Frequently Asked Questions | H.323 Architecture

Q: I’ve never heard of H.323. What applications do I use that rely on this?

A: Microsoft Netmeeting for one. Polycom and Tandberg videoconferencing clients are another.

Q: Do H.323 terminals have to explicitly send the H.225 call setup messages to the IP address of the gateway?

A: Yes, an H.323 endpoint must know the transport address—for example, the IP address and port number—for the Q.931 dialogue. Q.931 then provides the transport address for the H.245 control channel. This is how addresses are bootstrapped in H.323.

Q: In what layer of ISO you can put H.323 standard?

A: H.323 doesn’t map to just one layer, but is primarily implemented at layers 3 and 4.

Q: I’ve heard that H.323 uses more than one TCP/UDP port in order to transmit voice, video, and data. Are these ports fixed, or do they vary for each connection?

A: H.323 uses several ports and both TCP and UDP to signal and transport voice. H.225/Q.931 and H.245 use TCP and H.225/RAS and RTP/RTCP use UDP. Ports 1718–1720 are dedicated to H.323 traffic.

Q: Several dynamic port combinations are used per session as well.What is the best VoIP codec?

A: There are a number of factors to make that kind of determination. Probably most important is the nature of the network between the two ends. If you are connected of a LAN (high bandwidth, minimal delays, etc.), then G.711 generally provides the best voice quality.

Q: What’s an Application Layer Gateway?

A: ALGs peer more deeply into the packet than packet filtering firewalls but normally do not scan the entire payload. Unlike packet filtering or stateful inspection firewalls, ALGs do not route packets; rather the ALG accepts a connection on one network interface and establishes the cognate connection on another network interface. An ALG provides intermediary services for hosts that reside on different networks, while maintaining complete details of the TCP connection state and sequencing.

Q: What’s better, H.323 or SIP?What’s better, an apple or an orange?

A: Seriously, H.323 is based on SS7 and was designed to internetwork efficiently with the PSTN. SIP is based on HTTP and was not designed with interconnecting to the PSTN in mind. So, major carriers tend to use H.323 because it translates ISDN and SS7 signaling to H.323 VoIP signaling easily. SIP does not. On the other hand, SIP supports IM, is text-based, and is implemented more cheaply than H.323.