an example of such a network 100 is illustrated in fig1 , wherein n nodes 102 are connected by spans , or links , 104 . the bandwidth for network 102 will be equal to the sum of all traffic bandwidth that use the span as shortest path . graphically , the minimal traffic bandwidth usage for each span , i . e ., the effects for each span , is the sum of the bandwidth of communications that each arrow 106 crosses . in a ring size n , even some of communications between different nodes will use n / 2 spans ( odd ring sizes do not verify such condition ). it is indifferent to use one of two possible paths , so the minimum traffic bandwidth usage should not consider the bandwidth of those communications . fig1 a illustrates the case for the case where the number of nodes are odd . fig1 b illustrates the case when the number of nodes is even . since the main elements are the same in the figures , the reference numerals remain the same . in equation format the traffic for each span , between nodes n and n − 1 , can be given as follows : ts ( n - 1 , n ) = ∑ k = n - n - 1 2 n - 1 ⁢ ∑ l = n k + n - 1 2 ⁢ t ( k , l ) ts ( n - 1 , n ) = ∑ k = n - n - 1 2 - 1 n - 1 ⁢ ∑ l = n k + n - 1 2 - 1 ⁢ t ( k , l ) we can express the traffic for any type of network by putting both equations together : ts ( n - 1 , n ) = ∑ k = n - n - 1 + ( - 1 ) n 2 - 1 2 n - 1 ⁢ ∑ l = n k + n - 1 + ( - 1 ) n 2 - 1 2 ⁢ t ( k , l ) the resulting value is the minimum usage needed for the span . in other words , it is guaranteed that a span bandwidth will be higher than this value . the goal now is to know the effect of not using one span for traffic . the traffic of that span must follow the other spans . this affects not only the nodes adjacent to the span but all traffic that uses that span as the shortest path . therefore , the traffic increment introduced in the ring when a span s ( n − 1 , n ) is not used is given as follows : tis ( n - 1 , n ) = ∑ k = n - n - 1 2 n - 1 ⁢ ∑ l = n k + n - 1 2 ⁢ t ( k , l ) × n - 2 ⁢ ( l - k ) tis ( n - 1 , n ) = ∑ k = n - n 2 - 1 n - 1 ⁢ ∑ l = n k + n - 1 2 - 1 ⁢ t ( k , l ) × n - 2 ⁢ ( l - k ) and putting both equations together as before in order to obtain the following : tis ( n - 1 , n ) = ∑ k = n - n - 1 + ( - 1 ) n 2 - 1 2 n - 1 ⁢ ∑ l = n k + n - 1 + ( - 1 ) n 2 - 1 2 ⁢ t ( k , l ) × n - 2 ⁢ ( l - k ) so total bandwidth usage in a ring when the span s ( n − 1 , n ) is blocked , or idle , is given by : trs ( n - 1 , n ) = ∑ k = 1 n ⁢ ts ( k - 1 , k ) + tis ( n - 1 , n ) trs ( n - 1 , n ) = ∑ k = 1 n ⁢ ts ( k - 1 , k ) + tis ( n - 1 , n ) + n 2 × ∑ k = 1 n / 2 - 1 ⁢ t ( k , k + n / 2 ) trs ( n - 1 , n ) = ∑ k = 1 n ⁢ ts ( k - 1 , k ) + tis ( n - 1 , n ) + n × ( 1 + ( - 1 ) n ) 2 × ∑ k = 1 n / 2 - 1 ⁢ t ( k , k + n / 2 ) ( 1 ) the resulting span bandwidth owing to an idle span may be illustrated by fig2 a and 2 b , which illustrate the odd and even node n cases , respectively . in a network 200 , nodes n 202 are connected by links , or spans , 204 . as illustrated in fig2 a , an hatched - rings 206 , 208 and 210 represent the number of spans needed when communicating between two nodes . for example , those spans falling within the inner - hatched ring 206 , such as the span between nodes 2 and 5 , require one span . if the path of communication is now changed to go through node 7 , we see that the spans are within the middle - hatched ring . as the figure indicates , those communications in the middle ring , require 3 additional spans . in other words , an additional 3 spans are now needed to communicate between nodes 2 and 5 . for the outer ring , the number of additional spans needed is 5 . similarly , fig2 b illustrates the case for an even ring , wherein the additional spans needed are 2 for the inner ring and 4 for the outer ring . the above equations are used in the present invention to calculate the optimal protecting span that minimises the bandwidth utilization in the ring . this is done here by applying the expression ( 1 ) for each span and selecting the one that yields the lowest value . another important issue is to know what the impacts of failures or changes in the configuration are on the other spans in the ring . we now turn to resolving the problem of full resilience confirmation . that is , ensuring that the impact on the bandwidth of any other span does not cause that span to be overloaded . when a span is not used it introduces traffic to others spans in the ring . the traffic existing in a span s ( j - 1 , j ) due to a failure in a span s ( i - 1 , i ) is given as : ts ( j - 1 , j ) ⁢ fs ( i - 1 , i ) = ∑ k = i j - 11 ⁢ ∑ l = j i - 1 ⁢ t ( k , l ) ( 2 ) graphically , this can be represented in fig3 a and 3b