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Efficient Scheduling Disciplines for next Generation QoS-Aware GPON Networks Konstantinos Kanonakis, Student Member, IEEE, Ioannis Tomkos, Member, IEEE High-Speed Networks and Optical Communications Group, Athens Information Technology Center 19002 Athens, Greece Tel: +30 210 6682823, e-mail:
[email protected];
[email protected] ABSTRACT Future broadband access networks are expected to rely more and more on fiber-based solutions. To this end PON networks are regarded as one of the most prominent alternatives. This paper proposes a novel MAC scheduling algorithm for GPON networks that can offer efficient operation, combining QoS differentiation with high network utilization. It achieves improved performance by making use of both fixed and dynamic bandwidth allocations along with predictive allocations and the novel concept of offset-based scheduling with flexible intervals, being thus able to effectively adapt to fluctuations of traffic and meet quality of service requirements. The concept of operation is described in detail and the system performance is evaluated using computer simulation. Keywords: gigabit passive optical networks, dynamic bandwidth allocation, quality of service. 1. INTRODUCTION Passive optical networks (PONs) have long been studied and deployed and are generally considered as one of the most scalable and cost-effective solutions for the majority of FTTx (Fiber to the x) architectures. The predominant protocols are GPON (Gigabit PON) and EPON (Ethernet PON), standardized by the ITU-T [1] and the IEEE [2] respectively, providing symmetrical data rates of up to 2.488 Gbps (GPON) and 1Gbps (EPON) while updated standards specifying PON networks with capacities of 10Gbps, spanning distances of up to 100 km are expected to emerge in the next couple of years from both bodies. At the same time, consumer demands are also steadily increasing, with new emerging services and applications imposing strict Quality of Service (QoS) requirements. The MAC layer of the GPON network is responsible for offering those QoS guarantees, however offering QoS in PONs has always been a crucial and challenging issue. Most solutions proposed in the literature ([3], [4]) focus on how the Optical Line Termination (OLT) unit distributes slots of upstream frames among the queues of Optical Network Units (ONUs) in a periodic basis, with the period (scheduling interval) being fixed for the lifetime of each queue. This paper instead proposes a method of using flexible scheduling intervals with an offset-based scheduling mechanism to achieve QoS differentiation and high network utilization. The paper is structured as follows: In chapter 2, the network architecture and parameters are described. Chapter 3 explains the categorization of ONU queues into a number of types in order to provide QoS differentiation, while chapter 4 focuses on the concept of offset-based scheduling with flexible intervals and explains the way in which the proposed system works to achieve efficient operation with the aforementioned QoS differentiation. System performance is evaluated in chapter 5 and finally conclusions are drawn. 2. THE NETWORK UNDER STUDY The network under study is a common tree-shaped PON employing the standardized GPON protocol (however everything can be directly applied to any future update, e.g. the upcoming 10G GPON). Information is organized in frames of large size (the frame duration in [1] is 125 μs, leading to approximately 39 kbyte frames for 2.5 Gbps). The frame duration in our model is f t (expressed in s), while the sizes of downstream and upstream frames are f d and f u respectively. Downstream frames are broadcast to all k ( k is also called the split ratio) ONUs by means of a passive splitter (security is achieved by means of encryption) and each ONU collects only the data destined to it. The OLT includes in the downstream frames both user data and control information. The latter can be slot allocations for specific queues residing at the ONUs, which are characterized using the socalled Alloc-IDs, and queue status report requests. The proposed mechanism as will be explained in the following paragraphs handles Alloc-IDs individually, hence the piggybacked reporting mode as described in [1] is regarded as most suitable. In the upstream direction, frames are formed by data sent from the various ONUs in a TDMA fashion, guided by information sent by the OLT. Collision-free operation in the upstream, as well as a common timing among ONUs are achieved by using a ranging procedure during activation and registration and, if necessary, by imposing additional delay at the ONU side, so that the round-trip propagation delay can be regarded as fixed and common for all OLT-ONU pairs. We denote the one-way propagation delay as τ .
