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					       Introduction of Signaling


(**++**Register-g**++**)




Introduction of Signaling




  Contents
             1             Introduction                         2
             2             Signaling Network                    9
             2.1           Components of a Signaling Network    9
             2.2           Modes of Signaling                  11
             2.3           Signaling Network Structure         15




                                                                    1
                                                                   Introduction of Signaling




    1Introduction
    Why do we need signaling?
    Communication networks connect terminal equipments by using nodes (exchanges)
    to communicate speech, data, text, images etc. The nodes have to exchange some
    information in order to control the setup and clear down of these connections and
    also to maintain the network itself, this information is called signaling.

    Basically we have two kinds of signaling information:
       • Signaling between the terminal equipment and the nodes. {Discussed in the ISDN
           part}
       • Signaling between two nodes.


    Signaling between two nodes is divided into Two different types.


       •   Type 1: Channel-associated signaling (CAS)
    In such a system the 32 channels are divided as follows:

           •   30 channels available for up to 30 voice calls and also can carry Register
               signals.
           •   Channel (0) dedicated to carrying frame synchronization information.
           •   Channel (16)dedicated to carrying signaling information (Line signals).

    All 30-speech channels have to share the capacity of this one signaling channel. Time slot 16
    of any one frame is always assigned to two different speech channels simultaneously, with
    each speech channel being allocated 4 bits respectively.
    Channel-associated signaling systems re used mainly in networks employing
    preferably analog exchanges.




2
Introduction of Signaling




              Exchange A                                          Exchange B




                      Speech channel
                      Common channel




Figure 1.Channel-Associated signaling

                                                      User
                               Synchronization   Info.+Register
                                                    Signals

              PCM 1


               Ch.0     Ch.1       Ch.2             Ch.16           Ch.30   Ch.31




                                                  Line Signals


   Exchange                                                                         Exchange


     A                                                                                B
                                                      User
                               Synchronization   Info.+Register
                                                    Signals

              PCMn


               Ch.0     Ch.1       Ch.2             Ch.16           Ch.30   Ch.31




                                                  Line Signals




Figure 2.Connection between two exchanges using CAS signaling.

                                                                                               3
                                                                      Introduction of Signaling




      Frame 0 FAS     TS 1   TS 2                       TS 16 TS 17                  TS 30 TS 31




                                               FAS            SW




      Frame 1   SW    TS 1   TS 2                       TS 16 TS 17                  TS 30 TS 31




                                               TS 1          T S 17




      Frame 2 FAS     TS 1   TS 2                       TS 16 TS 17 TS 18            TS 30 TS 31




                                               TS 2          T S 18




     Frame 15   SW    TS 1                   TS 15 TS 16 TS 17 TS 18                       TS 31




                                               T S 15        T S 31




    Figure 3. Frames of one of the PCMs in case of using CAS signaling.


4
Introduction of Signaling




Type 2: Common channel signaling (CCS)
In such a system one common signaling channel is provided for a number of speech
channels. Thus, the capacity of the signaling channel is available as a common pool
and is used by the speech channels according to the dynamic demand, i.e. there is
no permanent assignment of signaling channel to speech channel.
The common signaling channel (often referred to as signaling link) carries out the
signaling information transport for a number of speech channels.
The signaling link can be viewed as a tunnel, which connects two exchanges,
possesses a typical transmission rate of 64 kbit/s and accepts and conveys all
signaling information.


Signaling information transfer is made possible by sending messages. A message is
an information block whose structure and meaning of the single elements in the block
are defined by specifications. The control of signaling information transfer is
separated from the control of speech-channel through-connection.




             Exchange A                                    Exchange B




                 Speech channel
                 Common channel




Figure 4.Common channel signaling




                                                                                       5
                                                                Introduction of Signaling




                                 Synchronization   User Info.



                  PCM 1


                   Ch.0   Ch.1       Ch.2            Ch. n           Ch.30   Ch.31




                                                    Signaling
                                                     Chanel


       Exchange                                                                      Exchange


         A                                                                             B

                                 Synchronization   User Info.


                  PCM n


                   Ch.0   Ch.1       Ch.2                            Ch.30   Ch.31




    Figure 5.Connection between two exchanges using CCS signaling.




6
Introduction of Signaling




What are the advantages of common channel signaling systems?
The separation between speech channel network and signaling network is the key
to the more flexible communications networks of the future (ISDN).
The creation of common signaling channels allows unrestricted communication
and flexible data transfer between two exchanges and/or their processors. This
data transfer can also be used for network management, operating and
administration functions.
The common signaling channels can also be used to exchange non-circuit-related
control information between exchanges (e.g. CCBS, CCNR, IN applications, etc.).
Signaling information can be exchanged without regard to the speech channel or
circuit status, and without disturbing the calling or called party.
Reduced call setup times thanks to the high transmission capacity (normally 64
kbit/s) and the usage of message structures (one message can include all called
party digits).
Processor-friendly message structure (multiples of 8 bits).
Common channel signaling also supports services such as
   User-to-user signaling
          Messages are exchanged directly between two terminals and pass
through
         the network in transparent mode.

   End-to-end signaling
          Messages are exchanged between the originating and destination
         exchange without being evaluated in the transit exchanges.
Reliability is high because error detection and correction measures provide for
error-free message transmission. If one signaling link fails, Rerouting guarantees
that the signaling information will still be transferred.
Like most modern protocols, the SS7 protocol is layered. The layered structure of
the system gives us the ability to change a level without affecting the other levels.
This means future services and applications can be implemented fast and cost-
effectively
The first two common channel signaling systems specified internationally by ITU-T
were
ITU-T signaling system No. 6 (CCS6) and
ITU-T signaling system No. 7 (SS7)


Only SS7 will be dealt with in this description.




