ISDN User Part (ISUP) and the Data User Part (DUP). Along with SCCP, which providesend-to-end signaling capability, MTP constitutes the Network Services Part (NSP) which

provides the Network Layer functionalities of the OSI model. The user parts of NSP are

Operations and Maintenance Application Part (OMAP) and Mobile Application Part (MAP).

4.4.6 SIGNALLING CONNECTION CONTROL PART (SCCP)

The Signaling Connection Control Part (SCCP) has the functions of the network as well as

the transport layers of the CCS7 protocol stack. Together with the MTP, it provides the OSI

transport layer capabilities. Unlike MTP, which provides only data gram service, SCCP

provides connection-oriented, and connection less services as well.

Thus, while MTP is sufficient for circuit switched application like TUP and ISUP, for non-

circuit related applications, such a database querying, the enhanced addressing capability of

SCCP is required. SCCP has a unique scheme of addressing and routing based on Global

Titles. SCCP utilizes the services of MTP to route its payload from one node to other.

In addition to routing transaction related messages submitted by the Transaction Capabilities

Application Part (TCAP), SCCP also segments and sequences large TCAP messages to fit

into the MTP packet size. At the distant node it is the responsibility of the peer SCCP to re-

assemble the segmented message.

4.4.7 TRANSACTION CAPABILITIES APPLICATION PART (TCAP)TCAP is an application part in the CCS7 stack and is responsible for establishing dialogue

with remote databases. It carries the date of higher layers like INAP and MAP and invokes

remote operations. An operation at remote and requires a series of queries and response as

part of a TCAP dialogue.

4.4.7.1 Management of a dialogue requires:

Establishing a dialogue.

Continuing the dialogue.

Terminating the dialogue.

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guaranteed with the channel associated signaling systems in analog communication networks,

but the systems do not meet requirements in digital, processor and controlled communication

network. Such networks offer a considerably larger scope of performance as compared with

the analog communication networks due, for instances, to a number of new services and

facilities. The amount and variety of the information to be transferred is accordingly larger.

The conventional channel associated signaling systems can no longer economically transportthe information. For this reason, a new efficient signaling is required in digital, processor-

controlled communication networks.

The CCITT has therefore specified the common channel signaling system no. 7 (CCS-7)

CCS-7 is optimized for application in digital networks. It is characterized by the following

main features. :

Fig : 4 Application of the Level Structure

Internationally standardized (national variations possible)

Suitable for the national, international and intercontinental network level.

Suitable for various communication services such as telephony, test services, data

services, and other services.

Suitable for service-specific communication networks and for the integrated services

digital networks (ISDN)

High performance and flexibility along with a future-oriented concept which well

meet new requirements.

High reliability for message transfer.

Processor-friendly structure of messages (signal units of multiples of

8 bits)

Signaling on separate signaling links; the bit rate of the circuit is therefore exclusively

for communication.

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SWITCH

BLOCK

EXCHANGE

A

LEVEL

1

LEVEL2

LEVEL

3

LEVEL4

SWITCH

BLOCK

EXCHANGE

B

LEVEL

1

LEVEL2

LEVEL

3

LEVEL4

X XX X

MTP

MT

P

SIGNALLING

LINK


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Signaling links always available, even during existing calls.

Use of the signaling links of 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.

4.6.2 SIGNALLING 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.24). One signaling

link can convey the signaling messages for many circuits.

The CCS signaling links connect signaling points (SPs) in a communication network. The

signaling points and the signaling links form an independent signaling network, which is

overlaid over the circuit network.

Fig : 5 Signaling via a common channel signaling link

4.6.3 SIGNALLING POINTS (SP)

A distinction is made between signaling points (SP) and signaling transfer points (STP).

The SPs are the sources (originating points) and the sinks (destination points) of signaling

traffic. In a communication network these are primarily the exchanges.

The STPs switch signaling messages received to another STP or to a SP on the basis of the

destination address. No call processing of the signaling message occurs in a STP. A STP can

be integrated in a SP (e.g. in an exchange) or can form a node of its own in the signaling

network. One or more levels of STPs are possible in a signaling network, according to the

size of the network.

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All SPs in the signaling network are identified by means of a code within the framework of a

corresponding numbering plan and therefore can be directly addressed in a signaling

message.

