Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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Chapter 7: Replacing Call Signaling with VoIP

Traditional telecom engineers have balked about the wordiness, or bulkiness, of

SIP’s message structure. Instead of using compact, machine-friendly message pack-

ets like H.323, SIP uses lengthy, human-readable headers like SMTP or HTTP. Pro-

ponents of SIP counter that this human readability makes SIP easier to troubleshoot,

and I tend to agree.

SIP is currently in Version 2.0. Its definition is found in RFCs 3261 through 3265.

The defined purpose of SIP is to coordinate and facilitate monitoring of media ses-

sions on the network. It supports a variety of addressing schemes and can be

designed as a centralized or distributed topology.

SIP Nodes

SIP endpoints and servers are called nodes. A SIP phone is a node. SIP phones can com-

municate directly with each other in order to establish media sessions, just as H.323

terminals can establish direct channels. But more often than not, especially in an

enterprise setting, SIP is used with a SIP server. SIP phones normally report to a dedi-

cated SIP server node called a registrar upon boot-up.

SIP registrar

The SIP registrar is a database server that communicates with SIP nodes in order to

collect, store, and disperse information about the whereabouts of SIP users. When a

SIP node registers