Abstract:
Typically paths are provisioned in a network with a margin which allows for component variations and failures in the network and so on. By providing more information about the requirements of the link in a path request, it is possible to more efficiently calculate a route for the path through the network. The efficiency gain allows greater network utilization which in turn saves costs for the network service provider.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of determining a path in a communications network in response to a path setup request including parameters which specify characteristics relating to a service level requirement for the path, and to corresponding apparatus and software. 
     BACKGROUND OF THE INVENTION 
     When users wish to communicate across a communications network such as an optical network, it is necessary for a path to be provisioned through the network between the two desired end points at the user locations. Thus a typical first step in setting up a connection between the users would be to issue a path set up request to the control layer of the network. Typically such a request would include information about the start and end locations of the path, and the kind of path required for example, and the bandwidth requirement. The request may also specify the type of protection which is required i.e. the method which is used to provide an alternative path should the primary path fail. 
     The control layer of the network then proceeds to determine whether a path between the start and end points is available and if so what is the optimum path. 
     Clearly the routing algorithm for the path should attempt to determine an optimum route taking into account factors such as routing policies set by the network management layer, available links, or any existing traffic on links and available capacity on those links and the resilience of those links particularly with reference to the desired protection (for example are the links following a shared path). These constraints are typical for all networks. 
     For analogue networks such as optical networks however, additional constraints must be taken into account. For example, ageing of components and fibres must be taken into account and margin built into the system to allow for these effects during the lifetime of the path, when provisioning the path. 
     Thus, for example with reference to  FIG. 1 , in an optical network, a margin of several dB, (maybe of the order of 12 dB), must be left to allow for degradation of the signal in the analogue domain. Typical components of the margin break down into random effects such as polarisation mode dispersion (PMD) and polarisation mode loss (PML), configuration tolerances such as optical signal to noise ratio (OSNR) equalisation mismatch, and long term OSNR degradation, component characteristics such as ageing of components and potential fibre losses such as normal signal degradation with length of transit in fibre and an allowance for losses through in service splicing of the fibre. These margins must all be added together to provide a total system margin which must be built into the path during routing. 
     Particularly initially, these margins are, to some extent and for some wavelengths, unused. Thus, additional equipment, especially regenerators, has been inserted into the network and gives no immediate benefit. Indeed, calls may be routed less-directly than they need to be in order to go via a regenerator site. 
     Thus in a typical network, the network must effectively be over-provisioned with unused capacity in order (amongst other things) to ensure that these margins and appropriate protection paths are in place. Furthermore, the network has little flexibility in how to provision the path. For, example in addition to over provisioning, the path setup request typically will force the network to use a particular type of protection which may not be appropriate for the current network state. It will be understood that any steps that may be taken to improve network utilisation (i.e. to reduce the requirement for over-provisioning) will provide cost benefits since the financial return produced from a particular investment in network equipment can be improved. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided a method of determining a route for a connection across an optical communications network comprising:—(a) receiving data specifying a service level requirement for the connection, (b) determining an optical signal margin requirement on the basis of the service level requirement, and (c) determining a route to use when provisioning the connection dependent on the determined optical signal margin requirement. 
     In accordance with a second aspect of the invention there is provided a method of determining a route for a connection across a communications network comprising receiving data specifying a service level requirement for the connection and determining a route on the basis of the service level requirement, the service level requirement being selected from a group containing a required longevity for the connection, a required resiliency for the connection, a required bit error rate for the connection, a required maximum duration of outage for the connection and a required maximum time before the connection can be provisioned. 
     In accordance with a third aspect of the invention there is provided a method of routing a path in a communications network comprising receiving a path setup request, the path setup request including a plurality of parameters which specify characteristics of the desired path and which include at least one parameter relating to a service level requirement for the path. 
     Preferably, the path setup request does not specify a protection technique. This allows the protection technique to be chosen by they network based on the user requirements (as specified in the path setup request) and the network information. Because this improves flexibility, this may well provide at least as good protection as the user expects but at lower network cost. 
