T1 Modems

T1 modems are not exactly existent items. The closest thing to a T1 modem would be a mux unit used in multiplexing T1s. T1s are digital 1.544Mhz telecom circuits. Also known as a "Hi-Cap" for High Capacity circuit, and more formally as a DS1, for "Digital Service level 1", a T1 is its most common name. Modems, by definition, perform a digital to analog conversion for a given input signal. For example: PC Modems convert digital information used by the computer to an analog signal that is compatible to the PSTN (Public Switched Telephone Network). Since T1s are digital carriers (that may carry analog signals multiplexed on to them for transport), modems are not used to terminate T1s.

Some may be confused by this, but suffice it to say, a Modem serves as a translator between a digital signal and an analog network service. Since T1 speeds are 24 times greater than analog network services (dial-up), a modem is not a viable option to terminate the T1 itself. Modems can be used to terminate channels of a T1 that have been de-multiplexed to an analog signal.

For example: XYZ Internet Service Provider uses T1 access for local lines due to cost savings. To terminate the T1, they use a Channel Bank with 2wDPT cards installed, with each of the 24 cards tied to a modem. In this case, though modems are used in the overall system, they do not actually terminate the T1 itself, only the services that are carried by the T1 access facility.

What can Terminate a T1? T1s are typically terminated into a combination of devices, each performing a specific function. Some devices can combine multiple functions within the same physical unit.

CSU: Channel Service Unit: This unit is either provided by the carrier or the customer and performs two basic functions.

1. Line equalization (i.e.: line build-out)

2. Remote Testing

• Carriers can place the unit in "loop-back" and can run a variety of performance tests on the unit without a technician present
• Customers can force the unit in "loop-back"

3. Alarming: CSUs display current line states

4. Is carried by 4 copper wires (or via fiber and converted back to copper at the customer site) and can support a wide range of services to include voice, data, and video. Through multiplexing (explained in detail later), a T1 circuit can carry a combination of all of these services into a single circuit, reducing monthly access costs as opposed to buying individual DS0 (analog or digital) circuits.

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