Simulation model and network configuration

Approximate analytical techniques have been considered for the evaluation of the First- Fit performance in terms o f the blocking probability [Karl], [Mokl]. However, these techniques assume statistically independent link-blocking events, which can lead to inaccurate results for a wide range of traffic loads [Zhul]. Additionally, an analytical model, accurate for realistic network topologies with paths formed by a high number o f hops, would be further complicated by combining the wavelength assignment policy with the burst scheduling discipline, introduced in section 3.9. Therefore, a discrete- event WR-OBS simulation system was developed in C++ to study WR-OBS QoS performance throughout this thesis. In particular, this tool is capable o f analysing burst aggregation delay as a function of the blocking probability as well as o f investigating the relationship between the maximum request scheduling delay and burst blocking probability for multiple classes of service and in terms o f the wavelength requirement. The reported results were obtained with the degree of confidence equal to 95%. The correctness o f the dynamic routing and wavelength assignment algorithm, incorporated in this system, is evaluated in the appendix B.

The ARPANet physical topology [Rami] (see Figure 3.6) with 20 nodes and 1 = 31 links was used in this chapter to investigate the WR-OBS QoS performance on a realistic network. As mentioned in section 3.1, the demonstrated relationships are then analysed in chapters 4 using a set o f randomly-connected networks, as well as a number of real optical backbones. A single-fibre links were assumed throughout this work.

THE DESIGN OF WA VELENGTH-ROUTED OPTICAL BURST-SWITCHED NETWORKS________ ^

To take into account traffic burstiness, bursts o f each traffic class were aggregated out of packets generated with the ON-OFF Pareto model used in [Hunl], with the PDF function given by

# ) = ^ (3-15)

X

where 1 < a.< 2 , Xq>x are the conditions for traffic to exhibit self-similarity. In the above equation, Xq represents minimum value o f x. The effect o f traffic self-similarity in OBS is currently hotly debated (see [Hul], [Diis4] and references therein). Burst aggregation was recently reported to smooth this effect over time-scales, significantly exceeding average packet length [Düs4]. At the same time, the reduction o f self­ similarity over large time-scales was shown to be held only in case o f pure time- threshold based burst assembly techniques [Hul]. Although the FBAT method is also based on a time-threshold, controlled by the burst aggregation delay ^aggr, the Pareto model was critical to ensuring the accuracy of burst aggregation for the analysis of low traffic loads (< 0.3) and low /aggr values (< 20 ms). This is because the latter result in significant decrease in the average burst sizes (i.e. they lead to bursts, not exceeding 1000 single packets in size). According to the Pareto model, the ON-OFF periods were generated as follows:

UMa ^OFF ( ^ ) “

Xq 1 - p

u

where p is the input traffic load, Don is the packet size in bits, Dqff is the number of void bits in the inter-arrival time, [/is a random variable uniformly distributed between 0 and 1, a =1.5 and^o = 1333 {Xq represents minimum packet size in bits). This way, the average packet size is about 500 bytes [Xiol].

The following should also be noted with regards to the interaction o f WR-OBS with higher-layer network protocols. An example of the latter is the v^dely-adopted Transmission Control Protocol (TCP), which supports a congestion control mechanism to minimise packet loss per connection. TCP implements a feedback system based on acknowledgements, controlling packet transmission rate [Stel]. Therefore, TCP can significantly affect traffic statistics, generated according to Eq. (3.15) [Arvl],