     The plurality of parameters may for example specify the longevity of the connection which in turn (and as described in more detail below) allows a less conservative OSNR link margin to be applied. For example, with reference again to  FIG. 1 , the component ageing and in-service splice margins may be reduced for a shorter term path since these effects will have little impact for a short term path. 
     Other parameters passed with the path request may include resiliency, typically in terms of a maximum outage period and/or an outage occurrence rate per annum. These parameters allow the network to recognise a path which can easily be re-routed compared to a path (for example with a very short permitted outage time) which cannot easily be re-routed “on the fly”. Reduced flexibility and re-routing (as explained below) increases the cost of carrying the path since it may result in non-optimal use of the network for other paths. 
     Another parameter typically specified is the bit error rate (BER). Using this information in a routing algorithm allows further adjustments to be made to the link margin since the OSNR is usually the deciding factor in producing a ceiling for the BER. A reduced BER requirement allows a reduced OSNR margin to be applied to a particular path. 
     Another parameter which may be considered during path set up is an execution delay for provisioning the route. A carefully calculated route taking full account of all parameters may take several days to provision and even a typical provisioning time will be several hours. However, it may be necessary to provision a route within a few fractions of a second. These factors may be taken into account when deciding how to calculate the route as explained below. Generally, short provisioning times equate with inefficient (and therefore expensive) provisioning. 
     In another aspect, the invention provides a processor for a communications network arranged to provision a path through the network in response to a path setup request, the processor being further arranged to accept a path setup request including a plurality of parameters which specify characteristics of the desired path and which include at least one parameter relating to a service level requirement for the path. 
     In a further aspect the invention provides a communications network including a processor arranged to provision a path through the network in response to a path setup request, the processor being further arranged to accept a path setup request including a plurality of parameters which specify characteristics of the desired path and which include at least one parameter relating to a service level requirement for the path. 
     In a yet further aspect the invention provides a computer program which when executed on a processor in a communications network causes the processor to establish a path in the communications network in response to a path setup request, the path setup request including a plurality of parameters which specify characteristics of the desired path and which include at least one parameter relating to a service level requirement for the path. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompany figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing parameters which are typically taken into account when designing margin in a communications path; 
         FIG. 2  is a schematic block diagram showing a typical network management configuration; 
         FIG. 3  is a schematic diagram showing potential routes between clients in a network; and 
         FIG. 4  is a schematic diagram showing wavelength provisioning across a network. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 2 , a typical network may have a management layer  2 , a control layer  4  and a bearer layer  6 . It will be appreciated that the logical separation of the network into these layers is somewhat arbitrary and that in practice parts of the control layer may be distributed, for example, between the bearer layer and the management layer. Nevertheless, for ease of explanation, these functions will be considered to be separate layers in the description below. 
     The management layer  2  typically contains an operations support system (OSS)  8  which carries out the co-ordination network functions by providing overall policy information to nodal controllers  10 - 1  to  10 - 4  in the control layer, and other functions such as monitoring for faults and locating faults. 
     The control layer has a plurality of nodal controllers  10 - 1  to  10 - 4  which share information such as link topology updates, available resources and physical layer analogue optical parameters (in an optical network). Nodal controllers typically also act as “owner” of the link resources of their nodes and are typically integrated with switching/terminal elements in the bearer layer. Nodal controllers carry out routing algorithms and are able to make the final “decision” to determine whether or not the node resources can be allocated to a new path. 
     In the bearer layer  6 , switching/terminal facilities  12 - 1  to  12 - 4  are provided for provisioning purposes by the nodal controllers. It is through these elements that the data actually flows to traverse the network once a path has been created. The bearer layer interacts with the control layer in both directions. The bearer layer performs measurements of the network performance and notifies the control layer of information such as link failures and current signal quality of all or a subset of existing connections. In the other direction, configuration changes are sent from the control layer to the bearer layer to adjust, tear down or set up new paths. 
