SYSTEM CONTROL

             H.323 utilizes the following protocols to provide the call control and framing functions.

H.225

            H.225 defines the lowest layer that formats the transmitted video, audio, data, and control streams for output to the network, and retrieves the corresponding streams from the network. It also handles the call control to initiate and tear down calls between terminals, gateways, MCUs, etc. H.225 also utilizes the packet format specified by the Internet Engineering Task Force (IETF), Real-time Transport Protocol (RTP) and Real-time Control Protocol (RTCP) for the following tasks:

1. Logical framing - Defines how the protocol frames and packages the data for transport over a selected communications channel.

2. Sequence numbering - Determines the order of data packets transmitted over a channel.

3. Error Detection - After initiating a call, one or more RTP or RTCP connections are established. Multiple streams allow H.225.0 to send and receive different media types simultaneously, each with their own frame sequence numbers and quality of service options. With RTP and RTCP support, the receiving node synchronizes the received packets in the proper order, so the user hears or sees the information correctly.

RTP is the Internet-standard protocol for the transport of real-time data, including audio and video. It can be used for media-on-demand as well as interactive services such as Internet telephony. RTP consists of a data and a control part. The latter is called RTCP.

 RTP and RTCP

             Real-time Transport Protocol, or RTP, is an IP based protocol which provides services for the transport of real-time data, such as audio and video. Those services   include payload type identification, sequence numbering, timestamping, and source identification. RTP is designed primarily for multicast of real-time data, but it can also be used in unicast.    

RTP Services 

1. Timestamping – The most important information in a RTP header is the timestamp. The sender timestamps each RTP packet with the point in time the first sample in the packet was encoded.

2. Payload type identification – RTP specifies the payload format as well as the encoding/compression scheme. Payload refers to the data transported by RTP in a packet.

3. Sequence numbering – Is used to place the incoming data packets in the correct order. Sequence numbers may also be used to determine if packets are lost during transmission.

4. Source Identification – Allows the receiver to know where the data is coming from.

Real-time Transport Control Protocol, or RTCP, is the control part of RTP. It monitors the quality of data transmission.  It also keeps track of participants in a session and distributes information about all participants in a session.

There are five types of RTCP packets which carry control information. These are:

1.                  Receiver Report (RR) – Participants that are not active senders generate this type of packet. RR packets contain reception quality feedback about data delivery, including the highest packets number received, the number of packets lost, inter-arrival jitter, and timestamps to calculate the round-trip delay between the sender and the receiver.

2.                  Sender Report (SR) – Active senders generate this type of packet. In addition to the reception quality feedback, they contain a sender information section, providing information on inter-media synchronization, cumulative packet counters, and number of bytes sent. 

3.                  Source Description Items (SDES) – These packets contain information to describe the sources.

4.                  BYE – This packet indicates the end of participation.

5.                  APP – Application defined RTCP packet.  This packet is intended for experimental use as new applications and features are being developed.

These packets are utilized by RTCP to provide the following services:

1. Quality of Service monitoring and congestion control – The primary function of RTCP. RTCP provides feedback on the quality of data distribution.  The control information is useful to the senders, the receivers and third-party monitors. The sender can adjust its transmission based on the receiver report feedback.The receivers can determine whether a congestion is local, regional or global. Network managersc an evaluate the network performance for multicast distribution.
2. Source Identification – The RTCP packet SDES contain textual information called canonical names as globally unique identifiers of the session participants.
3. Inter-media synchronization – Sender report packets contain an indication of real time and the RTP timestamp. This information may be used for inter-media synchronization such as synchronization of audio and video.
4. Control Information Scaling – Control information packets are sent to all participants regularly. When the number of participants increases, a tradeoff ensues between sending up-to-date control information and limiting control traffic. RTP limits the control traffic to at most 5% of the overall session traffic. This is done by adjusting the RTCP generating rate according to the number of participants.

