📄 rfc2885.txt
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MG Media Gateway
MGC Media Gateway Controller
NFAS Non-Facility Associated Signalling
PRI Primary Rate Interface
PSTN Public Switched Telephone Network
QoS Quality of Service
RTP Real-time Transport Protocol
SCN Switched Circuit Network
SG Signalling Gateway
SS7 Signalling System No. 7
5. CONVENTIONS
In this recommendation, "shall" refers to a mandatory requirement,
while "should" refers to a suggested but optional feature or
procedure. The term "may" refers to an optional course of action
without expressing a preference.
6. CONNECTION MODEL
The connection model for the protocol describes the logical entities,
or objects, within the Media Gateway that can be controlled by the
Media Gateway Controller. The main abstractions used in the
connection model are Terminations and Contexts.
A Termination sources and/or sinks one or more streams. In a
multimedia conference, a Termination can be multimedia and sources or
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, the null Context, which contains
all Terminations that are not associated to any other Termination.
For instance, in a decomposed access gateway, all idle lines are
represented by Terminations in the null Context.
Cuervo, et al. Standards Track [Page 11]
RFC 2885 Megaco Protocol August 2000
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context +-------------+ | |
| | | Termination | | |
| | |-------------| | |
| | +-------------+ +->| SCN Bearer |<---+->
| | | Termination | +-----+ | | Channel | | |
| | |-------------| | |---+ +-------------+ | |
<-+--->| RTP Stream |---| * | | |
| | | | | |---+ +-------------+ | |
| | +-------------+ +-----+ | | Termination | | |
| | | |-------------| | |
| | +->| SCN Bearer |<---+->
| | | Channel | | |
| | +-------------+ | |
| +-------------------------------------------------+ |
| |
| |
| +------------------------------+ |
| |Context | |
| +-------------+ | +-------------+ | |
| | Termination | | +-----+ | Termination | | |
| |-------------| | | | |-------------| | |
<-+->| SCN Bearer | | | * |------| SCN Bearer |<---+->
| | Channel | | | | | Channel | | |
| +-------------+ | +-----+ +-------------+ | |
| +------------------------------+ |
| |
| |
| +-------------------------------------------------+ |
| |Context | |
| | +-------------+ +-------------+ | |
| | | Termination | +-----+ | Termination | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| ___________________________________________________ |
+------------------------------------------------------+
Figure 1: Example of H.248 Connection Model
Cuervo, et al. Standards Track [Page 12]
RFC 2885 Megaco Protocol August 2000
Figure 1 is a graphical depiction of these concepts. The diagram of
Figure 1 gives several examples and is not meant to be an all-
inclusive illustration. The asterisk box in each of the Contexts
represents the logical association of Terminations implied by the
Context.
The example below shows an example of one way to accomplish a call-
waiting scenario in a decomposed access gateway, illustrating the
relocation of a Termination between Contexts. Terminations T1 and
T2 belong to Context C1 in a two-way audio call. A second audio call
is waiting for T1 from Termination T3. T3 is alone in Context C2.
T1 accepts the call from T3, placing T2 on hold. This action results
in T1 moving into Context C2, as shown below.
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ +-------------+ | |
| | | Term. T2 | +-----+ | Term. T1 | | |
| | |-------------| | | |-------------| | |
<-+--->| RTP Stream |---| * |------| SCN Bearer |<---+->
| | | | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ | |
| | +-----+ | Term. T3 | | |
| | | | |-------------| | |
| | | * |------| SCN Bearer |<---+->
| | | | | Channel | | |
| | +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 2: Example Call Waiting Scenario / Alerting Applied to T1
Cuervo, et al. Standards Track [Page 13]
RFC 2885 Megaco Protocol August 2000
+------------------------------------------------------+
|Media Gateway |
| +-------------------------------------------------+ |
| |Context C1 | |
| | +-------------+ | |
| | | Term. T2 | +-----+ | |
| | |-------------| | | | |
<-+--->| RTP Stream |---| * | | |
| | | | | | | |
| | +-------------+ +-----+ | |
| +-------------------------------------------------+ |
| |
| +-------------------------------------------------+ |
| |Context C2 | |
| | +-------------+ +-------------+ | |
| | | Term. T1 | +-----+ | Term. T3 | | |
| | |-------------| | | |-------------| | |
<-+--->| SCN Bearer |---| * |------| SCN Bearer |<---+->
| | | Channel | | | | Channel | | |
| | +-------------+ +-----+ +-------------+ | |
| +-------------------------------------------------+ |
+------------------------------------------------------+
Figure 3. Example Call Waiting Scenario / Answer by T1
6.1 Contexts
A Context is an association between a number of Terminations. The
Context describes the topology (who hears/sees whom) and the media
mixing and/or switching parameters if more than two Terminations are
involved in the association.
There is a special Context called the null Context. It contains
Terminations that are not associated to any other Termination.
Terminations in the null Context can have their parameters examined
or modified, and may have events detected on them.
In general, an Add command is used to add Terminations to Contexts.
If the MGC does not specify an existing Context to which the
Termination is to be added, the MG creates a new Context. A
Termination may be removed from a Context with a Subtract command,
and a Termination may be moved from one Context to another with a
Move command. A Termination SHALL exist in only one Context at a
time.
Cuervo, et al. Standards Track [Page 14]
RFC 2885 Megaco Protocol August 2000
The maximum number of Terminations in a Context is a MG property.
Media gateways that offer only point-to-point connectivity might
allow at most two Terminations per Context. Media gateways that
support multipoint conferences might allow three or more terminations
per Context.
6.1.1 Context Attributes and Descriptors
The attributes of Contexts are:
. ContextID.
. The topology (who hears/sees whom). The topology of a Context
describes the flow of media between the Terminations within a
Context. In contrast, the mode of a Termination (send/receive/_)
describes the flow of the media at the ingress/egress of the
media gateway.
. The priority is used for a context in order to provide the MG
with information about a certain precedence handling for a
context. The MGC can also use the priority to control
autonomously the traffic precedence in the MG in a smooth way in
certain situations (e.g. restart), when a lot of contexts must
be handled simultaneously.
. An indicator for an emergency call is also provided to allow a
preference handling in the MG.
6.1.2 Creating, Deleting and Modifying Contexts
The protocol can be used to (implicitly) create Contexts and modify
the parameter values of existing Contexts. The protocol has commands
to add Terminations to Contexts, subtract them from Contexts, and to
move Terminations between Contexts. Contexts are deleted implicitly
when the last remaining Termination is subtracted or moved out.
6.2 Terminations
A Termination is a logical entity on a MG that sources and/or sinks
media and/or control streams. A Termination is described by a number
of characterizing Properties, which are grouped in a set of
Descriptors that are included in commands. Terminations have unique
identities (TerminationIDs), assigned by the MG at the time of their
creation.
Terminations representing physical entities have a semi-permanent
existence. For example, a Termination representing a TDM channel
might exist for as long as it is provisioned in the gateway.
Cuervo, et al. Standards Track [Page 15]
RFC 2885 Megaco Protocol August 2000
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