THE DESIGN OF WA VELENGTH-ROUTED OPTICAL BURST-SWITCHED NETWORKS________ ^

especially when the core network introduces additional packet losses and/or latencies. These are inherent to OBS, due to the burst loss in the optical core and burst aggregation delays. In particular, WR-OBS architecture, although supporting loss-free burst end-to-end transmission, assumes significant burst edge-delays (in the tens o f ms range, as will be shown in section 3.7). The latter factor can potentially result in the increased number of time-outs in TCP acknowledgements, leading, in turn, to the increased number of packet re-transmissions (delay penalties) and, consequently, to changes in traffic pattern. On the other hand, as reported in [Lawl], the impact of WR- OBS on the TCP delay penalties can be relaxed, especially for small network diameters (up to 100 km), by properly adapting such TCP parameters as the time-out duration for which the re-transmission timer is set. Further improvement in TCP throughput can be expected by explicitly coupling the OBS and TCP mechanisms, so that a TCP sender can respond to a feedback from the WR-OBS burst aggregation mechanism [Lawl]. The implementation o f such technique could minimise TCP re-transmissions through the optical core due to the delay penalties, thus also minimising the impact o f TCP on traffic pattern, described by Eq. (3.15). This is particularly desirable for WR-OBS networks with long inter-nodal distances (> 1000 km), implying long TCP time-outs. It should be emphasised, however, that the implications o f the above WR-OBS/TCP coupling technique on overall network throughput require further investigation.

Nevertheless, it was assumed throughout this work that a mechanism, effectively alleviating the impact o f TCP time-outs caused by the burst edge delays, on the traffic patter described by Eq. (3.15), was supported by the WR-OBS edge-routers. This assumption enables to consider the WR-OBS performance in a protocol-independent way, which is of special importance to the analysis o f non-TCP types o f traffic, such as VIP/VoIP applications.

In addition, the TCP time-outs under this assumption are mostly determined by the burst loss, and not by the burst delay penalties. In WR-OBS, the former arise from the burst blocking due to wavelength unavailability, and their minimisation can be thought of as one of the objective of this thesis. The preliminary investigation o f the impact of the burst loss in conventional OBS on TCP performance was carried out in [Detl]. Using a single TCP connection with send rates o f up to 200 Mb/s and the burst aggregation time o f 3 ms, it was shown that loss, exceeding 10"^, significantly degrades

THE DESIGN OF WA VELENGTH-ROUTED OPTICAL BURST-SWITCHED NETWORKS________87_

TCP throughput. When the delay penalties in WR-OBS are relaxed, it is possible to conclude that the loss o f bursts, assembled with the input bit-rate o f lOGb/s by multiplexing different TCP connections, would have similar impact on the throughput of each TCP connection. Therefore, it was assumed throughout this work, that the maximum burst blocking, allowed by QoS requirements in WR-OBS, is 10“^.

The assumption o f infinite-size buffers for the burst aggregation was considered throughout this work. In this way, the burst size is only limited by the aggregation delay /aggr(Q), whose optimal values will be analysed in the following section.

The propagation time per link was initially set to 1 ms, resulting in the link size o f 200 km. Then, a constraint on the maximum source-destination distance was imposed to represent a backbone with the maximum shortest-path o f 1000 km. This was achieved by reducing the size of links, forming those shortest paths whose end-to- end propagation time initially exceeded 5 ms, such that the latter decreased to exactly 5 ms. It was also assumed throughout this work that the acknowledgement time, /ack,ctri-s, is 2.5 ms for all the control channels between the edge-routers and the control node, representing the distance o f 500 km between them.

Additionally, uniform distribution o f traffic loads between the edge-routers was assumed. Finally, it was assumed that both the calculation time o f the dynamic routing and wavelength assignment solver and the time o f the re-configuration o f the lasers and cross-connects are negligible, i.e. /caic = 0 and ^tuning = 0 throughout the analysis. This way, it can be demonstrated how the controllable delays, i.e. those imposed by the burst aggregation and scheduling, affect the QoS performance. It should be said, however, that whilst neglecting /caic is valid for most current backbones with the number o f nodes not exceeding a few tens [Zapl], the calculation time raises scalability issues when the number o f nodes further increases (see section 4.3).

THE D E SIG N O F WA VELENGTH-ROUTED OPTICAL BU RST-SW ITC H ED N E T W O R K S

© Edge-router

X Optical cross-connect Control node with request server

Figure 3.6. ARPA Net as centralised W R-O BS. Dashed lines show an exam ple o f the control topology with links assumed to have equal propagation delay.