CCS#7 Signalling

CCS#7 Signalling
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  Common Channel Signaling System No.7  __________________________________________________________________________________ _ 7COMMON CHANNEL SIGNALLING SYSTEMNO. 7 OVERVIEW OF SIGNALLING SYSTEMS One of the major factors influencing the development of signaling systems is therelationship between signaling and the control function of exchanges. Earlytelecommunication networks used analogue step-by-step exchanges. In such systems, thecontrol and switch functions are co-located, and when a call is made, the signaling and trafficfollows the same path within the exchange. his is known as !hannel ssociated #ignaling$! #%. In this case, the signaling and traffic also follows the same path external to theexchange, i.e. on the transmission link. he next stage trough, which the exchanges evolved, is shown in &ig. '.(. In suchexchange the control mechanism for setting-up and releasing calls is separated from theswitch block. he techni)ue allows much more flexibility in controlling calls and it alsoreduces costs. gain, ! # systems are typically associated with this type of exchange.*hereas signaling information is carried on the same path as it is associated speech circuitexternal to the exchange. he two are separated within the exchange. his is shown in &ig. +in which the speech traffic circuits $denoted by solid lines% are routed by the switch block butthe signaling information $denoted by dotted lines% is routed via the control function.etween Exchanges and , the signaling and traffic are carried over the same path. hisapproach was primarily designed to allow optimiation of functions within exchanges, but itseffectiveness is constrained by the need to combine signaling and speech traffic external tothe exchange. *ith !ommon !hannel #ignaling $!!#% systems, the philosophy is to separate thesignaling path from the speech path. he separation occurs both within the exchange andexternal to the exchange $&ig. +(%, thus following optimiation of the control processes,switch block and signaling systems. &ig. +( illustrates that, in a !!# environment, the switch block routs the speech paths as before, and however, a separate path internal to the exchangerouts the signaling $denoted by a dotted line%. his approach allows maximum flexibility inoptimiing exchange and signaling development. he approach gains maximum benefitswhen adopted in parallel with the introduction of digital exchange and digital transmissionsystems. !!# system being particularly efficient in these circumstances. Advantages Of Common Canne! S gna!! ng !ommon !hannel #ignaling is being adopted throughout the world in national andinternational networks for numerous reasons. he main reasons are/ ã he rapidly changing control techni)ues of exchanges. ã he limitations of ! # systems. ã he evolutionary potential of !!# systems. ''00 I, 1abalpur  Common Channel Signaling System No.7  __________________________________________________________________________________ _ F g # $ !ommon channel signalingOne result of the evolutionary process of exchange described above is to change therelationship between signaling and call control. In the early exchange systems, exchangescould communicate, but in a limited and inflexible manner, thus limiting the flexibility of callcontrol. In a !!# environment, the objective is to allow uninhibited communication betweenexchange control functions, or processor, thus tremendously broadening the scope andflexibility of information transfer. &urther advantages result from the evolutionary process of !!# and call control. hedrive to provide an unrestricted communication capability between exchange processorseliminates per-circuit signaling termination costs. hese costs are inevitable in per-circuit! # systems, but for funneling all signaling information into a single common-channel, onlyone signaling termination cost is incurred for each transmission link. here are costs penaltiesfor !!# systems, e.g. the message received by an exchange have to be analyed, resulting ina processing overhead. 2owever, these cost penalties are more than covered by theadvantages of increased scope of inter-processor communication and more efficient processor activity. '300 I, 1abalpur  Common Channel Signaling System No.7  __________________________________________________________________________________ _ he separation of !!# from traffic circuits, and the direct inter-connection of exchange processors, is the early steps in establishing a cohesive !!# network to allowunimpeded signaling transfer between customers and nodes and between nodes in thenetwork. he concept of a cohesive !!# network opens up the opportunity for theimplementation of a wide range of network management administrative, operation andmaintenance function. major example of such function is the )uasi-associated mode of operation. his mode of operation provides great deals of flexibility in network security,reduces the cost of !!