Flexible Bandwidth Provisioning in WDM Networks by Fractional Lambda Switching Achille Pattavina, Donato Grieco Politecnico di Milano, Milano, Italy e-mail: pattavina@elet.polimi.it Yoram Ofek Synchrodyne Networks, Inc., New York, U.S.A. e-mail: ofek@synchrodyne.com Abstract A new approach is introduced in this paper to make possible a flexible utilization of WDM networks using current technology. It is shown that the bandwidth made available end-toend by a single wavelength can be simply broken up into smaller fractions by relying on worldwide free synchronization systems, such as GPS, previously deployed for different applications. It is shown how this new approach, called Time Driven Switching, behaves in terms of call blocking when the basic parameters of the scheme are varied. I. INTRODUCTION Modern telecommunication networks use light to transport information; in WDM systems multiple colours travel on optical fibers increasing the total bandwidth. All optical networks are being studied to avoid conversion from optical to electric signal every time a switch is crossed, in order to forward it towards the proper destination. Present optical switches however can only divert entire wavelengths: all the data on a colour must go from the same source to the same destination (see for example [] and []). This leads to some constraints in the network itself, that is: a source needs a different colour for each destination; if the network has N access points and they have to be all connected to each other, the number of wavelengths needed can grow up to N (N problem, []). no aggregation/separation of multiple flows on/from a single wavelength can be operated: the wavelength travels unchanged switch by switch. Fast sub-networks cannot be connected to slow ones (unless using multiple colours) as the capacity of a single wavelength cannot fit into that of the crossed link. All these problems limit the extension of an all-optical network core, because of wavelengths growth and bandwidth mismatch between sub-networks. Therefore, it is advisable to provide directly the availability of fractions of the wavelength capacity, so as to support sub-lambda end-to-end connections [4]. Time Driven Switching provides a solution to make available a capacity equal to a fraction of that transported by a wavelength in order to permit the above mentioned operations; for this reason, a switch operating according to Time Driven Switching will be called the Fractional Lambda Switch [5]. Today there is a broad agreement that in the future the backbone network will consist of at the edges while the interior of the network will consist of an optical core. However, with wavelength switching () the optical core Fig.. (Multiple Optical Cores) Fractional Lambda Switching (Single Optical Core) with many optical core islands (a), one optical core (b). will consist of multiple islands, as shown in fig. a. The main reason for having multiple islands is due to the need of N wavelengths, N being the number of access points to the optical core. To further illustrate this problem assume that (i) the optical core has 5 access points, (ii) the average transmission distance for each wavelength is km, and (iii) each optical fiber carries DWDM channels and weighs. kg/km (a 5 km spool of Corning fiber weighs about.5 kg), then the optical core would require 5 5./ = kg or million tons of optical fiber - almost twice the weight of the great Khufu pyramid in Giza. The problems arising with are solved by TDS, which dynamically allocates fractions of an optical channel or a lambda over predefined routes in the network, thereby solving the N problem. Each lambda fraction is equivalent to a leased line in circuit switching. Consequently, it is possible to realize an optical network with a single optical core (rather than with multiple separate islands), as shown in fig. b, and thereby to extend the optical core all the way to the edges of the network. The principles of Time Driven Switching and allocation procedures are described in section II and III, respectively. Some elements about switching fabric implementation are given in section IV, whereas section V provides traffic performance results for a single switching fabric. (a) (b) GLOBECOM - 75 - -78-7974-8//$7. IEEE
TC second sw itch TF link b: TF 99 99 99 UTC contentofthe TF Beginning of a UTC second Beginning of a UTC second link a: TF data on this SVP enterb in TF 4 and exitin TF 9 Fig.. Time division in Time Driven Switching. Time (TF) TABLE I TYPICAL CAPACITY IN BYTES OF A TIME FRAME. TF 4 5 6 7 8 9 4 5 6 Delay Link bandwidth TF duration.5 µs 5 µs 5 µs 4 Gbit/s 65 65 5 Gbit/s 565 565 65 Gbit/s 56 565 65 55 Mbit/s 4 4 968 45 Mbit/s 7 7 8 II. TIME DRIVEN SWITCHING Time Driven Switching uses time division multiplexing and framed structures in order to give flexibility to optical networks. Time is actually divided into Time Frames (TFs) with equal duration (dt ), grouped in Time Cycle (TCs), all containing the same number (T ) of TFs (see fig. ). According to the extent of these temporal intervals and to the link bandwidth, the amount of data which can be sent during a time frame can be simply calculated. Table I gives some examples