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3. ALLOC-ID TYPES FOR QOS DIFFERENTIATION In order to provide QoS differentiation, it is necessary to categorize the ONU Alloc-IDs into several types, treating each of them in a different manner according to their individual demands. The proposed framework makes use of three general Alloc-ID types. The frequency of upstream slot allocations for an Alloc-IDi is defined by its scheduling interval SI i , with an amount of SRVi Bytes being allocated during each interval. Below we provide a description of each type and the way it is handled in terms of bandwidth allocation: Fixed Type: These Alloc-IDs correspond to the highest priority traffic (e.g. leased line emulation or other very demanding services) and have their guaranteed rate GRi allocated in the form of fixed periodic allocations, i.e. the OLT assigns to them a specific amount of data in each downstream frame without waiting for any queue reporting from the ONU side, therefore the amount of Bytes allocated in each upstream frame for a fixed-type Alloc-IDi is SRVi = GRi ⋅ f t and of course SI i = f t . Apart from the guaranteed bandwidth, this Alloc-ID type also enjoys minimal packet delay and jitter, assuming that proper traffic dimensioning has taken place when choosing the amount of fixed allocations. It could correspond to the “Guaranteed Bandwidth” traffic class of Diffserv. Flexible Type: This type covers the middle ground between the absolute QoS of the fixed type and the nonexistent QoS of best-effort traffic. All Alloc-IDs belonging to this type are characterized by an SLA-contracted guaranteed rate GRi and a maximum surplus rate SRi in order to offer statistical multiplexing or exploit free upstream bandwidth when available (the exact use of this surplus bandwidth is up to the provider). Bandwidth is assigned to the flexible Alloc-IDs in a Dynamic Bandwidth Allocation manner using status reporting. The shortest possible scheduling interval for a flexible-type Alloc-ID, which we define as the basic scheduling interval and denote as SI b , is bounded by the OLT-ONU round-trip propagation 2τ and the processing delay at the OLT and the ONUs, OLT p and ONU p respectively, hence SI b ≥ 2 τ + OLT p + ONU p . The basic scheduling interval is rounded up to become an integer multiple of the frame duration f t . The SI value for a flexible-type Alloc-ID has to be equal or larger than the basic scheduling interval. time
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Figure 1. Control message exchanges between OLT and ONUs. It must be stressed at this point that we do not regard the scheduling intervals for flexible-type Alloc-IDs as fixed during network operation (as is done in [3], [4] for example) but we instead allow the OLT to dynamically choose the optimal value for each SI i , based on criteria that will be detailed below and this is one of the strongest features of the proposed scheme. In that respect, the mechanism could be parallelized to the IPACT method of EPONs [5], employing however an entirely different mechanism for the allocation of bandwidth and, most importantly, for the differentiation among QoS classes. The flexible nature of the scheduling interval duration should of course not affect the contracted rates offered to each Alloc-IDi. Hence the number of bytes SRVi of Alloc-IDi served during each scheduling interval must obey the equation (of course as long as there are always bytes to be served at the queue): GRi ⋅ SI i ≤ SRVi ≤ (GRi + SRi ) ⋅ SI i .
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The process of updating the SI i values takes place once every f t , and the scheduling interval for a specific Alloc-ID will only be updated if a new report for it has arrived. It is obvious that the OLT must keep track of the time left for the next allocation/report request for each Alloc-ID. This is achieved using a vector with its elements containing integer values that are interpreted as countdown timers (the time unit being f t ) and which is updated once every f t . Obviously the reset for these timers happens when the corresponding scheduling interval has been updated and their starting value equals the updated SPi minus one. In addition, the OLT must keep a scheduling matrix containing allocations that have been scheduled in advance. Its columns denote future frames (column 0 corresponds to the allocation that must be done within the next frame) and the matrix must be shifted by one column to the left, once every f t . As will become clear later, a higher number of columns can provide finer differentiation among Alloc-IDs, at the cost of increased memory and computational requirements. Note that the same scheduling matrix is shared by all Alloc-ID types. Figure 1 depicts the control message exchanges between the OLT and an ONU, with the latter possessing two flexible-type Alloc-IDs i, j (and a number of fixed-type ones). The minimum possible basic scheduling interval has been chosen. It can be seen that the scheduling intervals are different for the two Alloc-IDs, while SI i changes during network operation. Best-Effort Type: This type does not claim any guaranteed bandwidth and can only allocate the bandwidth that is left unused by the previous types, up to a specified peak rate PRi . 