ZyXEL Communications MSC1000G Series, User Manual

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Chapter 16 VoIP
Management Switch Card User’s Guide
603
3
Proxy 2 sends a SIP INVITE request to User Agent 2 .
4
User Agent 2 sends a response back to Proxy 2 indicating that the phone is ringing. The response
is relayed back to User Agent 1 via Proxy 1 .
5
User Agent 2 sends an OK response to Proxy 2 after the call is answered. This is also relayed
back to User Agent 1 via Proxy 1 .
6
User Agent 1 and User Agent 2 exchange RTP packets containing voice data directly, without
involving the proxies.
7
When User Agent 2 hangs up, he sends a BYE request.
8
User Agent 1 replies with an OK response confirming receipt of the BYE request, and the call is
terminated.
16.1.4 Introduction to H.248
The H.248 protocol, also known as MEGACO (MEdia GAteway COntrol) or the Gateway Control
Protocol, defines a VoIP network in which basic functions (such as voice coding and decoding) are
performed by one device, and higher functions (such as setting up and managing calls) are
performed by another.
The H.248 protocol was developed jointly by the IETF (Internet Engineering Task Force) and the ITU
(International Telecommunication Union). It is defined by the IETF in RFC 3525, and by the ITU in
ITU-T H.248-1.
While SIP (Session Initiation Protocol) VoIP networks have no separate control unit, relying instead
upon intelligent endpoints, an H.248 VoIP network has a single intelligent control unit, which
manages a network of dumb endpoint devices.
An H.248 VoIP network consists of one or more media gateways and a media gateway controller.
• Media gateways (MGs) encode and decode voice data, transmitting it from one network to
another (for example, from the PSTN to an IP network, and vice versa). The H.248-enabled VoIP
line cards are MGs.
• Media gateway controllers (MGCs) are intelligent devices that manage the media gateways. They
set up, manage and tear down calls by providing instructions to the MGs.
The figure that follows compares a basic SIP network with an H.248 network.
• In the SIP example, analog telephones A and B each connect to a SIP ATA (Analog Telephone
Adaptor). The ATA s not only encode and decode the voice data between the analog and IP
networks, but also set up, maintain and end the call.
• In the H.248 example, the MG allows the users of analog telephone C and IP telephone D to
communicate by reporting events to the MGC (for example, if the user of phone C dials a
sequence of numbers). The MGC interprets the information according to its programming and
issues the relevant commands to the
MG (for example, telling the MG how to connect phones C
and D in a call, or telling the MG to play a busy tone to phone C ). See Section 16.1.4.3 on page
608 for a detailed example of H.248 call progression.