So far in this series I have described two telephony protocols (SIP and MGCP) that feature prominently on the CCIE Voice and CCVP exam blueprints. In this blog entry, I am going to describe another, H.323.

H.323 is a collection or framework of ITU standards for interactive multimedia communications over a packet network. Not only does it appear in a large proportion of Cisco's IP telephony/voice related exams, but there is also a huge installed base of H.323 enabled devices out in the field - so, whether you are focused on passing exams or doing a great job (hopefully both!), you're going to need to have a good understanding of it.

Some of the most important protocols and standards that make up the H.323 framework are:

• H.225 Registration, Admission, and Status (H.225 RAS): this enables communication between H.323 endpoints and H.323 gatekeepers (described later).
• H.225 call control and signalling: this is used to setup connections between H.323 endpoints.
• H.245: this protocol is used to perform functions such as negotiating capabilities and setting up logical channels that are used for the transmission of media such as voice or video traffic.

H.323 network elements include:

Terminals (endpoints): these are endpoints such as phones or video conference units. In Cisco unified communications networks, H.323 endpoints could include devices such as Cisco CallManager/Unified Communications Manager systems or video conferencing units.

Multipoint Control Units (MCUs): these are devices that control multipoint conferences (conference bridges), and are comprised of two logical elements - multipoint controllers (MC) and multipoint processors (MP).

Gateways: these are devices that interface between H.323 networks and other networks such as the Public Switched Telephone Network (PSTN). A Cisco voice gateway enabled for H.323 can perform this function.

Gatekeepers: these devices perform a plethora of functions. They can be responsible for call admission control (CAC), zone management, bandwidth control, dial plan, address translation and resolution, bandwidth management, call authorization, call control signaling, and so on. Cisco routers can be configured as H.323 gatekeepers.

Next time, I'll take a look at H.323 protocols and message types in more detail.

Mark