Slides for Week 4 - Lec 2

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  • 1.Application layer (continued)Week 4 Lecture 2

2. Internet Architecture Application layer HTTP, SMTP etcW3C Transport layer TCP, UDPIETF Network layer - IP Data link & Physical layers ITU the territory of the LANs/Telcos/ISPs IEEE 3. Variables in Internet service Bandwidth Latency delay sensitivity Error free loss tolerance 4. Application requirements ApplicationLoss Delay Band-Tolerant Sensitive Width WebNone NoMedium E-Mail None NoLow Video streamingSome SomeHigh Telephone callsSome Yes Low Video conferencing Some Yes Medium/ high 5. Characteristics Applications Video streaming Voice over IP Multicasting 6. Multicast routing 7. Multicast routing We have only talked about single host to single destination messages Some applications require a host to send messages to many destinations: Upgrade of software Streaming of an on-line lecture or movie Stock price information to subscribers 8. Three approaches At the application level where the application establishes a separate unicast transport connection to each receiver. This is not usually efficient and not scalable Broadcast controlled reverse path & spanning tree An explicit multicast where the network layer makes copies of the message as close to the recipient as possible 9. How multi cast works 10. How does it address the one message to all receivers? It is not practicable to: Attach a list of addressees to the message The addressees may not be known Uses a broadcast and subscribe approach Subscribers are added to a D class address See Kurose for more details IGMP (Internet Group Management Protocol) provides services for managing the group 11. Video and audio Streaming Streaming stored video Streaming live video Real time interactive video - video conferencing Voice over IP 12. Streaming stored video Media pre-recorded and held on server Ideally server near client (remember WEB caching discussion) Important that it plays at recorded speed Uses buffering to prevent jitter effect of delays and loss packets Because of buffering and one way (predominantly) latency not as big a problem as with live video streaming Can fast forward and rewind (not usually multicast) 13. Streaming live video Can still buffer and build in a delay Because it is live, cannot fast forward but some software will allow rewind Can be multicast or unicast Multicast overlay uses servers scattered through the broadcast area 14. Protocols for streaming WEB http uses TCP to make the request But TCP not satisfactory for streaming Browser kicks off a media player process Web server then instructs Steaming server to send to the media player post on the browser host probably using RTPWEB TCP WEB BrowserserverMediaUDPStreaming playerserver 15. Control streaming stored video Real Time Streaming Protocol (RTSP) The equivalent of the Remote control Allows the Media player and the Streaming server to communicate over two additional ports RTSP keeps track of the state of the streaming process including the session number and sequence number of the segment 16. Compression Video and audio must be compressed before transmission Speech normally requires 64kbps Stereo CD requires 1.4mbps Video much more again Various standards (codecs) for compression GSM for voice MP3 for stereo music MP1 and MP4 and others for video H.323 for Video conferencing 17. Voice over IP Delay sensitive but large bandwidth not required Range of user facilities required to simulate facilities available in PBX 18. Private WAN PSTN Call ManagerGateway Office 1 Private link Office 2PublicInternetGateway 19. Advantages Maintain one network instead of two One British bank estimate gives 25% cost saving Inter-office calls over own network fixed cost Application extensions On-line office directory Single messaging system Display product information Telephony options on screen Video conferencing 20. Risks Security could be open to Public internet New technology QoS over IP not totally standardised Power failure etc, not just network but phone system as well all eggs in one basket 21. Techniques for avoiding jitter in VoIP Three problems Variable latency Lost packets Packets out of order Approaches Sequence numbers & timestamps Fixed play out delay Error correction Redundant data to allow reconstruction of one packet Two audio streams- normal and a back-up low resolution Interleaving 22. Video conferencing Like a phone call, delays must be less than second Places real constraints delay sensitive and needs bandwidth