with typical values. On setting up a connection for a new couple sourcedestination, a free time frame is searched in the cycles associated to each link between them (time frames may have been reserved by other connections; the pattern of reserved time frames is the same in every cycle). If found, they are entirely reserved for all the connections between that couple. Up to C connections can be set up in the same time frame; this quantity can be easily found dividing the link bandwidth with the connection bit-rate and the number of time frames per cycle. As an example, more than 6 video-on-demand (VoD) streams (at.5 Mbit/s) can be transmitted on a Gbit/s link: in fact, by dividing a cycle into time frames, each of them could contain 6 VoD connections. If the number of simultaneous connections exceeds C, a new sequence of time frame is searched. Time frames are released if all their connections are cleared. The connection is lost if free time frames cannot be found, otherwise the sequence of time frames on the links will form a Synchronous Virtual Pipe (SVP): each packet put by the source on the SVP will pass from time frame to time frame until the destination is reached. Every crossed switch will be set to route the content of the arriving time frame to the proper one in the outgoing link. The whole network must be synchronized to recognize the time division, using a worldwide global system providing the absolute time reference with given accuracy [6][7][8][9]. A Fig.. Full Forwarding. typical example is given by the GPS system, which globally provides the standard time with an accuracy up to µs [6]. In this way switches will forward all packets contained in a time frame in the right direction, without needing to read any header. Dividing one wavelength into independent temporal fractions solves the problems mentioned about optical networks []: each fraction of the same wavelength can be used for a different destination; time frames can come from different up-links and go to different down-links (multi/demultiplexing); fast links can use short time frames and slow links longer ones, so that the amount of data transmitted is the same and the content of a time frame can be exchanged between them. The sequence of time frames on each link from source to destination can be chosen according to two modes: Immediate Forwarding: data arriving at a switch during a time frame must exit during the next one; Full Forwarding: data arriving at a switch during a time frame can leave the switch in any time frame, that is during a time frame between the next one and the corresponding one in the next cycle (fig. ). Immediate Forwarding gives more restrictions on finding free time frames and hence causes larger losses. Nevertheless, it grants minimum delay between transmission and reception, as packets don t have to wait in the switches until the proper time frame. For this same reason it minimizes buffer capacity within the switches. Full Forwarding has the complementary advantages: minimum loss and larger delay. Switches must also store packets for a longer time (larger buffers). An intermediate forwarding technique (called D-Frame Forwarding) can be used: in this case a time frame can be forwarded up to D time frames later. Its performance ranges from those of Immediate Forwarding, that is -F, to those of Full Forwarding, that is D-F: the same reasoning applies also to buffer requirements and delay. We assume here that switches do not support any wavelength conversion capability; hence wavelength continuity must be guaranteed end-to-end. GLOBECOM - 76 - -78-7974-8//$7. IEEE
III. ALLOCATION PROCEDURES Every time a new Synchronous Virtual Pipe has to be set up between two nodes, a free time frame has to be found for each crossed link. With Immediate Forwarding this sequence is made of consecutive intervals. Note that data entering a link during time frame i will exit in time frame i+t, where t dt is the amount of time taken by the signal to cross the link itself. To make the system feasible the length of each link must be such that it contains an integer multiple of time frames (an error of m would cause a time uncertainty of µs). Allocation procedures can be operated both by a central unit and by a distributed algorithm. We will discuss the latter: a central controller would be best where loops or alternative paths are present. We will discuss Immediate Forwarding and Full Forwarding with a single wavelength. These methods use a vector with T elements (one for each time frame), passed and processed from switch to switch. The final state of this vector will show the most suitable time frames to be reserved. A. Immediate Forwarding Each node n computes a binary vector (V n ) whose elements are if the corresponding time frame on the output link is reserved, otherwise. When a node receives a vector from the previous switch, it is rotated towards increasing times by an amount of TF given by the length of the incoming link plus TF to take into account the minimum delay in the switch. Then: ) the source node creates ( receives ) the vector (X = V ) containing the allocation pattern at its inlet; ) each node n, upon receiving a vector X n, rotates it, constructs