Voice Protocols: H323


H323 Components



H323 Components are to the right hand side underneath ‘Terminal Control and Management’

3 major components:

  1. H.225.0 Call Signaling – Call setup and teardown
  2. H.245 – Feature Exchange – Phone A calls Phone B or Gateway A and Gateway B, they have to agree on a CODEC.
  3. H.225.0 RAS – Registration, Admission, Status – GK – Gatekeeper focus..


H.323 is Peer to Peer which has scalability issues.

Gatekeeper provides Registration, Admission and Status. Also Bandwidth Control, network can be divided into ‘zones’.

H.323 Components

  • Gatekeeper
  • Gateway to FXS
  • Gateway to H.323 Terminal (Speaks H.323 directly. Very rare these days! (Netmeeting/Windows 95) Multipoint Control Unit (Audio/Video Conf Calling)

Cisco Router itself speaks H.323/DSPs used for Transcoding etc… H.323 is common today

H.323 Call Setup/Call Flows

  • Direct: Between 2 VGs directly: H.225 Call Setup -> H.225 Proceeding/ACK -> H.225 Alerting -> H.225 Connect -> H.245 (CODEC/RTP/RTCP etc….) When call is hang up… H.225 sends an END message
  • With GK: GK in between 2 VGs (Peer to Peer Topology)

Dial peers by default use H.323…

Gatekeeper registered as a POTS endpoint

Dial-peer sends everything to GK -> Admission Request -> Admission Confirm -> If failed Admission Reject sent. Destination GW checks in with the GK to confirm am I ok to speak to this GW? Is there enough BW? When call is in place, subtract BW… when call is disconnected, bandwidth is put back into the pool.

All governed by H.225.0 RAS

Location Requests are sent when GKs speak to other GKs offsite

RTP directly between GWs not GK



default-technology = default-gateway essentially.. can be confusing! You specify a VG to act as a DG so the Gatekeeper can send calls to it when it doesn’t have a dial-peer in place. I think.. :/