rfc2705.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   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.






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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.



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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:























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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.








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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.





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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
   MGCP.

2.1.1.2.  Analog line

   Analog lines can be used either as a "client" interface, providing
   service to a classic telephone unit, or as a "service" interface,
   allowing the gateway to send and receive analog calls.  When the
   media gateway also supports a NAS service, the gateway shall be
   capable of receiving audio-encoded data (modem connection) and
   convert them into data packets.



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RFC 2705         Media Gateway Control Protocol (MGCP)      October 1999