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T1 Basics – a Primer (cont’d) April 13, 2010

Posted by TelUS Consulting Services in Data Networking catagory.

T1 – Applications

Well, then, what do we do with these DS-1/DSX-1/T-1 signals? There are several applications and specific equipment that can be applied.

  • PBX
  • CSU
  • T1 Muxes
  • Fractional T1

The most important issue to see is that there can be T1 networks that are customer owned and T1 networks that use the AT&T Accunet T1.5 system. The applications will be the same but the constraints on the equipment are more stringent using the AT&T connection.

PBX (Private Branch Exchange)

Clearly the intended use of T1 was to bring in as many telephone lines using voice as possible through a digitized technique (PCM Pulse Code Modulation). Tie lines between PBXs account for many private T-1 network applications. This is supported through 2 and 4 wire E & M (Ear and Mouth) signalling techniques through the T1 Mux. A 2w FXS (Foreign Exchange Subscriber) function (dedicated line to a distant CO) and 2w FXO (Foreign Exchange Office) function (the CO version) can also be supported by the T1 trunk. In the latter mode, the T1 line acts as an “extension cord”. The primary way in which customers use this function is through the T1 Multiplexor.

CSU (Channel Service Unit)

This may be the easiest to explain. A DS-1 comes from the phone company to the customer. This line must be given the proper termination, line protection (vis-a-vis FCC Part 68), and message handling capability. In the old days, the phone company supplied this equipment but today this probably will be CPE (Customer Premise Equipment). The output of the CSU is the DSX-1 signal. The most common CSU is found in a T1 Mux however they can stand alone with various added functionality.

The bipolar output of the CSU can be connected to a DSU (Digital Service Unit) which converts the bipolar signals to unipolar and vice versa at the data rate gleaned from the bipolar signals.

The DCB T-Driver, for example, is a DSU. It takes unipolar data from the terminal and coverts it to a DS-1 signal. In many ways it also acts as a CSU and its transition to a CSU/DSU is quite possible. AT&T Pub 62411 requires that a CSU perform the following functions:

  • regeneration
  • loopback
  • keep alive

The regeneration part is part of the T-Driver functionality. Loopback is commanded from the Carrier in one of two ways:

  • in line data pattern with D4 (SF) formatting
  • using the FDL with ESF formatting

As the FDL is already being used in T-Driver, it would be rather straightforward to incorporate the appropriate responses to the command structure of the loopback from the carrier. The interface is already surge protected and meets FCC Part 68. The conclusion is that we have with relatively small impact an “ESF CSU” in the T-Driver product that can connect directly to the carrier. To incorporate an “SF CSU” which is still quite prevalent in use with D4 channel banks, would be a more significant undertaking requiring hardware and software changes.

As a matter of note, DDS (Digital Data Service) also requires a CSU but most units are sold as a CSU/DSU with a V.35 or RS-530 connector right on the device. DCB’s T1 and fractional T1 CSU/DSUs are examples.


This is actually a family of devices dedicated for customer use. They are normally T1 or fractional T1 TDMs which comply with format constraints , DACS interfaces, and often have an optional CSU. Their purpose, depending on the number of ports, is to allow transmission of data, image, and voice form many different sources of a single network link.

Many T1 Muxes are also Subrate Data Muxes (SRDMs). By this identification they are able to accommodate synchronous data rates of 2.4, 4.8, 9.6, and 19.2 Kbps. Asynchronous data rates are also allowed in some devices. SDRM operates per DS0.

Since T1 muxes are also DACS compatible at the DS0 level, Fractional T-1 service is also compatible with the devices. They also comply with the D4 channel bank requirements of bit density, zero density, and the provision of clear channel. FT1 is like SRDM only at the DS1 level. Hence, data may be at multiples of 64Kbps.

Also many T1 Muxes allow for the integration of the AT&T Switched 56 service. These are important month-end transfers, CAD/CAM files and teleconferencing.

FT Series Fractional T1 DSU/CSU

The FT DSU/CSU’s have a DS-1 output signal, and are FCC registered DSU’s. They take data at a configured speed via an RS-530/V.35 interface and convert the data to a T-1 data stream. The format of the data is can be D-4 or ESF. The transmitter is configured with a selectable signal attenuator (LBO) of 0, 7dB, and 15 dB per AT&T spec. The FT series is available in a single channel units (FT-1), two channel unit (FT-2) and a 4 channel unit (FT-4). Each port can be configured to use from 1 to 24 of the DS-0’s (56 or 64 Kbps each DS-0). The FT-2 and FT-4 units also have drop and insert capability.


T-Extender is a T1 repeater designed to AT&T specifications. This device takes a DS-1 signal and regenerates it as a DS-1 signal.  For example, a BPV is passed through just a readily as a normal signal.

In my next blog I will discuss a little about T1 troubleshooting.

Joe Buck, N.C.E.

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