Abstract:
A wavelength to be utilized to transfer a data flow amongst different wavelengths available to an adaptive path switched optical network is determined. A measure of average traffic intensity on a particular wavelength is determined. A breakable connection, a connection that is currently transmitting unprotected data packets on a wavelength for which bandwidth has been allocated and reserved, that has a least amount of average traffic intensity is selected. There may be an optical switch that determines the measure of the average traffic intensity on a particular wavelength and that selects the breakable connection for transmitting the data flow.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the European application No. 04021449.6, filed Sep. 9, 2004 and which is incorporated by reference herein in its entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to wavelength scheduling in APSON networks.  
       SUMMARY OF INVENTION  
       [0003]     APSON (Adaptive Path Switched Optical Networks) networks, similarly to OBS (Optical Burst Switching) networks, makes use of signalling protocols such as JET, Horizon or JIT in order to reserve a certain bandwidth for the transmission of a data flow. Adaptive optical networks consider alternative routes to determine the shortest path depending on the state of the network. The data flow in an Adaptive optical network may be a burst, plus possibly some IP packets.  
         [0004]     However, at the time the reservation request arrives at a switch, there might be several available (free) wavelengths to reserve. Therefore, it remains for each switch to decide to which wavelength an outgoing data flow should actually be assigned. This is known as the wavelength assignment or the wavelength scheduling problem.  
         [0005]     A wavelength scheduling algorithm decides which wavelength should be used to transfer a data flow from the different wavelengths which are temporarily available (i.e., unused) in a switch. Problematically, wavelength scheduling algorithms have a great impact on optical network performance since they can severely influence the blocking probability and, therefore, the throughput of the network.  
         [0006]     Several scheduling algorithms for OBS networks have been described in the literature. The most trivial wavelength scheduling algorithm randomly chooses a wavelength in order to transmit the data flow from the set of available wavelengths. Another very simple method is the first fit (FF) algorithm, which performs a round-robin search of available wavelengths and assigns the first free wavelength found. These two algorithms are rather primitive and may lead to a high blocking probability.  
         [0007]     Other algorithms for OBS networks which lead to a lower blocking probability have been described in the literature such as the latest unscheduled channel (LAUC) algorithm or the latest available unused channel with void filling (LAUC-VF) algorithm. Both of these aim at reducing the gaps between consecutive bursts. Generally speaking, in OBS networks the shorter the gaps between consecutive bursts, the better the scheduling algorithm is.  
         [0008]     Bandwidth gaps are fractions of bandwidth and, therefore, are dead zones, since they cannot be used for the transmission of a burst. Naturally, the less gaps there are and the shorter the gaps are, the more efficient the bandwidth use is. This leads to a lower blocking probability and to a higher throughput. This can be seen from timing diagram  100  of  FIG. 1 , wherein a Burst  1  ( 102 ) is separated from a Burst  2 ( 104 ) by a bandwidth gap  106 .  
         [0009]     APSON networks create a whole new problem to the wavelength scheduling issue. In actuality, the gaps between consecutive bursts are naturally filled with IP packets. In fact, conceptually speaking there is no such thing as gaps. This is more clearly seen in the timing diagram  200  of  FIG. 2 , wherein the Burst  1  ( 202 ) is separated by the Burst  2  by IP packets sent “on-the-fly” ( 206 ). From this it is clearly seen&#39;that APSON networks do not have bandwidth gaps in the sense that “gaps” no data have transmitted in the gaps.  
         [0010]     Therefore, even the most efficient scheduling algorithms developed for OBS networks (LAUC, LAUC-VF) cannot be used in APSON networks since they are focused on the reduction of such bandwidth gaps between consecutive bursts. Such an implementation would delete the IP packets transmitted in the “gap”. Certainly, the traditional random and FF scheduling algorithms could be used in APSON as well, but as in OBS networks, they would lead to a poor network performance in terms of blocking probability and throughput.  
         [0011]     Therefore in order to improve this situation, a new generation of APSON-specific scheduling algorithms is required. This invention aims at setting the basis and basic concepts for this new generation of algorithms.  
         [0012]     The idea of the invention is to design a wavelength scheduling algorithm that breaks the connection with the lowest traffic intensity in order to establish the requested incoming connection. Therefore, the traffic intensity on a link is identified as the most sensitive factor in terms of network performance that should be taken into account by a wavelength scheduling algorithm in APSON. By contrast, the length of the gaps was identified as the most sensitive factor in OBS networks.  
         [0013]     The broken connection is chosen according to the invention from the set of connections which are sending traffic for which no bandwidth has been allocated. That is, the set of connections which are sending unprotected IP packets in the x-switching regime (see  208 ,  FIG. 2 ). This is in contrast to the protected IP packets  210  ( FIG. 2 ). The idea is to break the connection that sends less information, since this will have the smallest impact on the network performance in terms of blocking probability and network throughput.  
         [0014]     The invention further defines a specific wavelength scheduling algorithm (referred to herein as LUC), which implies the definition of a suitable method to measure the traffic intensity in a link (so that the wavelength scheduling algorithm can take the decision) and of forwarding this information across the optical switches in the network.  
         [0015]     The invention, thus, realizes that the most efficient wavelength scheduling algorithms for OBS networks do not perform well on APSON. To elaborate a new generation of wavelength scheduling algorithms based on a novel concept of choosing the wavelength with the lowest instantaneous connection throughput (ICT). The invention report goes further and defines in detail one of these new generation wavelength scheduling algorithms (the LUC). This comprises the following inventive steps: to define exactly the ICT and to describe a method to easily measure it. Using the ICT it is possible with the invention to describe a mechanism to make the ICT information available to the network switches. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows an exemplary timing diagram,  
         [0017]      FIG. 2  shows an other exemplary timing diagram, and  
         [0018]      FIG. 3  shows an example of the operation of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     In an APSON network which has been running long enough to reach a steady-state, at any given time there are two kinds of connections which may be running through a switch, namely, unbreakable and breakable connections. Unbreakable connections are those connections which are still transmitting the data for which bandwidth has been allocated and reserved (e.g. through the JET, JIT or Horizon protocols). These connections cannot be touched by the wavelength scheduling algorithm. Breakable connections are those connections which already had sent the data for which bandwidth had been reserved and which are currently sending unprotected packets on the A-switching regime (see  208 ,  FIG. 2 ).  
