Local Loop FAQ

What is the local loop?

A pair of wires, moderately twisted for the entire length between the telephone company's end office and the user premises (the common telephone set) form a loop, so it is referred to as the local loop. This loop provides a user with access to the global telecommunications infrastructure that is installed all over the world. The local loop has been historically designed to provide voice grade audio service.

The circuit is powered from the central office with 48V (open circuit voltage) limited in current to a value somewhat higher than 20mA. This current is used for signaling phone access, burning off moisture, breaking through metallic oxides caused by corrosion, and powering a carbon microphone. The original telephone equipment contained no active electronics.

The actual wiring of the local loop may be considered to be a lossy transmission line. xDSL uses whatever frequencies will propagate on this line for purposes of digital data transmission. T1 modulation (alternate mark inversion) has been doing this for years. xDSL extends the capability by using modern technology to increase the data rates and distances spanned.

What is a bridge tap?

A bridge tap is an accidental connection of another local loop to the primary local loop. Generally it behaves as an open circuit at DC, but becomes a transmission line stub with adverse effects at high frequency. It is generally harmful to xDSL connections and should be removed.

Extra phone wiring within one's house is a combination of short bridge taps. A POTS splitter isolates the house wiring and provides a direct path for the xDSL signal to pass unimpaired to the ATU-R modem.

What are loading coils?

Loading coils are used to extend the range of a local loop for voice grade communications. They are inductors added in series with the phone line which compensate for the parallel capacitance of the line. They benefit the frequencies in the high end of the voice spectrum at the expense of the frequencies above 3.6kHz. Thus, loading coils prevent xDSL connections.

What are echo suppressors and echo cancellers?

These are active devices used by the phone company to suppress the reflection of an analog signal or positive feedback (singing) on the phone network. The effect of the echo on a voice connection is undesirable. Imagine that as you spoke into the phone's microphone, there was a short delay and you hear your own voice back over the earpiece. A soft echo that comes back fast enough is not bothersome to the average person. A more delayed echo is annoying.

A echo suppressor works by allowing only one direction to transmit at a time so as to entirely eliminate the effect of an echo. An echo suppressor is able to switch between each end very rapidly, typically within 5msec. Network echo suppressors make full-duplex communication impossible. However, modems can deactivate these devices by sending the 2100 Hz answer tone at the beginning of the connection.

An echo canceller subtracts a locally generated replica of the predicted echo based on the signal propagating in the forward direction. Echo cancellers do allow full-duplex operation and are generally preferred over echo suppressors in voice calls. But when network echo cancellers compete with echo cancellers within the modem they are problematic. Typically they reduce data rates to 9.6Kb/s or lower. Network echo cancellers are deactivated by placing 180 degree phase reversals every 450msec on answer tone. As long as carrier is maintained, they are supposed to remain deactivated.

xDSL is not affected by network echo suppressors/cancellers because they are part of the CODEC signal processing.

What is a CODEC?

CODEC is an abbreviation for coder/decoder. Specifically it converts a voice grade analog signal to u-law or A-law encoded samples at an 8 kHz sampling rate. xDSL bypasses the CODECs at the central office by separating the xDSL signal and voice frequencies in a POTS splitter. The voice signal is passed to a CODEC while the xDSL signal terminatesin a DSLAM, the xDSL equivalent of a CODEC.

How do I determine how far I am from my CO?

You can call your service provider and ask them for the address of your local CO. Using a map, you get an approximate distance from your residence to the CO.

However, these are very rudimentary measurements because you can never be sure exactly what route your line takes between the two points. It may not be a direct route. If you're interested in whether you will qualify for high speed broadband service or if you're just wondering what the potential speed you could attain may be, there are other factors to consider (i.e. wire gauge, element continuity, environments, etc.) In a nutshell, just knowing the where the CO is, may not tell you much at all.

What do people mean by a "truck roll"?

Anytime a service technician needs to be dispatched in order to install, configure or troubleshoot a line installation, it is referred to as a "truck roll". The significance of this term implies a real cost to the service provider whenever a technician's time is required. The term derives from the scene of a technician driving the familiar "company truck" and pulling up to the curb of your premises with the intention to install, configure or troubleshoot a line.

What is dry copper?

Dry copper refers to twisted pairs that are not connected to a telephone switch, battery or anything else between customer locations. They are merely cross-connected in between. The term "dry" actually originated over 100 years ago, when batteries were first used to power telephones. A dry pair had no power applied to it from the CO and a "wet" one did.

Some folks have been able to implement xDSL via dry copper connection between two sites. By simply placing xDSL modems at each end of the dry copper connection, a xDSL may be possible with little intervention from the perspective of the CO. However, this is a risky method of deploying xDSL, especially asymmetrical versions. The problems occur when there is interference between the dry copper xDSL lines and other lines nearby, such as T1 and POTS. Typically dry copper has been used for low speed alarm circuits. By implementing xDSL service over dry copper, you run the risk of future problems. You may disrupt service at the CO and hence, the CO's customers. Unless you have specifically contracted for this method of xDSL service in advance, beware.

What are binder groups and why are they important?

A binder group is just a bunch of wires. More correctly in the telco world, a collection of twisted pair wires will share a common "sheath". The implementation of services within a binder group needs to be considered so that the effect of interference between services does not degrade nearby signals. Interference between wire pairs in a binder group can be a major issue in xDSL deployment.