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Review Notes: Gateway Signaling Protocols

H.323
  • The H.323 protocol was designed by the ITU-T and initially approved in February 1996. It was developed as a protocol that provides IP networks with traditional telephony functionality.
  • In Cisco IP Communications environments, H.323 is widely used with gateways, gatekeepers, and third-party H.323 clients, especially video terminals. You configure connections between devices using static destination IP addresses.
  • Because H.323 is a peer-to-peer protocol, H.323 gateways are never controlled by Cisco Unified CallManager. Therefore, H.323 gateways are never registered at the Cisco Unified CallManager. Only the IP address is seen by the Cisco Unified CallManager to confirm that communications is possible.
MGCP
  • MGCP is a plain-text protocol used by call-control devices to manage IP telephony gateways. MGCP was defined under RFC 2705, which was updated by RFC 3660, and superseded by RFC 3435, which was updated by RFC 3661.
  • is a client/server protocol that allows a call agent (such as Cisco Unified CallManager) to take control of a specific voice port on a gateway.
  • With this protocol, the Cisco Unified CallManager knows of and controls individual voice ports on the gateway. It allows complete control of the dial plan from Cisco Unified CallManager, and gives CallManager per-port control of connections to the PSTN, legacy PBX, voice mail systems, POTS phones, and so forth.
  • This is implemented with use of a series of plain-text commands sent over User Datagram Protocol (UDP) port 2427 between the Cisco Unified CallManager and the gateway.
  • It is important to note that for an MGCP interaction to take place with Cisco Unified CallManager, the gateway must have Cisco Unified CallManager support.
  • A PRI and BRI backhaul is an internal interface between the call agent (such as Cisco Unified CallManager) and Cisco gateways. It is a separate channel for backhauling signaling information. A PRI backhaul forwards PRI Layer 3 (Q.931) signaling information via a TCP connection.
SIP
  • SIP is a protocol developed by the Internet Engineering Task Force (IETF) Multiparty Multimedia Session Control (MMUSIC) Working Group as an alternative to H.323.
  • SIP features are compliant with IETF RFC 2543, published in March 1999, RFC 3261, published in June 2002, and RFC 3665, published in December 2003.
  • Because it is a common standard based on the logic of the World Wide Web and very simple to implement; SIP is widely used with gateways and proxy servers within service provider networks for internal and end-customer signaling.
  • SIP is a peer-to-peer protocol where user agents (UAs) initiate sessions, like H.323. But unlike H.323, SIP uses ASCII-text-based messages to communicate. Therefore, you can implement and troubleshoot it very easily, and analyze the incoming signaling traffic content very simply.
  • Because SIP is a peer-to-peer protocol, the Cisco Unified CallManager does not control SIP devices, and SIP devices do not register with Cisco Unified CallManager. As with H.323 gateways, only the IP address is available on Cisco Unified CallManager to confirm that the communications between the Cisco Unified CallManager and the SIP voice gateway is possible.

SCCP (Skinny Call Control Protocol)
  • SCCP is a Cisco proprietary protocol that is used for the communications between Cisco Unified CallManager and terminal endpoints (based on Selsius prorocol, that was bought by Cisco).
  • SCCP is a stimulus protocol, meaning any event (such as on-hook, off-hook, buttons pressed, and so on) causes a message to be sent to the Cisco Unified CallManager.
  • The Cisco Unified CallManager then sends specific instructions back to the device to tell it what to do about the event.
  • Therefore, each press on a phone button causes data traffic between the Cisco Unified CallManager and the terminal endpoint. SCCP is widely used with Cisco IP phones.
  • The major advantage of SCCP within Cisco Unified CallManager networks is its proprietary nature, which allows you to make quick changes to the protocol and add features and functionality.

