The following is some information regarding some of the terms that you may have encountered in relation to loop qualifications for ADSL services. Part of this information was drawn from the Newton’s Telecom Dictionary, which is a good resource for data terminology. For your reference, another good on-line source of information is www.whatis.com .
Assymetrical Digital Subscriber Line, also commonly referred to as the generic term DSL, Digital Subscriber Line. High Speed DSL is a high-bandwidth Internet connection utilizing ordinary copper telephone lines. Depending on the location, DSL modems can train anywhere from 1.5 Mbps to 4.0 Mbps downstream, and 128 Kbps to 1.0 Mbps upstream. There are various other DSL technologies available, some training as fast as 7.0 Mbps and upwards. But for the purposes of this information, we will be sticking with the 1.5 Mbps to 4.0 Mbps service to which the terms below will apply.
A bridge tap is a branch of two-wire cable that is connected to the two-wire loop at one end, and is open (unterminated) at the other end. Bridge taps cause signal loss in the attached cable under two conditions:
There is an impedance discontinuity at the point where the cable is attached to the two-wire loop.
The impedance mismatch presented by the unterminated end of the bridge tap reflects the signal back to the two-wire loop.
The length of a bridge tap and the quantity of bridge taps determine how much signal rejoins the two-wire loop, and at what time relationship to the transmitted signal the reflections rejoin the two-wire loop. Bridge taps are acceptable within certain limits because the Datapath system distinguishes and masks most interfacing signals. If the length and quantity of bridge taps are not within the limits described below, they can produce reflected signals that interfere with the original signal. Bridge taps will not work with ADSL signals.
The short explanation is that the pair of wires they are trying to use is shared with other circuits (either voice or data). This is done where there is a shortage of cable or the distance is great. What is done is a electronic carrier is put on the line that carries as few as 2 or maybe 500 voice or data circuits. It is not ADSL compatible.
Note that Carrier Cable is referring to ‘Subscriber Carrier cable’ rather than ‘cable from another carrier’.
Central Office (CO)
A central office (often referred to as CO) is a building which houses the telephone switches and ADSL components for a neighborhood or town area. All telephone lines from the customer’s premises are routed to the CO, where it is then terminated and jumpered to the rest of the phone or ADSL network for a region. In some locations, a remote CO is used to extend the distance of the central office serving area.
Centrex is a bundled voice communications service, designed to provide key and PABX system equivalent functionality through central office switching hardware and software located on TELUS premises and station equipment located at the customer’s premise. ADSL will not work on Centrex services.
Extended Reach ADSL (ERA)
ERA is a new ADSL system which can reach extended distances beyond the current ADSL system. Since ADSL is a distance sensitive and copper-based technology, some locations were disqualified because they were too far from an ADSL-serving central office or were fed by a fibre-based technology. The ERA system is not CO-based, and can be used on both copper-based and fibre-optic based telephone lines. ERA systems can reach locations of around 10 kms or more. The ERA rollout is expected to hit Alberta sometime in the last half of 2002.
A facility is another term used by telephone companies and technicians to simply describe the telephone lines or cables, and the components making up the phone line from the CO to the customer’s premise.
This is a premise fed by fibre cable from the network. ADSL is compatible only with copper facilities only.
An extra long run of telephone cable (local loops exceeding 18,000 feet in length) equipped with load coils, to improve the voice quality of a phone line. What this loading does is to insert inductance in a local loop circuit to offset the effect of capacitance in the cable. Loading “tunes” the circuit to the voice frequency band (500 to 2500 Hz) and thus improves the quality at the expense of overall bandwidth. Digital signals and ADSL signals cannot pass through the load coils.
Measures of resistance. A resistance of one Ohm allows one Ampere to flow when a potential difference of one volt is applied to the resistance.
In simple terms, distance diminishes the strength of the signal.
The Ohms Resistance Range for ADSL has been currently defined as follows:
1001 – 1100 ohm – would qualify for 1.5M service
801 – 1000 ohm – would qualify for 2.5M service
0 – 799 ohm – would qualify for 4.0M service
Overline service is a network offering that consists of more than one line and allows incoming calls to rotate in sequence to the next available line. Overline is also referred to as a rotary line. (For example, if you call into 301-0001, if it is busy it rotates to 301-0002, if that is busy it rotates to 301-0003, etc, etc…) Because the cable is strapped, meaning connected with a jumper with the other lines in rotary, it is not ADSL compatible. If an overline is unstrapped from the rest of the rotary group, the line may qualify for ADSL.
Rural Cable or Line
Distance is a major factor with rural lines. With a rural line, the service is too far from any Central Office (CO) or Remote CO for ADSL to work properly. Note that rural addresses are often a legal land description (quarter, section, township, range and meridian – ie. SW4, 8, 6, W5).
A tadiran is installed at the customer’s premise at the demarc, to give the customer additional lines. Tadirans are often used to add a 3rd line, in locations that only have the capability for 2 lines. This device is only used as a temporary solution until a permanent facility (telephone cable) is placed or a defective line is repaired. Tadiran is not compatible with DSL.