Voice Over ATM VOA

1
Voice Over ATM (VOA)

• Raj Jain
• Professor of Computer and Info. ScienceThe Ohio
  State UniversityColumbus, OH 43210Jain_at_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
Network Configuration

• n voice sources sharing a T1 link

8
Max Acceptable Delay
Switch
Switch
Switch
Switch
Switch
S
D
x
x
x
x
x
6
6

• Cross-country propagation delay 24 ms
• Total delay 5x 6 6 24 lt 100
• Per-switch delay x lt12.8 ms
• 1-hop network delay x 24 lt 36.8 ms ? 40 ms

9
Performance Metrics

• Cell Loss Ratio (CLR)
• Delayed Cells Network delay gt 40 ms
• Degradation of Voice Quality DVQ (Lost cells
  Delayed cells)/Cells sent
• Useful cell ratio 1 - DVQ
• Fairness

10
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
Two-Class Algorithm
16
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
Low-Bit Rate Voice

• Time to fill 48-byte payload _at_8 kbps 48 ms

CellHeader
Payload 1
PktHdr
Payload 2
Payload 3
PktHdr
PktHdr
21
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
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/t1s1.5/6s
  151860.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_voi
  ce.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.

29
Thank You!
30
AAL2 Features

• Multiplexing and demultiplexing of multiple users
  on a single virtual circuit.
• No retransmission of lost packets.
• All channels are bi-directional

User 1
User 2
User n
User 1
User 2
User n
ATM Network
Node 1
Node 2
31
CPS format (Cont)

• CID Channel Identifier (8 bits)
• LI Length Indicator Payload Length - 1 (6
  bits)
• UUI User-to-User Indication (3 bits)
• PPT Packet payload type (3OAM, 0,1,2UUI)
• HEC Header error control (5 bits)
• CPS-INFO Information (1 to 45/64 bytes)

32
CPS Header (Cont)

• CID 0 Not used, 1 Reserved for layer
  management, 2-7 Reserved, 8-255 Identifies
  users
• UUI Transparently sent end-to-end by CPS
• PTT Transparently sent end-to-end by CPS
• HEC x5 x2 1
• Detects 1- and 2-bit errors in CPS header
• No error correction

33
Padding/Splitting

• A CPS packet may be split
• Last cell may be padded

34
ANP

• AAL2 Negotiation Procedure
• Assign, remove, status-poll AAL2 channel
• VCC setup uses standard B-ISDN signaling
• Same ANP for PBX trunking and mobile applications
• Will be decided by Sep 97

35
ANP Messages

• Assignment Request
• Assignment Confirm
• Assignment denied
• Removal Request
• Removal confirm
• status poll
• status response

36
SSCS

• SSCS for Voice
• SSCS for long data

37
OAM

• F6 Flow CPS common
• F7 Flow Per channel, F7s-CP, F7e-AAL, F7e-CP

F7e-AAL
Short PacketMux
Short PacketRelay/Mux
Short PacketMux
F7e-CP
F7s-CP
Switch
VC
VC
VC
38
Voice over ATM

• Two ways to connect PBXs
• Circuit Emulation Services (CES) - 1.5 Mbps CBR
  PVCs between PBXs
• Voice Trunking - one SVC per voice call

39

• After standards are set,
• PBX's can be eliminated (Proprietary).
• Phones can be replaced by headsets connected to
  PCs.
• Server based voice applications (voice mail,
  interactive voice response, text-to-speech
  conversion, faxback) can be used.
• Video conferencing.

40
Voice over LANs

• Process with low software delays
• Clock synchronization
• Signaling
• Supplementary services Call forwarding, redial,
  3-way calling, ...
• Compression is not required

41
Required QoS

• 50-100 ppm clock accuracy
• 1-2 ms CDV
• Max 1 slip/10 sec ? 10-5 clock accuracy or 10-5
  CLR
• 10 to 100 times lower slip rate required for data

42
Voice Desktop Requirements

• Multiplexing of voice and data in the same VC
• Save bandwidth by
• Compressed voice
• Other applications (ABR/UBR) can use unused
  bandwidth
• Using switched calls instead of leased lines

43

• Include timestamps in AAL5 cells 1-40 byte data,
  4-byte sequence number, 4-byte wall clock time,
  8-byte trailer
• Multipoint operation
• Interworking with voice over N-ISDN, Frame relay,
  wireless, and IP
• Inband signaling channels

44

• Default G.711 64 kbps 3.1 kHz audio. But allow
  other rates and coding.

45

• Resynchronize under loss or errors\
• Allow clock recovery
• Allow inband signaling

46
Guaranteed
Standby
Joy Riders
Confirmed
47
Classes of Service

• CBR (Constant bit rate) User declares required
  rate. Throughput, delay and delay variation
  guaranteed.
• VBR (Variable bit rate) User declares average
  and max rate.
• rt-VBR (Real-time variable bit rate)
  Conferencing. Max delay and delay variation
  guaranteed.

48

• nrt-VBR (non-real time variable bit rate) Stored
  video. Mean delay guaranteed.
• ABR (Available bit rate) Follows feedback
  instructions. Network gives maximum throughput
  with minimum loss.
• UBR (Unspecified bit rate) User sends whenever
  it wants. No feedback mechanism. No guarantee.
  Cells may be dropped during congestion.

49
Traffic Management on the Information
Superhighway
CAC
Shaping
1
2
UPC
3
Scheduling
4
Selective
5
6
FrameDiscard
7
Traffic Monitoring and feedback
50
Traffic Management Functions

• Connection Admission Control (CAC) Can
  requested bandwidth and quality of service be
  supported?
• Traffic Shaping Limit burst length. Space-out
  cells.
• Usage Parameter Control (UPC) Monitor and
  control traffic at the network entrance.

51

• Network Resource Management Scheduling,
  Queueing, virtual path resource reservation
• Selective cell discard Cell Loss Priority (CLP)
  1 cells may be dropped Cells of non-complient
  connections may be dropped

52

• Frame Discarding
• Feedback Controls Network tells the source to
  increase or decrease its load.