Signaling Protocols Comparison (H.323, MGCP, SIP, SCCP):

H.323
  • The H.323 protocol suite is a peer-to-peer protocol. The necessary gateway configuration is relatively complex, because you need to define the dial plan and route patterns directly on the gateway.
  • Examples of H.323-capable devices are the Cisco VG224 Analog Phone Gateway and the Cisco 2600XM Series, Cisco 2800 Series, 3700 Series, and 3800 Series routers.
  • The H.323 protocol is responsible for the entire signaling between the Cisco Unified CallManager cluster and the gateway. The ISDN protocols, Q.921 and Q.931, are only used on the ISDN link to the PSTN.
    • NOTES:
      • Q.921 – Also referred to as LAPD (Link Access Protocol – D Channel) and a close cousin of HDLC, is the Data Link protocol used over ISDN’s D channel. [ Reference: http://www.freesoft.org/CIE/Topics/125.htm ]
      • Q.931 –  is ISDN’s connection control protocol, roughly comparable to TCP in the Internet protocol stack. Q.931 doesn’t provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbps. Q.931 does manage connection setup and breakdown. Like TCP, Q.931 documents both the protocol itself and a protocol state machine. [ Reference: http://www.freesoft.org/CIE/Topics/126.htm ]

MGCP
  • The MGCP protocol is based on a client/server architecture.
  • That simplifies the configuration because the dial plan and route patterns are defined directly on the Cisco Unified CallManager within the cluster.
  • Examples of MGCP-capable devices are the Cisco VG224 Analog Phone Gateway and the Cisco 2600XM Series, 2800 Series, 3700 Series, and 3800 Series routers. Non-IOS MGCP gateways include the Cisco Catalyst 6608-E1 and Catalyst 6608-T1.
  • MGCP is used to manage the gateway. All ISDN Layer 3 information is backhauled to the Cisco Unified CallManager. Only the ISDN Layer 2 information (Q.921) is terminated on the gateway.

SIP
  • SIP is a peer-to-peer protocol.
  • The configuration that is necessary for the gateway is relatively complex because the dial plan and route patterns need to be defined directly on the gateway.
  • Examples of SIP-capable devices are the Cisco 2800 Series and 3800 Series routers.
  • The SIP protocol is responsible for the entire signaling between the Cisco Unified CallManager cluster and the gateway. The ISDN protocols, Q.921 and Q.931, are only used on the ISDN link to the PSTN.

SCCP
  • SCCP works in a client/server architecture in the same way as MGCP does. Therefore, it simplifies the configuration of SCCP devices such as Cisco IP phones and Cisco ATA 180 Series and VG200 Series FXS gateways.
  • SCCP is used on Cisco VG224 and VG248 analog phone gateways.
  • ATA’s enable communications between Cisco Unified CallManager and the gateway.
  • The gateway then uses standard analog signaling to the analog device connected to the FXS port. Recent versions of Cisco IOS voice gateways, for example, the 2800 series, also support SCCP controlled FXS ports.

IP-to-IP Gateways
  • Cisco Multiservice IP-to-IP Gateways (IPIPGWs) are the next-generation gateways within unified IP communications networks.
  • They facilitate connectivity between independent VoIP networks by enabling VoIP and videoconferencing calls from one IP network to another.
  • The Cisco Multiservice IPIPGW performs most of the functions of a PSTN-to-IP gateway, but typically joins two IP call legs, rather than a PSTN and an IP call leg.
  • Media packets (RTP packets) can flow either through the gateway (thus hiding the networks from each other) or around the gateway, if configured to do so.
  • Because the Cisco Multiservice IPIPGW is usually used to interconnect two independent networks like service provider networks or an enterprise network to a VoIP service provider, the gateways are generally configured to terminate the RTP. That makes it possible to have a single point of contact between those two networks, which leads to more security between those networks because the IP-to-IP gateway then functions as a proxy for signaling and voice traffic.
  • The Cisco Multiservice IPIPGW also allows the use of two different protocols on both sides. That makes interconnections between two different networks easier and allows simple upgrades from PSTN links to IP links because it doesn’t matter which protocols are already used within the existing network.

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