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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
4-2
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.)
4-3
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
4-6
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.
4-9
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
4-12
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
4-13
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.
4-14
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).
4-16
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
4-18
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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