Evaluation of Priority and Scheduling Schemes for an IEEE 802.14 MAC Protocol Loaded by Real Traffic Milosh V. Ivanovich
Moshe Zukerman
Department of Computer Science Monash University Wellington Road, Clayton Victoria 3168, Australia Tel & Fax: +61 3 9803-0595 e-mail:
[email protected]
E.E.E. Department The University of Melbourne Parkville, Victoria 3052, Australia Tel: +61 3 9344-9209 Fax: +61 3 9344-9188 email:
[email protected]
Abstract: We study a new scheme for provision of priorities within the framework of a MAC protocol which is a potential candidate of the (as yet unpublished) IEEE 802.14 standard. A comprehensive simulation study, based on real traffic traces, shows that this new method provides better protection for the high priority traffic than an earlier proposed scheme. Except for physical and MAC layer overheads, the scheme exhibits comparable behaviour to that of an Ideal Multiplexer, for traces with very diverse correlation structures. The significance of using realistic (correlated) traffic streams for modelling purposes is demonstrated and discussed.
1. Introduction Future Hybrid Fibre/Coax (HFC) networks will provide an access medium for the provisioning of pay TV and broadband multimedia services. The IEEE 802.14 Working Group is moving towards publishing a draft version of the MAC protocol standard for efficient transfer of data over HFC networks. Many of the contributions for this standard are based on a tree architecture with central control at the Head-End, and with a general philosophy employing a contention resolution phase and a reservation phase [IVAN 97b] [SALA 95] [DOSH 96]. Nodes contending for access inform the Head-End of their needs for capacity using signalling mini-slots, and the Head-End satisfies their needs in an efficient and fair way. While the signalling mini-slots may be subject to collision, the actual data is transmitted, based on the capacity allocated by the Head-End, collision free. The need for supporting many users with different quality of service (QoS) requirements (e.g. Internet, Voice, Video) while maintaining the guaranteed QoS levels, motivates the introduction of support for multiple traffic priorities and scheduling schemes as part of the MAC protocol. This paper studies by simulation the performance of two such priority-scheduling schemes implemented as additional intelligence on top of a typical MAC protocol (based on CPR [SALA 96]), using real Ethernet measurements as input traffic.
We focus on a certain version (implementation) of the Centralised Priority Reservation (CPR) protocol proposed by Sala and Limb [SALA 95] [SALA 96]. CPR is quite similar to the MAC Level Access Protocol (MLAP), submitted by Bisdikian et al. [BISD 96] to the IEEE 802.14 committee. We call our version of the protocol Fair CPR (F-CPR). We use the F-CPR as a “testbed” on top of which to apply different priority and scheduling algorithms, because it shares the same philosophy of operation (and many of the specifics) of a number of the most significant IEEE 802.14 contributions - MLAP [BISD 96] and ADAPt+ [DOSH 96] in particular. The CPR has already been shown in [IVAN 97a], [IVAN 97b] to be as efficient and as fair as an ideal ATM multiplexer, even under real Ethernet traffic traces which exhibit a high degree of correlation and burstiness. In addition, previous research [SALA 96], [IVAN 97a], and [IVAN 97b] has indicated that the problem of congestion collapse which may occur in certain access protocols based on data slot collision (rather than signalling slot collision) or in end-to-end window flow control, is largely avoided in protocols like CPR and MLAP by allowing sufficient capacity for the signalling traffic which is the only type of traffic subject to collision and is much lower in volume than the data traffic. We say largely avoided because, as our work in [IVAN 98b] illustrates, there are certain conditions which do cause signalling deadlock. In this paper, we consider a new scheme for provision of priorities called: Just-in-time Exit Timestamp (JET). JET provides better queueing performance for the high priority traffic than the method of [SALA 95]. JET is applicable to FCPR, CPR, MLAP and to ADAPt+. We present results of a comprehensive simulation study, where we compare between the two priority schemes, and study many performance aspects of multiple priority support within the IEEE 802.14 standard. We provide insight into the effect of correlation of performance using real traffic traces. According to [BISD 96], the specific traffic prioritisation and scheduling algorithm will quite unlikely be a part of the IEEE 802.14 standard. Hence such performance studies are important for the development of an efficient IEEE 802.14 priority scheme.
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The remainder of the paper is organised as follows: in Section 2 we briefly describe the F-CPR protocol as implemented in our simulation. In Section 3, we clarify the assumptions made. In Section 4, the priority and scheduling algorithms which we tested are presented. Sections 5 and 6 explore modelling issues and queueing insights respectively. Finally, in Section 7 we will present simulation results.
