H.323 – a signaling protocol used in a VoIP network. It is a packet-based multimedia communication system that is a set of specifications. These specifications define various signaling functions, as well as media formats related to “packetized” audio and video services.
H.323 standards were generally the first to classify and solve multimedia delivery issues over LAN technologies. However, as IP networking and the Internet became prevalent, many Internet RFC standard protocols and technologies were developed and based on some of the previous H.323 ideas. Today there is cooperation between the ITU and IETF in solving existing problems, but it is fair to say that the RFC process of furthering the standards has had greater success than the H.323 counterparts.
H.323 networks consist of call processing servers, (media) . Call Processing Servers provide call routing, and communication to VoIP gateways and end devices. Gateways serve as both the H.323 termination endpoint and interface with non-H.323 networks, such as the PSTN. Gatekeepers function as a central unit for call admission control, bandwidth management and call signaling. Although the gatekeeper is not a required element in H.323, it can help H.323 networks to scale to a larger size, by separating call control and management functions from the gateways.
H.323 specifications tend to be heavier (due to chattiness, in terms of control signaling) and with an initial focus in LAN networking. These standards have some shortcomings in scalability, especially in large-scale deployments. Primarily, limitations are due to chattiness or the heavy signaling required to establish H.323 sessions. H.323 is dependent on TCP-based (connection-oriented) signaling. There is a challenge in maintaining large numbers of TCP sessions because of the substantial overhead involved. However, most H.323 scalability limitations are based on the prevalent version two of the specification. Subsequent versions of H.323 have a focus on solving some of these problems.
Let’s look at the main H.323 process:
- With each call that is initiated, a TCP session (H.225.0 protocol) is created, using an encapsulation of a subset of Q.931 messages. This TCP connection is maintained for the duration of the call. Complete call setup process is shown in figure 5.
- A second session is established using the H.245 protocol. This TCP-based process is for capabilities exchange, master-slave determination, and the establishment and release of media streams. This group of procedures is in addition to the H.225.0 processes.
- The H.323 quality-of-service (QoS) delivery mechanism of choice is the Resource Reservation Protocol (RSVP). This protocol is not considered to have good scaling properties due to its focus and management of individual application traffic flows.
- Although H.323 may not be well suited in service provider spaces, it is well positioned to deploy enterprise VoIP applications. As a service provider, it might be necessary to bridge, transport or interface H.323 services and applications to the PSTN.