         [0020]     When a connection request arrives at a switch, the wavelength scheduling algorithm chooses among the breakable connections one connection to break. The wavelength associated to the broken connection is then used in order to transfer the data of the incoming requested connection. The decision of the wavelength scheduling algorithm should minimize the blocking probability and therefore maximize the network throughput.  
         [0021]     The main idea of the invention is to identify the instantaneous connection throughput (ICT) as the most sensitive factor to be taken into account by a wavelength scheduling algorithm in APSON, just as the length of the gaps was identified as the most sensitive factor in OBS networks. The ICT is a measure of the average traffic intensity on a certain wavelength. The goal of the wavelength scheduling algorithm is to choose among the breakable connections the connection with the lowest ICT. In other words, the philosophy is to break the connection that sends less information, since this will have the smallest impact on the network performance in terms of blocking probability and network throughput.  
         [0022]     In the example  300  illustrated by  FIG. 3 , the wavelength scheduling algorithm chooses between two wavelengths ( 302 , 304 ) in order to send an incoming connection request. Wavelength  1  ( 302 ) transfers a high traffic load and, therefore has a relatively high ICT  306 , whereas wavelength  2  ( 304 ) is transfers less traffic and, therefore, has a relatively low ICT  308 . Consequently, the wavelength scheduling algorithm decides to use wavelength  2  ( 304 ) for the incoming connection. This will lead to a lower blocking probability and a higher network throughput than if wavelength  1  ( 302 ) had been chosen to carry the traffic of the incoming connection.  
         [0023]     Now, an example of the operation of the invention according to  FIG. 3  will be provided. Wavelength  1  ( 302 ) carries higher traffic intensity  306 . Therefore, Burst  1  ( 310 ) is formed in less time than Burst  2  ( 312 ) and/or it is bigger, which leads to the following inequality: ICT 1 &gt;ICT 2 . According to this, the wavelength scheduling algorithm decides to use the resources of wavelength  2  ( 304 ), since this decision leads to a higher network performance in terms of blocking probability and throughput. Advantageously, if both wavelengths carry the same traffic intensity blocking one of them leads to more IP packets which cannot be transmitted, compared to the case in which one wavelength carries heavy traffic and the other almost no traffic.  
         [0024]     Next, the details of the least used channel wavelength scheduling algorithm will be discussed in the sense described above, which is referred herein as the least used channel wavelength scheduling algorithm (LUC). The first task is to develop a method to measure the ICT and, for that matter, a more precise definition of the ICT itself.  
         [0025]     As mentioned above, the ICT should reflect the average traffic intensity over a certain wavelength. The average traffic intensity can be calculated with a moving average algorithm with a predetermined window size. It is important that ICT is defined at the right time scale, that is, that the size of the window of the moving average is not too large or too small. If the ICT defined is too large, it would not capture the variability of the traffic intensity correctly. If the ICT is too small, it would be excessively sensitive to local changes on the traffic intensity.  
         [0026]     A connection in APSON begins with the transmission of a burst. The proposed method to measure the ICT of a connection consists on calculating the quotient of the size of the burst which is sent at the beginning of the connection divided by the time it took the edge node to accumulate the burst. The ICT is defined according to the invention by Equation 1 as follows:  
               ICT   =     B     t   B         ,           Equation   ⁢           ⁢   1             
 
 where B is the burst size and t B  is the burst formation time. Thus, Equation 1 provides a measure of the average traffic intensity calculated with a moving average with a window equivalent to the burst formation time. 
 
         [0027]     Now, with an Equation for the ITC, the invention proposes the following LUC wavelength scheduling algorithm. When an edge node generates a burst of size B in a time t B , the ICT is calculated (ICT=B/t B ). Next, when the edge node sends a connection request, the header of the request contains the ICT. With the header request the ICT information is sent to the optical switches along the path. As a next step, each switch keeps track of a lookup table of the ongoing connections and of their respective ICT values (equivalently in a centralized solution, the central unit keeps track of this information). The table contains as well whether a connection is breakable or unbreakable. At the beginning all wavelengths are initialized with a breakable connection with an ICT=0. When an optical switch receives the connection request it breaks the breakable connection with the lowest associated ICT. The entry on the lookup table is replaced with the values of the incoming connection.  
         [0028]     The instant invention provides a new manner of determining the ITC and providing a new wavelength scheduling algorithm solution. The invention provides merely one definition of the ICT and other methods to measure the ITC are certainly within the present invention. It follows that other ICT definitions would yield other wavelength scheduling algorithms.  
         [0029]     Advantageously, the invention is simple and does not require complex processing at the optical cross connectors, since the decision is made based on finding the minimum value of a set of values. Further, the invention is based on information easy to obtain and measure, since the ICT can be very easily calculated at the edge nodes by measuring the burst size and the burst formation time. The invention also reduces the blocking probability and increases the throughput in the network compared to other state-of-the-art wavelength scheduling algorithms. The state-of-the-art algorithms were specifically designed for OBS networks which have a different functionality than APSON and, therefore, are obsolete. The invention is also advantageous as it profits from variable (e.g. self-similar) traffic as described above with reference to  FIG. 3 .