Multimedia networks
- 1. Information and Communication Technologies(ICT) Principles and Perspectives Lecture : Distributed multimedia systems Study guide(17) essential reading (7)
- 2. Protocols for real time apps
- 3. Real Time protocol • Real Time protocol ( RTP ) • can be used for transporting common formats such as PCM, ACC, and MP3 for sound and MPEG and H.263 for video • Packet structure for carrying audio/video data • RTP provides • Payload identification • Packet sequence numbering • Time stamping
- 4. Real Time protocol ( RTP ) • RTP packet has UDP segment and media chunk • RTP is for end systems only, intermediate routes don’t examine it • RTP in end systems and RTP libraries provides transport layer interface that extends UDP with • Port numbers, IP address • payload type identification • packet sequence numbering • time-stamping.
- 5. RTP packet header • Version • Padding • Extension • CSRC count (CC) • Marker
- 6. RTP headers • Synchronization source identifier (SSRC). The SSRC field is 32 bits long. It identifies the source of the RTP stream. Typically, each stream in an RTP session has a distinct SSRC. • The SSRC is not the IP address of the sender, but instead is a number that the source assigns randomly when the new stream is started. • The probability that two streams get assigned the same SSRC is very small. Should this happen, the two sources pick a new SSRC value
- 7. Real-Time Control Protocol (RTCP) • Used in combination with RTP • All participants send reports periodically to all others , number of packets lost/sent, inter arrival jitter. Timestamps at receiver Vs RTP media timestamps etc. • Scaling issue is in multicasting. Too many packets by receivers • RTCP modifies the rate with which participants send traffic into the multicast tree as a function of the number of participants in the session
- 8. Session Initiation protocol (SIP) • Mechanisms for establishing calls over an IP network. • Allows the caller to notify the callee that it wants to start a call. • Allows the participants to agree on media encodings. • Allows participants to end calls. • Mechanisms for the caller to determine the current IP address of the callee. • Users do not have a single, fixed IP address because they may be assigned addresses dynamically (using DHCP) and because they may have multiple IP devices, each with a different IP address.
- 9. Session Initiation protocol (SIP) • Mechanisms for call management • such as adding new media streams during the call • changing the encoding during the call • inviting new participants during the call • call transfer, call holding.
- 11. Key Characteristics of SIP • First, SIP is an out-of-band protocol: The SIP messages are sent and received in sockets that are different from those used for sending and receiving the media data. • Second, the SIP messages themselves are ASCII-readable and resemble HTTP messages. • Third, SIP requires all messages to be acknowledged, so it can run over UDP or TCP
- 12. Scenarios • Bob doesn’t have proper codec • Will send list of available codecs, Alice will choose one and resend invite with that codec • Busy • Gone • Payment required • Forbidden etc.
- 13. SIP • IP not known to Alice • Will send request on email id like bob@domain.com • SIP Proxy will respond with IP address of Bob or voicemail box or a URL saying Bob is sleeping • SIP registrar. • Every SIP user has an associated registrar. Whenever a user launches an SIP application on a device, the application sends an SIP register message to the registrar, informing the registrar of its current IP address