which illustrate n nodes 302 connected by spans , or links , 304 for odd and even rings 300 , respectively . equation 2 is illustrated as the sum of the traffic that crosses an arrow 306 which connects the span with the failure and the span that is desired to know the bandwidth . if there is a need for resilience in the ring , it should be provisioned for the worst case , that is a maximum bandwidth in case of a failure . another interesting finding is that the bandwidth in a certain span when another span is not used is exactly the same when configuration inverts their roles ( independently of the traffic flow matrix ), that is : thus , only half of the calculations are needed to know the required span bandwidth , or in another way : the importance of such a type of network analysis is apparent when trying to provision for the possibility of assuring full services resilience within the ring . in the same perspective , determining if a particular span copes with the traffic demand for resilience can also be considered in the same way . to confirm if the ring infrastructure is full resilient the following methodology may be applied . the methodology is iterated for all spans . next , for each span , the methodology is iterated from the span being used in the iteration to the last span . in each iteration , expression ( 2 ) is calculated and the result is stored if the result is greater than any previous one . then , it is confirmed that the existing ring infrastructure can accommodate , for each span , the maximum value obtained in the last step . now , it is desired to minimise the maximum span bandwidth . until now , the focus on optimisation was made considering that the goal was to minimise the sum of the bandwidth required in each span . due to maximum span bandwidth capability , it is sometimes more important to minimise the bandwidth required in a span rather than the overall bandwidth in the ring . this situation may arise , for example , due to interface capability constraints . to continue , in order to find the protecting span that minimises the maximum value in any other span , the following methodology should be applied . again , the process is iterated for all spans . for each span , iterate from the span being used in the previous step to the last span . in each span in the instant iteration , expression ( 2 ) is calculated and the result is stored if the value is greater than any previous one . then , that span is chosen as the protecting span that minimises the maximum value obtained in the previous step . thus far , three optimization techniques have been described . each of these techniques may be used separately or in any combination or permutation . it shall be appreciated from the above that bandwidth usage optimisation in the ring allows the telecom operator to assure best usage and return on investment from its main resource while improving the quality of service that is being offered . knowing the effect of failures and consequent network behaviour provides the telecom operator the ability to earlier ascertain such events as well as earlier decide how to overcome them . methodically employing the invention provides a proactive maintenance that avoids the foregoing problems of overloading a ring network . such a capability is especially significant to assure a quality of service to clients and accomplish and agreed upon service level agreements . this translates into avoidance of profit erosion from sla penalties . the present invention shall now be discussed in terms of employing the above optimization techniques in order to configure a network . the effects of configuration changes give the ability to earlier prepare the required reconfigurations of the equipments or even accurately forecast necessary investments to support new traffic demand . this knowledge becomes especially useful when operators face traffic profiles that vary significantly according to different time schedules , for example , different configurations according to the hours of the day . in one configuration optimization , the required infrastructure is verified to assure full ring resilience committed to the several domains specific requests . consider the situation where there are several vlans , each one with specific requests for bandwidth . the configuration process is shown in fig4 . in a first step 402 , the granted traffic between nodes for all vlans is aggregated , in other words summned . for each span of the ring , the next step 404 is to calculate the maximum span bandwidth usage without using this span using expressions ( 1 ) and ( 2 ) from above . the ring bandwidth minimal requirement is the value for all spans as