Some aspects of ATM Network Performance Simon Crosby

University of Cambridge Computer Laboratory

In ATM networks the V i u a l Channel Identifier (VCI) for a connection is used at switches to route cells belonging to the connection along the correct channel. The VCI is also used to provide the appropriate rwurces for the connection and to check that the connection does not exceed the resources allocated to it. The resource management functions in an ATM network are responsible both for allocation of resources and for guarding the network against badly behaved users. In addition they allow the network to decide whether it can enter into new connection “contracts” with its users and guarantee their required Quality of Service (QoS). This summary highlights some of the resource management issues in ATM networks.

1 What are the most useful qualities of service for a network to provide? Some examples might include:

0 Fixed delay, fixed bandwidth, with a given cell loss rate probability, in other words, a circuit,

0 Fixed delay, variable bandwidth with cell loss probabiIity as a function of the bandwidth

0 No loss (except for transmission error) variable delay variable bandwidth channels.

Quality of Service and Contracts

wed,

Information about the distribution of delays might be another candidate. Contracts can be more specific, reflecting the sources’ likely behaviour, perhaps for changes in bandwidth. Understanding what constitutes a sensible contract not only involves understanding the sources to be attached, but also requires that we understand how to meet the contract, that is, how to deliver a particular

Contracts must be expressible in a way that allows them to be enforced. This requires that they be expressed in terms of observables (and controllables). In general terms we will want to express these contracts in terms of a few parameters such as peak demand, average demand, loss rate, and maximum latency. Once we have decided what the parameters for a channel are, we can ask the network to provide us with such a channel.

Part of the difficulty in expressing the requirements of sources is characterizing their behavior in a way which is meaningful to the network. Should we require users to be aware of the intricate details of their source processes [e.g. various moments of the inter-cell time distribution) or should we allow them to request services such as ‘%ideo on demand”, “colohr fax” etc? We also need to know the effects of the different source parameters on the network resources, both to allow us to control the network and to charge for the resources used.

- quality of service.

2 What-are the Resources? In a circuit switched network resources are link bandwidths. Link bandwidth is dedicated to a call when the call is established. It is easy to decide if sufficient resources are available. In traditional packet switching, as in ATM, both bandwidth and buffer space are resources. It is possible to have dedicated allocation of bandwidth to a virtual circuit. It is also possible to have dynamic allocation which is constrained in some way, for example by a minimum bandwidth. It is also

0 1995 The Institution of Electrical Engineers. Printed and published by the IEE. Savoy Place, London WCPR OBL, UK.

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possible to have dynamic allocation which makes use of priority and fairness, and finally it is possible to have purely random contention for bandwidth. Buffers in ATM are used for a variety of traffic types with different time-scales (and buffer requirements):

1.

2.

3.

4.

5.

For delay insensitive traffic which has no guaranteed bandwidth (large buffers)

To smooth out delay sensitive variable bit rate (VBR) traffic which may instantaneously exceed available bandwidth (medium buffers)

To allow constant bit rate traffic to synchronise to the frame structure used by scheduling algorithms (tiny buffers)

To smooth out simultaneous bursts from independent streams, that is when the instantaneous arrival rate is greater than capacity (medium bufEers)

To smooth out simultaneous cell arrivals from independent streams (small buffers)

The fact that there are two critical resources in ATM is not surprising, nor should it be viewed as a problem of ATM: using ATM as a pure circuit switching network would render buffer space uninteresting and using ATM purely for delay insensitive traffic would render bandwidth less interesting.

The above discussion makes clear that there are a number of aspects to resource management in ATM networks. These are dealt with in more detail in the following sections.

3 Call Acceptance Control The Call Acceptance Control function is responsible for deciding whether the network can accept a new call of a particular type or not. For calls which require some real guarantee from the network, the network must be aware of its outstanding commitments.

3.1 Effective Bandwidth Allocating bandwidth using the peak demand for bursty traffic reduces the problem of bandwidth allocation to a circuit switching problem. This allows the vast body of knowledge and experience associated with circuit switched networks to be used, but has the drawback that it results in ineffcient use of bandwidth. Another approach, if one knows something about the statistics of the traffic, is to allocate bandwidth on a statistical basis. This is often proclaimed as the advantage of ATM - its ability to statistically multiplex multiple bursty connections. For a given aggregate loss requirement it is possible to derive a4 efjktiwe bandwidth for each source which represents its “real” resource needs. The effective bandwidth of a bursty source lies somewhere between its peak and mean rates, and can be derived either from a knowledge of the statistical properties of the source or by observation (though the latter is still somewhat primitive). Given the effective bandwidth of each source the call acceptance problem is greatly simplified -the sum of the effective bandwidths should be less than the total capacity. Given n~ calls of type 1, n2 calls of type 2 etc we canderive an acceptance surface (which in practice is almost linear) which can be used to drive a CAC algorithm, in a similar way to a pure circuit switched network with constant bitrates. More detail on the effective bapdwidth concept can be found in [Kelly 911 and [Gibbens 911.

