IP Telephony Cookbook by Saverio Niccolini, Jorg Ott, et al - HTML preview

RCF

ARQ:x@tzi.org

LRQ:x@tzi.org

LCF + IP

ACF + IP

Setup: x@tzi.org

Figure 2.9 External address resolution using LRQs

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[IP Telephony Cookbook] / Technological Background

A location request can be sent via unicast or multicast. If sent via multicast, only the gatekeeper that can resolve the address replies. If a gatekeeper receives a unicast LRQ, it either confirms or rejects the request.

This mechanism can have a list of peer gatekeepers to ask, in parallel or sequentially. It is also possible to assign a domain suffix or number prefix to each peer so that an address with a matching number prefix of a neighbouring institution will result in a request to the gatekeeper of that institution. By defining default peers, one could also build a hierarchy of gatekeepers (see Chapter 7 for further details).

{ 2.2.1.7 A sample call scenario

Figure 2.10 depicts an example of an inter-zone call setup using H.323 with one gatekeeper (A) using direct signalling while the other uses routed signalling.The calling party in zone A contacts its gatekeeper to ask for permission to call the called party in zone B (1).The gatekeeper of zone A confirms this request and provides the calling party with the address of zone B's gatekeeper (2).1 The calling party establishes a call signalling channel (and subsequently/in parallel the conference control channel) to the gatekeeper of zone B (3), who determines the location of the called party and forwards the request to the called party (4).

Zone A