OpMiGua – Comparing two Transport Network Architectures

In this paper we compare with Optical Burst Switching (OBS) and Optical Migration Capable Networks with Service Guarantees (OpMiGua) two all-optical network architectures. While delay is analyzed qualitatively, a quantitative performance evaluation based on simulations shows differences regarding loss probabilities. In order to achieve a maximum of comparability both models are fed with identical traffic on the packet level. Results show that OpMiGua has a better performance. *This work is based on results of a Short Term Scientific Mission funded within COST Action 291 “Towards Digital Optical Networks”. 8. ITG-Fachtagung Photonic Networks, May 7. 8. , 2007, Leipzig, Germany Fig. 1 OBS node architecture Fig. 2 OpMiGua node architecture 2 Architectures for dynamic all-optical networks 2.1 Optical Burst Switching In the commonly considered approach of Optical Burst Switching (OBS) [1], packets from client networks are assembled at the edge of the network into so called bursts. Hereby, each burst only consists of packets for the same forwarding equivalence class. These bursts are transmitted through the network towards their destination. At the egress, bursts are disassembled and packets are forwarded to the client network. To allow sophisticated processing of the burst’s control information while keeping the complexity low, control information is separated and processed electronically while data remains in the optical domain. Thus, a control header packet precedes with a certain offset time the actual burst. The content of the control header packet, which is amongst others burst length and offset time, allows to select a path through a node and to reserve the required resources for the necessary time period. Hereby, all node resources like wavelength converters or fiber delay lines (FDLs) must be considered. This one-pass reservation does not guarantee the successful delivery of a burst. Contention may occur and as final consequence bursts have to be discarded, if all available contention resolution mechanisms fail. Fig. 1 shows the architecture of a basic OBS node. It consists of an optical switch fabric which switches bursts to the desired output fiber based on the control header packet information. Further, wavelength converters are placed at each output of the switch fabric to convert the burst to the appropriate wavelength for transmission to the next node. However full wavelength conversion, i.e., a converter on each output wavelength, is not mandatory for OBS as also a shared pool with a limited number of wavelength converters leads to substantial performance improvements in comparison to the total absence of converters [5]. Though further decreases of the blocking probability by contention resolution mechanisms like buffering via fiber delay lines are possible in OBS [6], they are not considered in the following. As already stated resources are reserved for a burst at the instance of arrival of the control header packet. This offset time can be efficiently used to introduce different priorities. The greater the offset time, the greater is the likelihood to find the necessary resources available. If the offset time is larger than the maximum burst duration plus offset time of bursts with lower priority even an absolute prioritization is achieved. Without this criterion in place, absolute prioritization can only be achieved via use of preemptive techniques [7]. In summary OBS is a switching paradigm supporting highly dynamic traffic in future networks. By switching on a burst level in the optical data plane it provides on the one hand a much greater flexibility than a network based on circuit switching. As information about bursts is processed in the electrical domain, OBS avoids on the other hand severe technological challenges of an optical packet switched network, as for example, optical signal processing and optical switching on a tiny time scale. 2.2 Optical Migration Capable Networks with Service Guarantees Optical Migration Capable Networks with Service Guarantees (OpMiGua [4]) have an inherent separation of different traffic classes. The so called Guaranteed Service class Traffic (GST) has high requirements 8. ITG-Fachtagung Photonic Networks, May 7. 