Cisco Collaboration: H.323 Protocol Explained

Modern communications are powered by digital services. That means we need apps and protocols to make modern telephony services work. One of those protocols happens to be H.323. Understanding the Cisco H.323 protocol is vital for managing enterprise-level VoIP communications. So, we’re going to discuss what H.323 is.

While this article covers H.323 at a very high introductory level, it’s only a lengthy definition of what H.323 is. If you need to start learning how to manage the H.323 protocol right now, we highly recommend looking at Lalo Nunez’s online training course.

A Video Overview of H.323 Protocol

CBT Nuggets trainer Lalo Nunez explains what the H.323 protocol is and how it can benefit you. 

What is the H.323 Protocol?

Most aspiring network engineers are likely familiar with SIP. It’s a protocol that provides telephony communications with applications like Microsoft Skype and Teams. Before SIP became standard, there was H.323, and at a high level, H.323 provides a lot of the same functionality as SIP. Think of H.323 as the older brother to SIP. 

What is H.323?

The H.323 protocol is itself based on the Q.931 protocol and is also a suite of protocols. It bundles the H.225 and H.245 protocols together, amongst others. The H.225 protocol handles call setup and teardown (over port 1720), while the H.245 handles media codec negotiation.

H.245 plays an essential role in the H.323 call process. As mentioned above, it handles media codec negotiation. It’s otherwise known as a capabilities exchange. H.245 is responsible for figuring out which codecs are compatible with all the devices connected to the call. 

H.245 performs logical signal processing (opens and closes the channels used for media transmission). Likewise, it uses RTP and RTCP to monitor call stats such as bandwidth and quality. 

Originating in 1996, H.323 was initially intended to enable video conferencing over LAN. This is in the early days of IP-based telephony services when many businesses still depended on old-fashioned phone systems. 

It was quickly adapted to handle transmitting audio over all sorts of IP networks and gained support with enhancements for both audio and video over packet-switched networks. It was later re-titled in 1998 to “Packet-Based Multimedia Communication Systems” to better emphasize what the protocol supports. 

The current version of H.323 was ratified in 2009. 

Why Does H.323 Default to Fast Start?

Expanding on H.323, it has both fast start and slow start capabilities. The difference between both is subtle but important. While using fast start, all the requirements for the call are transmitted from call initiation. On the other hand, slow start waits until the call is initiated before negotiating call requirements (such as which media codec to use). 

By default, H.323 gateways use fast start. That’s because slow start uses more messages to create calls and imposes more delay while waiting for calls to connect. Therefore, slow start isn’t used unless there is a reason to do so. 

Does H.323 Depend on CUCM?

It’s important to note that the H.323 protocol is not reliant on CUCM. In fact, the dial plan needs to be configured at the gatekeeper. When an H.323-based call is initiated, it reaches out for a gatekeeper to establish a connection. Once a gatekeeper is found and responds, it negotiates the call with the client. All call processing is done at the gatekeeper.

This is different from MGCP. MGCP depends on CUCM to control the voice gateway, so the voice gateway needs to be registered with CUCM to function. For this reason, H.323 is considered a peer-to-peer connection. 

Ready to Learn More About Cisco Collaboration Technologies?

This article covered a lot of information, yet it barely scratched the surface of H.323. For example, we didn’t discuss what controls compression in voice or media calls, how dial plans are managed, or why you would even use H.323 when SIP is readily available today. Likewise, we didn’t have time to define any of those pesky acronyms either. 

If you’re ready to take a deep dive into the H.323 protocol, how it’s different from SIP, and how to manage Cisco calls using H.323, look at Lalo Nunez’s training at CBT Nuggets. He covers every technical detail regarding H.323 and how to manage it properly. Lalo also offers lessons from his personal experience, explaining the gotchas he and his colleagues have found along the way.