4.6.4 SIGNALLING LINKS

A signaling link consists of a signaling data link (two data channels operating together inopposite directions at the same data rate) and its transfer control functions. A channel of an

existing transmission link (e.g. a PCM 30 link) is used as the signaling data link. Generally,

more than one signaling link exists between two SPs 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 SPs

can differ. All the signaling links between two SPs are combined in a signaling link set.

4.6.5 SIGNALLING MODES

The different signaling modes can be used in the signaling networks for CCS7 viz. associatedmode and quasi-associated mode.

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 SPs

which are also the terminal points of the circuit group (see fig.25) This mode of

recommended when the capacity of the traffic relation between the SPs A and B heavily

utilized.

Fig : 6 Associated mode of Signaling

In the quasi -associated mode of signaling, the signaling link and the speech circuit group run

along different routes, the circuit group connecting the SP A directly with the SP B. For this

mode the signaling for the circuit group is carried out via one or more defined STPs (see

fig.26). This signaling mode is favorable for traffic relations with low capacity utilization, as

the same signaling link can be used for several destinations.

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Fig : 7 Quasi-associated mode of signaling

The route defined for the signaling between an originating point and a destination point is

called the signaling route. The signaling traffic between two SPs can be distributed over

several different signaling routes. All signaling routes between two SPs are combined in a

signaling route set.

4.6.6 (F) NETWORK STRUCTURE

The signalling network can be designed in different ways because of the two signalling

modes. It can be constructed either with uniform mode of signalling (associated or quasi-

associated) or with a mixed mode (associated and quasi-associated).

The worldwide signalling 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 SPs.

4.6.7 PLANNING ASPECTS

Economic, operational and organizational aspects must be considered in the planning of the

signalling network for CCS 7. An administration should also have discussions with the other

administrations at an early stage before CCS 7 is introduced in order to make decisions, for

examples, on the following points.

(a) Signalling network

- Mode of signalling

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Signaling point ASignaling point B

Signaling links

Circuit group with

associated signaling

Signaling Point C/

Signaling transfer Pint

Circuit group with

associated signaling

Circuit group with Quasi

associated signaling A-C-B


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- Selection of the STPs

- Signalling type (en bloc or overlap)

- Assignment of the address to SPs

(b) Signalling data links, e.g. 64 kbit/s digital or 4.8 kbit/s analogue

(c) Safety requirements

- load sharing between signalling links

- diverting the signalling traffic to alternative routes in event of

faults error correction.

(d) Adjacent traffic relations

4.7 STRUCTURE OF CCS 7The signaling in CCS 7 are distributed among the following parts

message transfer part (MTP, See Sec 3.12.1)

Function-specific user parts (UP, see Sec. 3.12.2).

The MTP 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

MTP.

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

control the set-up release of circuit connections, the processing of facilities as well as

administration and maintenance functions for the circuits.

The functions of the MTP and the UP of CCS7 are divided into 4 levels. Level 1 to 3 is

allotted to the MTP while the UPs form level 4 (see fig.27)

Fig : 8 Functional Levels of CCS7

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ISDN-UP

Level 4

SCCP

Level 4

TUP

Level 4

Signaling network functions level 3

Signaling Link functions level 2

Signaling Data link functions level 1

Examples ofuser parts

Messagetransfer part


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4.8 MESSAGE TRANSFER PART (CCITT BLUE BOOK.

RECOMMENDATIONS Q. 701 TO Q. 707)

The message transfer part (MTP) is used in CCS by all user parts (UPs) as a transport

system for message exchange. Messages to be transferred from one UP to another are

given to the MTP. The MTP ensures that the message reach the addressed UP in the

correct order without any bit errors.

4.8.1 FUNCTIONAL LEVELS

Level 1 (Signalling data link) defines the physical, electrical and functional

characteristics of a signalling data link and the access units. Level 1 represents the

bearer for a signalling link. In a digital network, 64-kbit/s channel are generally used

as signalling data links. In addition, analogue channels (preferably with a bit rate of

4.8 kbit/s) can also be used via modems as a signalling data link.

Fig : 9 Message exchange between two signaling points with CCS

Level 2 (signaling link) defines the functions and procedure for a correct exchange of user

message via a signaling link. The following functions must be carried out 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.

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User part(e.g. ISDN -

UP)

Message

Transferpart

User

Message

User part(e.g. ISDN -

UP)

Message

Transferpart

User

Message

Circuit

s

Signal unit

Signaling Data Link

MTPComponen

t

UserInformation

MTP

Component

Address

UserMessage

Signaling part ASignaling part B


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- 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.

