ISDN User Part [Under SS7]
ISDN stand for Integrated Services Digital Network. Basic idea of ISDN was to provide digital link directly to subscriber and provide both voice and data over same (circuit switched) link. Link itself will have multiple channels which can be dynamically allocated depending on data rate. ISDN also separated signaling from transmission, D channels carrying signaling and B channel carrying data. B channels will typically be multiples of 64 kpbs. D channel would be 16 kbps (Basic Rate Interface/BRI) or 64 kbps (Primary Rate Interface/PRI) or multiples of it.
Though powerful, ISDN lacked the standardisation on network side. Without end-to-end ISDN connectivity, complete potential of ISDN was not realised until SS7. With SS7, ISDN was able to provide end-to-end (digital) connectivity. ISDN was accommodated in SS7 as ISDN User Part.
ISUP provide signaling required for voice/data connections between SSPs. (D channel) Signaling goes over (STP) signaling network whereas SSPs are connected via transmission network (B channels).
Let us see how ISUP setup the call.
A calls B. A and B are located on different exchange. The call is required to go through OE (originating exchange), TE (transit exchange) and DE (destination exchange). Exchanges are nothing but SSPs.
1) OE translates the digits (dialed by A) and finds out that the call need to routed to TE. To do it, it sends (ISUP) Initial Address Message (IAM) - containing called number, calling number, type of call etc. information - to TE. OE also activates backward direction transmission link to A.
2) TE again translates on called number received in IAM and finds out that the call need to be routed to DE. Note that the network operator need to make sure that the appropriate "translation tables" are present in exchanges. To do it, it sends out another IAM - most of the information transparently taken from IAM received from OE - to DE. At the same time, TE may reserve or connect a transmission link between OE and TE in backward or both directions. In practice, a backward direction link is connected.
3) DE finds out - from IAM received from TE - that called number is within its area. As B is not busy, it sends Address Complete Message (ACM) back to TE and reserve or connect the transmission link between DE and TE in backward or both directions (in practice backward direction link). After receiving ACM, TE also sends ACM back to OE. ACM can actually be considered as "called party B ringing". At this point DE may also provide a "ringing tone" over transmission link (if connected). B also sends out ringing to B.
4) Once B answers, DE sends out Answer Message (ANM) to TE and simultaneously completes the transmission path on both directions if not done already. TE also sends out ANM to OE and completes the transmission path. OE too completes transmission link to A. Now A and B are in conversation.
Release (REL) message is used during call teardown (reason would be called party busy, called party did not answer, or normal call release etc.). Reply to REL message Release Complete (RLC).
In next article, we will see ISUP message formats.
References: Q.700 (SS7), Q.764 (ISUP signaling procedures), SS7 book by Russell.
© Copyright Samir Amberkar 2010
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