Note that while the concept of effective bandwidth enables us to reduce the call acceptance problem to that used in circuit switching, the effective bandwidth of a source depends upon the details of its statistical behaviour and on the capacity of each link it will use.

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4 TrafFic policing and shaping Imagine a scquence of cells belonging to a virtual channel. The rate at which these cells are sent, or the inter-arrival time between cells, or the duration of and intervals between bursts may be part of the contract describing the virtual channel. By discarding or delaying cells within the sequence, we may change the sequence from which violates the contract to one which does not.

There are a host of definitions. but we will describe this process as tra@ shaping. We can distinguish shaping from policing - policing is the related process of seeing whether the stream

violates the contract and simply discarding or marking cells which are in violation. In general we think of shaping being performed by the user and policing being performed by the network.

As well as delaying or dropping cells, the shaping function can also mark cells as being in violation by setting the cell loss priority (CLP) bit. Inside the network when congestion occurs, a cell is more likely to be discarded if its CLP bit is set. 'IkafEc shaping is performed in isolation on a virtual channel; no account is taken of other virtual channels sharing the same link. Most traffic shaping attempts to smooth traffic out, that is change bursty traffic into constant bit rate traffc. This is achieved by delaying cells within a burst and playing them out at a fixed rate.

outgoing cells

.

Figure 1: A Leaky Bucket Algorithm

One scheme for doing this is the Leaky Bucket algorithm. There are many variants of this; we will describe the one shown in figure 1. Cells arrive into an input queue which has some fixed size. Cells arriving and finding the queue full cause a cell to be thrown away, either the first one, the arriving one or possibly a cell with a CLP bit set. Tokens are generated at a fixed rate and enter the token queue. If the token queue is full, the token is lost.

The dispatcher will transmit the cell at the head of the cell queue to be transmitted if there is a token in the token queue. This action takes the cell from the cell queue and the token from the token queue. In fact, if we ignore simultaneous arrivals and assume transmission is instantaneous, either the cell queue or the token queue will be empty. The length of the token queue represents the maximum burst size the dispatcher will allow into the network, the token generation rate is the average constant bit rate we are trying to achieve, and the m h u m size of the token pool is the maximum burst size which the network will tolerate. There are variations on this scheme where cells are not discarded but are marked as being in violation (virtual leaky bucket). Other schemes did not have a token queue so the traffic would be smoothed to always be within a rate determined by the token generation rate.

Leaky bucket schemes have been analysed a great deal and their properties are reasonably well known; more details can be found in [Roberts 921. Policing variations of the leaky bucket are simpler in that they do not have to actually have a queue. Cells which would have been queued can be allowed through immediately, the leaky bucket just has to keep track of the size the queues would have been. (See for example [ITU-T 931.)

5 Cell Scheduling As cells from different VCIs contend for the same resource, some arbitration must be made as to which has priority. This resource may be the switching fabric, buffer space, or transmission bandwidth on an output port. Scheduling cells for transmission depends on the switch architecture - which may be input or output buffered.

Input ports at input-buffered switches, may on the basis of the VCI, determine fabric priority and queueing strategy for a connection at the input, possibly based on the load of the switch. Different discard policies may also be in place - for example certain VCs may wish to have old cells discarded rather than new cells. Some VCs may wish to have all cells which have been in a queue for a certain length of time discarded. Scheduling at the input can be performed

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in isolation from other input ports or it can be coordinated. One way of delivering constant rate circuits is to define a complete periodic schedule of the switch, ensuring that no fabric and no output contention occurs. If traffic which has a constant rate allocation (possibly based on peak) has absolute priority over traffic with no bandwidth allocation then there is no danger in attempting to get the unguaranteed traffic through should there be no bandwidth allocated traffic to send when its turn comes.

Most manufacturers favour output-buffered switches, in spite of their increased complexity and cost, because their performance is more predictable. Non-blocking switching fabrics ensure that there is an extremely low probabdity of contention for switching resources by enabling one cell from every input to contend for a single given output in a single cycle. Such switches require buffering at the outputs. At the outputs, the switch faces the problem of allocation of buffers to different connections and the scheduling of cells onto the output link. An enormous amount of analytical and simulation work has been devoted to the study of cell scheduling policies for output buffered switches. Proposed mechanisms can be broken down into two broad classes - work conserving and non-work conserving mechanisms. The former are never idle when there are cells to transmit, whereas the latter permit cells to remain in the buf€er until they meet a delay requirement, for example a mean transmission rate. Many policies can be implemented in hardware at reasonable cost. An important consideration in assessing different scheduling mechanisms is whether they protect connections from one another. Fair queueing or round robin scheduling can be used to build “firewalls” between different connections which ensure that if a particular connection is badly behaved it only affects itself. A good survey of cell level scheduling techinques can be found in [Zhang 911.

6 Flow and Congestion Control Most of the discussion above has been concerned with bandwidth as the critical resource. Effective bandwidth, allocation on peak, traffic shaping, policing and deterministic cell sceduliig are largely bandwidth oriented.