     In order to set up a path, typically a user will provide a path setup request to a local nodal controller such as the controller  10 - 1  of  FIG. 1 . The control layer uses its knowledge of the current state of the network to carry out routing. Typically a routing algorithm will execute in the nodal controller  10 - 1  based on information provided in the path setup request, information provided by other nodal controllers and information provided by the nodal controllers associated bearer layer fabric  12 - 1 . Having determined a suitable path, (or a plurality of paths which need testing), information is communicated to the other nodes in the control layer to set up the path using the elements available in the bearer layer. 
     Thus with reference to  FIG. 3 , the control layer may achieve provisioning of routes (solid lines) between clients A and B and between clients C and D via the network  14 . Additionally, protection paths (dotted lines) may be provisioned using alternative elements in the bearer layer in order to provide some redundancy should the primary (solid line) routes fail. 
     Thus far, what has been described is generally conventional. However, by modifying the path setup request to include additional information which is not conventionally included, the routing algorithm may be made to provide more efficient network utilisation as described below. 
     Some of the possible parameters passed to the control layer and advantages conferred on the functioning of the routing algorithm are set out below. 
     Longevity and Network Fragmentation 
     The provision of a longevity parameter (i.e. an anticipated length during which the path is required) provides several advantages. 
     Firstly, in an analogue network such as an optical network, a path which is to be provisioned for a relatively short time does not need a large margin to allow for ageing of components in the network. A reduced noise margin allows more signals to be put onto the same portion of the network thereby increasing network utilisation or allows a lower quality (cheaper) set of links to be used. 
     Secondly, if a link is to be deprovisioned very quickly the margin provided in the link budget to ensure that the link will only suffer a small amount of drop out/disruption permitted within a service level requirement may be reduced since the probability of disruption, for example from connections added later, reduces with duration of the link. It will be noted that this requires the desired resilience of the link (as explained in more detail below) also to be provided within the path setup request. 
     Having knowledge of the longevity also allows long term connections to be routed along the shortest or most direct paths in order to avoid over using network capacity and also to ensure efficient long-term resource utilisation to reduce network fragmentation. Short term paths are not relevant to long term resource usage unless they cause a long term path to be sub-optimally routed. 
     Thus longevity of the link may be used to improve network utilisation by keeping the short terms paths away from popular links which are good for providing the best routing for longer term paths. 
     This may be achieved by providing a link weighting which includes a “path_time_to_live” parameter. For example, cost (link, path time to live)=cost (link)^(1 over path time to live). Thus by making more congested links weighted more heavily the relative distinction in weightings is increased so that longer term links cost less to put on popular routes. 
     If the path request also contains a maximum permissible latency parameter, short term paths may be constrained from being caused to be routed through tortuous paths in order to meet the weighting constraints described above. 
     Resiliency and r Source Fragmentation 
     The resiliency parameter may be used to overcome the problem of resource fragmentation on links in the network. 
     This problem occurs when there is a resource continuity requirement such as in the example shown in  FIG. 4 . In this optical network, unless wavelength converters are used (which are expensive) a wavelength service within a wavelength routed optical network will be of the same frequency/wavelength throughout all of the node-node hops that it traverses. This is a significant constraint when trying to find a possible route for such a wavelength. 
     In  FIG. 4 , four wavelength services are shown, wavelength  1 , wavelength  2 , wavelength  3 , wavelength  4 . It will be noted that approximately half the network resources are being used and yet it is not possible to provision a fifth service across all links of the network. 
     In order to provision a fifth service, it is either necessary to re-arrange some of the existing traffic, potentially causing service interruption (for example moving wavelength  1  to be at 1558 nm) or a wavelength converter function must be inserted (for example allowing the new service to be transmitted at 1559 nm and converted to 1558 nm anywhere after the first link). Clearly the provision of a wavelength converter increases the cost of provisioning the link. 
