L.R.He, B.M.G. Cheetham Mobile Systems Architecture Group, Department of Computer Science, University of Manchester, Oxford Rd, M13 9PL, U.K.

Application of real time multimedia communications to distance learning over IP networks.. Current RTP systems are based on comparing frame loss rate, as reported by RTCP, with thresholds. We presents a flow/congestion control mechanism for adapting to congestion using measurements of time delay, jitter & speech packet loss rate. We propose a dynamic assignment of priority to the speech, image & data as appropriate to distance learning activity.

To improve the flow/congestion control mechanism based on RTP and RTCP To allow dynamic priority assignment determining how the available capacity is divided between speech / image / data.

Distance learning is applied in these fields: * Providing open learning environments * Offering more information for traditional teaching * Providing continuing education after graduation * Developing academic co-operation Three types of distance education -Paper based learning -TV and radio based learning -Internet based learning

Real Time Multimedia has the following advantages for Distance Learning : * Liveliness * Efficiency * Interactivity Internet Browser (student 1) Browser (student N) Browser (teacher 1) Browser (teacher M) Web server Database (Course Information, Content, Assignments, Problems and Solutions, Audio-video Information, Examinations, Announcements, Student Records)

Camera & microphone Data RTCP sender report Internet Receiver 1 Receiver 2 RTCP Receiver Report Check result Most of the real-time applications are based on the end-to-end real time transport protocol (RTP) and real time control protocol (RTCP).

Real time transport protocol is used for end-to end data transfer. RTP transports real-time media data along with synchronization information over a datagram protocol. G.711,G.722,G.723.1, G.728,G.729 H.261, H.263 RTP RTCP User Datagram Protocol Network Layer Link Layer Physical Layer

Real-Time control Protocol has a feedback function. Feedback from the receivers is necessary for diagnosing distribution faults. RTCP produces sender and receiver reports. Stream statistics Packet counts Sender identification Quality of service Lost packets

Packet loss probability Loss rate(%) 100% c 0 Congested--- Decrease Unloaded--- Increase Loaded--- Hold Receiver classifies loss error Bit-rateNetwork state u Problems lie in dynamics of bit-rate adaptation n

Packet loss probability Speech packet unusable probability Average jitter “loaded” Management Priority weighting “unloaded” “congested” high priority weighting or low priority weighting

1 Determination of the network states As before, the network will be defined to have three different states, “congested”, “loaded”, and “unloaded”. If 1  λ n  λ c or (λ n  λ u and λ sn >  λ sc ), the network state is “congested”. λ sc is a threshold. If λ n  λ u, then the network is “unloaded”. If (λ n  λ u and λ sn  λ sc ), the network is “loaded”.

The probability of a received speech packet being unusable λ sn as follows: n th control period T  (400  P d ) ms P d is the processing delay.

If the network state is “congested”, H = H –  H. If the network state is “unloaded”, H = H +  H. If the network state is “loaded”, the sender hold the image and other packets transmission rate, checks the average jitter, J n, to decide how to adjust the speech packet transmission rate. 2 Management The priority of a given packet Average JitterSpeech transmission rate Decreasing H s +  ConstantHsHs Increasing Hs  Hs   * H smin  H s    H smax Table: Determination of the requested sending rate of speech packet

A flow/congestion control mechanism based on RTP and RTCP has been investigated. A more appropriate variation in transmission rate is achieved by the new method. It can be appropriately applied to distance learning over internet.