Transporting Voice by using IP Chapter 2 The

Transporting Voice by using IP Chapter 2

The IP Protocol Suite • IP is a routed protocol for passing data packets • Other protocols invoke IP for the purpose of getting these data packets from origin to destination • So IP must work with higher layer protocols for any application to work properly • Remember the OSI 7 -Layer model?

Internet Standards • The Internet Society : Non-profit body with overall objectives to keep the internet alive and growing • The Internet Architecture Board (IAB): Technical advisory group of the Internet Society. • The Internet Engineering Task Force (IETF): Volunteers who cooperate in the development on Internet standards; equipment vendors, network operators, research institutions.

Internet Standards ctd. . . • Internet Engineering Steering Group (IESG): Manages and controls IETF’s activities, can approve a particular specification. • Internet Assigned Number Authority (IANA): Responsible for unique numbers, parameter values and meanings.

Internet Standards Process • Begins life as an Internet draft • Once it is considered complete it can be published as an RFC (Request for comments) • The RFC is given a number and becomes a draft standard. • To achieve this it must have at least 2 independent successful implementations and interoperability must have been demonstrated.

The IP Datagram Format

Routed vs Routing Protocols • Routed: IP, IPX, Novell IPX, Open Standards Institute networking protocol, DECnet, Appletalk, Banyan Vines, Xerox Network System (XNS). • Routing: Ø Routing Information Protocol (RIP and RIP II) Ø Open Shortest Path First (OSPF) Ø Intermediate System to Intermediate System (IS-IS) Ø Interior Gateway Routing Protocol (IGRP) Ø Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP) Ø Border Gateway Protocol (BGP)

Transmission Control Protocol (TCP) • Ensures that packets are delivered to destination in sequence • Primary function is to overcome the limitations of IP through an end-to-end confirmation • Port Numbers: Is a means of identifying a specific instance of a given application. • Other header fields?

TCP Header Format

Real-time Transport Protocol

User Datagram Protocol (UDP) • Passes data from and application to IP to be routed to the far end. • At the far end it simply passes incoming data from IP to the application. • Provides no acknowledgement functionality • What happens if a UDP packet is lost? • Checksum simply checks that received data is error free

UDP Header Format

Voice over UDP, not TCP • Speed is more important than loss of data • Voice packets are smaller so drop of a few will not be noticeable in the overall context. • Packet loss of about 5% is generally acceptable • Provided that loss is fairly evenly divided • What happens if they arrive out of sequence? • QOS techniques can involve establishing a set pattern through the network

Real Time Protocol (RTP) • A Transport Protocol for Real Time Applications • Sits on top of UDP • Helps address some of the problems associated with UDP in terms of packet loss • RTP contains a companion protocol (RTCP) • RTCP provides exchange of messages between sessions to ensure some sort of reliability

Fast-forward to the Year 2021 • Director of Development for MME, Inc. You Common Service Video Conferencing Streaming Audio Movies ?

Two Goals of RTP’s Common Service • General enough to be truly “common” – Who knows what applications are coming? – Throughout history, communication has changed: • Oral (traditions passed between generations) • Written • Visual • Specific enough to actually be useful

RTP can deliver • • Multimedia applications requirements RTP architecture RTP details RTP does meet the requirements

Requirements (1) • Timing – Time-stamping for buffered playback • to minimize jitter – Synchronization of multiple streams – Dynamic frame boundaries • Video: frame length varies due to compression • Audio: “talkspurts”

Requirements (2) • Network issues – Dealing with packet loss – Dealing with congestion • Even with multicast – Bandwidth utilization • Minimize header bits

Requirements (3) • Miscellaneous – Interoperability • Encoding • Compression – ID of source • To whom am I listening? • Useful especially in video-conferencing

Requirements Summary • This is not TCP! – Who cares if we lose a packet or two? – Who cares if we have jitter? • Calls for a different protocol. . .

RTP Architecture “ALF” and “ILP” • Application-level framing: – The application best knows its own needs – May not ask for retransmission, but for lower resolution • Integrated Layer Processing – Tightly coupled layers – Keeps data presentation from being the bottleneck • Gives the app. access to the data ASAP!

RTP: Summary • A very thin protocol – Usually built into application • No hard QOS guarantees – Designed for soft real-time apps – Depends on underlying network – Can run over ATM • Two components: – Media(data) transport: RTP – Control: RTCP

RTP Concepts • Port numbers for both RTP and RTCP • Participant IP addresses – Strength is multicast • Relays – Mixers – Translators

RTP Header Format

RTCP • ID of sender • Provides various reports for use in: – Qo. S and congestion control • so an app can change resolution or compression strategies – Session size and scaling • conferencing

Mixers • Mixer: An application that enable multiple media streams from different sources to be combined into one overall RTP stream – Could receive and combine various sources in an effort to reduce bandwidth

Translators • Used to manage communications between entities that do not support the same media formats or bit rates: e. g. TDM to STDM – Keeps incoming sources separate – To transform to a lower quality format to broadcast on lower-speed networks – To send through firewalls

Compression • Can use various types – JPEG – MPEG – H. 261 • Provided by application • Negotiated using RTCP

Calculation Round-Trip Time (RTT) • • • This is another function of RTCP Useful metric when measuring voice quality T 1, T 2, T 3 and T 4 RTT = T 4 - T 3 + T 2 - T 1 or T 4 - (T 3 - T 2) - T 1

Calculation Jitter • Jitter is defined as the mean deviation of the difference in packet spacing at the receiver compared to packet spacing at the sender for a pair of packets. • If Si is timestamp for packet i and Ri is the time of arrival in RTP timestamp units for packet i then for 2 packets i and j the deviation in transmit time D is given by: • D(i, j) = (Rj-Ri) – (Sj-Si) = (Rj-Sj) – (Ri-Si)

IP Multicast • An example of this with Vo. IP is a conference call • Send a packet to a single destination address associated with all listeners • 224. 0. 0. 1 All hosts on a local subnet • 224. 0. 0. 2 All routers on a local subnet • 224. 0. 0. 5 All routers supporting OSPF • 224. 0. 0. 9 All routers supporting RIP v 2

Summary • Multimedia applications have much different needs than http or ftp! • RTP meets those needs: • • Minimized jitter Synchronized sources Dynamic, payload-specific frame length Adaptation in the face of congestion Interoperability Effective use of bandwidth Support for video-conferencing (multicast, IDs)