Signaling System No.7 Protocol Architecture And Sevices part 20

Summary MTP3 provides reliable message delivery for signaling traffic between SS7 nodes. The network structure provides for a hierarchical design
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Signaling System No.7 Protocol Architecture And Sevices part 20SummaryMTP3 provides reliable message delivery for signaling traffic between SS7 nodes.The network structure provides for a hierarchical design, using the point code todiscriminate between hierarchy levels.Signaling Message Handling uses the Point Code to send messages to the correctdestination and discriminate incoming messages to determine whether they havereached their destination. The message handling functions use static routinginformation maintained at each node to populate the MTP Routing Label and toselect the correct link for sending the message.SS7s Signaling Network Management procedures provide a mechanism to handlenetwork failures and congestion with minimal loss, duplication, or mis-sequencingof messages. Due to the critical nature of SS7 signaling, the procedures forhandling failures and congestion are comprehensive. SNM uses the exchange ofmessages between nodes to communicate failure and recovery events as well as thestatus of routes. Timers monitor SNM procedures and messages to ensure thatappropriate action is taken to maintain network integrity.Because MTP3 adheres to the modularity of the OSI model, the user parts candepend on the MTP3 transport without being aware of the underlying details. Thetwo levels exchange a simple set of primitives to communicate status. < Day Day Up > < Day Day Up >Chapter 8. ISDN User Part (ISUP)The ISDN User Part (ISUP) is responsible for setting up and releasing trunks usedfor inter-exchange calls. As its name implies, ISUP was created to provide corenetwork signaling that is compatible with ISDN access signaling. The combinationof ISDN access signaling and ISUP network signaling provides an end-to-endtransport mechanism for signaling data between subscribers. Today, the use ofISUP in the network has far exceeded the use of ISDN on the access side. ISUPprovides signaling for both non-ISDN and ISDN traffic; in fact, the majority ofISUP-signaled traffic currently originates from analog access signaling, like thatused by basic telephone service phones.The primary benefits of ISUP are its speed, increased signaling bandwidth, andstandardization of message exchange. Providing faster call setup times thanChannel Associated Signaling (CAS), it ultimately uses trunk resources moreeffectively. The difference in post-dial delay for calls using ISUP trunks is quitenoticeable to the subscriber who makes a call that traverses several switches.NOTEPost-dial delay is the time between when the originator dials the last digit and theoriginating end receives an indication (or audible ringback).In addition to its speed efficiencies, ISUP enables more call-related information tobe exchanged because it uses Common Channel Signaling (CCS). CAS signalingseverely limits the amount of information that can be exchanged over trunksbecause it shares a small amount of space with a calls voice stream. ISUP definesmany messages and parameters, therefore, allowing information about a call to beexchanged both within the network and between end-users. Although messagesand parameters do vary between different countries, a given variant provides astandard means of exchanging information between vendor equipment within thenational network, and to a large degree, at the international level.For the reader who is unfamiliar with the PSTN and how switching exchangeswork, Chapter 5, The Public Switched Telephone Network (PSTN), explains thePSTN, describes the basic concepts of call processing at an exchange, andintroduces the concepts of trunks, trunkgroups, and routing.ISUP consists of call processing, supplementary services, and maintenancefunctions. This chapter is divided into the following sections, which describe thespecific components of ISUP: • Bearers and Signaling • ISUP and the SS7 Protocol Stack • ISUP Message Flow • Message Timers • Circuit Identification Codes • Enbloc and Overlap Address Signaling • Circuit Glare • Continuity Test • ISUP Message Format • Detailed Call Walk-Through• Circuit Suspend and Resume• ISUP and Local Number Portability• ISUP–ISUP Tandem Calls• Interworking with ISDN• Supplementary Services• Additional Call Processing Messages• Maintenance Messages and Procedures < Day Day Up > < Day Day Up >Bearers and SignalingISUP allows the call control signaling to be separated from the circuit that carriesthe voice stream over interoffice trunks. The circuit that carries the voice portion ofthe call is known within the telephone industry by many different terms. Voicechannel, voice circuit, trunk member, and bearer all refer to the digital time slotthat transports the voice (fax, modem, or other voiceband data) part of a call. Theterm voice circuit can be somewhat ambiguous in this context becausesometimes it is used to refer to the trunk span that is divided into time slots, or toan individual time slot on a span.The signaling component of the call is, of course, transported over SS7 signalinglinks. This creates two independent paths for call information between nodes: thevoice path and the signaling path. The signaling mode describes the signalingrelation between the two paths. Following is a brief review of the associated andquasi-associated signaling modes as they relate to ISUP, which we discussed inearlier chapters.If the signaling travels on a single linkset that originates and terminates at the samenodes as the bearer circuit, the signaling mode is associated. If the signaling travelsover two or more linksets and at least one intermediate node, the signaling mode isquasi-associated. In Figure 8-1, part A shows quasi-associated signaling be ...