Kommunikationssysteme [KS] Dr.-Ing. Falko Dressler Computer Networks and Communication Systems Department of Computer Sciences University of Erlangen-Nürnberg http://www7.informatik.uni-erlangen.de/~dressler/ dressler@informatik.uni-erlangen.de [KS], WS 2005/2006 2-4.1
Course Overview Network Infrastructure Basics of communication networks, packet- / circuit switched networks, telephone networks, ATM Quality of Service and Multimedia Policing, scheduling, AQM, audio/video streaming, conferencing and VoIP, IntServ, DiffServ, ATM, MPLS Simulation Tools, accomplishment, measures Protocol Engineering FSM, SDL, MSC, UML Wireless / Mobile Communications GSM, UMTS, mobility issues, WLAN, WPAN, WSN Network Security Threats and security goals, basic mechanisms [KS], WS 2005/2006 2-4.2
Multimedia Networking [KS], WS 2005/2006 2-4.3
Multimedia Internet Applications Remote Local Interactive Continuous Playback Streaming Local Playback Storage Download [KS], WS 2005/2006 2-4.4
Multimedia Application Types Storage/Download Capturing/or downloading multimedia sequences to/from storage devices Local playback Playback of multimedia sequences from a local disk Streaming On-line playback of multimedia sequences stored on remote servers May pause during playback to account for network congestion Continuous playback Continuous on-line playback of remote multimedia sequences No pausing allowed Interactive Multi-participant interactive multimedia sessions [KS], WS 2005/2006 2-4.5
Multimedia Applications cont d Application Type Storage Download Example Video Production Off-line video editing Application QoS Requirements Bandwidth Delay Jitter Errors / Losses High Medium-Hi Medium Medium-Hi Medium None High None Local Playback DVD Playback N/A N/A N/A None Streaming Real Audio/ Real Video Low - high High Medium Medium Continuous Playback Live Broadcast High Medium Low Medium Interactive Audio/Video Conference Low Low Low High [KS], WS 2005/2006 2-4.6
Multimedia protocol stack MGCP/Megaco H.323 SDP SIP Signaling Quality of Service Reservation Measurement RTSP RSVP RTCP TCP UDP IPv4, IPv6 Media Transport H.261, MPEG PPP AAL3/4 AAL5 PPP RTP Application daemon kernel Sonet ATM Ethernet V.34 [KS], WS 2005/2006 2-4.7
Timing Recovery [KS], WS 2005/2006 2-4.8
Timing Recovery for Synchronous Services Synchronous source sends periodic information blocks Network output not periodic Network Applications that involve voice, audio, or video can generate a synchronous information stream Information carried by equally-spaced fixed-length packets Network multiplexing & switching introduces random delays Packets experience variable transfer delay Jitter (variation in interpacket arrival times) also introduced Timing recovery re-establishes the synchronous nature of the stream [KS], WS 2005/2006 2-4.9
Introduce Playout Buffer Packet Arrivals Packet Arrivals Playout Buffer Packet Playout Packet Playout Sequence numbers help order packets T max Delay first packet by maximum network delay All other packets arrive with less delay Playout packet uniformly thereafter [KS], WS 2005/2006 2-4.10
Time Arrival times Playout clock must be synchronized to transmitter clock Send times Playout times Receiver too slow; buffer fills and overflows T playout time Time Receiver too fast buffer starvation Time Receiver speed just right Many late packets [KS], WS 2005/2006 T playout time T playout time 2-4.11
Clock Recovery Timestamps inserted in packet payloads indicate when info was produced t 4 t 3 t 2 t 1 Timestamps + - Buffer for information blocks Error signal Add Smoothing filter Counter Adjust frequency Playout command Recovered clock Counter attempts to replicate transmitter clock Frequency of counter is adjusted according to arriving timestamps Jitter introduced by network causes fluctuations in buffer & in local clock [KS], WS 2005/2006 2-4.12
