A novel delay-bounded traffic conditioner for optical edge switches
TL;DR: An on-line variable-rate conditioner is developed that approximates the off-line optimum, and requires O(1) amortised computation per packet arrival, making it amenable to efficient hardware implementation at the high data rates required by optical edge switches.
Abstract: Optical packet switched (OPS) networks provide very limited contention resolution resources such as fibre delay lines (FDLs) and wavelength converters. Effective use of these resources in minimising contention losses within the all-optical core requires conditioning of traffic aggregates by the optical edge switches. Traditional rate-based shapers such as the leaky-bucket fail to provide acceptable delay performance for real-time traffic aggregates: this paper therefore explores novel conditioning mechanisms for OPS networks transporting traffic aggregates with time constraints. Using as a theoretical basis a known off-line optimum smoother for stored video traffic, we develop an on-line variable-rate conditioner that approximates the off-line optimum, and requires O(1) amortised computation per packet arrival, making it amenable to efficient hardware implementation at the high data rates required by optical edge switches. We also demonstrate via simulation of short and long range dependent traffic that our conditioner allows losses in the optical core to be reduced by orders of magnitude at the expense of a bounded and relatively low increase in end-to-end delays. We believe that our conditioner can deliver significant performance benefits when employed at the edge of an all-optical network.
Summary (2 min read)
- The situation is particularly acute in optical packet switched (OPS) networks, where contention resolution resources such as fibre delay lines (FDLs) and wavelength converters are used sparingly due to cost and size limitations.
- The authors previous work in  , and similar studies in  , have shown that the conditioning of traffic at the ingress to the optical network helps contain losses by allowing more effective utilisation of the sparse contention resolution resources.
- With traditional rate-based shapers such as GCRA or leaky-bucket, this delay performance is difficult to characterise, and requires assumptions about the traffic model.
- Smoothing has been studied extensively in the context of video transmission.
- The authors therefore develop an online real-time conditioning algorithm that approximates the off-line optimum and has constant amortised computational complexity per packet arrival.
II. PROBLEM SPECIFICATION
- The deadlines specify the times by which each packet must have been placed on the output link.
- Packets are assumed to enter the conditioner according to an arbitrary arrival process and are released onto the output link by the server.
- Traffic burstiness can be reduced by a rate-based (e.g. leaky-bucket) shaping mechanism, but this results in a traffic-dependent delay which may be unacceptably large.
- Amongst all feasible exit curves, the one which corresponds to the smoothest output traffic flow has been shown  to be the shortest path between the origin and the point (T, A(T )), as shown in figure 2 .
- The problem of determining good service schedules in online applications has been studied in the context of transmitting those video streams in which delays of seconds to minutes are tolerable, for example in some news and sports broadcasts.
III. A PRACTICAL ON-LINE TRAFFIC CONDITIONER
- The off-line optimum identified above is useful in contexts where the workload arrival is known in advance.
- In step 1 of the algorithm, the length of the incoming packet is determined, along with its deadline.
- At this stage the hull is convex and the backward scan can stop, resulting in the new convex hull.
- In spite of a constant amortised cost per packet arrival, a packet arrival in the worst-case may cause all hull points to be scanned (steps      in order to restore convexity.
- This sequence has O(1) complexity per slot.
IV. SIMULATION STUDY
- And the corresponding impact on loss and delay in a simple OPS network.the authors.
- Each link operates at 10 Gbps per wavelength, and optical packets have fixed length of 1250 bytes such that they fit exactly in one slot.
- The figure also plots the burstiness of the traffic released by the conditioner, for smoothing delay bounds d of 1µsec, 10µsec, 100µsec, 1msec, and 10msec.
- Figure 9 plots the loss at the core OPS as a function of core FDL buffer capacity and edge conditioning delay budget.
- Figure 10 plots the burstiness β(s) versus s on log scale for the input traffic, and the slope of −(1 − H) = −0.15 validates the Hurst parameter setting of 0.85 (the different slope at lower time-scales is explained by the discretization of the ideal fluid model required to generate packets).
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Cites methods from "A novel delay-bounded traffic condi..."
...Conditioning Delay-Controlled Traffic [ 33 ] — This technique implements a traffic conditioner in which the packet transmission rate to OPS network may be variable, but the packet delay is bounded....
Cites background from "A novel delay-bounded traffic condi..."
...What is therefore required is a means of smoothing traffic at short-time-scales without adversely impacting on end-to-end delays....
...Typical routers today have sufficient electronic buffers to absorb such short time-scale burstiness, and longer time-scales rate fluctuations are protected against by means of rate-based shaping methods such as leaky-bucket or GCRA....
Cites background from "A novel delay-bounded traffic condi..."
...Thus at time , the arrival curve considered to the right of is a horizontal line (since future arrivals are not known yet), and the shortest-path exit curve degenerates to the convex hull of the deadline curve [ 33 ]....
...from our own [ 33 ], ) on the use of traffic pacing techniques for alleviating contentions in OPS networks with very small buffering resources....
"A novel delay-bounded traffic condi..." refers background in this paper
...Our second scenario feeds each edge node with long range dependent (LRD) traffic which has in recent years been shown to be more representative of real traffic in data networks ....
"A novel delay-bounded traffic condi..." refers methods in this paper
...We generate LRD traffic using Norros’ self-similar traffic model , with our filtering method developed in  that can generate very long sample paths of fractional Gaussian noise....