Following are the functional areas:

- Message handling, i.e. directing the message to the desired signaling link, or to the

correct UP.

- Signaling network management, i.e. control of the message traffic, for example by

means of changeover of signaling links of a fault is detected and change back to

normal operation after the fault is corrected.

4.8.2 SIGNAL UNITS (SU)

The MTP transports messages in the form of SUs of varying length. A SU 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 SUs.

- Message Signal Units (MSU)

- Link Status Signal Units (LSSU)

- Fill-in Signal Units (FISU)

Using MSUs the MTP transfers user messages, that is, message from UPs (level 4) and

message from the signaling network management (level 3). The structure of the three types of

message units is shown in fig.

The LSSUs contain information for the operation of the signaling link (e.g. of the alignment).

The FIUs 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.

4.8.3 PROTOCOL INFORMATION BITS

Flag (F): (8 Bits) the SUs are of varying length. In order to clearly separate them from one

another, each SU begins and ends with a flag. The closing flag of one SUs is usually also the

opening flag of the next SU. 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

big pattern of a flag is 01111110.

Backward sequence number (BSN): (7 bits) The BSN is used as an acknowledgment carrier

within the context of error control. It contains the forward sequence number (FSN) of a SU in

the opposite directive whose reception is being acknowledged. A series of SUs can also

acknowledged with one BSN.

Backward Indicator Bit (BIB): (1 bit) The BIB is needed during general error correction with

this bit, faulty SUs are requested to be retransmitted for error correction.

Forward sequence number (FSN): (bits) A FSN is assigned consecutively to each SU to be

transmitted. On the receive side it is used for supervision of the correct order for the Sus and

for safeguarding against transmission errors. The number 0 to 127 is available for the FSN.

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Forward indicator bit (FIB): (1 bit) The FIB is needed during general error correction. It

indicates whether a SU is being sent for the first time of whether it is being retransmitted.

Length indicator (LI): (6 bit) The LI is used to differentiate between the three SUs. It gives

the number of octets between the check bit (CK) field and the LI field. The LI field contains

different values according to the type of SU, it is 0 for FISU, 1 or 2 for LISU and is greaterthen 2 for MSU.

The Maximum value in the length indicator fields is 63 even if the signaling information field

(SIF) contains more than 63 octets.

Fig : 10 Format of various signal units

Check bits (CK) : (16 bit) The CKs are formed on the transmission side form the

contents of the SU and are added to the SUs as redundancy. On the receive side, the

MTP can determine with the CKs whether the SU was transferred without any errors.

The SUs acknowledged as either positive or faulty on the basis of the check.

4.8.4 FIELDS SPECIFIC TO MSUS:

Service information octet (SIO): (8 bits) it contains the Service Indicator (SI, 4 bits) and

Subservice fields (SSF, 4 bits) where last 2 bits are Network Indicator (NI).

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Message Signal unit

Link Status Signal unit

Fill in Signal unit

CK

FSIF

SIO

LI

FSN

BSN

F

I

B

B

IB

F

C

KF

S

F

L

I

FS

N

BS

NF

I

B

B

IB

F

CK

FL

I

FS

N

BS

N

B

I

B

FF

IB


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An SI is assigned to cash user of the MTP. It informs the MTP which UP has sent the

message and which UP is to receive. Four SI bits can define 16 Ups (3-SCCP, 4TUP, 5-

ISUP, 6-DATAUP, 8-MTP test, etc.) The NI indicates whether the traffic is international (00,

01) or national (10, 11). In CCS7 a SP can belong to both national and international network

at the same time. So SSF fields indicate where the SP belongs.

The MTP evaluates both items of information.

Signaling information fields (SIF): (2 to 272 octets) it contains the actual user message. The

user message also includes the address (routing label, 40 bits) of the destination to which the

message is to be transferred. The maximum length of the user message is 62 octets for

national and 272 octets for international networks. (One octet= 8 bits). The format and coding

of the user message are separately defined for each UP.

4.8.5 FIELDS SPECIFIC TO LSSUS:

Status field (SF): (1 to 2 octets) It contains status indications for the alignment of the transmit

and receive directions. It has 1 or 2 octets, out of which only 3 bits of first octet are defined

by CCITT, indicating out (000), normal (001), Emergency (010) alignments, out of service

(011), Local processor outage (100) status etc.