                                                                                        7
                                                               Introduction of Signaling




                     Exchange A                                  Exchange B




                                                Exchange C




                              Speech channels
                              Common channel signaling links




    Figure 6.Alternative paths through the signaling network




8
Introduction of Signaling




2 Signaling Network
2.1 Components of a SS7 Signaling
    Network

Basic concepts

        A telecommunications network served by common channel signalling is
composed of a number of switching and processing nodes interconnected by
transmission links. To communicate using SS7, each of these nodes requires to
implement the necessary “within node” features of SS7 making that node a signalling
point within the SS7 network. In addition, there will be a need to interconnect these
signalling points such that SS7 signalling information (data) may be conveyed
between them. These data links are the signalling links of SS7 signalling network.
The combination of signalling points and their interconnecting signalling links form the
SS No. 7 signalling network.


Signaling network components


Signaling points

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


The signaling points are the sources (origination points) and sinks (destination
points) of the signaling traffic. In a communications network both these points are
usually exchanges.

The signaling transfer points forward received signaling messages to another
signaling points. No call processing of the message takes place in a signaling
transfer point. A signaling transfer point may be integrated in a signaling point (e.g.
an exchange) or may be a separate node in the signaling network.

All signaling points in a SS No. 7 network are identified by a unique code known as a
point code (Signaling Point Code (SPC)) defined by a corresponding numbering
scheme and can therefore be addressed specifically in a signaling message.

Signaling link

                                                                                           9
                                                                   Introduction of Signaling




     The common channel signalling system uses signalling links (time slots belonging to
     an existing transmission route [e.g. a PCM30 link] ) to convey the signalling
     messages between two signalling points.
     For redundancy purposes, more than one signaling link generally exists between two
     signaling points. If one signaling link fails the SS7 functions cause the signaling traffic
     to be diverted to functioning alternative links.
     A number of signalling links that directly interconnect two signalling points forms what
     is called a signalling link-set.
     Two signaling points that are directly interconnected by a signaling link are, from a
     signaling network structure point of view, referred to as adjacent signaling points.




                           Switching                               Switching
                            network                                 network

                                                Circuits




         Signaling link                                                         Signaling link
           terminal                                                               terminal
                                              Signaling link


            Control                                                                 Control




     Figure 7.Signaling and circuit network




10
Introduction of Signaling




2.2 Modes of signaling
Two different signaling modes can be used in the signaling network.
If the associated mode of signaling is used, the signaling link is routed together with
the associated circuit group. That is to say, the signaling link is connected with those
signaling points, which are also the end points of the circuit group. This signaling
mode is recommended in cases where the traffic relation between the signaling
points A and B carries high traffic loads.
In the quasi-associated mode of signaling the signaling links and the circuit group
follow different routes. Although the circuit group connects the signaling points A and
B directly, one or more signaling transfer points handle the signaling for the circuit
group. This mode is advantageous for less busy traffic relations as it allows one
signaling link to be used for several destinations simultaneously.




                                                                                           11
                                                                   Introduction of Signaling




      Signaling point A                                                   Signaling point B

                                       Circuit group



                                      Signaling link




     Figure 8.Associated mode of signaling




          Signaling point A                                              Signaling point B

                                             Circuit group




                                          Signaling links




                                        Signaling point C/
                                        signaling transfer point




     Figure 9.Quasi-associated mode of signaling




12
Introduction of Signaling



Signaling point modes
A signaling point, at which a message is generated, is the originating point of that
message.
A signaling point to which a message is destined, is the destination point of that
message.
A signalling point at which a message is received on one signalling link and is
transferred
to another link, is a Signal Transfer Point (STP).

Signaling routes
The path determined for the signaling between an origination point and a destination
point is termed the signaling route. Between these two signaling points the signaling
traffic can be distributed over several different signaling routes. All the signalling
routes that may be used between an originating point and a destination point by a
message traversing the signalling network is the signalling route set for that signalling
relation.



In Figure 10 the signaling routes from A to B are LSET1, LSET2 and LSET3. These
routes comprise the signaling route set for B.




                                                                                            13
                                                   Introduction of Signaling




                                       T1                T5




                      LSET1




        Originating                                                Destination
        point A                LSET2                               point B
                                       T2   T4




                               LSET3




                                       T3
                                                 Tx: Signaling transfer point




     Figure 10.Signaling route set




14
Introduction of Signaling




2.3 Signaling Network structure
The definition of two different modes of signaling allows various signaling network
designs. A network can be structured with a uniform mode of signaling (associated or
quasi-associated) or else with a mixed mode (associated and quasi-associated).
The worldwide signaling network is categorized by two functionally independent
levels, the first one is the international level and the second one is the national level.
Each network has a separate numbering scheme for its own signaling points.

The circuits between two adjacent signaling points are combined to form a circuit
group.




                                                                                             15
                                                            Introduction of Signaling




        NAT0                                                    NAT0

                       D




                                                        E
                 C                        NAT1
                                                                           F




                                                                 G

       NAT0                                      NAT1

                                      A


                  B

                                    Gateway
                                                   INAT0                       H



                                                              Gateway




                                                                          NAT0



     Figure 11. Network Structure

16
Introduction of Signaling




                            17
     Introduction of Signaling




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