5.3. Multimedia Data Transmission

Delivery of Multimedia Data
IP-Multicast and the MBONE
Video over ATM Networks

5.3.1. Delivery of Multimedia Data

1. Quality Measuring Parameters

• Bandwidth -- hundreds of Mbps

• Latency (maximum frame/cell delay) -- important for Interactive TV, audio/video synch, etc.

• Frame/Cell Loss or Error

• Jitter -- Playback Jitter is standard deviation between frame playback

  

  • Intuitively, it is a measure of smoothness.

  • Video prioritization algorithms set priorities to minimize playback jitter in the event of loss.

2. Prioritized Delivery

• Transmission algorithms provide prioritized delivery

  e.g., higher priority to audio than to video

Prioritization for MPEG

• Highest priority -- I frame, lowest -- B frame.

• Choose proper levels in Progressive mode or hierarchical mode.

Prioritization for Audio

• Break into groups of every nth sample, prioritize and send groups. Receiver interpolates lost groups.

  

• Example: if groups 3 and 4 are lost, effective sampling rate is 22 kHz instead of 44 kHz.

  Loss is perceived as change in sampling rate, not dropouts.

5.3.2. IP-Multicast and the MBONE

1. The Internet Multicast Backbone (the MBONE)

• Used for Audio and Video conferencing on the Internet.

  The MBONE today has over 20,000 users, 1500 routers, 30 countries.

• Based on IP-multicast technology

  

2. IP-multicast

• Technology for IP-multicast was built in 1988

  In March, 1992, Internet Engineering Task Force meeting was broadcast (audio only) in the Internet.

• IP-multicast supports multicast -- good for video conferencing, mailing lists, bulletin board, updates to replicated databases

• IP-multicast has anonymous membership. Source multicasts to special IP address, doesn't know who will receive.

• FDDI and Ethernet have hardware multicast

  

Tunneling

• MBONE uses a network of routers (mrouters) that support multicast.

• Augmented by tunneling -- encapsulate multicast packets inside regular IP packets, so that they can be sent among the islands of the MBONE that do not support IP-multicast.

IP-multicast details

• Recall: IP addresses are 32 bits. If first 4 bits are "1110", then message is an IP-multicast message.

  Covers IP address range 224.0.0.0 to 239.255.255.255

• Host software maps IP-group addresses into list of recipients, then either hardware multicasts (if available), or sends multiple unicasts to next node on tree.

• IP has time-to-live (TTL) field: each router decrements, discards message when zero. Used to limit packet lifetime.

• Problem: How to do/maintain mapping?
  Solution: Internet Group Management Protocol (IGMP)

3. IGMP

• 2 Message types: Query and Report

  Query: multicast to special group (224.0.0.1) by routers to all local host to inquire group membership. Set TTL = 1

  Report: used to respond to query, and to join groups.

• On receiving query, members wait a random time before responding. (Seed for random number generator is IP address of machine)

• If a member hears another response, then it stops its timer.

• Routers periodically query group membership, and declare themselves group members if it gets a response to at least one query. If no responses after a while, they declare themselves as non-member.

5.3.3. Video over ATM Networks

1. Types of QoS in Video

1. CBR (Constant Bit Rate) -- e.g. uncompress video
   
2. VBR (Variable Bit Rate) -- suitable for packetized video (compressed video)
   
3. ABR (Available Bit Rate) -- can be backed off or buffered due to congestion, can only specify frame/cell loss and error
   
4. UBR (Unspecified Bit Rate) -- no guarantee on any quality parameter

2. Native ATM Support for Video

• ABR is currently supported by most existing networks.
  CBR needs AAL 1 which supports Class A, i.e., real-time, constant bit rate, connection-oriented traffic
  VBR needs AAL 2 which takes care of variable bit rate

• MPEG2 Convergence to ATM -- MPEG will be supported over AAL5 (ATM Adaptation Layer 5). A new Audio-Visual Service-Specific Convergence Sublayer (AVSSCS) will be developed.