T1 Physical Characteristics

DB-15

RJ-48C RJ-48X

Pin

Signal

Pin

Signal

1

Tx Tip

1

Rx Ring

9

Tx Ring

2

Rx Tip

3

Rx Tip

4

Tx Ring

11

Rx Ring

5

Pin 1

Pin 8

RJ-48 8-Pin Modular Connector

Another interface that finds its origin in the days when DSUs and CSUs were separate devices is the DSX-1. This was the interface between the CSU and DSU and was considered the demarcation interface, with the CPE on one side and the service provider's equipment on the other. The physical connection was usually 8-pin modular or DB-15. Today, with the CSU and DSU functionality being integrated more often than not in one device, the demarc tends to be the opposite side of the CSU, its DS-1 interface with the provider's network.

Although the DSX-1 signal is the same as the DS-1 signal, the DSX-1 is a shorter-distance interface, which is not able to be deployed to the service provider's network without interfacing locally to a device with CSU functionality. The DSX-1 interface often connects two devices, such as PBXs or a PBX and a multiplexer, to each other locally, which simulates an actual T1-network connection. In COs the DSX-1 signal and interfaces interconnect pieces of equipment within the same office. Based on the distance between devices, line equalization (short-haul build-out) must often be applied to the DSX-1 interface to add gain to the signal, which allows it to be recognized and accepted by the opposing device. Line equalization is usually set in terms of feet, ranging from 0 to 655 feet, in roughly 133-foot increments.

Related to line equalization (inversely), but found on DS-1 connections to the service provider's network, is long-haul line build-out (LBO). LBO serves at least two main purposes. A more antiquated purpose stems from older repeaters requiring at least 7.5 dB of attenuation, with respect to the nominal signal level. If the CPE supplies a stronger signal than the repeater expects, pulses are not interpreted as the 1s they are intended to represent, and an all-0s signal results. More common today is the use of long-haul LBO to equalize signal strength between two or more customers whose circuits meet at a non-repeated line junction. This is a mutual cross-connect point or other such facility, whereby multiple circuits convene for redirection to the customers' respective first repeaters. If one customer is closer to the junction than the other, without LBO applied to the closer customer's DS-1 signal, the potential for crosstalk interference to affect the more distant customer's signal is great. Long-haul LBO is generally configurable ranging from 0 to -22.5 dB, in -7.5 dB increments.

The intent of the parenthetical comment in the first sentence of the last paragraph, regarding the two build-out schemes being inversely related, was to point out that while moving away from the 0 setting with line equalization, the signal level becomes stronger. Conversely, while moving away from 0 in the setting of long-haul LBO, a weaker signal level results.

Bridged taps and loading coils are common, everyday occurrences in the world of analog telephony distribution. Bridged taps are lengths of cable that are spliced into the primary distribution run, at a point as close to the subscriber as the facility can reasonably be tapped into. This is done to provide service without having to run a new pair all the way from the CO. Loading coils are sort of like non-powered analog amplifiers for frequencies in the voice range, which block frequencies outside of this range. Both bridged taps and loading coils are destructive to the quality of the T1 signal and must be removed before placing a T1 circuit into service.

The 6000-foot span between repeaters comes from the specifications for signal loss budget. Although different service providers vary as to where they decide to install a repeater, the customary rule of thumb is to install a repeater where the signal is measured to have a level of -30 dB (30 dB less than the original signal). A dB is a measurement of signal strength, with respect to a known value (relative power level). For audio levels, a dB is measured against a level of sound that is equivalent to the volume threshold of human hearing. For electrical signals, as in the case of a T1 signal, a dB represents the ratio of the observed signal strength, with respect to the initial signal strength from the source.

The mathematics behind dB values employs the science of logarithms. The logarithmic function results in a positive value when based on an argument greater than or equal to 1. For arguments less than 1 (but greater than 0), the logarithm is negative. Therefore, if gain has been introduced, and the resulting signal is stronger than the original signal, the logarithm is computed for a value greater than 1, and the result is positive. Conversely, if the signal experiences loss, in comparison to the original signal level, the ratio is less than 1 and the result of the logarithmic function is negative. It is important to understand that the negative sign in front of the dB level does not imply a negative signal level. In fact, the logarithmic function is undefined for values less than or equal to 0. The following examples illustrate the concept of the logarithmic function as it relates to dB of signal loss or gain.

• Formula for calculating dB loss or gain:

— 10 log (observed signal level/initial level) = loss or gain in dB

• For an observed signal level equal to the original level:

• For an observed signal level 10 times the original level:

• For an observed signal level 1,000,000 times the original level:

• For an observed signal level one tenth the original level:

You now can compute that a loss of 30 dB represents a signal level observed at one thousandth of the initial signal level.