2. F-CPR - A Brief Overview Under the F-CPR (as well as CPR and certain IEEE.14 proposals), nodes contending for access inform the Head-End of their needs for capacity using two types of signalling minislots: Contention MiniSlots (CMS) and Data MiniSlots (DMS). While the CMS’s may be subject to collision, the DMS’s are collision-free. The DMS’s may only be used when a station has already successfully reserved the data channel and has one or more further messages enqueued in its local message buffer. The user data is transmitted collision-free based on the capacity allocated by the HeadEnd. This capacity allocation is performed at the Head-End according to some efficient and fair scheduling algorithm. Due to space limitation, we shall not elaborate any further on the details of the access protocol and HFC architecture. These can be found in [IVAN 97a], [IVAN 97b], [IVAN 98], [BISD 96], [SALA 96] and [DOSH 96]. [IVAN 97b] illustrates the various data slot fields associated with the upstream and downstream paths respectively, and introduces the standard acronyms used when describing any variant of the CPR protocol (e.g. CMS, DMS, ACK/GR messages etc.), while Table 1 provides details and insights into their functionality. Slot/Field Data Field CMS Field DMS Field
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Details For the F-CPR we have chosen an ATM cell format (53 Bytes). CMS Field may be “contested”. If a collision has occurred the CMS is re-transmitted. Once a station completes transmission of its current message, it may write a further request (if its message queue is non-empty) into this reserved field.. An ACK message signifies a station’s success in the contention process, and tells it to shortly expect a GRANT message. The GRANT may even be in the same downstream data slot as the ACK, and informs the station how many contiguous upstream data slots have been allocated to it, starting from the next upstream time slot. Table 1: Slots and Fields
It was found in [IVAN 97a] that when using a traffic load whereby there is a small number of active stations (e.g. 10 stations, very unevenly distributed load), the F-CPR delay performance was severely affected by the “go/stop
phenomenon” - the name given to the adverse effect of the F-CPR’s round trip delay (RTD) on the throughput of a single station. In [IVAN 97a] we explained that F-CPR is subject to this effect since a station must always stop transmitting after it has sent the last cell of its current message, and then be forced to wait for a subsequent grant to use more bandwidth. Even in a system not loaded by any other stations, the minimum turn-around time from when the station sends its last cell (and with it a request for more bandwidth) to when the Head-End can deliver the next grant, equals the RTD of the HFC system. Note that the go/stop phenomenon is only expected to impact data applications, because delay sensitive applications such as voice or broadcast quality video will use the streaming feature of a MAC protocol that can periodically allocate guaranteed timeslots without the need for ongoing station to Head-End signalling [DOSH 96] [SALA 95].
3. Assumptions We make the following three key assumptions: (1) Regarding the HFC medium, it is assumed that the MAC protocol operates in an error- and loss-free environment. The effects of errored and lost signalling (CMS) messages are studied in [IVAN 98a]. (2) We use the very simple p-persistence algorithm [SALA 96] for contention resolution instead of more complicated algorithms. (There is no agreement on which algorithm is best to use.) In p-persistence, a probability, p, is communicated to the stations, which signifies the probability with which they will re-try to access the contention-based signalling channel after a collision. Therefore, from the instant when a collision is deemed (by the station’s timeout counter) to have occurred, a station will re-send its request, on average, within 1/p timeslots. (3) It must be made clear that the focus of this paper is on the MAC layer. When we conclude that the protocol is efficient, we mean that the MAC layer operation is efficient. Of course, there may be inefficiencies due to effects of other layers. These effects are not studied here. Nevertheless, given that we demonstrate that the protocol performs almost like an ideal multiplexer (IM) in terms of its efficiency and fairness, studies such as TCP over ATM (which assumes ATM to behave like an ideal multiplexer) can be used to evaluate performance of TCP over an IEEE 802.14 MAC layer.