4.9 ADDRESSING OF THE SUS (IN SIF)

A code is assigned to each SP in the signaling network according to a numbering plant. The

MTOP uses the code for message routing. The destination of a SU is specified in a routing

label. The routing label is component of every user message and is transported in the SIF.

The routing label in a MSU consists of the followingDestination Point Code (DPC): (14 bits) identifies the SP to which this message is to be

transferred.

Originating Point Code (OPC): (14 bits) specifies the SP from which the message originates.

The coding of OPC and DPC is pure binary and using 14 bits linear encoding, it is possible to

identify 16,384 exchanges. The number of exchanges in DOT network having CCS7

capability is expected to be within this limit.

Signalling Link Selection (SLS) field: (4 bits): the contents of the SLS field determine the

signalling route (identifying a particular signalling link within a link set or link sets) along

which the message is to be transmitted. In this way, the SLS fields are used for lead sharing

on the signalling links between two SPs.

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Fig : 11 Distribution of functions in message transfer part

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Level 4

Level 2

Level 3

Level 1

TUP ISDN-UP Other UPs

Message

Distribution

Message

Routing

Message

Discrimination

Signaling

traffic

Mgmt.

Signaling

route

Mgmt.

Signaling

Link

Mgmt.

Signaling network managementSignaling Link

management

Signaling link status

control

Error rate monitoring

Control for initial

alignment

Transmission

controltransmission

buffer

ReceiveControl

Check bit and

flag generator

Flag,

alignment

and error

detection

Other Signaling

Links

Signaling data link


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The SIO contains additional address information. Using the SI, the destination MTP identifies

the UP for which the message is intended. The NI, for example, enables a message to be

identifies as being for national or international traffic.

LSSUs and FISUs require no routing label as they are only exchanged between level 2 of

adjacent MTPs.

The message sent from a user to the MTP for transmission contains: the user information, the

routing label, the SI, The MI and a LI. The processing of a user message to be transmitted in

the MTP begins in level 3.

The MTP is responsible for (a) transmitting and (b) receiving SUs (c) for correcting

transmission errors, (d) for the signalling network management and (e) for the alignment. Its

functions are spread over the functional levels, 1, 2, and 3.

The message routing (level 3) determines the signalling link on which the user message is to

be transmitted. To do this, it analyzes the DPC and the SLS field in the routing label of the

user message, and then transfers the message to the appropriate signalling link (level 2)

The transmission control (level 2) assigns the next FSN and the FIB to the user message. In

addition, it includes the BSN and the BIB as an acknowledgement for the last received MSU.

The transmission control simultaneously enters the part of the MSU formed so far in the

transmission and retransmission buffers. All MSUs to be transmitted are stored in the

retransmission buffer until their fault-free reception is acknowledged by the receive side.

Only then they are deleted.

The check bit and flag generator (level 2) generates CKs for safeguarding against

transmission errors for the MSU and sets the flag for separating the SUs. In order that any

section of code identical to the flag (01111110) occurring by chance is not mistaken for the

flag, the user message 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 stream along a signalling 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 sequencebetween two flags corresponds to one SU. The alignment detection (level2) monitors the

synchronism of transmit and receive sides with the bit pattern of the flags.

Using the CKs also transmitted error detection (level 2) checks whether the SU was correctly

received. A fault free SU is transferred to the receive control, while a faulty SU is discarded.

The reception of a faulty SU is reported to error rate monitoring, in order to keep a

continuous check on the error rate on the receive side of the signalling link. If a specified

error rate is exceeded, this is reported to the signalling link status control by error rate

monitoring. The signalling link status control then takes the signalling link out of service and

sends a report to level 3.

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The receive control (level 2) checks whether the transferred SU contains the expected FSN

and the expected FIB. If this is the case and if it is a MSU, the receive control transfers the

user message to level 3 and causes the reception of the MSU to be positively acknowledged.

If the FSN of the transferred MSU does not agree with that expected, the receive control

detects a transmission error and causes this and all subsequent MSU 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

UPs or to be transferred to another signalling link (quasi-associated message). This

preselection is achieved in the message discrimination by evaluation of the DPC. A user

message that only passes through a SP (STP) is transferred by the message discrimination to

the message routing, where its treated as a user message to be transmitted.

If a received user message is intended for one of the connected UPs (SP), it is transferred to

message distribution (level 3). The message distribution evaluates the SIO, thereby

determining the UP concerned, and delivers the user message there.

