Transporting Voice by using IP

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Transporting Voice by using IP

• Chapter 2

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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?

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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.

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Internet Standards ctd ...

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

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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.

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The IP Datagram Format
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Routed vs Routing Protocols

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

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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?

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TCP Header Format
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Real-time Transport Protocol
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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

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UDP Header Format
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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

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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

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Fast-forward to the Year 2021

• Director of Development for MME, Inc.

Common Service
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Two Goals of RTPs 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

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RTP can deliver

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

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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

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Requirements (2)

• Network issues
• Dealing with packet loss
• Dealing with congestion
• Even with multicast
• Bandwidth utilization
• Minimize header bits

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Requirements (3)

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

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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...

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RTP ArchitectureALF 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!

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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

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RTP Concepts

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

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RTP Header Format
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RTCP

• ID of sender
• Provides various reports for use in
• QoS and congestion control
• so an app can change resolution or compression
  strategies
• Session size and scaling
• conferencing

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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

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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

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Compression

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

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Calculation Round-Trip Time (RTT)

• This is another function of RTCP
• Useful metric when measuring voice quality
• T1, T2, T3 and T4
• RTT T4 - T3 T2 - T1
• or T4 - (T3 - T2) - T1

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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)

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IP Multicast

• An example of this with VoIP 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 v2

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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)