4. Multiple Priority Schemes There exists a clear need to support many users with different QoS requirements. Hence the need for multiple priority implementations. In this paper, we study and compare between two schemes: the one we call the Scheduling Advance (SA) scheme of [SALA 95] and the Just-in-time
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Exit Timestamp (JET). Under SA, upon arriving at the HeadEnd, a lower priority request will be allocated bandwidth, so that its related data will be transmitted after all higher priority data known to the Head-End at the time of the lower priority bandwidth allocation. In other words, requests of all priorities are arriving at the Head-End and are served by a single server queue with a non-preemptive priority discipline. The JET on the other hand is based on the idea that the lower priority station will not be notified of its allocation until the last moment (taking into account the propagation delay from the station to the Head-End), and if high priority requests arrive before the allocation (GRANT) is sent to the low priority station, this GRANT will be further delayed. An Ack on the other hand must be sent immediately. The JET also provides FIFO fairness among all messages of the same priority arriving at the Head-End. In some cases, this latter requirement may conflict with the JET “last moment” idea. The fairness criteria will prevail in such a case. Let us take as an example two low priority requests arriving at the Head-End - one for a far away station and one from a nearby station, so that the nearby station request arrives first. Under the JET “last moment” paradigm, the closer station’s last moment will actually come later on in time than the further station’s last moment, because of the longer propagation delay in sending the GRANT. Given their different propagation delays, we could have provided better service for further stations by allowing the “last moment” concept to override the FIFO rule (since in the example given, the far away station would be the first to achieve access, even though its request arrived later). It should be clear to the reader however, that allowing this to take place would mean that FIFO-based fairness of access would vanish, since in some situations the scheme would discriminate against those stations which are closer to the Head-End. Namely, in the example given, if we were to send the further station’s GRANT first, there could potentially be a number of intervening higher priority messages which would repeatedly block the scheduling of the nearby low priority station. If FIFO order was maintained, then even in the case of higher priority arrivals, both the further and closer stations would be equally disadvantaged. Therefore, in our JET scheme implementation we clearly sacrifice better performance of some low priority stations over others, for the sake of achieving FIFO fairness among all low priority stations, irrespective of their position along the coaxial access medium. In any case, JET provides better queueing performance for the high priority traffic than SA. The extensive discussion of [IVAN 97c] on JET and SA includes a detailed description and insight into their operation.
5. Modelling Issues
5.1 Traffic Modelling Ethernet measurements counting the number of Ethernet frames arriving per second were used to create a “realistic” traffic model. We considered a batch arrival process where the inter-batch times are independent and identically distributed (i.i.d.), and each batch represents a message with the length of the message (in ATM cells) being the number of Ethernet frames recorded per second according to the measured traffic. In this way we maintain the Long Range Dependence (LRD) property of the original traffic trace. Alternatively, the LRD property can be eliminated by randomly shuffling the batches. This uncorrelated process will be called the shuffled process while the former will be called the unshuffled process.
5.2 Choice of Priority Assignment Mechanism There are two approaches to the generation of multi-priority traffic, each based on its own assignment mechanism. The first is the one used in [SALA 95], which we call Randomly Mixed Priorities (RMP) priority assignment. Under RMP, any station in the system can generate messages of any priority; the probability density function (of generating a certain priority message) within each station can be arbitrary. The second approach to multi-priority traffic generation is based on the principle of having every individual station transmits only at a given priority. This type of system is termed Priority Groups (PG) assignment. PG assignment is justified by the fact that it is realistic to assume that there will be variable-size subsets of stations (e.g. individual homes), each of which is using a given type of application and thus requiring a given level of traffic priority. Notice that although we maintain the overall correlation of the original arrival stream, the intra-correlation of a particular stream (of a given station for a given priority) under the RMP is lower than that of PG. This of course leads to gap in performance which will be demonstrated in our simulations. In our simulation we equalise the loads of each of the three priorities for both RMP and PG.
6. Queueing Insights We shall discuss here important traffic and protocol effects which affect queueing performance. Then in the next section we shall present a wide range of quantitative simulation results which demonstrate the significance of these effects. One interesting observation is that strong intra-stream correlation (which is known to have an adverse effect on queueing performance) actually helps, in some sense, to improve performance. Long bursts of consecutive packets allow stations request further bandwidth through collision free DMS’s, thus avoiding the need for time consuming
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In principle, we also know from [IVAN 97a] and [IVAN 97b] that except for the RTD due to the go/stop effect, FCPR almost behaves as a work conserving IM. Nevertheless, it is expected that the JET, which treats the high priority traffic better than the IM, due to its preemptive scheduling nature, will compensate its high priority traffic for the extra delay incurred by the F-CPR’s RTD. Low priority traffic could also enjoy some gaps in high priority RTD periods to get closer to the IM performance (to compensate their own RTD effects). This is an example of circumstances which delay high priority traffic and compensate lower priority traffic.
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Therefore, all modelling and architecture issues which reduce intra- as well as inter-station correlation improve performance. We already mentioned that shuffled PG as well as RPM traffic streams are less correlated then the unshuffled PG, and hence are expected to provide better performance (at least for the high priority traffic). Multiplexing of many stations also has a smoothing effect on the traffic, reducing correlation in the multiplexed traffic, hence improving queueing performance. High multiplexing also allows more stations to use the Round Trip Delay (RTD) periods of other stations (created as a result of the go/stop phenomenon) thus improving efficiency and reducing delay (see [IVAN 97a] and [IVAN 97b]).