illustrated in step 406 . it shall be appreciated that the invention has application in the real world . for example , erp does not necessarily have a uniform bandwidth all over the ring . each span may have a different bandwidth . this probably may be achieved by selectively setting the committed information rate ( cir ) for each vlan or peak information rate ( pir ). for example , this may be achieved according to operator &# 39 ; s choice and sla agreements between the operator and provider . in another aspect of the invention , this may be used as a kind of connection admission control ( cac ) to decide whether another vlan should be added to the ring or not . in any event , the invention would be very useful in these networks . turning now to a second configuration optimization , we desire now to determine how domains should be set accomplishing lowest ring overall bandwidth usage as possible . with reference to fig5 , the first step 502 treats all vlans as being separated and not aggregated in domains as before . these domains are used inside a ring and not have any influence in traffic separation . in step 504 , we minimise ring bandwidth usage for each vlan according to the expressions ( 1 ) and ( 2 ). thereafter , we aggregate all vlans in step 506 using the same protecting span in a given domain . the ring bandwidth is by this way minimised . j now , we turn to the third configuration optimization technique of the present invention , wherein it is desired to creating domains with minimal ring bandwidth . in other words , we determine how domains should be set in order to accomplish bandwidth requirements for each span . with reference to fig6 , we again begin by treating ( step 602 ) each individual vlan . for each vlan , we minimise ring bandwidth usage in step 604 . thus far , the third configuration is similar to the process in fig5 . in the third process , instep 606 , the configurations are then sorted by ring bandwidth usage using all vlans . if this configuration does not accomplish span bandwidth requirements , as tested in step 608 , then the process flow proceeds to the next minimised configuration ( step 610 ) and step 608 is repeated , thereby , continue the searching for the span that accomplishes the minimum bandwidth requirements . when a solution is finally found , the process flow then proceeds to step 612 to look for another solution . in step 612 , the process aggregates all vlans using the same protecting span in a domain . this configures ring bandwidth optimised according to span requirements as indicated in step 614 . now turning to three concrete examples , we consider the three configuration optimizations above in turn . consider a simplified example comprised of a ring with 3 nodes ( a , b , and c ) and spans ( 1 , 2 , and 0 . in the example , there are 10 vlans and the following bandwidth requirements ( values are in mbit / s ). it is assumed for this example that there are two requirements cir and pir , and that it is mandatory that cir requirements are accomplished even if a link fails and the ring has enough bandwidth to assure pir requirements when all links spans are operational . the cir and pir requirements for this example are shown in fig7 a and 7 b , respectively . of course , these requirements may be other than those shown . the first configuration optimization , i . e ., verifying the required infrastructure to assure full ring resilience committed to the several domains specific requests , is now considered . to find the required infrastructure , we first aggregate all spans bandwidths as shown in fig8 a . then the maximum span bandwidth usage without using each span is calculated as explained above with reference to fig4 . as will be seen , the increment in the ring for not using span 1 in fig8 a is 29 mb . as we see from fig8 b , the required bandwidth for each span in order to meet the cir requirements are 62 , 63 and 63 mbits for each of the spans 1 , 2 and 3 , respectively . in this example , it is seen that span 1 should be reserved as the idle span since it has been calculated that the maximum bandwidth without using this span is 62 mbits , which is 1 mbit less than using the other spans 2 or 3 as the idle span . now , we turn to the second configuration optimization example . in this case , it is desired to determine how the domains should be set accomplishing lowest ring overall bandwidth usage as possible . the process shown in fig5 is executed , yielding the tables in fig9 a - c . each table corresponds to each span 1 , 2 or 3 maintained idle , respectively . the traffic for each vlan 1 - 10 is indicated for each of the remaining spans . for example , in fig9 a , the