Understanding the Capabilities
of Gateway Protocols
The three voice gateway protocols supported in Cisco’s AVVID architecture are
Skinny Station Protocol (SSP),H.323, and MGCP. Skinny Station Protocol allows a
Skinny client to use TCP/IP to transmit and receive calls and RTP/UDP/IP packets
for audio.An example of a Skinny client is an IP phone or gateway. The Skinny
clients communicate with a Cisco CallManager over TCP on ports 2000–2002. SSP
was developed by Cisco (formerly Selsius) as a low-bandwidth gateway protocol.
H.323 is the most supported gateway protocol from Cisco, and is an ITU-T
(Telecommunication Standardization Sector of the International Telecommunications
Union) standard for packet-based audio, video, and conferencing. It is the
umbrella standard for the conferencing standard (made up of others such as H.245,
H.225, and Q.931), and is the only gateway that provides full routing capabilities. It
transmits and receives media streams via RTP with Real-Time Control Protocol
(RTCP), carried over UDP, thereby providing status and control information.
Signaling, such as Registration,Admission, and Status (RAS), H.245, and Q.931 is
transported over TCP.Q.931 signaling, meanwhile, is for call setup and termination.
Capabilities, however, are exchanged by utilizing H.245, which is for call control,
and establishes multimedia communication or call services between the H.323
clients. Some new features to be supported in H.323v2 are H.235, H.450.x, Fast
Connect, alternative gatekeepers,Q.931 forwarding, and integration of T.120. For
its part, H.235 allows security and authentication, such as registration passwords,
while H.450.x offers supplementary services.
The MGCP protocol functions in an architecture where the call control
intelligence is removed from the gateway. Level3, Bellcore, Cisco, and Nortel
developed MGCP (described in Request for Comments [RFC] 2705), which is a
master/slave protocol, where the gateway is the slave servicing commands from
the master, which is the call agent. In the Cisco AVVID environment, the
CallManager functions as the call agent.Two benefits MGCP provides over
H.323 are centralized dial plans and dynamic dial plan updates versus statically
configuring each H.323 gateway. (The MGCP protocol communicates over UDP
port 2427 and TCP port 2428.)
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68 Chapter 3 • AVVID Gateway Selection
The skinny gateways are the DT-24+, DE-30+, and Catalyst 4000/6000
modules, which provide CallManager access to digital gateways. An example of
an H.323 gateway is a Cisco IOS router like the 2600 and 3600.The VG-200 is
an MGCP gateway with future support for the 2600, 3600, 3810, and Catalyst
modules.
Another protocol being implemented in Cisco gateways is the Session
Initiation Protocol. SIP (described in RFC 2543) is an application-layer control
protocol that can establish, modify, and terminate multimedia sessions or calls.
These sessions include IP conferences, telephone calls, and multimedia distribution.
A Cisco VoIP solution for SIP consists of a SIP agent, 7960 IP Phone, SIP
gateway, and a SIP proxy server.
SIP supports five elements of establishing and terminating communications:
User location
User capabilities
User availability
Call setup
Call handling
Currently, the VoIP world is dominated by H.323; the emergence of SIP and
the increasing number of applications supporting this new technology means the
interoperability of SIP with existing H.323 networks. SIP was first available in
IOS version 12.1(1)T. SIP was enhanced in IOS version 12.2(2)XA, supporting
the Cisco 2600 and 3600 Series router-voice gateways, and the Cisco AS5300,
AS5350, and AS5400 access server-voice gateways now allow both SIP and H.323
to fully coexist, as well as allowing interoperability between the two protocols.
NOTE
An example of new software utilizing the functionality of SIP is the application
Windows Messenger, which is part of Windows XP. Windows
Messenger is real-time communications software that provides end-toend
IP telephony. SIP is a multipart communication protocol, but the first
version of Windows Messenger will only support a two-way conversation.
Video gateways are used to convert form H.320 devices to H.323 devices.
The Cisco IP/VC products allow companies to utilize their legacy H.320 ISDNwww