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collision resolution phase. However, in most cases, the latter effect is less significant than the former, and the net effect of intra-stream correlation on queueing performance is negative.
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7. Simulation Results
Figure 1: Priority Assignment Mechanism Effect: 10 Stations, High Priority
We now present simulation results highlighting the effect of priority scheduling schemes, number of stations, traffic correlation, and priority assignment mechanisms on access delay and utilization. The results are compared with those obtained by the work conserving IM which is used as a benchmark.
As expected, from the utilisation graphs one can observe that the nature of the JET scheme allows it to reserve a greater proportion of the channel for its high priority messages, than that possible by SA. As was the case with the delay curves, the impact of JET’s high priority bias is more pronounced in the smaller system (Figure 5) because of the increased influence of the go/stop phenomenon with only 10 transmitting stations.
7.1. JET Versus SA Figures 1 and 4 illustrate that the high priority traffic delay performance for the JET scheme is better than that of the SA scheme, whether the systems are PG or RMP based. The SA scheme's worse high priority performance in comparison to the JET is quite noticeable for the RMP system (20% higher delay on average), the unshuffled PG system (delay increases to infinity at much smaller load, 0.85 instead of 1.15), and finally, for the shuffled PG system where the JET scheme yields tolerable delays even at a load of 1.5 while SA goes to infinity at a load of only 1.3.
In general go/stop generates a certain deviation (overhead) from the from the IM curve. This is true for all priorities. (See in Figure 5 the deviation of SA from IM for all priorities.) On the other hand, the JET brings the high priority average delay curve closer (as compared to SA) to that of the IM. In the case of any of the lower priorities there are three effects: (1) their own go/stop effect which is detrimental, (2) higher priorities’ go/stop RDT which provides them with access opportunities and is beneficial, and (3) the JET preemptive scheduling which is detrimental. Note in Figure 5 how the medium priority traffic behaves differently when carried by the F-CPR (SA or JET) as opposed to when handled by the IM. This comes as a result of the impact of go/stop on traffic of the medium priority
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class, for this small system (only 10 stations). That is, the particular arrival profile of the medium priority traffic is such that long periods of single-station activity occur, nonconcurrently with high priority traffic activity, meaning that there are long periods of simulation time when medium priority messages see no other “enemy” to their throughput performance, other than their own RTD overhead. The particular scheduling scheme does not play a big role here, and so the result is a significant difference between the IM and the F-CPR medium priority curves, without much difference between the curves of the JET and SA schemes (except for high priority traffic). 200
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traffic model we use it in Figure 8 to illustrate the effect of the number of stations in the system, on the average access delay. All three systems (F-CPR:JET, F-CPR:SA and IM) are shown, and the figure has a graph for each of the priorities. The first observation is that, like in the single priority systems, the delay performance of all three 10 station systems is significantly worse than that of the 50 station systems - and priority has no effect at all. This is consistent with the discussion in Section 6: more stations - less go/stop wastage and a reduction in traffic correlation and burstiness. The second observation we make is related to the RTDrelated wastage as measured by the proximity of the F-CPR delay curves to the IM curves. We see that in the case of the 10 station systems (where the go/stop effect is stronger) this wastage is much greater than in the case of the 50 station systems, for all priorities except the lowest. In the case of the lowest priority, the level of waste for the 50 and 10 station systems is similar. This is consistent with our discussion in Subsection 7.1 where we explain that the lower priorities are affected less by the go/stop effect because they enjoy RTD gaps of higher priorities.
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Comparing Figures 1 and 4, we can observe the improvement in performance when the number of stations is increased from 10 to 50. This result is expected due to the fact that multiplexing reduces correlation and variance. This is also noticed by comparing the two parts of Figure 7 and by comparing Figures 5 and 6. This improvement is especially significant for the Unshuffled Trace as highlighted in Figure 8. Since the Unshuffled PG system is the most realistic
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Figures 1-4 show that the unshuffled PG traffic gives the worst queueing performance for the high priority traffic in all cases. This is of course expected. It is well known that correlation causes bad queueing performance. Given that the unshuffled PG traffic is also the most realistic, these results provide certain warnings to those using uncorrelated processes or those based on the RMP principle. Under the unshuffled PG traffic modelling, where the Head-End typically sees only a few stations transmitting large bursts at the same time, the go/stop RTD effect is more significant, and hence low priorities can take advantage of RTD gaps belonging to higher priorities and improve their relative performance to be like that of RMP (see Figure 3). Notice that in systems based on RMP priority assignment, the high priorities’ traffic streams take advantage of each others RTD gaps, thereby blocking out lower priority traffic from entering such gaps.
