Slides for Week 4 - Lec 2

Slides for Week 4 - Lec 2

• 1.
  Application layer (continued) Week4 – Lecture 2
• 2.
  Internet Architecture Application layer– HTTP, SMTP etc W3C Transport layer – TCP, UDP IETF Network layer - IP Data link & Physical layers – ITU the territory of the LANs/Telcos/ISPs IEEE
• 3.
  Variables in Internetservice • Bandwidth • Latency – delay sensitivity • Error free – loss tolerance
• 4.
  Application requirements Application Loss Delay Band- Tolerant Sensitive Width Web None No Medium E-Mail None No Low Video streaming Some Some High Telephone calls Some Yes Low Video conferencing Some Yes Medium/ high
• 5.
  Characteristics Applications • Videostreaming • Voice over IP • Multicasting
• 6.
  Multicast routing
• 7.
  Multicast routing • Wehave 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 • Atthe 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 castworks
• 10.
  How does itaddress 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 audioStreaming • 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 RTP WEB TCP WEB Browser server Media UDP Streaming player server
• 15.
  Control streaming storedvideo • 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 andaudio 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 Manager Gateway Office 1 Private link Office 2 Public Internet Gateway
• 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 avoidingjitter 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 • Likea phone call, delays must be less than ½ second • Places real constraints – delay sensitive and needs bandwidth