RTP only provides end-to-end delivery services for data with real-time characteristics, such as interactive audio and video. It does not provide any mechanism to ensure timely delivery. It relies on support from lower layers that actually have control over switches and routers.

Q.931

This protocol defines how each H.323 layer interacts with peer layers, so that participants can interoperate with agreed upon formats. The Q.931 protocol resides within H.225.0. As part of H.323 call control, Q.931 is a link layer protocol for establishing connections and framing data. Q.931 provides a method for defining logical channels inside of a larger channel. Q.931 messages contain a protocol discriminator that identifies each unique message with a call reference value and a message type. The H.225.0 layer then specifies how these Q.931 messages are received and processed.

H.225 RAS (Registration, Admission, Status)

            H.225 RAS messages define communications between endpoints and gatekeepers. RAS is not used if a gatekeeper is not present. Unlike H.225.0 call signaling and H.245, H.225.0 RAS uses unreliable transport for delivery. In an IP network H.225.0 RAS uses UDP.

H.225 RAS communications include:

1. Gatekeeper Discovery - Gatekeeper discovery is used by endpoints to find out their gatekeeper. An endpoint that needs to find the transport address of its gatekeeper(s) will multicast a gatekeeper request (GRQ) message. One or more gatekeepers may reply with a GCF message containing the gatekeeper transport address.
2. Endpoint Registration - Once a gatekeeper exists all endpoints must be registered with it. This is necessary because gatekeepers need to know the aliases and transport addresses of all endpoints in its zone to route calls.
3. Endpoint Location -  Gatekeepers use this message to locate endpoints with a specific transport address. This process is required, for example, when the gatekeeper updates its alias-transport address database.
4. Other Communications -  A gatekeeper performs many other management and control duties such as admission control, status determination, and bandwidth management which are all handled through H.225.0 RAS messages.

H.245

This standard provides the call control mechanism that allows H.323-compatible terminals to connect to each other. H.245 provides a standard means for establishing audio and video connections — the series of commands and requests that must be followed for one component to connect and communicate with another. This standard specifies the signaling, flow control, and channeling for messages, requests, and commands.

            H.245 software maintains a single control channel to handle many system functions including:

1. Master-Slave Determination – This function is used to prevent conflicts that may arise when two terminals involved in a call initiate similar events simultaneously, and only one such event is possible or desired. The terminal status may be re-determined at any time during a call, as long as no procedure that is locally active is dependent upon the result.
2. Capability Exchange – This procedure is intended to ensure that the only multimedia signals transmitted are those that can be received and treated appropriately by the receiving terminal. The total receive and decode capabilities are transmitted in a message known as a capability set.

Receive capabilities describe a terminal’s capability to receive and decode incoming information streams. Transmitters are required to limit the content of the information they transmit within the capabilities of the receiver. Transmit capabilities, on the other hand, describe a terminal’s ability to transmit information streams. Transmit capabilities serve to offer a receiver a choice of modes of operation, so that the receiver may request the transmitter to transmit in a mode that the receiver prefers. The absence of transmit capabilities means that the transmitter is not offering the receiver a choice of preferred modes.

Terminals may re-issue capability sets at any time.

3. Opening and Closing of Logical Channels – H.323 audio and video logical channels are uni-directional end-to-end links (or multipoint links in the case of multipoint conferencing). Data channels are bi-directional. A separate channel is needed for audio, video, and data communication. H.245 messages control the opening and closing of such channels. H.245 control messages use logical channel 0 which is always open.
4. Round-trip delay determination – This feature provides the ability to calculate the round-trip delay between transmitting and receiving terminals. It can also be used to determine if a terminal is still functional.
5. General Commands and Indications - Commands are defined as messages, which force an action at a remote terminal. Indications are merely bits of information, and unlike commands, do not force any action.

The built-in framework of H.245 enables codec selection and capability negotiation within H.323. Bit rate, frame rate, picture format, and algorithm choices are some of the elements negotiated by H.245.