VOICE-OVER PACKETS IN A PACKET NETWORK
to the gateways under their command. There is no mechanism defined in MGCP for
synchronizing "call agents." It is, in essence, a master/slave protocol where the gateways
are expected to execute commands sent by the "call agents."
In the MGCP protocol an assumption is made that the connection model consists of
constructs that are basic end-points and connections. End-points are sources or sinks of
data and could be physical or virtual. The following are two examples of end-points:
1. An interface on a gateway that terminates a trunk connected to a PSTN switch
(e.g., local or toll-connecting, etc.). A gateway that terminates trunks is called a
2. An interface on a gateway that terminates an analog POTS (plain old telephone
service) connection to a telephone, a key system, PABX, and so on. A gateway that
terminates residential POTS lines (to telephones) is called a residential gateway.
An example of a virtual end-point is an audio source in an audio-content server.
Creation of physical end-points requires a hardware installation, while creation of virtual
end-points can be done in software .
Megaco or ITU-T Rec. H.248 (Ref. 13)
Megaco is a call-control protocol that communicates between a gateway controller and
a gateway. It evolved from and replaces SGCP (simple gateway control protocol) and
MGCP (media gateway control protocol). Megaco addresses the relationship between a
media gateway (MG) and a media gateway controller (MGC). An MGC is sometimes
called a softswitch or call agent.
Both Megaco and MGCP are relatively low-level devices that instruct MGs to connect
streams coming from outside the cell or packet data network onto a packet or cell stream
governed by RTP (real-time transport protocol). A Megaco (H.248) connection model
is illustrated in Figure 15.8. There are two principal abstractions relating to the model:
terminations and contexts. A termination acts as sources and/or sinks for one or more
data streams. In a multimedia conference a termination can be multimedia, and it sources
and sinks multiple media streams. The media stream parameters, as well as modem, and
bearer parameters are encapsulated within the termination.
A context is an association between a collection of terminations. There is a special type
of context called the null context, which contain all terminations that are not associated
with any other termination. For example, in a decomposed access gateway, all idle lines
are represented by terminations in the null context.
Let's look at three context possibilities. (1) A context with just one termination is call
waiting. The caller does not hear anyone else. (2) A context with two terminations is a
regular phone call. Of course each person is expected to hear the other. (3) An example
of more than two terminations is a conference call. Each party hears each and every
The maximum number of terminations in a context is a media gateway (MG) property.
MGs that offer only point-to-point connectivity might allow at most two terminations per
context. MGs that support multipoint conferences might allow three or more terminations
The attributes of contexts are:
The topology (who hears/sees whom).