Data Communications Networking Devices - 4th Ed.pdf

6.2 T1/ E1 MULTIPLEXERS _________________________________________________________ 651character to the terminal, while the multiplexer connected to the computer discardscharacters echoed by the computer. This enables data ¯ow through the multiplexersystem to be more responsive to the terminal operator. Since error detection andcorrection is built into all statistical multiplexers, a character echo from the computeris not necessary to provide visual transmission validation and is safetyeliminated by echoplexing.The other options listed in Table 6.4 should be self-explanatory, and the usershould checkvendor literature for speci®c options available for use on differentdevices.Utilization considerationsAlthough networking requirements govern the type of multiplexer that will resultin the best price-performance, several general comparisons can be made betweendevices. In comparison to FDM, the principal advantages of TDM include theability to service high input data sources, the capacity for a greater number ofindividual inputs, the performance of data compression intelligent multiplexers),the detection of errors, and the request for retransmission of data statistical andintelligent multiplexers). The key differences between multiplexers are tabulated inTable 6.5.Table 6.5Multiplexer comparisonsFDM TDM STDM ITDMEf®ciency poor good better bestChannel capacity poor good better bestHigh-speed data very poor poor better bestCon®guration changeData rate good fair good goodNumber of channels poor good better betterInstallation ease poor poor good goodProblem isolation poor poor good goodError detectionretransmission n/a n/a±good automatic automaticMultidrop capability good n/a possible possible6.2 T1/E1 MULTIPLEXERSThe primary utilization of T1 and E1 multiplexers is to concentrate both data anddigitized voice on a `T series' or `E series' digital facility. To obtain an appreciationof the utilization of T1 and E1 multiplexers it is necessary to examine the process bywhich voice conversations can be digitized as well as the relationship between voicedigitization and the data formats of the T1 and E1 carriers.