text: fig3 a through 3c illustrate an exemplary method of assigning wdm channels in an exemplary link so as to mitigate the effects of pmd . the entire wavelength range of frequencies 310 supported by the link contains multiple wdm channels 320 . in accordance with the present invention , when traffic is assigned to wdm channels , wdm channels belonging to a first set of channels 320 . 1 , shown with solid shading in fig3 b , are deployed first . the first set of channels 320 . 1 are selected so as to be spaced apart by a frequency ( or wavelength ) spacing 350 . the spectral spacing 350 is preferably selected so that the wdm channels 320 . 1 are statistically uncorrelated with each other . wdm channels are statistically uncorrelated when their spectral separation exceeds the pmd correlation bandwidth of the link . depending on the definition of the correlation bandwidth , the pmd correlation bandwidth is approximately between two and six times the bandwidth of the principal state of polarization ( b psp ) of one quasi - static link section . as such , in the exemplary channel assignment scheme illustrated in fig3 a - 3c , the spectral spacing 350 is preferably greater than approximately 6b psp . the bandwidth of the psp ( b psp ) is given by : for links with n quasi - static sections of equal mean section dgd , the mean static section dgd determined from the mean link dgd as follows : mean static section dgd = mean link dgd / n 1 / 2 ( eq . 2 ) table i below shows the frequency spacing for statistically independent wdm channels of an exemplary link having various numbers of quasi - static sections for the case of a mean link dgd of 5 ps , chosen as an example for a 40 - gb / s system . the spacing between the wdm channels 320 . 1 can be uniform ( i . e ., the spacing between all adjacent pairs of channels 320 . 1 is the same ) or not , as long as the spacing is at least as large as the spacing 350 . furthermore , the channels 320 . 1 need not be deployed in any particular order . in the exemplary embodiment illustrated in fig3 a - 3c , the spectral spacing 350 spans five wdm channels . as can be appreciated , the number of wdm channels spanned by the spectral spacing 350 can vary depending on the size of the spectral spacing 350 and the spacing of the wdm channels . in accordance with the exemplary method of the present invention , once all of or at least most of the wdm channels 320 . 1 have been deployed , a second set of channels 320 . 2 ( see fig3 c ) can then be deployed . as with the wdm channels 320 . 1 , the channels 320 . 2 are selected so as to be spaced apart from each other by at least the spacing 350 . ( for illustrative purposes , fig3 c shows a portion of the channels 320 . 2 deployed ; i . e ., four of a possible six channels .) the second set of wdm channels 320 . 2 is selected so as to preferably maximize their spacing from the first set of channels 320 . 1 already deployed . thus , for example , in the exemplary scheme illustrated in fig3 a - 3c , the set of wdm channels 320 . 2 can be shifted one channel to the left from the positions shown . the remaining wdm channels 320 are deployed as described above with respect to the sets of channels 320 . 1 and 320 . 2 . this scheme reduces the link &# 39 ; s susceptibility to pmd - induced outages in the framework of the hinge model by minimizing , to the extent possible , the correlation among the wmd channels deployed . this substantially reduces the possibility of having 100 % of the channels of a link fail because of a pmd - induced outage , as would be the case if all channels deployed were closely spaced , i . e . within a band , and thus correlated . under the scheme of the present invention , only a fraction of the deployed channels would likely be affected by a pmd - induced outage , as quantified by the ncr . note , however , that after the first set of channels 320 . 1 has been completely deployed , additional channels deployed as part of subsequent sets will be within the spacing 350 from the already deployed channels . in other words , there will now be channels that are correlated ( e . g ., each of the four deployed channels 320 . 2 shown in fig3 c will be correlated with the adjacent channels 320 . 1 ). correlated wdm channels can be said to belong to a correlation group or band , such as the groups of channels 370 shown in fig3 c . when deploying channels in accordance with the present invention , it is preferred that the number of wdm channels within the bands be as equal as possible . during channel growth , however , some sub - bands will have n channels , while others will have n + 1 channels . knowing that a certain fraction of wdm channels will always violate the outage specification , a certain number of pmd protection channels can be deployed together with appropriate monitoring and switching mechanisms to provide good transmission quality for all primary channels , even in the presence of non - compliance . thus , for example , provisioning 133 channels will provide for 100 compliant channels when the ncr is approximately 25 %. in order for the ncr to apply and for such “ over - provisioning ” to be effective , however , the correct ratio between correlated and uncorrelated wdm channels should be maintained as channels are assigned . in case of a transmission system with unequal wdm channel groupings within different sub - bands , the over - provisioning must reflect the proportion of the groupings . ( this addresses the unequal channel deployment problem mentioned above .) thus , for example , in a link having ten primary bands and three protection bands , where the ten primary bands consist of six bands with three assigned channels each and four bands with two assigned channels each , the three protection bands will consist of two bands with three channels each and one band with two channels . in a further embodiment , shared pmd compensation can be employed as an alternative to over - provisioning . within a band of correlated wdm channels , pmd compensators can be deployed for the maximum number of deployed wdm channels within the fraction of bands that are non - compliant . thus , for example , in a system with an ncr of 25 % having ten bands of five wdm channels each , out of which three bands are equipped with three wdm channels each and seven bands are equipped with two wdm channels each , three pmd compensators can be deployed to which the pmd - affected channels can be switched . the pmd compensators within a band can be implemented , for example , as multiple , first - order compensators or as a single higher - order compensator . a compensator can be regarded as a reversed emulator of the hinge model . this could be done with one dedicated compensator for each band , or with compensators for only an ncr of the bands , including monitoring and switching as needed . it is understood that the above - described embodiments are illustrative of only a few of the possible specific embodiments which can represent applications of the invention . numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention .