Gatekeeper Basics

Gatekeepers act like central nervous system in an H.323 network. They provide network access, bandwidth management, address translation,accounting, and dial plans. Gatekeepers allow the simple configurations of the majority of devices on your network, while requiring only a few devices to hold the majority of configurations. This architecture keeps call routing, security, and administration centralized to a few devices.

Admission Control: Controls endpoint admission into the H.323 network using H.225 Registration, Admission, and Status (RAS) messages, as follows:

  1. Admission Request (ARQ)
  2. Admission Confirm (ACF)
  3. Admission Reject (ARJ)

Bandwidth control: Provides endpoint bandwidth management requirements using H.225 RAS messages. When an endpoint decides it
needs a certain amount of bandwidth for the call, it sends a BRQ message to the GK asking for an amount of bandwidth to be aside for the call. The Gatekeeper responds with BCF (Bandwidth Confirm) or BRJ (Bandwidth Reject)

Zone management: Manages all registered endpoints in the zone. This provides centralized control for many devices within the zone.

Address Resolution: H323 ID and E.164 numbers can be resolved to an IP address for the destination gateway

H323 Call Setup and Control:

  • H225 messages flow between the gateway and gatekeeper using port 1719 over UDP
  • RAS messages are passed  through the GK to each device
  • If no GK then H225 messages are sent between endpoints
  • H225 Q931 setup message between the gateways over UDP
  • H245 for media control and capabilities negotiation between gateways
  • Once call is established, RTP stream flows between the gateways over UDP

Reference: Cisco Book Gateways & Gatekeepers

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