ECS5365 Lecture 6 ATM Traffic and Network Management

1. ECS5365 Lecture 6ATM Traffic and Network Management Dr Richard Nelson Centre for Telecommunications and Information Engineering (CTIE) Monash University Richard.Nelson@eng.monash.edu.au

2. Outline • Approaches to Traffic Control • ATM Service Classes • Constant Bit Rate • Variable Bit Rate • Unspecified Bit Rate • Available Bit Rate

3. Approaches to Traffic Control • Admission Control • Priority Control • Usage Parameter Control (Traffic Policing) • Traffic Shaping • Rate Feedback Flow Control (ABR)

4. Connection Admission Control • Adequate resources reserved before connection setup • Long term control • Preventive • Complex algorithms for ATM Switches

5. Priority Control • Cell Loss Priority • CLP Bit set to 0 or 1

6. Limitation of Priority Schemes • Guaranteed bandwidth is low (typically 50%) • Need high probability of low priority cell getting through, otherwise won’t be used • Complexity of discarding cells from buffer • Not discarding head of line cell

7. Usage Parameter Control (Traffic Policing) • Network polices traffic • Usage Parameter Control • Can discard or reduce priority of non-conforming cells

8. Traffic Shaping • Shapes traffic to conform to contract • Controls peak rate and burstiness • Uses same algorithms as traffic policing • Reduces cell discard probability • Optional • Smooths traffic to reduce arrival time variation

9. Rate Based Control • Feedback from network to source • Very low cell loss rate • Complex algorithms and added expense in switch • Complex fairness problems

10. CBR Service Class • ATM cells sent at regular intervals • Defined by Peak Cell Rate (PCR) • 155 Mbps line rate • CBR connection at 155 Mbps gives an interval between cells of 2.73 microseconds • Traffic associated with VPI/VCI characterised by Tmin (minimum interarrival time) and Tavg (average interarrival time)

11. Effect of Multiplexing Multiple CBR Connections • Introduces cell delay variation • Require a cell delay variation tolerance associated with Peak Cell Rate

12. Cell Delay Variation for PCR Policing • Traffic policed on interarrival time • Problem of ‘slotted’ nature of ATM • Peak cell rates constrained to reciprocal of integral number of cell slot times • e.g. 150 Mbps provided to ATM layer • next possible PCRs are 75 Mbps, 50 Mbps, … • Can define finer rates with cell delay variation tolerance

13. Value of Cell Delay Variation Tolerance • Large enough to account for all multiplexing effects and provide range of PCR values • Small enough to guarantee QoS of other connections

14. Generic Cell Rate Algorithm Cell Arrival Time ta ta > TAT-L Non-Conforming Cell TAT =max (ta,TAT)+I Conforming Cell TAT Theoretical Arrival Time L Limit I Increment ta Cell Arrival Time

15. Traffic Shaping • Modify traffic at source to conform to GCRA • Traffic scheduled • Uses Leaky Bucket Algorithm Shaper Policing Network Source

16. Leaky Bucket Algorithm Input Overflow Output

17. Variable Bit Rate Service Class • Most applications have variable bandwidth demands • Data • Compressed video • MPEG • Motion JPEG • Need a contract that allows limited variable traffic into the network

18. VBR Traffic Contract • A constant rate of variable length cell bursts • Specified by • Peak Cell Rate (PCR) • Sustainable Cell Rate (SCR) • Burst Tolerance (BT) • Maximum Burst Size defined in terms of PCR and BT

19. Policing for VBR • Two generic cell rate algorithms in parallel • First polices PCR • Allows for Cell Delay Variation Tolerance • Second polices SCR • Allows for Burst Tolerance

20. VBR Traffic Shaping • Requires token bucket with leaky bucket Shaper Policing Network Source

21. VBR Traffic Contracts in UNI 4.0 • nrt-VBR : non real time VBR • maximum cell transfer delay • cell loss ratio • rt-VBR : real time VBR • peak to peak cell delay variation • maximum cell transfer delay • cell loss ratio

22. Usability of VBR contracts • Prerecorded compressed video • can VBR parameters • Live video • CAC difficult • shaper acts as a buffer, possibly interfering with QoS • Bursty data • CAC impossible • Shaper possible

23. Unspecified Bit Rate Service Class • Limitations of CBR and VBR for data • Data • adaptable to available bandwidth • tolerant of delay and delay variation • intolerant of high loss rates • does not need resource reservation • is very bursty

24. UBR Characterisation • PCR physical line rate (usually) • CLP 0+1 • No resources reserved

25. Congestion with UBR • Will occur often • Assumed higher layers will avoid it (TCP) • If it occurs, higher layers will solve it

26. Congestion Control in TCP • TCP is the transport layer in TCP/IP • Uses packet loss as implicit feedback • Adjusts packet rate down when loss occurs • Adjusts packet rate up when no loss

27. Early Experiments with TCP over UBR • Very low throughput • 5 % • Led to Early Packet Discard

28. Early Packet Discard • AAL5 only • Switch buffer reaches a threshold • entire AAL-PDU is dropped • Uses 3rd bit of PTI field in ATM header to identify end of packet

29. Early Packet Discard Requires Careful Tuning • Two experiments with EPD • Switch buffer of 256 cells, PDU length 200, throughput 80 % • Switch buffer 100 cells, PDU length 50, throughput 22 %

30. ITU and UBR • ITU does not define UBR • Similar service with • VBR • SCR 0 • Cells CLP = 1

31. Available Bit Rate Service Class • Reactive Congestion Control • Network feedback to adjust traffic rate • Very low cell loss rate • Complex algorithms • Fairness issues

32. ABR design goals • Scalable (WAN and LAN) • Optimal • Fair • Robust • Usable in public network • no assumption as to user cooperation

33. Credit v. Rate Based • Credit based • Switches determine available resources in buffers • Send credits to sources based on free resources • Simplifies sources • Rate based • Switches determine fair sending rate for sources • Simplifies switches

34. Proportional Rate • Switches signal increase or decrease rate • Sources response proportional to current rate • Simplest system

35. Explicit Rate • Explicit rates sent to sources. • Switches determine rate

36. Explicit rate - Advantages • Straight forward policing • Fast converge to optimal operating point • Initial rate has less impact • Robust against errors or loss of RM cells.

37. Explicit rate - Disadvantages • Complexity • Cost • Switch memory requirements • Switch processor requirements

38. Source behaviour • An RM cell sent every n-th data cell. • RM cell contains • CCR (current cell rate) • desired cell rate • Sources adjust their rates.

39. Switch behaviour • Computes a fair share. • Sets or clears the reduce bit.

40. EPRCA • Combines ER and PRCA. • Allows both binary feedback and explicit feedback switches in the same network. • Congestion detection based on queue length - results in unfairness.

41. Virtual Source/Destination • Avoids large round trip delay • Switches acts as virtual sources and destinations. • Reduces size of the feedback loop. • Intermediate switches can use proprietary congestion control scheme. • Easy to isolate misbehaving users.

42. Questions • What cell delay variation tolerance is needed to define a peak cell rate of 80 Mbps over a line rate of 150 Mbps? • If cell delay variation tolerance is zero and the highest possible peak cell rate is 150 Mbps, the next peak cell rate is 75 Mbps, the next is 50 Mbps. What is the next lowest peak cell rate that can be defined?