rfc2705.txt
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An example of a virtual endpoint is an audio source in an audio-
content server. Creation of physical endpoints requires hardware
installation, while creation of virtual endpoints can be done by
software.
Connections may be either point to point or multipoint. A point to
point connection is an association between two endpoints with the
purpose of transmitting data between these endpoints. Once this
association is established for both endpoints, data transfer between
these endpoints can take place. A multipoint connection is
established by connecting the endpoint to a multipoint session.
Connections can be established over several types of bearer networks:
* Transmission of audio packets using RTP and UDP over a TCP/IP
network.
* Transmission of audio packets using AAL2, or another adaptation
layer, over an ATM network.
* Transmission of packets over an internal connection, for example
the TDM backplane or the interconnection bus of a gateway. This is
used, in particular, for "hairpin" connections, connections that
terminate in a gateway but are immediately rerouted over the
telephone network.
For point-to-point connections the endpoints of a connection could be
in separate gateways or in the same gateway.
Arango, et al. Informational [Page 6]
RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
1.1. Relation with the H.323 standards
MGCP is designed as an internal protocol within a distributed system
that appears to the outside as a single VoIP gateway. This system is
composed of a Call Agent, that may or may not be distributed over
several computer platforms, and of a set of gateways, including at
least one "media gateway" that perform the conversion of media
signals between circuits and packets, and at least one "signalling
gateway" when connecting to an SS7 controlled network. In a typical
configuration, this distributed gateway system will interface on one
side with one or more telephony (i.e. circuit) switches, and on the
other side with H.323 conformant systems, as indicated in the
following table:
___________________________________________________________________
| Functional| Phone | Terminating | H.323 conformant |
| Plane | switch | Entity | systems |
|___________|____________|_________________|_______________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through H.225/RAS and|
| | SS7/ISUP | | H.225/Q.931. |
|___________|____________|_________________|_______________________|
| | | | Possible negotiation |
| | | | of logical channels |
| | | | and transmission |
| | | | parameters through |
| | | | H.245 with the call |
| | | | agent. |
|___________|____________|_________________|_______________________|
| | | Internal | |
| | | synchronization| |
| | | through MGCP | |
|___________|____________|_________________|_______________________|
| Bearer | Connection| Telephony | Transmission of VOIP |
| Data | through | gateways | data using RTP |
| Transport | high speed| | directly between the |
| Plane | trunk | | H.323 station and the|
| | groups | | gateway. |
|___________|____________|_________________|_______________________|
In the MGCP model, the gateways focus on the audio signal translation
function, while the Call Agent handles the signaling and call
processing functions. As a consequence, the Call Agent implements the
"signaling" layers of the H.323 standard, and presents itself as an
"H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the H.323
systems.
Arango, et al. Informational [Page 7]
RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
1.2. Relation with the IETF standards
While H.323 is the recognized standard for VoIP terminals, the IETF
has also produced specifications for other types of multi-media
applications. These other specifications include:
* the Session Description Protocol (SDP), RFC 2327,
* the Session Announcement Protocol (SAP),
* the Session Initiation Protocol (SIP),
* the Real Time Streaming Protocol (RTSP), RFC 2326.
The latter three specifications are in fact alternative signaling
standards that allow for the transmission of a session description to
an interested party. SAP is used by multicast session managers to
distribute a multicast session description to a large group of
recipients, SIP is used to invite an individual user to take part in
a point-to-point or unicast session, RTSP is used to interface a
server that provides real time data. In all three cases, the session
description is described according to SDP; when audio is transmitted,
it is transmitted through the Real-time Transport Protocol, RTP.
