Multimedia Communications

1
Multimedia Communications

• Tutorial 3 (Chapter 6 -- 8)

2
Chapter 6

• Consider best effort service.
• This service is not ideal for multimedia
  communications. Why?
• The sender and the receiver can execute some
  methods to preserve good QoS. State some of these
  methods.
• These methods are usually not effective when the
  traffic is heavy. Why?
• Answer
• try its best to deliver packets, QoS not
  guaranteed Packets loss and delay cause delay
  jitter.
• execute packet recovery to reduce packet loss
  rate execute feedback rate control to reduce
  delay use buffering at the receivers side to
  handle jitter
• When the network traffic is extremely heavy, the
  loss rate and error rate of the packets grow much
  larger, making the receiver difficult to recover
  original packets. Buffering also takes a long
  time at this situation

3
Chapter 6

• Consider integrated services.
• Integrated services are ideal for multimedia
  communications. Why?
• What are the disadvantages of integrated
  services?
• Each router must keep track of the reserved
  resources for the ongoing sessions. Why?
• What is the purpose of RSVP?
• Answer
• Each multimedia session can get sufficient
  resources to fulfill its QoS requirements
• All the routers involved must agree to reserve
  resources for each flow Resources reservation
  involves additional processing and communication
  overhead Each router must maintain internal
  per-flow state.
• Subject to router crash, the router has to
  remember the state of all the ongoing sessions,
  and when a session ends, it can release the
  resource
• RSVP is used to reserve the necessary resources
  in each router in order to provide QoS guarantees
  to the session.

4
Chapter 6

• Consider RTP
• State the main functions provided by RTP.
• Is it possible that two RTP packets of a stream
  have the same sequence number?
• Is it possible that two RTP packets of a stream
  have the same timestamp?
• Answer
• sequence number (for loss/duplication detection)
  timestamp (receiver to playback) record the
  encoding method identifier to indicate the
  stream source.
• Possible. sequence number can circle.
• Possible. two packets might be sampled at the
  same time

5
Chapter 7

• What are the disadvantages of streaming from web
  servers.
• Answer
• By streaming from web servers, user has to first
  send a http request to server, the server returns
  a content-type header and the media file or the
  meta file which will be passed to the media
  player. The transmission of media file involves
  http which is not designed for multimedia
  communication. The major drawback is that we are
  not able to achieve some advanced management for
  playback control of multimedia streaming, such as
  fast forward, rewind, pause, etc.

6
Chapter 7

• You are going to provide a movie-on-demand
  service to many users, so you plan to run the
  streaming server on multiple computers. How would
  you design the system?
• Answer
• We have to separate the streaming server from the
  web server. At the web server, we can add the
  link pointing to the streaming server. After
  clicking the link, the user get a presentation
  description file and pass it to the media player,
  the player and the streaming server uses the RTSP
  protocol to communicate. If there are many user
  try to access the service at the same time and we
  are able to set up multiple computers running
  streaming server, then we can add more links at
  the web server, and the individual presentation
  description file may refer to one specified
  computer in order to stream out the multimedia
  sources to client which sends the service request.

7
Chapter 7

• Design a partial caching scheme such that the
  proxy can effectively support fast forward and
  rewind operations.
• Answer
• First, we can use Popularity-based Partial
  Caching (PPC) to allocate server resources to
  cache initial portions of a number of multimedia
  streams.
• Then, in order to provide playback control from
  proxy instead of the original server, we have to
  set up a streaming server at the proxy and use
  RTSP to stream out the stored multimedia streams
  to clients which ask for the corresponding
  portions of multimedia streams.

8
Chapter 7

• A movie-on-demand company provides a pay-movie
  service over the Internet. Each user must pay a
  service charge and then she can view a movie.
• If a proxy caches a movie, then the other users
  may be able to view this movie without paying
  service charge. Why?
• How would you tackle the above problem?
• Answer
• If some user request for this movie file, the
  proxy checks its cache and if the movie found, it
  will directly pass this movie to the relative
  user.
• We can add some authentication information tag to
  the movie file so that only legal user can access
  it no matter whether it is streamed form the
  original server or form the proxy.

9
Chapter 8

• The conventional radio and TV adopt broadcast
  communication, but Internet radio and Internet TV
  do not. Why?
• Answer
• To broadcast information, it is important to be
  clear how the signal being transmitted to the
  receivers for conventional radio and TV, they
  can send signal on air and air is the media to
  pass information to all receivers and air is
  almost everywhere. Hence it is easy to implement
  broadcast for radio and TV. Things totally
  different when we think use the Internet as the
  media to pass information. Routers do not provide
  the key functions to support multicast for the
  current Internet.

10
Chapter 8

• How would you apply the unicast-for-multicast
  approach to construct distributed servers for
  Internet TV/radio?
• Answer
• For a group of users who has the same bandwidth,
  we can dispatch a server to send stream to them
  at the speed no faster than their bandwidth.
• If there exists n groups, we can use n servers to
  fulfill our needs

11
Chapter 8

• How can Simulcast support bandwidth adaptability?
• Answer
• Except for the servers direct children, all the
  clients retrieve the media stream from another
  client. Each client can provide the stream with a
  variable speed. The server can dispatch a client
  as a new stream provider to a new added client
  based on the newcomers access speed.

12
Chapter 8

• In single-rate adaptation, each receiver reports
  its reception status to the source regularly.
• State one disadvantage of this method.
• How would you relieve the above disadvantage?
• Answer
• If the bandwidth of all the receivers differ
  dramatically, then the server will multicast with
  the minimum speed among the receivers.
• Provide different media streams adapted to
  different group of receivers.