The TimeLine Protocol

The TimeLine protocol provides the means of distributing multimedia information (including MPEG-2 encoded video clips) to the clients in the MediCAL project. The server protocol executes on SUN workstations, and clients are provided for PCs under NT and SUN workstations.

The protocol is designed to optimise transfer when a group of students in a classrroom view one from a number of CAL sessions. A typical multimedia session will consist of a sequence of video clips (each of approx. 10 minutes duration), separated by audio commentries (often accompanied by a still image). Each student is free to choose their own path through the presented material - but all syudents are encouraged to view all the material at least once.

The Timeline protocol provides the following benefits, when compared to traditional file transfer protocols:

• Low Overhead (Application Layer Framing)
• Transmission Scheduling
• Rate Control
• Scalable Reliability
• Multicast Capability

Brief Description of Protocol

The protocol has two components:

• Session Protocol - This coordinates the transfer of files from the server, each client indicates to the session server the video clips in which it has an interest, and the estimated time at which the clip will be needed. This indication is provided in a Session Request. The session client provides this information periodically to the session server. The session server executes from time-to-time to collect infromation from all clients and processes session requests to determine the order of transmission.
• Transfer Protocol - Each client also runs one or more client processes for reception of files containing the required multimedia information. The clients receive the data from a transfer server, which is started through a Transfer Request from the associated session server. There is one transfer server for each file being sent. Different transfer servers may coexist on the same hardware platform, or be distributed between a number of platforms. A transfer server maintains no data concerning the group of clients to which it may be communicating, it simply follows the instruction of the session server. The foile is sent as a sequence of Data packets. Should packets from a transfer be lost (by the server, network or (most probably) the client), the client assumes responsibility for requesteing repair of the file by sending a Repair Request to the Transfer Server.

The client part of the timeline system executes under either NT on a PC platform or under Solaris on a SUN platform.

The server part of the timeline system executes under only Solaris on a SUN platform.

The operation of the protocols is summarised in the following diagram, which shows a single file being sent to a single client.