first span 1 is idle and , therefore , no traffic is indicated in the first column corresponding to span 1 for any of the vlans . the remaining spans 2 and 3 carry traffic for the vlans 1 - 10 as shown in the second and third columns . the total bandwidth needed for each vlan is aggregated and indicated in the last column . from the tables , it can be seen that vlan 7 has the same bandwidth increment to the ring independent of which span is used for protection . similarly , vlan 10 has the same bandwidth increment to the ring independently if span 1 or span 2 are used for protection . the results for vlans 7 and 10 , as will be explained , can be used to simplify the configuration solution . of course , these results will be different in other scenarios . the invention then selects domains , or groups of vlans and corresponding configurations that minimize the bandwidth for the ring . in the example given , there are actually six different configurations that minimise the ring bandwidth . these combinations of the vlans are extrapolated according to the following observations of the tables in fig9 a - c . the configurations are determined by totalling the bandwidth for the remaining spans for each vlan and comparing which protection scheme offers the smallest bandwidth requirement . as can be observed , for example , the total ring bandwidth need for vlan 1 is smallest in the first protection scheme , i . e ., where span 1 is used for protection . using a similar process , it is determined from this example that vlans can be grouped into the various protection schemes into domains . therefore , in a domain 1 — using span 1 for protection — vlans 1 , 2 and 9 are selected . a domain 2 — using span 2 for protection — selects vlans 3 , 5 and 6 . domain 3 — using span 3 for protection — selects vlans 4 and 8 . vlan 7 may belong to any domain . vlan 10 shall belong to domain 1 or to domain 2 . there are six configurations because vlan 10 can be in either domain 1 or 2 and for each of these permutations vlan 7 can be in any of the three , which by permutational arithmetic makes 6 combinations . the 6 configurations are shown in fig1 a - f . consider case 1 , a domain 1 uses span 1 for protection and is assigned vlans 1 , 2 , 7 , 9 and 10 . a second domain , domain 2 , uses span 2 for protection and is assigned vlans 3 , 5 and 6 . a domain 3 uses span 3 for protection and is assigned vlans 4 and 8 . from fig1 a , we see that span 1 requires = 27 + 12 = 39 mbits . span 2 requires = 42 + 14 = 56 mbits . span 3 requires = 50 + 26 = 76 mbits . thus , of the spans , span 3 requires the maximum = 76 mbits . the total ring bandwidth can be seen from fig1 a as = 92 + 53 + 26 = 171 mbits . in case 2 , domain 1 uses span 1 for protection and is assigned vlans 1 , 2 , 7 and 9 . domain 2 uses span 2 for protection and is assigned vlans 3 , 5 , 6 and 10 . domain 3 uses span 3 for protection and is assigned vlans 4 and 8 . we see from fig1 b , that span 1 requires = 31 + 12 = 43 mbits . span 2 requires = 38 + 14 = 52 mbits . span 3 requires = 40 + 36 = 76 mbits . thus , the maximum span bandwidth is = 76 mbits for span 3 . the total ring bandwidth = 78 + 67 + 26 = 171 mbits . considering case 3 were domain 1 uses span 1 for protection and is assigned vlans 1 , 2 , 9 and 10 . domain 2 uses span 2 for protection and is assigned vlans 3 , 5 , 6 and 7 . domain 3 uses span 3 for protection and is assigned vlans 4 and 8 . as seen from fig1 c , span 1 requires = 39 + 12 = 51 mbits . span 2 requires = 30 + 14 = 44 mbits . span 3 requires = 38 + 38 = 76 mbits . thus , the maximum span bandwidth is = 76 mbits for span 3 . the total ring bandwidth = 68 + 77 + 26 = 171 mbits . in consideration of case 4 , domain 1 uses span 1 for protection and is assigned vlans 1 , 2 and 9 . domain 2 uses span 2 for protection and is assigned vlans 3 , 5 , 6 , 7 and 10 . domain 3 uses span 3 for protection and is assigned vlans 4 and 8 . we see from fig1 d that span 1 requires = 43 + 12 = 55 , span 2 requires = 26 + 14 = 4 and span 3 requires = 28 + 48 = 76 mbits . again , we see that the maximum span bandwidth is = 76 mbits . the total ring bandwidth is = 54 + 91 + 26 = 171 mbits . considering case 5 , domain 1 uses span 1 for protection and is assigned vlans 1 , 2 , 9 and 10 . domain 2 uses span 2 for protection and is assigned vlans 3 , 5 and 6 . domain 3 uses span 3 for protection and is assigned vlans 4 , 7 and 8 . we see from fig1 e that span 1 requires = 27 + 24 = 51 mbits . span 2 requires = 30 + 26 = 56 mbits . span 3 requires = 38 + 26 = 64 mbits . here , we see that that maximum span bandwidth is from span 3 = 64 mbits . the total ring bandwidth = 68 + 53 + 50 = 