# on small traffic routes and extends the data-transfer capabilities for non-circuit related signaling. ! # systems posses limited information-transfer capability due to/ ã he restricted number of conditions that can be applied $e.g. the limit variation thatcan be applied to a 4.!. loop or the limited number fre)uency combinations that can be implemented in a voice fre)uency system% ã he limited number of opportunities to transfer signals $e.g. it is not possible totransmit voice-fre)uency signals during the conversation phase of a call withoutinconveniencing the customers or taking special measures.% 5either of these restrictions applies to !!#. he flexible message-based approachallows a vast range of information to be defined and the information can be during any stageof a call. 2ence, the repertoire of !!# is far greater than channel-associated versions andmessages can be transferred at any stage of a call without affecting the calling and calledsubscribers. !!# systems transfer signals very )uickly, i.e. at 6' 7bps. his speedy signaling also permits the inclusion of far more information without an increase in post dialing delay. echni)ues used in modern !!# system can further improve the flexibility providedto customers. 89ser-to-user8 signaling and end-to-end signaling techni)ues are used wherebymessages can be transferred from one customer to another without undergoing a full analysisat each exchange in the network. *hilst forms of end-to-end signaling are possible using! # systems, the techni)ue can be more efficiently implemented with !!# systems. One of the problems that prompted the development of !!# systems was 8speechclipping8 in the international network. In some case ! # systems, it is necessary to split thespeech path during call set-up to avoid tones being heard by the calling customer. his resultsin a slow return of the answer signal and, if the called customer starts speaking immediatelyafter answer, then the first part of the statement by the called customer is lost. s the firststatement is usually the identity of the called customer, this causes a great deal of confusionand inconvenience. !!# systems avoid the problem by transferring the answer signal )uickly. s a result of the processing ability of !!# systems, a high degree of reliability can be designed into the signaling network. Error detection and correction techni)ues can beapplied which ensure reliable transfer of uncorrupted information. In the case of anintermediate exchange failure, re-routing can take place within the signaling network,enabling signaling transfer to be continued. *hile these features introduce extra'600 I, 1abalpur  Common Channel Signaling System No.7  __________________________________________________________________________________ _ re)uirements, the common channel approach to signaling allows a high degree of reliabilityto be implemented economically. major restriction of ! # is the lack of flexibility, e.g. the ability to add newfeatures is limited. One factor that led to the development of !!# was the increasing need toadd new features and respond to new network re)uirements. 0esponses to new re)uirementsin !!# can be far more rapid and comprehensive than for channel associated versions. !!# systems are not just designed to meet current needs. hey are designed to the asflexible as possible in meeting future re)uirements. One way of achieving the objective is todefine modern !!# systems in a structured way, specifying the signaling system in a number of tiers. he result is flexibility signaling system that reacts )uickly to evolving re)uirementsand future services can be incorporated in a flexible and comprehensive manner. !hanges toexisting services can be implemented more )uickly and at lower cost than with ! # systems. Ove%v e& Of S gna!! ng S'stem No. 7 #ignaling #ystem 5o. : $!!#:% is a message based signaling system between #tored;rogram !ontrolled $#;!% switches. *here the intermediate nodes may be used a #ignalransfer ;oints $#;s% !!#: network can be used for transmitting call related messages, aswell as show speed data packets between I#45 users. he #ignaling !onnection !ontrol ;art$#!!;% enables it to act like a packet network. hus it is an important pre-re)uisite toIntegrated #ervice 4igital 5etwork $I#45% and Intelligent 5etwork $I5% features. Enhancedservice for the public telephone network can also be provided using this message basedsignaling system. Some of te sa! ent feat(%es of CCS7 a%e# ã &ast, reliable and economical. ã it- oriented protocol. ã <abeled messages ã ssociated and )uasi-associated mode of working ã Error correction is supported at link level $level +% by transmission and se)uencecontrol. ã #ignaling message handler at level = supports message routing. ã 0edundancy and load sharing is possible on signaling links. !hange back on link restoration is possible. ã 0edundancy and load sharing is possible on signaling routes, along with diversion onroute failure. CCS7 )%oto*o! Sta*+ he !!#: protocol stack comprises of four layers. *ith reference to the O#I : layer model, the correspondence between the layers is depicted in &ig. '.=. he functions definedfor each layer or level are briefly described in the following paragraphs. ':00 I, 1abalpur
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