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VoIP signaling protocols are responsible for call setup, maintenance, and teardown. A number of different protocols are in
use—some standards-based, others proprietary, and each with advantages and disadvantages. The following sections
introduce the signaling protocols you should know about, including SCCP, H.323, MGCP, and SIP.
VoIP Signaling Protocols
VoIP signaling protocols handle the call setup, maintenance, and teardown functions of VoIP calls. It is important to keep
in mind that the signaling functions are an entirely separate packet stream from the actual voice bearer path (RTP). The
signaling protocol in use must pass the supervisory, informational, and address information expected in any telephony
system.
VoIP signaling protocols are either peer-to-peer or client-server; in the case of peer-to-peer protocols, the endpoints have
the intelligence to perform the call-control signaling themselves, Client-server protocols send event notifications to the
call agent (the Unified CM server) and receive instructions on what actions to perform in response. The following table
summarizes the characteristics of the four signaling protocols dealt with here.
Inter-Vendor Implemented Implemented
Protocol Standard? Compatibility on Gateways on Cisco IP Phones Operating Mode
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
environments, it is a long-established and stable protocol very suitable for intervendor compatibility. H.323 is supported
by all Cisco voice gateways and CM platforms as well as some third-party video endpoints.
MGCP
Media Gateway Control Protocol is a lightweight client/server protocol for PSTN gateways and some clients. It is simple
to configure and allows the call agent to control the MGCP gateway, eliminating the need for expensive gateways with
intelligence and complex configurations. The gateway reports events such as a trunk going off-hook, and the call agent
instructs the gateway on what to do; the gateway has no local dial plan because all call routing decisions are made at the
call agent and relayed to the MGCP gateway. MGCP is not as widely implemented as SIP or H.323. MGCP is not
supported by Unified CM Express or the Smart Business Communication System.
SIP
Session Initiation Protocol is an IETF standard that uses peer-to-peer signaling. It is very similar in structure and syntax
to HTTP, and because it is text-based, it is relatively simple to debug and troubleshoot. SIP can use multiple transport
layer protocols and can support security and proxy functions. SIP is an evolving standard that currently provides basic
telephony functionality; further developments and extensions to the standard will soon make it feature-comparable with
SCCP. One of its most important capabilities is creating SIP trunks to IP Telephony service providers, replacing or
enhancing traditional TDM PSTN connections.
SCCP
Skinny Client Control Protocol is a Cisco-proprietary signaling protocol used in a client-server manner between Unified
CM and Cisco IP Phones (and some Cisco gateways). SCCP uses TCP connections to the Unified CM to set up, maintain,
and tear down voice and video calls. It is referred to as a stimulus protocol, meaning that it sends messages in response to
events such as a phone going off-hook or a digit being dialed. SCCP is the default signaling protocol for all Cisco IP
phones, although many also support SIP; SIP does not yet support the full feature set available to SCCP phones. All
Cisco Unified Communications call agents (CM, CM Express, and the 500 Series) and some gateways support SCCP.
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