     Sensitivity to Outages 
     By including information concerning the sensitivity of a particular path to outages, for example, in terms of maximum duration of outage and repetition rate of outages, it is possible to determine whether links may be moved to defragment the network to provide additional capacity. Furthermore, the number of times each link can be moved may be deduced. The choice of movements used to perform the defragmentation may be arranged to take advantage of this. 
     Additionally, a path which is known to be tolerant to outages may be protected more easily with a protection path since protection paths may be rearranged as each fault occurs to ensure that every path is still protected. This allows increased sharing of protection paths between links. 
     BER 
     By including a BER parameter in the path request, it is possible to reduce the noise margin since the OSNR typically has a direct bearing on achievable BER. With knowledge of the BER, it is possible to tune the likely OSNR of a provisioned path to ensure that the BER is met. A high permissible BER may allow a reduced OSNR margin. Knowledge of a desired BER may also allow other parameters to be adjusted such as the power of adjacent wavelengths (which may have an impact on non-linear distortion of the wavelength of interest). 
     Execution Delay for Provisioning 
     By specifying in the path request how long the network may take before provisioning the path, it is possible to further optimise the cost of provisioning. For example, switches which may be configured quickly may need to be used for a path which must be provisioned quickly whereas slow switches (which would typically be cheaper) may be used for paths having a lower requirement in this respect. 
     As a further alternative, a network may be provisioned with sites for items such as optical regenerators which are not installed until actually required. Other articles suitable for this treatment may be channel filters or band or group filters etc. 
     During automatic route calculation, these “virtual items” may be treated as if they exist and if they are used in the route may cause a work order to be issued for the additional items to be provisioned. 
     Knowing how quickly the path must be established and knowing how long it would take to provision such virtual items, allows the use or otherwise of these items. Also, knowing the longevity of the path allows the automatic routing algorithm to take this into account. Clearly it is unlikely to be economically viable to provision a new item such as a regenerator for a path which is only required for a few days. 
     The virtual items may also have several sub-classes. For example, a virtual item may be one which can be provisioned using equipment which the carrier has in stock. Alternatively, a virtual item may be flagged as provisionable using equipment that the carrier needs to order. Clearly the second sub classification will take longer to provision than the first. 
     This allows the initial investment in the network to be minimised and additional investment to be focussed on parts of the network where it is required based on path requests. 
     Furthermore, it might be possible to roll over badly provisioned paths on to new equipment to more optimally route the path over time, for example by using different links or using different more cost effective elements in the bearer layer. 
     Tarriffinq 
     The control layer may also feed information to the management layer to enable a client to be charged more for different types of path requests. For example, for a path which must be provisioned quickly is likely to be more expensive than a path which can be provisioned more efficiently by taking more time in the route calculation. A path which is required for a longer time may also cost more and clearly a path which requires less resilience (and may therefore be more flexibly routed and re-routed or carried on cheaper equipment) is likely to cost less. 
     These factors may be considered by the routing algorithm to generate pricing information to be fed back to the management layer. This may be used for example to set a price to be offered to a customer by assessing the actual cost and then adding a suitable profit margin. 
     Alternatively, this information may be used to cause charging to occur according to a tariff pre-agreed with a customer. An exemplary basis for this might be cost=B*F*D where B is a basic cost, F is a factor for increasing the price for fast provisioning if required and D is a discount factor for reducing the price if the connection is not required for a long period. 
     Implementation 
     The control layer may also take action if, for example, a path remains provisioned beyond its projected longevity parameter. For example, the path may automatically be deprovisioned, may be reprovisioned along another route or may have its SLA downgraded so that it is allowed to be subject to increased outages for example. 
     The routing algorithm is typically implemented using software executing somewhere in the network. The software may operate using neural network techniques and/or using case based reasoning techniques. The inputs to these techniques may be the raw and/or pre-processed path request parameters.