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October 2013 – Ed.1.0

EQUITEL E400 Family – Control and communications protocol

Page 26

Chapter 4. SIP

The SIP protocol is defined under the standard RFC 3261 and encompasses the initiation, management,

modification and finalization of multimedia services exchange sessions among several participants, through

digital networks, telephone calls, video-conferences, delivery of multimedia contents…

The most common application of this protocol nowadays is the management of VoIP calls (in the point 1.2 we

have given a brief overview of this technology).

EQUITEL has incorporated a SIP agent to allow the equipments of the E400 family to receive and make calls

directly to IP telephones and, if the suitable PBX is used, even to analogue or mobile phones.

Both the board E401 and the intercom E451 have the possibility of using this protocol instead of EQUITEL’s

proprietary one by configuring the equipment in SIP mode with PBX or in P2P-SIP mode.

SIP is a very complex protocol encompassing several aspects. In the modules E401 and E451 we have focused on

the application for VoIP calls management. We have implemented a small part of the protocol that has allowed

us to develop an easy functionality for answering incoming calls and making calls to the configured device, by

means of pressing an available input (“push-to-call”).

In this chapter we will explain briefly some basic concepts to understand the application of this protocol in the

E400 intercom system.

4.1 Basic concepts

In a basic SIP managed VoIP network we find the following main components:

User Agent

It is any device capable of beginning or accepting any SIP session. It can be an IP telephone, a video-

telephone, a computer running the appropriate application, an E401 board, an E451 intercom, etc.

Registrar

It is a server that maintains the physical location (IP address) of every network user agent.

Each user agent has a logical address that remains unchanged, and a physical one that may change.

When connecting to the network, each agent must register in the server to inform of its logical address,

and the physical one at that moment.

In the most typical example of the SIP protocol (IP telephony), the logical addresses are the equivalent to

their extension numbers. Each telephone (user agent) has a fixed extension but its IP address may change

(it can be assigned dynamically with the DHCP protocol, for example)

In this case, the PBX contains a registrar agent maintaining the equivalence list between each extension

number and its corresponding IP addresses in the network.

Furthermore, this PBX may authorize each user agent to register or not by using security passwords, with

the aim of accepting only the authorized users.

Once an agent is registered, the rest can access it through the registrar server using the assigned logical

address (extension).

The link between the logical address and the physical one within the registrar agent has a fixed expiry

date. If the subscription is not renewed before that date, it expires and makes the agent inaccessible to

the rest of the units.

SIP server

A SIP server puts in contact an agent with another one, either as an intermediary by exchanging the

messages between itself and the agents (proxy server), or indicating to the others which is each agent

delivery address (redirect server)

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Equitel E451 Manual de usuario Pagina 29

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