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

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

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

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

Priority Control Cell Loss Priority CLP Bit set to 0 or 1

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.

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

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

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.

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)

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

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

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

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

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

Leaky Bucket Algorithm Input Overflow Output

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

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

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

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

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.

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

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

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

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

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

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

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

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 %

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

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

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

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.

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

Explicit Rate Explicit rates sent to sources. Switches determine rate

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.

Explicit rate - Disadvantages Complexity Cost Switch memory requirements Switch processor requirements

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.

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.

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.

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.

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?