The simulation results obtained for the packet loss ratio are shown in Figure 5.7 for different values of offered load. Two values are considered for TDC in the fully sub- scribed network. The first two curves in each plot in Figure 5.7(a) and 5.7(b) show packet loss ratio as a function of offered load for OBS using traditional methods of one way reservation scheme at data rates of 10 and 40 Gbps respectively. The third
5.5. RESULTS AND DISCUSSION ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 0.0 0.2 0.4 0.6 0.8 1.0 1e−06 1e−04 1e−02 1e+00 Load P ac
ket Loss Ratio
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Traditional OBS with 10Gbps Traditional OBS with 40Gbps Proposed OBS with 10Gbps Proposed OBS with 40Gbps Electrical Network with 10Gbps Electrical Network with 40Gbps
Medium Diversity Workload
(a) ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 0.0 0.2 0.4 0.6 0.8 1.0 1e−06 1e−04 1e−02 1e+00 Load P ac
ket Loss Ratio
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Traditional OBS with 10Gbps Traditional OBS with 40Gbps Proposed OBS with 10Gbps Proposed OBS with 40Gbps Electrical Network with 10Gbps Electrical Network with 40Gbps
High Diversity Workload
(b)
Figure 5.7. Load Vs Packet Loss Ratio measured in the fully subscribed network for:
(a) TDC= 10 and (b) TDC = 20.
and fourth curves show the equivalent performance of the proposed methods of OBS using a two-way reservation scheme, while the last two curves show the correspond- ing performance of the baseline electrical network. Packet losses are observed in the proposed and the baseline network only at a very high load while the packet losses in traditional OBS occur even at very low load. This is due to burst losses caused by con- tention in a traditional OBS. Similar results may be observed in a 2:1 oversubscribed network, although these are omitted from Figure 5.7 for clarity.
5.5.4
Performance of the Control Plane
In order to assess the performance of the control plane, the algorithm was run on an Intel host with a Core i7, 2.17 GHz processor and 16 GB RAM. The results were
5.5. RESULTS AND DISCUSSION
Table 5.3. Performance of the Control Plane in FOSA
Al g o r i t hm O v e r-s u bs c r i p t i on O p t i cal Sw i t ch(P) Deg ree o f ToR(X) E x ec.T
Routing and scheduling
4 : 1 ∀ P 10 < 0.15 µs 2 : 1 20 < 0.3 µs 1 : 1 40 < 1 µs Switch Configuration 4 : 1 10 10 ≈ 0.029 µs 2 : 1 20 20 ≈ 0.031 µs 1 : 1 40 40 ≈ 0.033 µs
obtained for several combinations of parameters. To ensure statistical significance, the results of 1000000 runs were averaged. The results are shown in Table 5.3.
When a control packet arrives at the controller, the controller performs the routing, scheduling and switch configuration operations described in Algorithm 7. The routing and scheduling operations are described from line 1 to 46 and switch configurations operations are described from lines 47 to 53 in Algorithm 7. The complexity of the routing and scheduling algorithm is O(2X +µ), where X is the degree of ToR switches
andµ represents the sum of processing time of all other instructions. The complexity
of the switch configuration operations is O(P + µ) where P is the total number of op- tical switches. Theµ is assumed to be a constant of negligibly low value and its value is in the range of a few nanoseconds. The execution time is measured in the fully subscribed, 2 : 1 oversubscribed and 4 : 1 oversubscribed networks as shown in Table 5.3. 40 servers per rack are considered. It can be noticed in Table 5.3 that the exe- cution time of routing and scheduling operations is in nanoseconds scale for all types of networks. It is minimum in 4:1 oversubscribed network but it increases slightly as we decrease network oversubsription. Similarly, the execution time of the switch configuration operations is minimum when P is minimum and it increases slightly with the increase of the number of optical switches. The overall execution time of switch configuration operations is negligible (at most a few nanoseconds). We get a total execution time of the control plane processing by adding up the execution times
of routing/scheduling and switch configuration operations which is in nanoseconds
range. So the proposed algorithms in the control plane demonstrate efficient perfor- mance across all types of network oversubscriptions.
5.6. CONCLUSION
5.6
Conclusion
In this chapter, a novel optical interconnect based on fast optical switches called FOSA was studied. The previous hybrid design HOSA with TDS has the limitation of the control plane. The control plane can only support applications that have high traffic stability, i.e. workloads that last several seconds. So for dynamically changing traffic patterns, the new architecture FOSA features fast optical switches in a single hop topology with a centralized, optical control plane. Similar to the HOSA, the single stage core topology can be easily scaled up and scaled out.
The OBS with two-way reservation is considered to get zero burst loss. The two- way reservation is not appropriate for conventional backbone optical networks due to the high RTT of the control packet but in a DCN, this RTT is not high. The network- level simulation is used to model different workloads with various data rates by con- sidering different edge to core network over-subscription and investigate the perfor- mance of such designs across various usage patterns. The results reveal that the pro- posed technique shows considerable improvement in terms of throughput and packet loss ratio as compared to the conventional methods of OBS while comparable perfor- mance in terms of delay with the conventional methods of OBS is also achieved. The proposed technique also demonstrates delay and throughput performance comparable to that of electrical data centre networks.
Apart from the better performance of the FOSA than the OBS using traditional methods and comparative performance with the baseline electrical network, it also has better performance than the HOSA and HOSA with TDS. Unlike HOSA with TDS, the FOSA has efficient control plane performance as well. This is because no additional overhead for maintaining statistics of traffic demand is required in the FOSA. In the next chapter, the performance of TCP in the FOSA is evaluated and its comparison with the traditional OBS and the baseline electrical network is provided.
CHAPTER 6
PERFORMANCE EVALUATION OF TCP
OVER FAST OPTICAL SWITCH
ARCHITECTURE FOR DCN
6.1
Introduction
In this chapter, the performance of TCP over FOSA is evaluated for use in a DCN using network-level simulation. The performance of TCP over OBS networks is degraded because of the wrong interpretation of congestion in the network by its flow control algorithm. The contention induced losses are responded to as if they were conges- tion induced losses. The former are burst losses occurring due to unavailability of a wavelength even at the low network load.
To evaluate the performance of TCP over FOSA, various workloads with different burst assembly parameters are used to compare TCP performance with two-way reser- vation to its performance with conventional methods of one-way reservation and to its performance in a conventional electronic packet switching DCN.