X n = X n AND V n and forwards it if it contains one or more ones (the value means shows that all its corresponding previous time frames are free); ) the last node chooses a free time frame in the vector (marked as ) and sends the choice back to the source node in rider to reserve the whole sequence corresponding to that time frame; 4) the sequence is reserved if, in the meantime, no other Synchronous Virtual Pipe has made an analogous request for any of these time frames. B. Full Forwarding The case of Full Forwarding is a little more complex: this method will find the sequence of free time frames minimizing the total delay. It differs from Immediate Forwarding in that each element X n (i) ( i T ) of the vector X n sent from node to node will be a number indicating the minimum accumulated delay in the best sequence terminating with the corresponding time frame. Upon initialization of X (X (i) = for i =,...,T ), the vector X n is computed from V n as follows: { reserved if Vn (i) = X n (i)= min (X n (j)+(i j)modt) otherwise j:x n (j) N λ, Fig. 4. 4 Node N ode N ode N ode 4 Reserved TF A location pattern rotated to the initialtc Accum ulated delay Free TF Furtherdealy BestSVP tilthistf BestTF (bestsvp) An example of trellis for Full Forwarding W λ, λ, W λ, λ, W λ, Fig. 5. Switching matrix Structure of a Fractional Lambda Switch. W W W The last node will choose the time frame whose element has the least value, and the path leading to that time frame will be reserved (if still available). This procedure can be more clearly illustrated by a trellis structure in which a path minimizing delay is constructed stage by stage: each time frame is connected with the time frame that, in the previous switch, has accumulated less delay. In the last step only few paths will be available, and the best one is chosen. Figure 4 shows an example of best path choice using the trellis structure. The algorithm is repeated for each colour, and the choice is made among them (random with Immediate Forwarding, minimizing delay with Full Forwarding). IV. SWITCH FABRIC Now we briefly discuss about possible implementations of a TDS switch. Basically it must be able to receive/send signals from/to WDM links, performing every possible combination between inlets and outlets, and it must store incoming data until the proper time frame. The basic structure of a TDS switch is depicted in Figure 5; buffers are used to enable the proper forwarding of time frames. In principle the switching core can be realized using either electrical or optical technology. Every time frame, each inlet can be connected to a different outlet, and this configuration is held for the whole interval; for this reason the switch fabric must have, at least, a rearrangeable non-blocking interconnection network (unless trading a simpler structure with internal loss). As an example, a Beneš GLOBECOM - 77 - -78-7974-8//$7. IEEE
topology can be used. In an electronic switching fabric wavelengths have to be converted into electrical signals to be sent to electrical interfaces. Data buffering with an arbitrary delay can be easily realized using the available technology. In an all-optical network, the optical buffers needed can be made using delay lines to keep packets inside the switch till the proper time frame. Note that a delay of µs is obtained with a m ordinary fiber: to avoid using extremely long lines, the maximum delay has to be quite short; so, short time frames and Immediate Forwarding are preferable (D-Frame Forwarding can be used to trade loss with complexity). V. SWITCH PERFORMANCE The most important aspect of Time Driven Switching is how time is considered: the time frame is the protocol basic tile, more than packets or connections. That is, the behavior of the switch can be seen as the forwarding of entire time frames rather than single packets. So, the number of time frames per cycle is what mostly influences its performance. Various parameters determine this quantity, first of all the number of simultaneous connections for a single input of the switch: on setting up a new path through the switch, a couple of time frames is searched in both sides of the connection, so it is useful to grant a time frame for each possible Synchronous Virtual Pipe passing through the same port, or for most of them. We have made the assumption that up to V different Synchronous Virtual Pipes are possible between the same inlet and the same outlet of the switch. Every inlet can be connected to all outlets, and so the total number of time frames on all the wavelengths in the fiber (T W ) must be larger than N V. We analyze now the behavior of a single-wavelength switch (W =), and assume the traffic pattern to be a Poisson process at each inlet, with randomly chosen destinations. All graphs in this section show the loss probability as a function of the offered load normalized to total capacity of an inlet (T C). Table I gives the amount of data which can be sent during a time frame: according to link bandwidth, packet size and number of packets in a single connection, the number of connections per time frame (C) can be computed. The value C should be large enough to store all connections in a Synchronous