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Voice/IP

Voice/IP. Geoff Huston Internet Society. Voice and Data. Analog voice transmission has dominated the communications industry for the past 100 years The entrance of multi-service digital networks is placing a new set of demands on the service profile of communications networks

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Voice/IP

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  1. Voice/IP Geoff Huston Internet Society

  2. Voice and Data • Analog voice transmission has dominated the communications industry for the past 100 years • The entrance of multi-service digital networks is placing a new set of demands on the service profile of communications networks • Will we see convergence to a single network platform?

  3. Voice Networks • Voice transmissions have an number of characteristics: • 3Khz bandwidth • limited amplitude (<25db) • time synchronization • limited average duration (200 seconds) • High localization (80:20 rule) • Strong traffic peaking characteristics

  4. Digitizing Voice • 8000 samples per second (Nyquist Theorem) • 125 second timebase • 256 discrete amplitude levels • 8 bits per sample • 64Kbps PCM data stream

  5. 64K Networks • Voice networks are built by multiplexing and switching synchronous 64K data streams • Time division multiplexing • 125 second time base • 8 bit symbols per time slot per voice channel • 2Mbps bearer is 32 x 64K slots • 30 data slots • 1 channel signaling slot • 1 frame sync slot = 2048Mbps

  6. Circuit-switched Networks • Time division switches • reorder the timeslots of a TDM data stream • impose 1 slot time constant delay • Space Switches • crossbar switching • 2 slot time delay due to muxing overhead • Supports dynamically switchable end-to-end synchronously clocked circuits

  7. A Voice Network Analogue Copper Loop Codec 64K PCM digital stream Mux TDM 2Mbps bearer SDH Switch SDH STM-16 bearers

  8. A Data Network • Switches Packets, not circuits • Each packet may be independently forwarded, delayed or dropped by each router • Each packet is independently switched to its addressed destination • There is no time synchronization between sender and receiver

  9. Data Networks • Highly cost effective infrastructure • low levels of network functionality • high potential carriage efficiency • Functionality pushed beyond the network edge • Assumption of adaptive data flow control by end hosts • No guarantees of service level by the network.

  10. Voice over IP • packetize the digital voice stream • add timing information • add IP headers • send across the network • strip IP headers • feed into playback buffer using timing information • playback analogue signal

  11. Packetizing Voice • Compress the digital stream • differential PCM • Linear Predictive Encoding • silence suppression • packetize the stream into fixed length payloads 00110101101010111 110101101101 101101

  12. Voice over IP • Insert RTP header • 12 bytes or more • Insert UDP header • 8 bytes • Insert IP header • 20 bytes or more • Payload size (packet rate) is a compromise between packet overhead and latency and jitter

  13. Voice over IP Modem 56Kbps async Network Access Server 100Mbps LAN Access Router 100Mbps LAN Core Router *

  14. Bounded End-to-End Delay - interaction requires delay to be under 500ms Jitter - high jitter causes large playback buffers Drop - signal quality VoIP Service Requirements OUT IN

  15. Delay, Jitter and Drop are all outcomes of router queue behaviour Queues are used to: resolve contention for a resource buffer speed differences within the network Why do Routers have queues?

  16. Resource Contention Queues 1 2 4 3 2 1 FIFO Output Queue 3 Forwarder 4 Scheduler Input Queues

  17. TCP is an adaptive data protocol TCP has no ‘fixed’ data transfer rate. Instead, TCP uses an adaptive flow control algorithm TCP uses a feedback loop to adjust the sending rate to the available network capacity TCP and Queues

  18. TCP rate control 100 Queue Saturation 90 80 70 60 TCP FLOW RATE (% of maximum send rate) 50 Congestion Avoidance 40 Slow Start 30 20 10 Single Session Two Sessions 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 TIME

  19. The Multi-Service Problem Real-Time flows require: short queues admission control priority queuing Congestion-Managed flows require: large queues no admission control explicit congestion notification

  20. Mixing TCP and UDP TCP flow rate UDP flow rate Time Buffer starvation period as a result of a TCP burst UDP Sessions TCP Session

  21. Mixing TCP and UDP UDP flow rate TCP flow rate Time TCP restarts due to tail drop packet loss UDP Sessions TCP Session

  22. Can you mix Voice and Data at the packet level? Voice over IP works - as long as: small proportion of total traffic queue lengths are kept short some network inefficiency is tolerated i.e. as long as the proportion of VOIP traffic is low compared to rate-adaptive traffic and the network is generally unloaded One Network Platform

  23. Does high quality service require resource reservations? Can resource reservation be provided? Is the cost of simulating time switching in a packet switched network higher or lower than the cost of operating a distinct time-switched network? Where is the cross-over point? Is service convergence and the mother-ship single platform operational model just a perverse throwback fantasy? The Multi-Service Network

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