The distributed gateway systems and MGCP will enable PSTN telephony
users to access sessions set up using SAP, SIP or RTSP. The Call
Agent provides for signaling conversion, according to the following
table:
Arango, et al. Informational [Page 8]
RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
_____________________________________________________________________
| Functional| Phone | Terminating | IETF conforming systems|
| Plane | switch | Entity | |
|___________|____________|_________________|_________________________|
| Signaling | Signaling | Call agent | Signaling exchanges |
| Plane | exchanges | | with the call agent |
| | through | | through SAP, SIP or |
| | SS7/ISUP | | RTSP. |
|___________|____________|_________________|_________________________|
| | | | Negotiation of session |
| | | | description parameters |
| | | | through SDP (telephony |
| | | | gateway terminated but |
| | | | passed via the call |
| | | | agent to and from the |
| | | | IETF conforming system)|
|___________|____________|_________________|_________________________|
| | | Internal | |
| | | synchronization| |
| | | through MGCP | |
|___________|____________|_________________|_________________________|
| Bearer | Connection| Telephony | Transmission of VoIP |
| Data | through | gateways | data using RTP, |
| Transport | high speed| | directly between the |
| Plane | trunk | | remote IP end system |
| | groups | | and the gateway. |
|___________|____________|_________________|_________________________|
The SDP standard has a pivotal status in this architecture. We will
see in the following description that we also use it to carry session
descriptions in MGCP.
1.3. Definitions
Trunk: A communication channel between two switching systems. E.g., a
DS0 on a T1 or E1 line.
2. Media Gateway Control Interface
The interface functions provide for connection control and endpoint
control. Both use the same system model and the same naming
conventions.
Arango, et al. Informational [Page 9]
RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
2.1. Model and naming conventions
The MGCP assumes a connection model where the basic constructs are
endpoints and connections. Connections are grouped in calls. One or
more connections can belong to one call. Connections and calls are
set up at the initiative of one or several Call Agents.
2.1.1. Types of endpoints
In the introduction, we presented several classes of gateways. Such
classifications, however, can be misleading. Manufacturers can
arbitrarily decide to provide several types of services in a single
packaging. A single product could well, for example, provide some
trunk connections to telephony switches, some primary rate
connections and some analog line interfaces, thus sharing the
characteristics of what we described in the introduction as
"trunking", "access" and "residential" gateways. MGCP does not make
assumptions about such groupings. We simply assume that media
gateways support collections of endpoints. The type of the endpoint
determines its functionalities. Our analysis, so far, has led us to
isolate the following basic endpoint types:
* Digital channel (DS0),
* Analog line,
* Annoucement server access point,
* Interactive Voice Response access point,
* Conference bridge access point,
* Packet relay,
* Wiretap access point,
* ATM "trunk side" interface.
In this section, we will develop the expected behavior of such end
points.
This list is not limitative. There may be other types of endpoints
defined in the future, for example test endpoint that could be used
to check network quality, or frame-relay endpoints that could be used
to managed audio channels multiplexed over a frame-relay virtual
circuit.
Arango, et al. Informational [Page 10]
RFC 2705 Media Gateway Control Protocol (MGCP) October 1999
2.1.1.1. Digital channel (DS0)
Digital channels provide an 8Khz*8bit service. Such channels are
found in trunk and ISDN interfaces. They are typically part of
digital multiplexes, such as T1, E1, T3 or E3 interfaces. Media
gateways that support such channels are capable of translating the
digital signals received on the channel, which may be encoded
according to A or mu-law, using either the complete set of 8 bits or
only 7 of these bits, into audio packets. When the media gateway
also supports a NAS service, the gateway shall be capable of
receiving either audio-encoded data (modem connection) or binary data
(ISDN connection) and convert them into data packets.
+-------
+------------+|
(channel) ===|DS0 endpoint| -------- Connections
+------------+|
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint support is a characteristic of the
gateway, and may in fact vary according with the allocation of
resource within the gateway.
In some cases, digital channels are used to carry signalling. This
is the case for example of SS7 "F" links, or ISDN "D" channels.
Media gateways that support these signalling functions shall be able
to send and receive the signalling packets to and from a call agent,
using the "back haul" procedures defined by the SIGTRAN working group
of the IETF. Digital channels are sometimes used in conjunction with
channel associated signalling, such as "MF R2". Media gateways that
support these signalling functions shall be able to detect and
produce the corresponding signals, such as for example "wink" or "A",
according to the event signalling and reporting procedures defined in
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