H.323 Protocol Pros and Cons

The H.323 protocol has some benefits and some drawbacks.


The following are some H.323 protocol benefits:

  • Caller ID H.323 provides caller ID from Foreign Exchange Office (FXO) and T1 channel-associated signaling (CAS) ports, whereas MGCP does not.
  • Fractional PRI H.323 supports the use of a fractional PRI.
  • Interoperability H.323 is widely used and interoperates well with applications and devices from multiple vendors. Because all H.323 devices must support the core protocols, a gateway and CallManager have no version dependence.
  • Granular call control H.323 allows a great amount of control over the treatment of calls to and from the gateway, such as for digit manipulation, load balancing, and call rerouting. You can use Toolkit Command Language (Tcl) and voice extensible markup language (VXML) applications.
  • Legacy systems support You can integrate legacy systems based on POTS or ISDN lines into your H.323 network. H.323 supports more types of TDM interfaces and signaling than MGCP.
  • Multimedia support You can use H.323 for both voice calls and video conferencing. H.323 also allows data transfer.
  • Non-Facility Associated Signaling (NFAS) support H.323 supports NFAS, which allows you to control multiple ISDN PRI lines with just one D channel, thus giving you more usable channels.
  • H.323 gatekeepers Gateways can point to a gatekeeper for call control and address resolution.
  • PRI call preservation Because the gateway terminates both Q.921 and Q.931 signaling, the loss of its CallManager does not require dropping calls using the PRI line.


The H.323 protocol drawbacks include the following:

  • Configuration Gateway configuration is more involved than with MGCP because of the need for dial plan information for the dial peers. H.323 gateways would need a complex configuration to achieve the same functionality as CallManager partitions and Calling Search Spaces. The use of gatekeepers can eliminate some of this complexity.
  • Lack of centralized dial plan When you make changes to the dial plan, you must reconfigure all the gateways to reflect that change. With MGCP, however, only the CallManager needs the dial plan information. Using a Gatekeeper alleviates this somewhat.
  • Security H.323 does not support the use of secure RTP in Cisco IOS versions before the later releases of 12.4T.
  • Call survivability The default configuration of H.323 does not support call survivability. By default, if the connection to the CallManager is lost, all active calls are dropped. When using Survivable Remote Site Telephony (SRST), CallManager terminates active calls when it comes back online.
  • Q Signaling (QSIG) facility IE support H.323 has no support for QSIG facility IE; therefore, information (such as calling name and redirect number) that is carried in the facility IE is lost. The integration of a QSIG PBX with Cisco IP Telephony requires the use of MGCP.

Part I: Voice Gateways and Gatekeepers

Gateways and Gatekeepers

Part II: Gateways

Media Gateway Control Protocol


Session Initiation Protocol

Circuit Options

Connecting to the PSTN

Connecting to PBXs

Connecting to an IP WAN

Dial Plans

Digit Manipulation

Influencing Path Selection

Configuring Class of Restrictions

SRST and MGCP Gateway Fallback

DSP Resources

Using Tcl Scripts and VoiceXML

Part III: Gatekeepers

Deploying Gatekeepers

Gatekeeper Configuration

Part IV: IP-to-IP Gateways

Cisco Multiservice IP-to-IP Gateway

Appendix A. Answers to Chapter-Ending Review Questions


Cisco Voice Gateways and Gatekeepers
Cisco Voice Gateways and Gatekeepers
ISBN: 158705258X
EAN: 2147483647
Year: 2004
Pages: 218

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