ADSL

In ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premises (downstream) than the reverse (upstream).

On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises.

It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end.

Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.

The marketing reasons for an asymmetric connection are that, firstly, most users of internet traffic will require less data to be uploaded than downloaded.

Secondly internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate a lot of uploads.

Under the usual discrete multitone modulation (DMT) scheme, each of these is further divided into smaller frequency channels of 4.3125 kHz.

The data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined.

The total maximum capacity derived from summing the bits-per-bin is reported by DSL modems and is sometimes termed sync rate.

[4] These choices can either be conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice that makes for a slower connection, or less conservative in which more bits per bin are chosen in which case there is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present.

A low SNR margin will mean high speeds, provided the noise level does not increase too much; otherwise, the connection will have to be dropped and renegotiated (resynced).

ISPs (but users rarely, apart from Australia where it's the default[5]) have the option to use interleaving of packets to counter the effects of burst noise on the telephone line.

It is therefore necessary to install appropriate frequency filters at the customer's premises to avoid interference between the DSL, voice services, and any other connections to the line (for example intruder alarms).

This way, the DSL signal is separated as close as possible to the central office and is not attenuated inside the customer's premises.

A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband (that is, POTS telephone-like) devices are connected to the line.

Telephone extension cords are typically made with small-gauge, multi-strand copper conductors which do not maintain a noise-reducing pair twist.

These effects are especially significant where the customer's phone line is more than 4 km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation.

On top of ATM, there are multiple possibilities of additional layers of protocols (two of them are abbreviated in a simplified manner as "PPPoA" or "PPPoE"), with TCP/IP providing the connection to the Internet.