Virtual Pipe: in this way only a time frame has to be reserved, and allocation procedures are called only once (unless all calls on the Synchronous Virtual Pipe are cleared). Providing a greater time frame helps maintaining its occupancy around the average offered load: less connections would exceed C and seldom new time frames would have to be reserved. This is clearly shown in Figure 6, which depicts the probability of call blocking P bl with Full Forwarding operations in the case of two time frames per Synchronous Virtual Pipe. For larger values of C the event that an idle time frame is not found occurs with a smaller probability. In the case of Immediate Forwarding it is important to distinguish between the two causes of call blocking. A requested call is not set-up either because there is no bandwidth available to serve it on the addressed output (event of call Fig. 6. Fig. 7. Influence of TF capacity with Full Forwarding. Rejection probability with Immediate Forwarding. refusal) or because the constraint of immediate forwarding does not allow the call set-up in spite of the availability of idle time frames (event of call rejection). In the former case we refer to refused calls, in the latter case to rejected calls.the probability of call rejection (P rej ) is shown in Figure 7: if we compare these results to those accounting for both causes of call blocking (see (Figure 8), it is clear that call refusal is the main cause of blocking. The figure shows that with Immediate Forwarding the difficulty in finding proper time frames makes loss probability almost independent of C. To reduce blocking it is then useful to provide more than one time frame per Synchronous Virtual Pipe: in this way blocking would occur only reserving the last time frames among them. This may be achieved, preserving cycle duration, decreasing C (see again Table I). Figure 9 illustrates this situation with Immediate Forwarding when the quantity T C (i.e. the capacity of a time cycle) is kept constant: larger T values provide better performance, despite of a smaller capacity C. Note that there are some intermediate values that are almost equivalent, when the advantages provided by larger T values are compensated by the smaller value of the capacity. A similar behaviour characterizes Full Forwarding too. The importance of the number of time frames for each GLOBECOM - 78 - -78-7974-8//$7. IEEE
Fig. 8. Influence of TF capacity with Immediate Forwarding. Fig.. Blocking performance for different N and T/NV values. Fig. 9. Performance of Immediate Forwarding for different TF capacities. Fig.. Blocking performance for different forwarding techniques. Synchronous Virtual Pipe is expressed by Figures and : in these graphs the performances of switches with the same ratio T/NV are very similar, despite of number of ports N (Figure ), or different numbers V of Synchronous Virtual Pipes in the same ports pair (Figure ). Moreover, as shown in Figure, forwarding doesn t affect loss for small values of the ratio T/NV. Only when the number of time frames per Synchronous Virtual Pipe is changed the traffic performance vary: larger values reduce the blocking events. VI. CONCLUSIONS Time Drive Switching has been shown to be an effective solution to the problem of providing end-to-end connections with arbitrary capacity in an optical network. Its simplicity relies on the utilization of worldwide synchronization systems such as GPS. Interworking among optical networks based on different transmission technologies is made easier. Traffic performance with immediate forwarding and full forwarding of time frame contents has been evaluated. Further study is needed to evaluate Time Driven Switching in a network-wide environment. REFERENCES [] Optical Networks Magazine, Special Section on Wavelength Routed Networks: Architectures, Protocols, and Experiments, vol., no., Jan./Feb.. [] Optical Networks Magazine, Special Section on Routing and Control in Optical Networks, vol., no. 4, Jul./Aug.. [] B. S. Arnaud, Current Optical Network Designes May Be Flawed, Optical Networks Magazine, vol., no., pp. 8, Mar.. [4] C. Assi, A. Shami, M. A. Ali, Y. Ye, and S. Dixit, Integrated Routing Algorithms for Provisioning Sub-Wavelength Connections in IP-over- WDM Networks, Photonic Network Communications, vol. 4, no. /4, pp. 77 9,. [5] M. Baldi and Y. Ofek, Fractional Lambda Switching, in IEEE ICC, Optical Networking Symposium, Apr.. [6] Global Positioning System data archive. National Institute of Standards and Technology (NIST). [Online]. Available: http://www.boulder.nist.gov/timefreq/service/gpstrace.htm [7] Global Navigation Satellite System GLONASS. Russian Federation Ministry of Defense - Coordination Scientific Information Center. [Online]. Available: http://www.rssi.ru/sfcsic/english.html [8] Transport-satellite navigation. European Union. [Online]. Available: http://europa.eu.int/scadplus/leg/en/lvb/l45.htm [9] Two-way Satellite Time and Frequency Transfer (TWTFT). National Physical Laboratory. [Online]. Available: http://www.npl.co.uk/npl/ctm/twstft.html [] M. Baldi and Y. Ofek, Realizing Dynamic Optical Networking, Optical Network Magazine, vol. 4, no. 5,. GLOBECOM - 79 - -78-7974-8//$7. IEEE