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

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
The two main problems of priority schemes are that a large part of the bandwidth needs to be allocated to the low priority traffic to ensure a reasonable probability of low priority traffic being transmitted. If the probability of loss is high, no applications will mark the cells as low priority.
The second issue is the complexity in discarding lower priority cells scattered throughout the buffer. This is very difficult, as most communication devices use FIFO queues where only the head of the queue is accessible. Consequently, the device may need to anticipate overflow and discard low priority cells as they arrive. The effectiveness of anticipatory schemes is still open to question.

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
The initial rate based control was based on a single bit end to end control. EFCI bit is set by congested switches. The destination sends Resource Management (RM) cells to the source. Sources use additive increase and multiplicative decrease of their rates.
In a positive polarity feedback, the sources keep reducing their rate until an RM cell is received. The decrease of rate is proportional to their current rate (hence the name Proportional Rate Control Algorithm).
Fairness problem in PRCA: If p=Probability of EFCI being set in a hop, then
1-(1-p)n = np = Probability of EFCI being set on at least one hop in a n-hop VC.
Long paths have thus fewer opportunities to increase the rate. This is known as the Beat Down Problem.

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
Source
Shaper
Policing
Network

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 Source
Shaper
Policing
Network

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
With the upcoming version of the UNI 4.0 specification, two types of VBR services are proposed: Real-Time VBR (rt-VBR) and Non-Real-Time VBR (nrt-VBR). Non-Real-Time VBR differs from Real-Time VBR in that cell delay variation and maximum cell delay time do not matter. Mean cell delay is specified instead.

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 Rate 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
In the explicit rate scheme, the switches determine the rate at which sources should send data.
The rates at which sources should transmit are explicitly told to the sources by the feedback mechanism.

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.
The policing mechanism is straight forward. The policing mechanism simple copies the rate information which is sent to the sources via the feedback signal.
The rate of convergence of the sources to the optimal operating point is much faster than the binary feedback scheme.
The initial rate of the source has much less impact on the operation of the network.
It is much more robust against errors or loss of RM cells. If an RM cell is lost, the source will get the information in the next RM cell.

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.
Each source sends an RM cell every n-th data cell. The RM cell contains the current cell rate (the rate at which the source is currently transmitting) and the rate at which the source would like to transmit.
On receipt of the backward RM cell, a source adjusts it rate according to the feedback information obtained in the backward RM cell.

39. Switch behaviour Computes a fair share. Sets or clears the reduce bit.
A switch computes a fair share for all the Virtual Circuits passing through the circuit. Different algorithms have been proposed to compute the fair share.
If the desired rate < fair share for a particular VC, the desired rate is granted.
If the desired rate > fair share, the desired rate = fair share and the reduce bit is set.
Setting the reduce bit implies that the desired rate could not be allocated and the source should transmit at the fair share.
Clearing the reduce bit means that the source can transmit at the desired rate.

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.
The Enhanced Proportional Rate Control algorithm combines the Explicit Rate and the PRCA algorithm.

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.
The virtual source /virtual destination technique can be used to avoid the large round trip delay encountered in end-to-end rate control.
Switches act as virtual sources and virtual destinations to generate and receive feedback information.
It has the advantage of reducing the feedback loop.
Moreover, the intermediate switches can use proprietary congestion control scheme within a loop.
Misbehaving users are isolated in the first control loop.

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?