8. , 2007, Leipzig, Germany concerning packet loss and jitter. Therefore, traffic of this class is transported in a connection oriented manner along preestablished end-to-end lightpaths and is given absolute priority. This ensures that there are no losses due to blocking and delay jitter is minimized. The other class is Statistically Multiplexed (SM) traffic, which has looser requirements and can be handled without reservations. For this class losses due to blocking and delay jitter due to buffering or deflection routing are allowed. Despite this inherent separation both traffic classes can share the capacity of the same wavelength. In Fig. 2 the architecture of a basic OpMiGua node is shown. On each wavelength, SM and GST packets enter multiplexed the node. The traffic classes are separated by a packet separator according to a specific labela. GST packets are forwarded to a circuit switch whereas SM packets are directed to a packet switch. After traversing the circuit and packet switch, SM and GST packets for the same direction are multiplexed on each wavelength. In this node architecture GST packets are circuit switched after separation. So, a wavelength can be used only by GST packets with the same source and destination node. Statistical multiplexing of GST packets from different sources is unfeasible. The treatment of SM packets is completely different. After separation they are forwarded to a packet switch. This can be realized all-optically or electronically with O/E/O conversion. Packets are switched towards their destination and scheduled onto a currently available wavelength on the correspondent output fiber. Hereby, the occupancy of the wavelengths by GST packets must be considered. As absolute priority is given to GST traffic, two options arise: first, preemption of SM packets is necessary in case of GST arrivals. Second, scheduling of SM packets has to be already aware of future GST arrivals within a sufficiently large time window. This is commonly realized by monitoring the occupancy of each wavelength before the traffic is delayed in a FDL as indicated in Fig. 2. Combinations of those schemes are also possible, i.e., the time window of known future GST arrivals is not as big as that in the case of total collision avoidance [8]. In consequence of this resource utilization, successful transmission of SM packets is only possible during gaps between GST packets. Blocking may occur on an output wavelength either if the output port is occupied by packets of any service class, or if the control unit a As distinguishing label it is one possibility to use the polarization of the light. However the actual realization is not of importance for the considerations in this paper and not considered any further. computes that a collision will occur between the SM packet to be scheduled and an incoming GST packet. As explained in the previous paragraph, every arrival of a GST packet/burst may cause an SM packet to be lost. Hence, reducing the number of arrivals should decrease the blocking probability of SM packets. Indeed, this is observed in [9] and aggregation of GST packets into bursts is used here too. However the aggregation time may not be arbitrarily long as the GST class is mainly considered for traffic with stringent timing requirements. In contrast, there is no need to aggregate SM packets. In the following we assume that the occupation of the wavelength by GST packets is known in advance for the maximum duration of a SM packet transmission. By this it is possible to achieve minimum losses of SM packets. Furthermore, we assume the packet switch to be all-optical with full wavelength conversion but without any buffering. Also, we assume that the GST class is used for high priority (HP) and the SM class for low priority (LP) traffic. 3 Qualitative comparison of OBS and OpMiGua Comparing the two architectures, two main differences can be seen, that have an impact on the system performance. First, while in OBS all traffic is aggregated into bursts at the network ingress, in OpMiGua only the HP traffic is aggregated. Second, while in OBS all traffic shares all wavelengths, in OpMiGua each HP packet is transported on an end-to-end wavelength and only LP traffic can use all wavelengths – in the ingress as well as each core node. In the following, we discuss the impact of these two differences on delay and capacity requirements while the impact on loss will be discussed in section 4. 3.1 Delay and delay jitter In OpMiGua high priority packets are aggregated based on limitation of time-out and size in order to defragment the free slots for low priority traffic. Therefore, those packets experience a non-constant aggregation delay. This is limited by the assembly time-out, i.e., the maximum time between the arrival of the first packet for a burst and the departure of the burst. Furthermore, high priority bursts destined for the same node have to be transmitted consecutively if there is only one appropriate high priority wavelength. That means if a HP bursts is presently transmitted and further aggregation units finish bursts, those have to wait. 8. ITG-Fachtagung Photonic Networks, May 7. 8. , 2007, Leipzig, Germany In the core nodes, for certain schemes of multiplexing HP and LP traffic after switching it is necessary to know in advance if a certain wavelength will be free within the time necessary for transmission of the LP packet. This is realized by delaying in each node high priority bursts after detection, e.g., by a fiber delay line, leading to an additional constant delay. Low priority traffic is not aggregated in OpMiGua. Thus, it is only marginally delayed at the network ingress other bursts. In core nodes, the delay depends only on the realizati