Specifications

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be the shortest distance between the CO and subscriber. Some ILECs are installing remote Terminals (RT)
to reduce cable distance allowing them to serve more customers.
2.2.1.2 Digital Loop Carrier
DLC is a technique that allows the phone company to use a single circuit to deliver more then one phone
line. This reduces the cost of delivering telephone service. Unfortunately it is incompatible with DSL.
If your phone is on DLC you will not be able to get DSL. Depending on the type of DLC it may also limit
analog modem speed to 33kbps or less. Newer versions of DLC called Remote Terminals eliminate the
limitation allowing more customers to obtain DSL.
2.2.1.3 Load Coils
As phone signals travel down the wire they become attenuated due to resistance and impedance effects.
This is more pronounced at high frequencies then low. The phone company extends the range of phone
circuits by placing loading coils periodically along the line. This cancels out some of the harmful effects
and result in a stronger voice signal over long circuits.
Unfortunately load coils are incompatible with DSL. Load coils are effective over the range used by human
voice at the expense of higher frequencies. This did not matter as long as the circuit was used solely for
voice but it severely attenuates DSL. The only solution is to have them removed. SDSL providers normally
pay the ILEC to do this as part of the circuit install. If you line is loaded you will not qualify for ADSL.
2.2.1.4 Bridge Taps
When telephone cable is installed the phone company does not know exactly how many circuits will be
needed at every location. The solution is to run a large cable down the road. As customers order phone
service the installer selects an unused pair in the cable and splices the local drop. This means the circuit
feeding your phone may continue past your house for hundreds or thousands of feet. This is called a bridge
tap. It is of no consequence for telephone service. DSL is designed to tolerate some amount of bridge tap,
but if your circuit is marginal it may cause problems or push you line over the distance limit. SDSL
providers normally pay the local Telco to remove bridge taps when the circuit is installed. This tends to be
expensive and is not done for low cost residential ADSL.
2.2.1.5 Noise and Crosstalk
The cable that carries your phone circuit also carries many other phone lines. These may be ordinary POTS
service, ISDN or T1. Imperfections cause some unintentional coupling from one circuit to another. This
raises the noise floor on the connection. If it gets excessive your speed is reduced or you see lots of
retransmissions to recover from data errors.
If you can hear noise on your phone you are much more likely to get the Telco to fix the problem. If it only
affects DSL it will be hard getting them to fix it, since in general consumer grade DSL is not warranted for
minimum speed. It is simply a best effort on the part of the Telco.
2.2.1.6 Half Ringer
Excerpted from ADSL Forum Technical Report 013.
It has been standard practice in many areas of the United States
to install, at the Network Interface Device (NID), a network
termination device that is called a half ringer. It is an
example of an AC type termination device since it is detected
using AC techniques.
A normal POTS mechanical ringer, in a residential telephone, is
made up of an inductor and a capacitor in series that is bridged