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A T1 is considered to be a f ractional T1 when it carries a circuit greater than DS0 speed, but something less than a full T1 or DS1. For example, since a T1 runs at 1.544Mhz, it is capable of carrying any circuit running at speeds up to its own speed of 1.536Mhz (8kb is required for T1 framing and line coding). Applications that require a 256kb circuit, a 512kb circuit would require a Fractional T1 for access. Fractional T1s can also be considered Integrated T1s, if the remaining unused bandwidth leftover by the > DS0 circuit(s) are used for voice services. Typical Examples:
ESF/B8ZS T1s also have the benefit of ESF monitoring, which allows the carrier to setup ESFMUs (monitoring units) to watch the performance of the T1 itself without disrupting the services carried by the T1. ESFMUs track performance using the 8kb ESF channel for statistics. ESF/B8ZS is required for Fractional T1s since the access T1 is not channelized, and contiguous bandwidth is required to allow a single circuit greater than DS0 speed. Termination Options. Fractional T1s will typically terminate at the customer premises directly into a multiplexer. (Note: if the multiplexer does not provide CSU-functionality, the T1 will terminate first into a CSU for testing and line build-out equalization, then into the mux)  
Other Termination Options. Fractional T1s may be combined with other T1s on a DS3 access facility. If this is the case, the fractional T1 line will typically be de-multiplexed (from the access DS3) at the customer facility using an M13. Once at the T1 level, a D/I mux or TDM mux may be used.  Common Names / Muxing Levels:
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Fractional T1
