Configuring G.726 on the Cisco HSI

Chapter 3 Provisioning the Cisco HSI

HSI Feature Configuration

The G.726 codec enables transcoding a PCM channel to or from an ADPCM data stream. The standard supports four data rates:16, 24, 32 and 40 kbit/sec.

G.726 capability is advertised by the Cisco HSI and other H.323 gateways/endpoints in H.225 fast-start elements, in H.245 (tunneled or a separate TCP/IP connection) terminal capability (TCS) messages, and open logical channel (OLC) messages.

Currently, H.323 devices use several different methods to advertise G.726. ITU G.726 Annex B defines one method, referred to in this document as g726-generic. Cisco H.323 gateways (for example, the Cisco AS5400) support an alternate method referred to as g726-cisco. There is another method used by the OpenH323 project; however, the Cisco HSI does not support that method.

MGCP gateways advertise G.726 capability using the method described in RFC 3551 (RTP Profile for Audio and Video Conferences with Minimal Control). The four data rates use dynamic payloads; however, the 32kbit/sec data rate, alternatively, can have a static payload value of 2 (this alternative value is being phased out).

You can configure the Cisco HSI for 32kbit/sec MGCP support using dynamic or static payload values. In addition, you can configure the Cisco HSI to support g726-generic and/or g726-cisco for the H.323 signaling. If possible, it is best to select g726-cisco for your network because it offers additional flexibility.

The g726-generic method cannot indicate the data rate in H.245 TCS messages. The ITU standard specifies that the data rate is only advertised in the OLC messages.

Note The H.245 ASN.1 syntax supports advertising the bitrate in TCS messages; however, G.726 Annex B prohibits advertising the bitrate in TCS messages. The Cisco HSI advertises the bitrate in the TCS messages as a “hint”; however, H.323 gateways/endpoints might not extract the field and take advantage of the presence of the bitrate in the TCS message.

The fact that the g726-generic method cannot indicate the data rate in an H.245 TCS message is not a problem if the MGCP gateway and your network are designed to support all data rates for this codec. However, if all data rates are not supported, it is possible for the remote endpoint/gateway to select a non-preferred or non-supported data rate in the OLC message.

Note For example, a data-rate preference list may establish the following order: G.726-16kbit/sec (highest preference), G.711-Alaw (second preference), G.726-24kbit/sec (lowest preference). In this case, a remote endpoint could select G.726-24kbit/sec in the OLC message; whereas, the Cisco HSI would prefer G.726-16kbit/sec. In this example, the next preferred codec ought to be G.711 A-law and not G.726-24kbit/sec. However, the g726-generic limitation enables the remote endpoint to select the least preferred codec.

If a data-rate preference list specifies only a single rate (for example, G.726-16kbit/sec), it is not possible to advertise this fact in the TCS message. Subsequently, the remote endpoint may attempt to open the media stream using an unsupported data rate (perhaps, G.726-24kbit/sec).

Whenever OLC messages are exchanged and a non-supported G.726 data rate is detected, to prevent unnecessary call clearing, the Cisco HSI always attempts to send the data rate selection to the MGCP gateway. If the MGCP gateway does not support the selected data rate, it sends a message to the Cisco PGW to clear the call.

	Cisco H.323 Signaling Interface User Guide
	Cisco H.323 Signaling Interface User Guide
OL-4806-01 Rev. A14		3-27
OL-4806-01 Rev. A14		3-27

H.323 specifications

Cisco Systems H.323 is a set of standards designed for transmitting audio, video, and data over packet-switched networks. It is a critical technology that enables real-time communication and has become integral to the world of VoIP (Voice over Internet Protocol) and video conferencing applications. Originally developed by the ITU-T, H.323 has been widely adopted and implemented by Cisco Systems, allowing organizations to leverage reliable and scalable communication solutions.

One of the standout features of H.323 is its ability to support interoperability among varying equipment and platforms. This means that devices from different manufacturers can communicate seamlessly, which is vital in a diverse technical environment. H.323 does this by defining protocols that handle signaling, media transport, and control, ensuring that various systems can work together effectively.

Key technologies utilized within H.323 include H.225 and H.245 signaling protocols. H.225 is responsible for call setup and management, establishing communication between endpoints. In contrast, H.245 manages the negotiation of media channels, allowing devices to configure themselves based on the capabilities of each other. This adaptability is crucial for dynamic network conditions typical in VoIP applications.

Another defining characteristic of H.323 is its support for multiple types of multimedia communication, including point-to-point and multipoint calls. This flexibility supports everything from simple audio calls to complex videoconferencing setups, where many participants can share video and audio streams concurrently. Cisco enhances this capability through its hardware and software offerings, optimizing performance for conference calls.

H.323 also features robust error resilience mechanisms. This ensures that real-time data, which is typically sensitive to delays and losses, can be transmitted with minimal disruption. Moreover, it facilitates quality of service (QoS) implementation, allowing organizations to prioritize voice and video traffic over less critical data services, thereby improving the overall user experience.

In conclusion, Cisco Systems H.323 stands out as a comprehensive solution for real-time communication over packet-based networks. Its main features, including interoperability, multimedia support, and enhanced error resilience, position it as a relevant technology in both corporate and personal communication settings. As organizations continue to adopt cloud and hybrid communication models, H.323 remains a reliable backbone for ensuring connectivity and collaboration across diverse platforms.