Chapter 15 Chapter 15 Multimedia and Networks Multimedia Systems

Key Points  IP is the Internet Protocol and transfers datagrams between hosts that are identified by IP addresses.  TCP is layered on top of IP, to provide reliable delivery of sequenced packets.  UDP is also layered on top of IP, and provides a more efficient method of delivery, but it is not reliable.

Key Points  RTP runs on top of UDP and provides additional services for real-time delivery of data of different payload types.  Multicasting cuts down network traffic by sending packets to host groups, only creating duplicates when it is necessary. (Contrast unicasting.)  HTTP is used to transmit data between Web servers and clients. The client sends a request and gets a response.

Key Points  A GET request is used to retrieve a Web page. The response includes the status and, if it was successful, the text of the page is included in the body of the response.  HTTP includes a cacheing mechanism to help reduce network traffic.  RTSP is like an `Internet VCR remote control', providing control functions for RTP streams used for video etc.

Key Points  Quality of Service provides a measure of how well a network can deliver streamed data.  Server-side scripting is used to enable an HTTP server to communicate with other resources, and pass the results back to a client. The Common Gateway Interface (CGI) provides a mechanism for this.

Introduction  The relationship between computer networks and multimedia is a contradictory one: –they are incompatible perfect partners

Multimedia  Files are often very large  Complex processing such as decompression may be required  Response times must be short and tight  Synchronization constraints must be respected

Networks  Networks are particularly poorly placed to satisfy those demands, because of –inherent limitations of present day technology –complex and unpredictable effects of the interactions between network components and patterns of network traffic

Protocols  Protocols are the rules governing the exchange of data over network  They are conceptually organized into layers, stacked on top of each other  The protocols on each layer are implemented using those on the layer below

Protocols  The lowest layer protocols deal with the actual physical signals transmission  Higher level protocols handle the transfer of packets of raw data and ensure they reach their correct destination  The highest layers implement more application-oriented services

TCP/IP Networks  Packet-switched Networks – all messages are split into small pieces, called packets, which are sent separately  Advantage – enable network bandwidth to be shared efficiently between many messages  Disadvantage – can’t guarantee network quality

Internet Protocol  IP is the Internet Protocol, which makes the Internet possible  All that IP does is attempt to deliver individual datagrams from one host to another. It doesn't even guarantee to succeed

Transmission Control Protocol  TCP, the Transmission Control Protocol, is layered on top of IP  TCP provides reliable delivery of sequence packets. It does this using a system of acknowledgement

Transmission Control Protocol  Acknowledgement –when the destination receives a packet, it sends an acknowledgement to sender –if the acknowledgement is not received within a specified period of time (the time-out), the packet is sent again –the mechanism of acknowledgement achieves the reliability of transmission

Transmission Control Protocol  Sliding window –instead of sending a single packet and waiting for an acknowledgement, it allows the sender to transmit multiple packets before waiting for an acknowledgement Initial window window slide Acknowledgement 1 2

Transmission Control Protocol  TCP is less efficient than IP, because it adds extra overheads in acknowledgements and retransmission  For some networked multimedia applications, the possibility of lost packet is more acceptable than the overhead of TCP

User Datagram Protocol  UDP, the User Datagram Protocol, is built on top of IP, like TCP, but is much simpler  Like IP, UDP only tries its best to deliver datagrams, it does not offer reliable delivery provided by TCP  UDP is a suitable basis for delivering data such as streamed video and audio, for which real-time constraints are more important than totally reliable delivery

Real-time Transport Protocol  The low-cost delivery of UDP is not enough for delivering streamed video and audio  The Real-time Transport Protocol (RTP) typically runs on top of UDP, adding extra features that are needed for sequencing identifying and synchronization

Real-time Transport Protocol  sequence number –Each packet has a sequence number to enable applications to reconstruct a sequence of packets and detect whether any are missing  timestamp –It records the instant at which the first byte contained in a packet was sampled –Synchronization can be collated with the timestamps to ensure that simultaneously sampled data is played back at the same instant

Multicasting  A common situation where data is unnecessarily duplicated arises when a group of Internet users require access to the same resource at the same time  Conventional (unicast) transmission require that the server from which the video is being streamed send a copy of it to everybody who has set up a connection

Multicasting  In multicasting, a single packet is sent and is duplicated along the way whenever routes to different users diverge

Application Protocols for Multimedia  Higher level protocols must run on top of the network and transport protocols to provide services suitable for distributed multimedia applications –HTTP (Hypertext Transfer Protocol)  the basis of the World Wide Web –RTSP (Real Time Streaming Protocol)  a newer protocol designed to control streamed media

HTTP  Interaction between a Web client and server over HTTP ： –Client/Server model  To start things off, the client opens a TCP connection to a server  the client sending requests, which are met by responses from the server

HTTP  HTTP requests and responses are collectively known as messages  Both consists of a string of 8-bits characters, so they can be treated as text by programs that read them

HTTP  Messages conform to a simple rigid structure –initial line  request line (for a request)  status line (for a response) –one or more headers  containing various parameters and modifiers –body  containing data, such as the contents of a file being sent by the server

HTTP  A request line comprises three elements –method identifies the service being requested, the most commonly used method is GET –identifier tells the server which resource is being requested –HTTP version indicates which protocol version the client is using GET /compbooks/chapman/index.html HTTP/1.1

HTTP  Headers take the form of a header name followed by a colon and some arguments Host: User-Agent: Mozilla/4.0  One of the most commonly seen headers in GET requests is Accept, which indicates the range of types of data that the browser can deal with Accept: image/gif, image/jpeg Accept: */*

HTTP  Status line also comprises three components –protocol version tells the client which HTTP version the server is using –status code –short phrase explains to human readers what the status code means HTTP/ OK

RTSP  RTP does not provide all the necessary functionality required for streamed data –we usually want to be able to  start, stop and pause them, and possibly go to a particular point in the stream (for streams that are not being transmitted live)  for live streams, we might want to schedule a time at which to start the display

RTSP  RTSP is intended to provide these services. It is often described as “Internet VCR remote control protocol”  Syntactically, RTSP closely resembles HTTP with request and status lines and headers

RTSP  In the simplest mode of operation, an RTSP client sends –PLAY requests to cause the server to begin sending data –PAUSE requests to temporarily halt it  Note that the media data is transmitted separately, often using RTP; RTSP merely coordinates the transmission

Relationships between TCP/IP protocols used for multimedia IP TCPUDP RTSPHTTP RTP

Quality of Service  Delay  Variation in delay is called jitter –Two problems  Variation in time between packets can result in time base errors  Loss of synchronization of several streams –Lip-sync  Loss  Measurable quantities –delay, jitter, loss

ATM  Asynchronous Transfer Mode –Offer guarantees about the quality of services –Much more like a circuit-switched network  Integrated Services Architecture –Different classes of services  Best effort  Guaranteed  RSVP –Resource Reservation Protocol

Server-side Computation  Client-side scripting  Server-side scripting –Common Gateway Interface (CGI)

CGI  A mechanism for server to pass on data in a client’s request to a script –CGI script  Three places –Headers in HTTP request –Query string –POST