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The result is that the delay performance of the RMP system, is immune from protocol artifacts and is solely determined by the queueing behaviour of the aggregated traffic profile at the Head-End. Figures 1-3 highlight this by showing the close proximity of the RMP system's F-CPR delay curves to its IM benchmark delay curves, as opposed to the much more severe delay increase (when comparing F-CPR versus IM) for both shuffled- and unshuffled-trace PG systems. Something to learn from this observation is that testing the protocol with unshuffled PG systems, regardless of the priority scheduling scheme employed or the shuffling status of the trace, is a more stringent examination because it takes into account both normal queueing and protocol-specific effects. Finally, in the case of the PG systems, it is the medium priority traffic which is more negatively affected than the low priority traffic. This occurs because of differences in the particular traffic arrival profiles of these two priorities (i.e. one is more susceptible to the go/stop phenomenon than the other); and, due to the fact that a greater portion of the low priority traffic delay consists of waiting behind higher priority messages, so that the effective limitation on these kind of messages becomes waiting and not the unavoidable RTD. The best example of this is to be seen in the set of medium priority curves in Figures 1-3 where, although the IM benchmark shows the RMP system to suffer worse performance from a queueing-only standpoint than the unshuffled-trace PG system, the go/stop throughputlimitation characteristic of F-CPR is significant enough to reverse this observation in both the JET and SA scheme cases, and show worse performance for the unshuffled-trace PG system!
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8. Conclusion We have considered a new multipriority scheduling scheme called JET, which was designed to perform above the future IEEE 802.14 standard, in terms of protocol stack functionality. A comprehensive simulation study of this new scheme under realistic traffic conditions has been carried out, and we have demonstrated the following: • As expected, JET provides better protection for the high priority traffic, than an earlier proposed scheme. • Except for physical and MAC layer overheads, F-CPRJET exhibits comparable behaviour to an IM, for traces with very diverse correlation structures. • We have demonstrated and discussed the significance of using realistic traffic models. • We have provided valuable insight into protocol artifacts and queueing performance of F-CPR.
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References [BISD 96] C. Bisdikian, B. McNeil, R. Norman, and R. Zeisz, "MLAP: A MAC Access Protocol for the HFC 802.14 Network," IEEE Communications Magazine, vol. 34, no. 3, pp. 114-121, March 1996. [CAPE 79] J. I. Capetanakis, “Tree Algorithm for Packet Broadcasting Channel”, IEEE Trans. Inform. Theory, vol. IT.25, pp. 505-515, September 1979. [DOSH 96] B. T. Doshi, S. Dravida, P. D. Magill, C. A. Siller Jr. and K. Sriram, "A Broadband Multiple Access Protocol for STM, ATM, and Variable Length Data Services on Hybrid Fiber-Coax Networks", IEEE 802.14 WG Doc. 802.14-96/222, September, 1996. [IVAN 97a] M. Ivanovich, M. Zukerman and R. G. Addie, "Performance Evaluation of an IEEE 802.14 MAC Protocol under Realistic Traffic Conditions", Proc. ITC 15, Washington, June, 1997. [IVAN 97b] M. Ivanovich, M. Zukerman and R. G. Addie, "Performance Investigation into an IEEE 802.14 MAC Protocol for HFC Networks", Proc. ICC ’97, Montreal, June, 1997. [IVAN 97c] M. Ivanovich and M. Zukerman, "IEEE 802.14 MAC Protocol with Priorities", Proceedings of APCC '97, Sydney, December 1997. [IVAN 98a] M. Ivanovich, Teletraffic Modelling, Analysis and Synthesis of a Generic Broadband Multi-service Access Protocol, Doctoral Dissertation, Computer Science Dept., Monash University, Australia, 1998. [IVAN 98b] M. Ivanovich and M. Zukerman, “Worst Case Signalling Traffic for a Multi-service Signalling Protocol”, Proc., IEEE ICC '98, June 1998. [SALA 95] J. O. Limb and D. Sala, “An Access Protocol to Support Multimedia Traffic over Hybrid Fibre/Coax Systems”, 2nd International Workshop in Community Networking, pp. 35-40, Princeton, July 20-22, 1995. [SALA 96] D. Sala and J. O. Limb, “A Protocol for Efficient Transfer of Data over Fibre/Cable Systems”, Proc., INFOCOM ‘96, San Francisco, March, 1996.
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