Data Communications Networking Devices - 4th Ed.pdf

6.2 T1/ E1 MULTIPLEXERS _________________________________________________________ 675Figure 6.33 Alternate routing.T-carrier multiplexers providea variety of methods to effect alternate routing to includeusing prede®ned tables and by the dynamic examination ofcurrent activity at the time of failuregeneration, consider the T-carrier networkillustrated in Figure 6.33. In thisillustration the T-carrier circuit connecting multiplexers A and B has failed. Withan alternate routing capability some or all DS0 channels previously carried bycircuit AB must be routed from path AC to path CB to multiplexer B.If the multiplexers employ alternate routing based upon prede®ned tablesassigned by operations personnel, DS0 channels previously routed on path AB willbe inserted into the T-carrier linking A to C and then routed onto path CB basedupon the use of those tables. As the DS0 channels from path AB are inserted intothe T-carrier linking A to C, DS0 channels on path AC must be dropped, a processreferred to as bumping. If calls were in progress on path A to C and C to B when thefailure between A and B occurred, those calls may be dropped depending uponwhether or not the multiplexers employ a priority bumping feature or have thecapability to downspeed voice digitized DS0 channels.Priority bumping refers to the ability to override certain existing DS0 subchannelsbased upon the priority assigned to DS0 channels that were previouslycarried on the failed linkand the priorities assigned to DS0 active channels on theoperational links. Downspeed refers to the capability of multiplexers to shift to adifferent and more ef®cient voice digitization algorithm to obtain additionalbandwidth to obtain the ability to carry DS0 channels from the failed linkon theoperational circuits. One example of downspeed would be switching from 32 kbpsADPCM to 24 kbps ADPCM, resulting in freeing up 12 kbps per operational DS0channel.When alternate routing and route generation is dynamically performed bymultiplexers, those devices examine current DS0 activity and establish alternaterouting based upon prede®ned priorities and the current activity of DS0 channels.If prede®ned tables are used, the multiplexers do not examine whether or not aparticular DS0 channel is active prior to performing alternate routing, however,some multiplexers may have the ability to perform forced or transparent bumpingregardless of whether they use ®xed tables or dynamically generate alternate paths.Under forced bumping, DS0 channels are immediately reassigned, whereas undertransparent bumping current voice or data sessions are allowed to complete prior totheir bandwidth being reassigned.RedundancySince the failure of a T1 or E1 multiplexer can result in a large number of voice anddata circuits becoming inoperative, redundancy can be viewed as a necessity similarto business insurance. To minimize potential downtime you can consider dual