General Parameter Settings

The network is simulated using the event-driven simulator OMNeT++. Table 5.1 tabulates some important network parameters utilised in simulations. In the model, the number of ToR switches in a cluster, M , is set to be 16 and the number of clusters, N , which is also the OPS switch size, is fixed to be 128, thereby the resultant datacentre network provides connectivity to 2048 ToR switches via 10 or 40 Gbps links. Each ToR switch is modelled as a traffic source/sink, which generates and sends traffic flows to other ToR switches. Note that the traffic generation process follows an Engset traffic model where the packet transmission times are exponentially distributed and the idle times between packet transmission periods are also exponentially distributed. The number of OPSs in the optical network is represented by P , and each input/output fibre of the OPS carries K transmission channels. This implies that each input port of the OPS is equipped with K TWCs, and each output port has K FWCs. The number of FDLs is denoted L, and each FDL has S TWCs. The normalised base delay of FDLs, which is computed as the ratio between the FDL base delay and the mean packet transmission time, is taken as 0.1, based on the FDL base delay analysis in Chapter 2. The maximum number of allowable FDL circulations is denoted by R. In packet retransmission, the maximum number of allowable packet retransmissions is defined by B. If B = 0, no packet retransmission is allowed in the network, whereas if B = ∞, the number of allowable packet retransmissions is unlimited.

The amount of (normalised) traffic transferred from a rack (ToR) to all other racks (ToRs), which is referred to as inter-rack traffic, is represented by a. Particularly, if a = 0, there is no inter-rack traffic, whereas when a = 1, all traffic generated by a ToR switch is destined for other ToR switches (either in the same local cluster or other remote cluster). According to previous datacenter traffic studies [4], a is typically observed to be around 0.2 (20%), but in this section the performance analysis is performed for inter-rack traffic loads far beyond 0.2, to test how the optical network will scale under much heavier load. Given a, the total traffic load generated inside a cluster is a × M , which can be classified into two types of traffic: intra-cluster traffic and inter-cluster traffic. The intra-cluster traffic is exchanged between the ToR switches within the same cluster, thereby it is handled by the cluster electronic switches without travelling through the core optical network.

Table 5.1: Table of Notation M Number of ToR switches in a cluster.

N OPS input/output port count. It also defines the number of clusters in the network P Number of Optical Packet Switches (OPSs) in the network

K Number of transmission channels per incoming/outgoing fibre of the OPS L Number of Fibre Delay Lines (FDLs)

S Number of channels (TWCs) per FDL

R Maximum number of allowable FDL circulations B Maximum number of allowable packet retransmissions

a Inter-rack traffic load, the (normalised) load sent out from a ToR, that is destined for other ToRs (in the same or in different clusters)

β Average proportion of the traffic load (a) generated by each of the 2048 ToR switches that needs to traverse through the optical network. In the uniform traffic pattern, β≈1, whereas in the generated non-uniform traffic pattern, β=0.835

ρ Optical network load, the proportion of load offered to the optical network, relative to its full load capacity

Conversely, the inter-cluster traffic, which represents the data traffic exchanged between two ToR switches residing in different clusters, needs to travel through the core optical network. The ratio between the inter-cluster traffic load and the total cluster traffic load (a×M ) is expressed as β. This ratio defines the average proportion of the traffic load (a) generated by each of the 2048 ToR switches that needs to traverse the proposed optical network. Note that β is determined by the particular traffic matrix used. In the case of a uniform traffic pattern where the traffic load from a ToR switch is uniformly distributed to all ToR switches, β is computed as, β =M N −M_{M N −1}≈1, that is, almost all traffic load generated by each of the ToR switches needs to traverse the optical network. In the case of our chosen hot-spot traffic matrix, β is still maintained at a high value, which is calculated as 0.835.

Based on β, the total traffic load sent from a cluster to the optical network is computed as β×a×M . Since the proposed network is composed of N (= 128) clusters, the total requested optical network capacity is β×a×M ×N . As the total optical network capacity is P ×K×N , the offered network load on the core optical network, denoted by ρ, is defined as ρ =β×a×M ×N_{P ×K×N} =βaM_{P K}, which is the ratio of the requested optical network capacity to the total optical network capacity. Thus, ρ is equivalent to the average traffic load on each transmission link of the OPS. We remark that as each input/output port of the OPS carries K transmission channels, the offered traffic load per switch port is ρK. Note that _{P K}M determines the over-subscription ratio of the core optical network. For the given dimensions of M , P and K, the network is oversubscribed by the ratio 4 : 3 in the uniform traffic pattern (β = 1). For non-uniform traffic (given the lower value of β) the over-subscription is

approximately 4β : 3. In either case, the optical network understudy is oversubscribed, so that by scaling a, its performance can be fully stress-tested in the simulation studies that follow.