4.10SIGNALLING NETWORK MANAGEMENTThe signalling network management is a function of level 3. It controls the operation and the

interworking of the individual signalling links in the signalling networks. To this end, the

signalling network management exchanges messages and control instruction with the

signalling links of level 2, sends message to the UPs and works together with the signalling

network management in adjacent SPs. For the interworking with other SPs the signalling

network management uses the transport function of the MTP. Management messages are

transferred in MSUs like user messages. For discrimination, the management messages havetheir own SI. The signalling network management contains 3 functions blocks.

a. The signalling link management controls and monitors the individual signalling links. It

receives the messages concerning the alignment and status of the individual signalling

link, or concerning operating irregularities and effects any changes in status which may

be necessary. It addition, the signalling link management controls the putting into service

of signalling links, including initial alignment and automatic realignment of signalling

links after failures or alignment losses due to persistent faults. If necessary, the signalling

link management transfers messages to the signalling traffic management or receives

instructions from there.

b. The signalling route management controls and monitors the operability of signallingroutes. It exchanges messages with the signalling route management in the adjacent STPs

for this purpose. The signalling route management receives, for example, messages

concerning the failure or reavailability of signalling routes or the overloading of STPs. In

cooperation with the signalling traffic management, it initiates the appropriate actions in

order to maintain the signalling operation to the signalling destinations involved.

c. The signalling traffic management controls the diversion of the signalling traffic from

faulty signalling links or routes to fault-free signalling links or routes. It also controls the

load distribution on the signalling links and routes. To achieve this it can initiate the

following actions.

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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

Change back; when signaling link becomes available again after a fault has been

corrected, the signaling traffic management reverses the effect of the changeover.

Rerouting; when SP 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 signalling traffic management sends messages to the users in

its own SP in order that they reduce the load. The management also informs the adjacent SPs

of the overloading in its own SP and requests them to also reduce the load.

The signaling traffic management accomplishes its functions by

Receiving messages from the signaling link and signaling route management.

Sending control instructions to signaling link and signaling route management.

Directly accessing the signaling links, e.g., during emergency alignment.

Modifying the message routing on failure of signaling routes.

Exchanging management message with the signaling traffic management in adjacent SPs.

4.11USER PARTS (UP)As earlier level 4 functions, which include formatting of messages based on the applications,

are allotted to UPs. Each UP provides the functions for using the MTP for a particular user

type. Some of the Ups as currently specified by the CCITT are:

Telephone user part (TUP)

Integrated services digital network user part (ISDN-UP)

The signaling connection control part (SCCP)

The transaction capabilities application part (TCAP)

For Intelligent Network (IN) application, Intelligent Application Part (INAP) and TCAP are

used. SCCP forms the interface between these UPs and MTP.

Fig : 12 Message transfer part users

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TCAP

Users

CCS#7

Users

Other MTP

Userse.g.MUP

TU

F

ISDN

UP

TCA

P

SCC

P

Message transfer part(MTP)


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Fig. 31 Shows the users of the MTP as well as their relationship to one another and to

the MTP. CCS7 can be adapted to all requirements due to the modular structures.

Expansion for future application is also possible. Each CCS user can specify its own

UP, for example, the mobile user part (MUP) is Siemen's own specification for the

mobile telephone network C450.

4.11.1 TELEPHONE USER PART (TUP)

Use of CCS7 for telephone call control singling requires (I) application of TUP function, in

combination with (ii) application of an appropriate set of MTP functions. The TUP is one of

level 4 users in CCS7. It is specified with the aim of providing the same features for

telephone signaling as other telephone signaling system. It exchanges signaling messages

through MTP. Signaling messages contain information relating to call set up and condition of

speech path. The TUP message consists of SIF and a SIO. This signaling information are

generated by the TUP of the originating exchange. The label is 40 bits long, comprises DPC,OPC and CIC. CIC indicates one of the speech circuits connecting the destination and

originating points. Level 3 identifies the user to which a message belongs by SIO, which

comprises a SI and SSF. For TUP SI value is 4. The SSF distinguishes the signaling message

is for national or international network.

4.11.2INTEGRATED SERVICES DIGITAL NETWORK USER PART

The ISDN- UP covers the signaling functions for the control of calls, for the processing of

services and facilities and for the administration of circuits in ISDN. The ISDN- UP has

interface to the MTP and the SCCP for the transport of MSUs The ISDN-UP use SCCP

functions for end-to end signaling.

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4.CCS7 Signalling

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