H.323 Settings
As we move into the essential functions of the Cisco Expressway, we begin with the multimedia communications over the packet-based network. Deriving from the ITU Telecommunication Standardization Sector (ITU-T) H.320 that was utilized over Integrated Services Digital Network (ISDN)-based networks, H.323 was published by the International Telecommunications Union (ITU) in November 1996 with an emphasis on enabling videoconferencing capabilities over a local-area network (LAN), but was quickly adopted by the industry as a means of transmitting voice communication over a variety of IP networks, including wide-area networks (WANs) and the Internet. H.323 also provides a framework that uses other protocols to describe the actual protocol:
H.225.0: Registration, Admission, and Status (RAS), which is used between an H.323 endpoint and a gatekeeper to provide address resolution and admission control services
H.225.0: Call signaling, which is used between any two H.323 entities to establish communication based on Q.931
H.245: Control protocol for multimedia communication, describes the messages and procedures used for capability exchange, opening and closing logical channels for audio, video, data, and various control and indication signals
Real-time Transport Protocol (RTP)/Real-time Transport Control Protocol (RTCP): Protocols for sending or receiving multimedia information (voice, video, or text) between any two entities
The Cisco Expressway supports the H.323 protocol and it is also an H.323 gatekeeper. As an H.323 gatekeeper, the Expressway accepts registrations from H.323 endpoints and provides call control functions such as address translation and admission control. For an endpoint to use the Expressway as its H.323 gatekeeper or SIP registrar, the endpoint must first register with the Expressway.
To enable the Expressway as an H.323 gatekeeper, ensure that the H.323 Mode setting is set to On (Configuration > Protocols > H.323), as shown in Figure 3-1. H.323 mode is a powerful option that enables or disables functionality of the Cisco Expressway as an H.323 gatekeeper.
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Figure 3-1 H.323 Mode
There are two ways an H.323 endpoint can locate an Expressway with which to register: manually or automatically. The option is configured on the endpoint itself under the Gatekeeper Discovery setting:
If the mode is set to automatic, the endpoint tries to register with any Expressway it can find. It does this by sending out a Gatekeeper Discovery Request, to which eligible Expressways will respond.
If the mode is set to manual, you must specify the IP address or the fully qualified domain name (FQDN) of the Expressway with which you want your endpoint to register, and the endpoint will attempt to register with that Expressway only.
You can prevent H.323 endpoints from being able to register automatically with the Expressway by disabling Auto Discovery on the Expressway (Configuration > Protocols > H.323).
While you are on the Configuration > Protocols > H.323 page, you can also configure the H.323 settings on the Expressway to fit your organization by utilizing Table 3-2 as a reference.
Table 3-2 H.323 Settings
Field |
Description |
Usage Tips |
|---|---|---|
H.323 mode |
Enables or disables H.323 on the Expressway. H.323 support is set to Off by default. |
You must enable H.323 mode if you are clustering the Expressway, even if there are no H.323 endpoints in your deployment. |
Registration UDP port |
The listening port for H.323 UDP registrations. |
The default Expressway configuration uses standard port numbers so you can use H.323 services out of the box without having to first set these up. The default port is 1719. |
Registration conflict mode |
Determines how the system behaves if an endpoint attempts to register an alias currently registered from another IP address. Reject: Denies the new registration. This is the default. |
An H.323 endpoint may attempt to register with the Expressway using an alias that has already been registered on the Expressway from another IP address. The reasons for this could include: Two endpoints at different IP addresses are attempting to register using the same alias. |
|
Overwrite: Deletes the original registration and replaces it with the new registration. |
A single endpoint has previously registered using a particular alias. The IP address allocated to the endpoint then changes, and the endpoint attempts to re-register using the same alias. Reject is useful if your priority is to prevent two users registering with the same alias. Overwrite is useful if your network is such that endpoints are often allocated new IP addresses, because it will prevent unwanted registration rejections. Note that in a cluster, a registration conflict is only detected if the registration requests are received by the same peer. |
Call signaling TCP port |
The listening port for H.323 call signaling. |
Default port: 1720. |
Call signaling port range start and end |
Specifies the port range used by H.323 calls after they are established. |
The call signaling port range must be great enough to support all the required concurrent calls. Default start and end: 15000–19999. |
Time to live |
The interval (in seconds) at which an H.323 endpoint must re-register with the Expressway to confirm that it is still functioning. The default is 1800. |
Some older endpoints do not support the ability to periodically re-register with the system. In this case, and in any other situation where the system has not had a confirmation from the endpoint within the specified period, it will send an IRQ to the endpoint to verify that it is still functioning. Note that by reducing the registration time to live too much, you risk flooding the Expressway with registration requests, which will severely impact performance. This impact is proportional to the number of endpoints, so you should balance the need for occasional quick failover against the need for continuous good performance. |
Call time to live |
The interval (in seconds) at which the Expressway polls the endpoints in a call to verify that they are still in the call. The default is 120. |
If the endpoint does not respond, the call is disconnected. The system polls endpoints in a call, whether the call type is traversal or non-traversal. |
Auto discover |
Determines whether the Expressway responds to Gatekeeper Discovery Requests sent out by endpoints. The default is On. |
To prevent H.323 endpoints being able to register automatically with the Expressway, set Auto Discover to Off. This means that endpoints can only register with the Expressway if their Gatekeeper Discovery setting is Manual and they have been configured with the Expressway’s IP address or FQDN. |
Caller ID |
Specifies whether the prefix of the ISDN gateway is inserted into the caller’s E.164 number presented on the destination endpoint. |
Including the prefix allows the recipient to directly return the call. |