Here we turn our attention to the control of buffering within the network. In talking about buffers as resources, we said that data paxkets could give rise to large demands on buffers. This of course depends upon our notion of how data is sent into the network. We could revert to a circuit switched mode of operation and allocate bandwidth to data streams based on peak demand. This rather igores the experience of statistical multiplexing which shows that allocating on peak for data sources is neither necessary nor efficient. For data transfer we are usually concerned to minimize the average latency across the network rather to minimize guaranteed maximum latency. Thus a user sending data, for example, wishes to get their information as quickly across the network as possible taking advantage of instantious bandwidth which may become available. The problem with this approach is that eventually buffer capacity somewhere in the network is exceeded and data is lost. This is a symptom of network congestion. The so-called Available Bit Rate (ABR) service has been proposed as the ideal service for data based transfer, offering low loss, but without any guarantees on end-to-end transfer delay. Congestion control must be aware of the traffic type which is being controlled: it is pointless to use a flow control mechanism to limit the transmission rate of a source with real-time constraints so that all data are successfully transmitted, when the late arrival of data is equivalent to loss. For such trafhc types simply discarding traffic when congestion occurs is the best policy- For tr&c which is loss sensitive but which has less strict constraints on end-to-end delay, such as the ABR service, flow control policies make sense.

To prevent or min i i se these congestion events, we need to implement a congestion control strategy. We can distinguish three broad classes of congestion control strategy:

1. Schemes in which the network takes no active part (end to end congestion control)

2. Schemes in which the network ensures that information is only sent from one hop to the

3. Schemes in which the network gives an indication of its state, to end systems (congestion

next if there is buffer available for it (hop by hop flow control), and

notification).

Flow control, at the ATM celI level, can be performed between the user and the network and between components of the network. B-ISDN currently only makes provision for flow control between the user and the network, and even here it looks likely that it will only be used to arbitrate among a number of users sharing the same link into the network.

Recently the ATM Forum standardized on a rate-based flow control mechanism, as opposed to hop by hop credit-based flow control. This is a type of end to end congestion control where the end systems use information about cell loss or congestion to modify the rate at which they send cells into the network. In this scheme the network can be involved since advantage can be taken of the congestion notification indication bit in the ATM cell header (EFCI).

The alternative, hop by hop credit-based flow control has been implemented by DEC in their ATM equipment. Credit-based flow control requires slightly more buffer space than when using rate-based flow control, however it can ensure that there is zero cell loss, over large bandwidth- delay product links. The DEC implementation is still the only commercially available rate/flow controlled ATM equipment. The motivation for the ATM Forum’s decision to adopt rate-based control was to avoid per VC queueing in the output ports of switches, regarded as costly and complex to implement, however recent announcements by ATM equipment manufacturers show the following trends in their flow control implementations:

All equipment implements/will implement the ATM Forum Explicit Forward Congestion Indication, because the Forum agreements require at least that much for a switch to claim ABR support;

0 They will also implement Per-VC queuing, to isolate ABR users in case the Forum’s rate feedback algorithm works poorly, and to isolate the VCs’ congestion for EFCI;

0 Finally they will provide generous buffering, since the ATM Forum ABR is quite imprecise at avoiding congestion, compared to credit-based flow control;

The avoidance of per-VC queuing was the major argument for which 100 vendors voted to persue rate-based control over credit, flow control. But EFCI performs poorly without per-VC queuing, so leading vendors implement per-VC queuing anyway, or will do so in their future models.

7 Conclusion Resource management is perhaps the single most challenging aspect of ATM. We have only outlined some of the problems and possible approaches. Many of the problems may never arise. In large networks the laws of large numbers may give rise to enormous statistical multiplexing gains. In small networks, providing more resource than might seem necessary and running at low utilisation may be a more appropriate way to proceed. We should not lose sight of what it is we are trying to optimise - high network utilisation is not a primary aim, it is a secondary aim of keeping costs down. If high network utilisation requires expensive resource management algorithms or even algorithms which must change as the traffc mix changes then it will very rapidly become a non-goal.

References [Kelly 911 Kelly,F.P. Effective Bandwidths at Multi-type Queues. Queueing Systems, Vol9.

[Gibbens 911 Gibbens,R.J. and Hunt,P.J., “Effective bandwidths for the multi-type UAS channel.” Queueing Systems, Vol9.

[ITU-T 939 International Telecommunications Union Telecommunication Standardization Sector, Recommendation 1.371 Tkafic Control and Congestion Control in B-ISDN, March 1993.

[Roberts 921 Performance evaluation and design of multiservice networks. COST 224 Project, Roberts,J.W.(ed), Commission of the European Communities, ISBN 92-826-3728-X.

[Zhang 911 Zhang,H. and Keshav,S, “A Comparison of Rate Based Service Disciplines”, Computer Communication Review, Vol21, No 4, Sept 91.

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