ADSL

Expected ADSL speeds

256/64 = above 20KBytes p/s
512/128 = above 40KBytes p/s
1500/256 = above 120KBytes p/s
 

Return to Top

Cable

Dial up

Standard analogue telephone lines can only transmit raw data at around 3430bps.
Some telcos only guarantee their lines for 28.80bps.
Data compression raised the data speeds to 9600, 14,400, 28,800 and finally 33,600 baud.
33,600 baud was thought to be the absolute speed limit on modem transmissions until two competing Digital Technology compression standards (X2 and KFLEX) were created. These two standards were then replaced by V.90 and now V.92.

Both the V.90 and V.92 standards default to the fastest possible connection, but impurities in the public switched telephone system may reduce the actual throughput of the modem. Physical defects in the makeup of the telephone line, electronic switching devices, filters, amplifiers etc, installed by the telco, extension leads and even the weather can reduce the effective bandwidth available to your modem.
Other devices (phones, fax and answer machines etc) sharing the modem line will also have a negative effect on data bandwidth.

The client modem tries to detect these impairments during the initial connection negotiation, if the line conditions change, the modem will retrain. A retrain can be initiated at either end of the connection by a modem detecting lost or corrupt data. During the initial connection "handshake", the host (ISP's) modem negotiates with the client modem, testing the capabilities of the phone line, by generating a DIL (Digital Impairment Learning) data burst. This "discovery" is a V.8 DIL data burst.
This is the funny sounding "BONG" during a modem handshake. The "BONG" sound is an equal mix of sound that is spread across 150 Hertz to 3750 Hertz. The client modem will listen to the sound and try to locate the "bad spots" (noise or static) or impairments and avoid placing data into them.

In most cases when a modem is losing data, it will train down. Line conditions are dynamic and will vary during the call as well as from call to call. In a case where a client modem makes an aggressive connection, there may be so much noise on the telephone line that the modem spends a large portion of the time re-sending data that was corrupted or damaged, or re-training to overcome this condition. This is one of the causes of low throughput and disconnections if the modems fail to renegotiate.

It is very common during peak telephone usage that a call to a number right next door be routed across town, or to a different city and back. While this helps the phone company deal with increased demand, it can create a potential for low-quality modem connections.

The more expensive (than internal) external modems are usually better at coping with less than perfect phone lines, they also don't relay on the computer hardware and software to maintain the link.

Telcos that use their television cable network as part of their analogue phone network also introduce problems due to the bandwidth used, amplifiers, compression etc. client modems can often only obtain V34 (33.600baud) connections.

Return to Top

Satellite

Return to Top

Return to Top

Please let me know if any of the links no longer work and any links you would like added.