Figure 4.7 Static individual trunking
A more likely solution is a model where a PBX ‘knows’ which numbers are located in the IP
world and the IP Telephony servers use a shared database that defines which number belongs to which server. If there are different types of IP Telephony servers, it may be that they are unable to share a common routing database in which case each server has its own database, resulting in administration overhead and risk of inconsistency.
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4.1.1.2.3 Dynamic individual routing
The previously introduced possibilities required some kind of static configuration to bind a number to a specific server.These scenarios usually end up using at least one routing database, but often more.The requirement for such a burden presumably has its roots in the classic PBX
mentality of static configuration. In the IP world, the network offers the possibility of carrying information from one server to another so that IP Telephony protocols provide the means to dynamically exchange routing information.The mechanisms can generally be divided into ‘Push’
and ‘Pull’ techniques.When using a ‘Push’ mechanism, a server informs its peers of every endpoint that registers or un-registers from the server, thus allowing its peer servers to immediately make routing decisions if necessary. On the other hand, care must be taken that servers that are integrated later, be synchronised.
‘Pull’ mechanisms are used when a server asks its peer servers for a target address when it needs to resolve an external target address.The peer giving a positive reply receives the routed call (see Figure 4.8).
In H.323, trunking is achieved by LRQ messages (see Section 7.1.1). SIP has no extra mechanism for address resolution but is able to fan out a call to multiple servers at the same time. Both signalling protocols may use TRIP (see Section 7.1.3) to distribute numeric addresses, but not names.
D
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