Introduction To H.248 - ZyXEL Communications MSC1024G Series User Manual

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.

18.1.4 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 ATAs 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 18.1.4.3 on page 548
MSC1000G/1024G/1224G Series User's Guide
for a detailed example of H.248 call progression.
Chapter 18 VoIP
543