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A T1 is considered to be an ISP T1 when it is used as an Access T1, direct from the customer premises to an ISP (Internet Service Provider). Since ISP T1s are used solely for Internet access, they will not be carrying voice or other data services in a multiplexed access arrangement. An Internet Service Provider T1 will provide access throughput of 1.536Mhz of symmetrical bandwidth (less 8kb required for framing and line coding). Symmetrical bandwidth refers to the ability for the circuit to carry the same speed in either direction (i.e.: 1.536Mhz upload or download, depending upon the application). Asymmetrical bandwidth refers to a circuit that is not capable of the same upload or download speeds. (DSL or Cable-based service is an example of asymmetrical service) Framing and Line Coding. Since ISP T1s carry only data, the T1 access facility must be provisioned using ESF/B8ZS framing and line coding. The first part, ESF, refers to Extended Super-Framing , which allows for a single 8kb channel for control of the entire T1 and is a requirement for services greater than 56kb in speed. B8ZS refers to Binary Eight, Zero Suppression. This line coding option allows for extended streams of zeros without losing timing on the T1 facility. Since ISP T1s only carry a single circuit running at 1.536Mhz, the framing and line coding options for Data T1s are further restricted since the service(s) running on the access T1 are faster than 56kb.
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 Internet T1s since the access T1 is not channelized, and contiguous bandwidth is required to allow a single circuit greater than DS0 speed. Termination Options. ISP T1s will typically terminate at the customer premises directly into a router capable of accepting a T1 circuit. (Note: if the router does not provide CSU-functionality, the T1 will terminate first into a CSU for testing and line build-out equalization, then into the router) Other Options. ISP T1s may be combined with other T1s on a DS3 access facility. If this is the case, the T1 for ISP line will typically be de-multiplexed at the customer facility using an M13.  Common Names / Muxing Levels:
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ISP T1