Synchronization to a Common Clock Transmitter Receiver f s M Network M f r M = # ticks in local clock In time that net clock does N ticks f n /f s =N/M N ticks f n N ticks Δf=f n -f s =f n -(M/N)f n f r =f n -Δf Network clock Clock recovery simple if a common clock is available to transmitter & receiver E.g. SONET network clock; Global Positioning System (GPS) Transmitter sends Δf of its frequency & network frequency Receiver adjusts network frequency by Δf Packet delay jitter can be removed completely [KS], WS 2005/2006 2-4.13
Real-time Transport Protocol RTP RTCP [KS], WS 2005/2006 2-4.14
Real-Time Protocol RTP (RFC 1889) designed to support real-time applications such as voice, audio, video RTP provides means to carry: Type of information source Sequence numbers Timestamps Actual timing recovery must be done by higher layer protocol MPEG2 for video, MP3 for audio RTP Control Protocol (RTCP) Monitors quality of service at receivers Conveys monitored info to senders Canonical Name CNAME for each participant RTCP Packets Sender Report Packet Receiver Report Packet Source Description (SDES) BYE: end of participation by sender APP: application specific functions [KS], WS 2005/2006 2-4.15
RTP Packet Format Version (2) Padding flag Extension Header Flag Contributing Source Count (# CSRC IDs) Marker (significant events, e.g. frame boundaries) Payload Type: e.g. PCM, MPEG, Sequence Number: detect packet loss Timestamp: sampling instant of first byte Synchronization Source: ID for sync source CSRC List: contributing sources to payload 0 8 16 31 V P X CC M Payload Type Sequence Number Time Stamp SSRC Identifier CSRC Identifier [KS], WS 2005/2006 2-4.16
RTP Packet [KS], WS 2005/2006 2-4.17
RTCP Packet [KS], WS 2005/2006 2-4.18
Session Control Protocols SIP H.323 [KS], WS 2005/2006 2-4.19
Session Initiation Protocol Session: association involving exchange of data between Internet end systems Internet telephone call multimedia videoconference instant messaging event notification Session Initiation Protocol Setting up, maintaining, terminating session People & media devices Multicast or mesh of unicast connections Support for user mobility Over UDP or TCP [KS], WS 2005/2006 2-4.20
SIP Protocol Text-based client-server protocol with syntax similar to HTTP Transaction: client request /server(s) response(s) Basic signaling through transactions SIP Request: method invoked INVITE, ACK, OPTIONS, BYE, CANCEL, REGISTER INVITE & ACK used to initiate calls [KS], WS 2005/2006 2-4.21
SIP Protocol registrar INVITE sip: tom@startup.com c= IN IPv4 192.168.12.5 m=audio 35092 RTP/AVP 0 (2) (3) INVITE sip: tom@192.168.15.17 c= IN IPv4 192.168.12.5 m=audio 35092 RTP/AVP 0 (5) ringing (1) (4) (7) SIP/2.0 200 OK proxy (6) SIP/2.0 200 OK (8) ACK (9) Media flow [KS], WS 2005/2006 2-4.22
SIP System Components User Agent: software in end system that acts on behalf of an user User Agent Client: to initiate a call User Agent Server: to answer a call Network Servers: call routing to establish a call Proxy Server: receives request, determines server to send it to, and forwards request; Response flows in reverse direction Redirect Server: returns message telling client address of next server Registrar: registrations on current user locations Example: exercises [KS], WS 2005/2006 2-4.23
H.323 Systems Support for real-time multimedia communications (voice, audio, video, data) H.323 provides call control, multimedia management, bandwidth management, interfaces to other networks Gateways handle signaling messages between packet network & other networks Gatekeeper handles call control inside H.323 net Multipoint control unit combine media streams Gatekeeper MCU Telephone IP network Gateway Public Telephone Network H.323 Terminal H.323 Terminal [KS], WS 2005/2006 2-4.24
Scope of H.323 Microphone Speakers Camera, Display Audio Codec Video Codec Receive Path Delay Data Equipment System Control H.225 Layer LAN I/F System Control User I/F H.245 Control Call Control RAS Control [KS], WS 2005/2006 2-4.25
Typical H.323 Stack H.323 Multimedia Applications, User Interface Data Applications Media Control Terminal Control and Management V.150 T.120 T.38 Audio Codecs G.711 G.723.1 G.729.. Video Codecs H.261 H.263 H.264.. RTCP H.225.0 Call Signaling H.245 H.225.0 RAS RTP UDP TCP TCP/UDP UDP TCP/UDP TCP UDP IP [KS], WS 2005/2006 2-4.26