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Introduction of Signaling
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Introduction of Signaling
Objectives The participant is able to
explain the differences between common channel signaling and channel associated explain the advantages of common channel signaling name the basic elements in the common channel signaling network
Contents 1 2 2.1 2.2 2.3
Introduction Signaling Network Components of a Signaling Network Modes of Signaling Signaling Network Structure
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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.
Exchange A
Exchange B
Speech channel Common channel
Figure 1.Channel-Associated signaling
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Synchronization
User Info.+Register Signals
Ch.2
Ch.16
PCM 1
Ch.0
Ch.1
Ch.30
Ch.31
Line Signals
Exchange
Exchange
A
B Synchronization
User Info.+Register Signals
Ch.2
Ch.16
PCMn
Ch.0
Ch.1
Ch.30
Ch.31
Line Signals
Figure 2.Connection between two exchanges using CAS signaling.
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Frame 0 FAS
TS 1
TS 2
TS 16 TS 17
FAS
Frame 1
SW
TS 1
TS 2
SW
TS 16 TS 17
TS 1
Frame 2 FAS
TS 1
TS 2
SW
TS 1
TS 30 TS 31
T S 17
TS 16 TS 17 TS 18
TS 2
Frame 15
TS 30 TS 31
T S 18
TS 15 TS 16 TS 17 TS 18
T S 15
TS 30 TS 31
TS 31
T S 31
Figure 3. Frames of one of the PCMs in case of using CAS signaling.
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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
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Synchronization
User Info.
Ch.2
Ch. n
PCM 1
Ch.0
Ch.1
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.
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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 costeffectively 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.
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Exchange A
Exchange B
Exchange C
Speech channels Common channel signaling links
Figure 6.Alternative paths through the signaling network
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Signaling Network
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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.
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Signaling link 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 network
Switching network Circuits
Signaling link terminal
Control
Signaling link
Signaling link terminal
Control
Figure 7.Signaling and circuit network
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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.
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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
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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.
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T1
T5
LSET1
Originating point A
LSET2
T2
T4
Destination point B
LSET3
T3
Tx: Signaling transfer point
Figure 10.Signaling route set
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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.
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NAT0
NAT0 D
C
E
NAT1
F
G
NAT1
NAT0 A
B Gateway
INAT0
H
Gateway
NAT0 Figure 11. Network Structure
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