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Voice Over ATM (VOA)

ATM Network. Voice Over ATM (VOA). Raj Jain Professor of Computer and Info. Science The Ohio State University Columbus, OH 43210 Jain@cse.ohio-State.Edu http://www.cse.ohio-state.edu/~jain/. Overview. Importance of Traffic Management Simulation Results OSU Scheduling Algorithm

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Voice Over ATM (VOA)

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  1. ATMNetwork Voice Over ATM (VOA) Raj Jain Professor of Computer and Info. ScienceThe Ohio State UniversityColumbus, OH 43210Jain@cse.ohio-State.Edu http://www.cse.ohio-state.edu/~jain/

  2. Overview • Importance of Traffic Management • Simulation Results • OSU Scheduling Algorithm • Recent Developments: New AAL for Voice

  3. Why No Voice on ATM? • Incomplete standards • No appropriate AAL • Cheaper alternatives • Traffic Management and SchedulingStandard committees will not work on internal switch/NIC mechanisms

  4. Delay Issue • 48 bytes at 64 kbps = 6 ms  Need Echo cancelers • 48 bytes at 16 kbps = 24 ms  too long • Can't fill a cell completely • Current AALs allow segmentation (long packets to multiple cells). • Do not allow blocking (short packets in one cell)

  5. Simulation Results • Voice Traffic Model • Performance Metrics • Network configuration • Simulation results

  6. Voice Traffic Model • On-off Model: • 352 ms on, 650 ms off • Times depend upon the sensitivity of the detector  On Off 

  7. Source 1 Destination 1 Switch Switch Source n Destination n 64 kbps 1.5 Mbps 64 kbps Network Configuration • n voice sources sharing a T1 link

  8. Max Acceptable Delay • Cross-country propagation delay = 24 ms • Total delay = 5x + 6 + 6 + 24 < 100 • Per-switch delay x<12.8 ms • 1-hop network delay x + 24 < 36.8 ms  40 ms Switch Switch Switch Switch Switch S D 6 x x x x x 6

  9. Performance Metrics • Cell Loss Ratio (CLR) • Delayed Cells: Network delay > 40 ms • Degradation of Voice Quality =DVQ = (Lost cells + Delayed cells)/Cells sent • Useful cell ratio = 1 - DVQ • Fairness

  10. 2 x  i f ( x )  x 2 n  i Fairness Index • Always lies between 0 and 1 • If k out of n users get k/n, others get 0, fairness index is k/n • Independent of scale

  11. Scheduling Policies • Earliest deadline first • Longest queue first • Round robin

  12. Simulation Results RR 0.25 EDF 0.20 LQF 0.15 DQF 0.10 0.05 10 20 30 40 50 60 70 80 Number of Voice Sources

  13. Simulation: Conclusions • Scheduling policy does affect voice quality and fairness • Voice traffic should only partially use the link. Remaining capacity used by data. • Average delay is not a good measure of voice quality. • Need a scheduling algorithm better suited for ATM.

  14. OSU Scheduling Algorithm • Conventional algorithms do not provide quantitative delay or bandwidth guarantees • Not ideal fit for ATM networks

  15. Time CBR ABR Sched- CBR ABR Credit Credit uled Credit Credit 0 0.6 0.4 - 1 1.2 0.8 CBR 0.2 0.8 2 0.8 1.2 ABR 0.8 0.2 3 1.4 0.6 CBR 0.4 0.6 4 1.0 1.0 ABR 1.0 0.0 5 1.6 0.4 CBR 0.6 0.4 Two-Class Algorithm

  16. CBR rt-VBR nrt-VBR ABR UBR n-Class Scheduler • Ensures no-starvation for all classes even under overload. • Each class has an allocation Guaranteed under overload

  17. n-Class (Cont) • Some classes need minimum delay  have priority. • Some classes are greedy: They will send more than allocated and will want to use all left-over. No left-over capacity. • Left-over capacity must be fairly allocated. • ERICA scheduler achieves all these goals.

  18. New AAL: Requirements • Allow transfer of short variable length packets • Allow packets with variable inter-arrival times • Allow packets to cross ATM cell boundaries • Allow for partially filled cells • Allow multiple connections to be multiplexed on one VC

  19. AAL2 • Objective: Support low bit rate (below 64 kbps) delay sensitive applications allows sending less than 48-byte payload • ITU-T SG13: ITU I.363.2 • Previously known as AAL-CU • 0 to 64-byte packets. Default 45 bytes.

  20. ATMSwitch BaseStation ATMCellAssemblerDissembler Leased T1/E1 Low-Bit Rate Voice • Time to fill 48-byte payload @8 kbps = 48 ms CellHeader PktHdr Payload 1 PktHdr Payload 2 PktHdr Payload 3

  21. SSCS SSCS SSCS SSCS Protocol Structure • Common Part specification (CPS) • AAL2 Negotiation procedure (ANP) • Service Specific Convergence Sublayer (SSCS) User 1 User 2 User 3 User n Layer MgmtANP CPS ATM

  22. PacketHeader PacketPayload Packet Format CellHeader Cell Payload StartField CPS PDUPayload PacketPayload PacketHeader

  23. CPS Packet Format • 3-byte header. UUI = User-to-user indication, PPT = Payload protocol type Channel Id Length Indicator PPT UUI Header Error Check Info

  24. Cell Format • STF: Start field = CPS PDU header • OSF: Offset of the first packet • SN: Sequence number mod 2, 0 or 1 • P: Parity (odd) of start field • Pad: Padding (0-47 bytes) CellHeader OSF SN P CPSPacket CPSPacket CPSPacket Pad STF

  25. Status • I.363.2 Text frozen, Feb 97 • Specifies CPS • ANP, SSCS, OAM, to be done

  26. Summary • Traffic management and scheduling is an important component for QoS • Scheduling makes a difference in quality of voice over ATM • AAL2 is being designed for low-bit rate voice

  27. References • ITU-T I.363.2, ftp://ftp.t1.org/pub/ts1s/t1s1.5/6s151860.doc • ATM Forum, "Voice and Telephone over ATM to the Desktop Specification," March 1997, Letter ballot, af-vtoa-0083.000.ps • ATM Forum, "Requirements for compressed voice to the desktop," LTD-VTOA-DSK-02.03, April 1997, • Madge Networks, "Voice over ATM: A Sound Assessment," http://www.data.com/tutorials/atm_voice.html

  28. References (Cont) • Beth Gage and Liza Henderson, "Voice Over ATM," Network World, March 11, 1996. • M. Jackson, "VTOA AAL5 format proposal," ATM Forum 96-955, August 1996. • G. Leijonhufvud, "Timing of Voice Services to the desktop," ATM Forum/96-953, August 1996 • D. Alley, "Echo cancellation for voice services in ATM Networks," ATM Forum/96-949, August 1996.

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