171 mbits . considering the final case 6 , domain 1 uses span 1 for protection and is assigned vlans 1 , 2 and 9 . domain 2 uses span 2 for protection and is assigned vlans 3 , 5 , 6 and 10 . domain 3 uses span 3 for protection and is assigned vlans 4 , 7 and 8 . we see from fig1 f that span 1 = 31 + 24 = 55 mbits , span 2 requires = 26 + 26 = 52 mbits and span 3 requires = 28 + 36 = 64 mbits . thus , as in the last case , the maximum span bandwidth is required by span 3 = 64 mbits . the total ring bandwidth = 54 + 67 + 50 = 171 mbits . it shall be noticed that all of the configurations 1 - 6 yield a total ring bandwidth of 171 mbits . thus , according to the methodology of fig5 , any of the configurations are acceptable . now , it will be determined how the domains should be set accomplishing bandwidth requirements for each span . however , in regard to the methodology given in fig6 , the effect on the individual spans of the above configurations are considered . from the above , it can be seen that the minimum span bandwidth , 64 mbits for span 3 , can be attained from configurations 5 or 6 . this may be an acceptable span bandwidth . however , consider the situation above where it was determined in fig8 b that the maximum span bandwidth for the spans are 62 , 63 and 63 mbits for the spans 1 , 2 and 3 , respectively . considering that the minimum bandwidth in configurations 5 and 6 for span 3 is 64 mbits , it is clear that configurations 5 and 6 have a negative impact on span 3 . in other words , configurations 5 and 6 , while satisfying total ring bandwidth requirements , do not meet the requirement for at least one of the span bandwidths , here span 3 . the third process shown in fig6 considers this situation . to resolve the conflict between overall bandwidth and the effect of the configuration on the individual spans , the invention adjusts the configurations and strikes a balance between the overall bandwidth needs and the impact on the spans . more particularly , the invention seeks to re - configure the domains in order to meet the span bandwidth requirements whilst finding the least increment to the overall ring bandwidth . observing the span bandwidth requirements in the specific example above , it is required at least 64 mbit / s ( if vlan 7 belongs to domain 3 ). considering that the available bandwidth is 62 mbit / s for span 1 and 63 mbit / s for spans 2 and 3 the solutions found for minimise the bandwidth usage are of no use . next configuration ( sorted by ring bandwidth usage ) if found by exchanging a protection span for one vlan . it should be select the exchange that brings the least ring bandwidth increment or increase . in the example here , the invention finds the increment from the above configurations as being achieved by one of the following exchanges : vlan 1 — using span 2 for protection instead of span 1 ; vlan 2 — using span 2 for protection instead of span 1 , vlan 3 — using span 1 for protection instead of span 2 ; vlan 4 — using span 3 for protection instead of span 2 ; vlan 5 — using span 3 for protection instead of span 2 ; vlan 6 — using span 1 for protection instead of span 2 ; in regard to the process in fig6 , the increment calculation is performed in step 606 . the invention then checks whether the exchange fulfils the minimum span bandwidth requirements in step 608 . the invention , checks this for all remaining configurations , namely the 6 configurations in the example that arise due to the permutations of vlans 7 and 10 . if the minimum bandwidth for the spans is not met ( step 610 ), the next exchange above is examined and the process in step 608 is repeated . after a few iterations , depending on the sorting mechanism , a solution will be found . in the particular example here , the best solution is to use the following configuration . domain 1 — using span 1 for protection — vlans 1 , 2 and 9 ; domain 2 — using span 2 for protection — vlans 5 , 6 and 10 ; domain 3 — using span 3 for protection — vlans 3 , 4 , 7 and 8 . it should be noticed in the above example that there was an optimal configuration that minimises the ring bandwidth according to each span available bandwidth . in the case that bandwidth requirements were too low ( below 59 mbit / s in this example ) a solution will not be found . however , an optimal solution would be found that best fits the bandwidth requirements . in this sense , the invention provides at least a best solution for all cases . it shall be appreciated that the present invention has been described with reference to specific examples , but that the invention may be practiced with modifications that are within the spirit and scope of the invention .