4. OFFSET-BASED SCHEDULING WITH FLEXIBLE INTERVALS As mentioned in the previous paragraph, the OLT must determine the optimal SI i value for each flexible-type Alloc-IDi, taking into account several contradicting criteria in terms of efficiency and utilization. First of all, the piggybacked upstream queue status report ( DBRu ) for an Alloc-ID covers a fixed amount of data per frame. For small scheduling intervals and low data rates, the bandwidth wasted for status reporting could be significant. Thus, we already meet the first constraint which dictates that the ratio of report bandwidth per Alloc-IDi against its guaranteed rate should be kept lower than a predefined value which we denote as MSRRi . Each flexible-type Alloc-IDi is associated with a set of QoS requirements which in most cases are: a maximum packet delay, a maximum packet delay variation and a maximum packet loss probability. Loss can only occur due to buffer overflow at the Alloc-ID queues. We will hereafter assume that all buffers are infinite, and use the packet queueing delay as a metric for the system performance. It is obvious that both delay and delay variation are affected by the choice of SI i . We follow the approach of setting a lower limit to the SI i for each Alloc-IDi, equal to SI i + offset i . Higher offset i values imply higher delays and the corresponding Alloc-IDs can use a narrower range of SI i values. These effects give the opportunity to fine-tune the performance of the various flexible-type Alloc-IDs (apart from the choice of GRi and SRi values). Another basic feature of the proposed mechanism is the use of reports for the dynamic rate estimation of Alloc-IDs. At every scheduling interval update, the rate estimate is computed using the new and old queue size reports as well as the old SI i value. A more advanced solution is to use even older values as well in an exponential averaging manner. Then, the estimated rate is used to make a linear prediction of the number of accumulated Bytes at the ONU queue by the time of the allocation arrival at the ONU side. This prediction is used during the allocation phase instead of the actual ONU queue report. In any case, at the time of the scheduling interval update, the OLT logic must determine the optimal scheduling interval values by executing the following steps for each flexible-type Alloc-IDi: Among all eligible SI i values (producing status reporting ratio less than MSRRi and beginning with the one dictated by the offset i parameter), choose the one which corresponds to the largest unallocated space (among values corresponding to the same space, choose the lowest) by looking at the relevant columns of the scheduling matrix. Then compute the maximum possible SRVi value based on the aforementioned prediction method, taking care not to violate the GRi value or exceed the upstream frame size. The order in which Alloc-IDs are examined in order to update their scheduling intervals affects their performance, since the ones that are updated first may block the targeted scheduling intervals of others. This issue can be solved by applying a simplistic prioritized discipline in the examination of the various Alloc-IDs, while among the ones with the same priority, a shifted round robin could be applied in order to provide longterm fairness. Note that the proposed mechanism will inherently distribute allocations in consecutive slots (even if the various SI i are of equal duration) achieving in this way higher utilization of the system, which would not be possible if a rigid, off-line assignment of scheduling intervals had been employed.
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After the aforementioned procedure, a second round of queue examinations takes place for the flexible-type Alloc-IDs, where any remaining space in the upcoming frames can be allocated to the queues in the same prioritized fashion, allocating extra bandwidth up to their surplus rates. Note however that the scheduling intervals have already been determined for each Alloc-ID. Finally, bandwidth is assigned to the best-effort Alloc-ID type and these Alloc-IDs can use any unassigned bandwidth up to their peak rate, choosing their scheduling intervals as described above. 5. PERFORMANCE EVALUATION The performance of the proposed scheme was evaluated using the OPNET Modeler network simulator. The upstream and downstream rates were both assumed to be 10 Gbps while the other network parameters were: τ = 500 μs (which corresponds to a maximum OLT-ONU distance of 100 km, assuming propagation speed equal to 2 ⋅ 105 km s ), f t = 125 μs , k = 8 and OLT p = ONU p = 100 μs . Each ONU possessed four Alloc-ID queues, one corresponding to the fixed type, one to the best-effort and the other two belonging to the flexible type, which we denote as flexible type 1 and 2 respectively. Packets arrived at each queue following a Poisson process, while the packet size equalled 1500 Bytes . Each ONU produced traffic which belonged by 10% to the fixed type Alloc-ID and by 30% to each of the other types. The GRi of each fixed type Alloc-ID was three times its average rate, for flexible type 1 it was 1.3 times, while for flexible type 2 it was set to zero. The SRi value was 0.2 and 1.5 times the average rate for the flexible type 1 and 2 respectively. Finally, the PRi for each besteffort type Alloc-ID was set to 1.5 times its average rate. 350
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Figure 2. (a) Average queueing delay (b) Probability density function of queueing delay for 50% load. Figure 2(a) depicts the average queueing delay for the Alloc-IDs of each type as a function of the average total upstream load (the latter expressed as a ratio of the 10 Gbps rate). The offset i parameters were 0, 15 and 30 for the flexible type 1, 2 and best-effort Alloc-IDs respectively. Finally, the probability density function (PDF) of queueing delay for 50% average total upstream load is shown in Figure 2(b) (the fixed type is not shown for practical reasons, as its PDF almost resembles an impulse function). 6. CONCLUSION A novel bandwidth allocation mechanism for GPON networks has been described and its ability to offer efficient performance along with QoS differentiation has been verified using computer simulation. REFERENCES [1] ITU Rec. G.984.3, Study Group 15, “Gigabit-Capable Passive Optical Networks (G-PON): Transmission Convergence Layer Specification”, Geneva, Switzerland, Oct. 21–31, 2003. [2] IEEE Draft P802.3ah (tm), IEEE Standard, 2004. [3] H.C. Leligou, Ch. Linardakis, K. Kanonakis, J.D. Angelopoulos, Th. Orphanoudakis, “Efficient medium arbitration of FSAN compliant GPONs”, International Journal of Communication Systems 19 (2006) pp. 603-617. [4] C. Hung, P. Kourtessis, J. M. Senior, "GPON Service Level Agreement based Dynamic Bandwidth Assignment Protocol", IET Electronic Letters, vol. 42, no. 20, pp. 1173-1174, Sept. 2006. [5] G. Kramer, B. Mukherjee, G. Pesavento, "IPACT: a dynamic protocol for an Ethernet PON (EPON)", IEEE Communications Magazine, vol. 40, no .2, pp. 74-80, Feb. 2002.
