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34 ADVANCES IN ELECTRONICS AND TELECOMMUNICATIONS, VOL. 1, NO. 2, NOVEMBER 2010 Fig. 4. Results chart for 20% traffic switched every 20 seconds. Fig. 5. Results chart for 20% traffic switched every 40 seconds. (different delays on different paths). Further considerations gave several interesting answers on the meaning of dynamic routing mechanisms. The proposed simulation model made it possible to answer some questions and to shed light on the scope of other problems. Using some proportions between classes in differentiated services domain packets reordering caused by path switching should be marked in end-to-end balance. It may not be skipped and omitted in the system analysis. The AF switched sequence changed order packets to all AF sendpacketsratiomaynotbeexplainedbyapplyingthe known analytical equations (for the non-preemptive priority system). The ratio value is significant for flexible services and should be taken into consideration. Furthermore, an important conclusion for EF traffic was found. The streaming services have lower switched sequence changed order than all EF sent packets ratio when EF share in the overall traffic amount is 20–40 %. Some additional remarks were also found for different time values between routing table recalculations. It turned out that the optimal time between routing table updates (in short term changes – seconds) was 35–40 seconds interval. This statement is based on simulation results but will not be discussed in this paper due to space limitation. For routing table switching time a local minimum of the 35–40 seconds was observed. For all analyzed situations residual time is important when packet length differs between given traffic classes (EF – 160 bytes, AF – 500 bytes, BE – 1,500 bytes). Further investigationswill be aimed at findingthe relationsfor AF traffic and explaining the issue using the newly developed analytical equations. REFERENCES [1] S. Chen and K. Nahrstedt, “An Overview – of – Service Routing for the Next Generation High – Speed Networks: Problems and Solutions,” IEEE Network Magazine, vol. 12, no. 6, pp. 64–79, Dec. 1998. [2] G. Feng, K. Makki, N. Pissinou, and C. Douligeris, “Heuristic and Exact Algorithms for QoS Routing with Multiple Constraints,” IEICE Trans. Commun., no. 12, pp. 2838–2850, Dec. 2002. [3] J. T. Moy, OSPF Anatomy of an Internet Routing Protocol, 2001. [4] ——, OSPF Complete Implementation, 2001. [5] J. N. Daigle, Queuing Theory with Applications to Packet Telecommunication, 2005. [6] [online], http://www.omnetpp.org. M. Czarkowski received the M.Sc. degree in telecommunication systems from Gdansk University of Technology (GUT), Gdansk, Poland, in July 2004. He is currently pursuing for the Ph.D. degree in Telecommunication Networks and Systems, GUT. His Ph.D. work focuses mainly on dynamic routing algorithms with Quality of Service (QoS.) S. Kaczmarek received the M.Sc./B.Sc. in electronics engineering, Ph.D and D.Sc in switching and teletraffic science from Gdansk University of Technology, Gdansk, Poland, in 1972, 1981 and 1994, respectively. His research interests include: IP QoS and GMPLS networks, switching, routing, teletraffic and quality of service. He has published more than 190 papers. Now he is the Head of Teleinformation Networks Department.
ADVANCES IN ELECTRONICS AND TELECOMMUNICATIONS, VOL. 1, NO. 2, NOVEMBER 2010 35 Packet dispatching schemes supporting uniform and nonuniform traffic distribution patterns in MSM Clos-network switches Abstract—In this paper new packet dispatching schemes for efficient support of the uniform as well as the nonuniform traffic distribution patterns in Memory-Space-Memory (MSM) Closnetworkswitchesare presented.Threesuchschemes, calledStatic Dispatching-First Choice (SD-FC), Static Dispatching-Optimal Choice (SD-OC) and Input Module (IM)-Output Module (OM) Matching (IOM), are proposed and evaluated. The algorithms are able to unload the overloaded input buffers employing a central arbiter. This effect is a desirable feature especially for effective support of the nonuniform traffic distribution patterns. We show via simulation that the proposed schemes deliver very good performance in terms of throughput, cell delay, and input buffers size under different traffic distribution patterns. The results obtained for the proposed algorithms are compared with the results obtained for selected request-grant-accept iterative packet dispatching schemes. Index Terms—Clos-network, packet scheduling, packet switching, virtual output queuing. Janusz Kleban I. INTRODUCTION THE switching fabric in high-performance packet switching nodes may be built as a single stage-switch (e.g. crossbar) or a multi-stage switch, such as the Clos switching fabric. The switching process in a multi-stage switching fabric consists of two activities, namely input-output matching and route assignment between the first and last stages. These two phases can be processed separately or simultaneously. Since the high-speed switching fabrics support fixed-length packets called cells, packets of variable size must be segmented into cells at switch input ports, and cells must be reassembled into packets at switch output ports [1]. While cells are being routed in a switching fabric, it is very likely that more than one cell is destined for the same output port or for a physical link inside the multi-stage switching fabric. Cells that have lost contention must be either discarded or buffered. Buffers may be placed at inputs, outputs, inputs and outputs,and/or within the switching fabric [2]. The virtual output queuing (VOQ) is widely implemented as a good solution for input queued (IQ) switches, to avoid the Head- Of-Line (HOL) blocking problem encountered in the inputbuffered switches. In VOQ switches every input provides a single and separate FIFO for each output. Such a FIFO is called a Virtual Output Queue. When a new cell arrives at the input port, it is stored in the destined queue and waits for transmission through a switching fabric. To solve internal blockingand output port contentionproblemsin VOQ switches, fast arbitration schemes are needed. The arbitration scheme decides which items of information should be passed from inputs to arbiters, and – based on that decision – how each arbiter picks one cell from among all input cells destined for the output. Algorithmswhich can assign the route between inputandoutputmodulesare usuallycalledpacketdispatching schemes. Considerable work has been done on scheduling algorithms for VOQ switches. Most of them achieve 100% throughputunder uniform traffic, but the throughputis usually reduced under nonuniform traffic [1], [3]–[14]. A switch can achieve 100% throughputunder uniform or nonuniformtraffic if the switch is stable, as was defined in [15]. In general, a switch is stable for a particular arrival process if the expected length of the input queues does not grow without limits. Multiple-stage Clos-network switches are a potential solution to overcome the limited scalability of single-stage switches, in terms of the number of I/O chip pins and the number of switching elements. Different dispatching schemes forthethree-stageClos-networkswitcheswereproposedinthe literature [4]–[6], [9]–[14]. The basic idea of these algorithms is to use the effect of desynchronizationof arbitration pointers and a common request-grant-accept handshaking scheme. All high speed switching fabrics implemented by the manufacturers of switches/routersare now based on SERDES technology. The signals passing through these serial links are within the range of several hundred nanoseconds. It is very difficult to implement the algorithms with multiple-phase iterations in a three-stage environment with currently available technologies, because of time constraints (one slot time in a 10 Gbps switching fabric lasts around 50 ns). In this paper SD-FC, SD-OC, and IOM packet dispatching schemes are presented. These algorithms give better performance results than other dispatching schemes proposed for the MSM Clos switching fabric, and can achieve 100% throughput for both the uniform and the nonuniform traffic distribution patterns. The remainder of this paper is organized as follows. Section II introduces some background knowledge concerning the MSM Clos switching fabric that we refer to throughoutthis paper. Section III presentsthe SD-FC, SD-OC, and IOM packet dispatching schemes. Section IV is devoted to performance evaluation of the proposed algorithms. The comparisonof cell delay betweenproposedalgorithmsand the selected multiple-phaseiterativepacketdispatchingschemesis also shown. We conclude this paper in Section V.
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