DSL Speed Technology Summary

Our DSL service can perform at speeds of up to 1.5 M download / 256 K upload (512K / 128K for DSL Lite). However, actual performance may be less than this maximum under certain circumstances. The following explanation detailing the DSL connection training process was provided by BellSouth, through whose facilities some of our DSL services are provided. For your reference, ADSL is Asymmetrical DSL, the particular kind of DSL service that PortBridge Internet provides. A DSLAM is the piece of phone company equipment that enables the DSL connection to the end-user's location.

When testing the speed of a DSL service, please note that internet-based speed tests are affected by general network traffic and latency between the testing server and the end-user location as well as by the actual speed of the DSL service. As a result, the results of these tests typically are not an accurate representation of the true DSL connection speed.

The ADSL signal between the DSLAM and the end-user is a set of 256 individual tones (electro-magnetic signals) in the 0 to 1104-kilohertz range. In a perfect world, most of these tones can be used to carry data (tone 1 is used for POTS and some tones are required for band separation), however, there are many other signals both inside (e.g. T1s, DAML and ISDN) and outside the cable (e.g. AM transmitters) that are using portions of the same frequency band. They compete with these signal-carrying tones and make their detection problematic. This is called noise (sometimes referred to as disturbers or interferers). The ratio of the strength of the data signal to the strength of the noise signal is called signal-to-noise-ratio (SNR) or noise margin (NM), and is expressed in decibels (dB). The higher this number, the more bits of data that can be carried within that specific tone.

If all of these paths provide a satisfactory level of expected error rate, then the available bandwidth for carrying the ADSL signal is very large (greater than 8 Megabits per second). The BellSouth Industrial class of ADSL service will limit the downstream sync-rate to 1,472 kbit per second, and the upstream rate to 256 kbit per second. The ADSL equipment — the DSLAM at one end and the end-user modem at the other — will negotiate to use only a certain sub-set of these tones that will provide for the smallest error rate (or largest noise margin) on the connection. This is called training.

In particular, the training will use only those tones that during the negotiation process have at least a 6db noise margin. If there are a number of noise sources, or if the data signal is diminished by the length of the circuit, it may not be possible to meet the maximum rates of 1,472 kbps downstream and/or 256 kbps upstream. When this happens, the equipment will offer to synchronize at a lower sync-rate. This is done by eliminating tones or carrying fewer bits in certain tones (in 32 kbit per second steps) until the remaining tones all meet (or are within 1 or 2 dB of meeting) the 6db target SNR margin.

The percentage of tones used for data communication is expressed as occupancy percentage. If there is an overall high signal-to-noise ratio across the spectrum, then the equipment will sync up at maximum speed (as determined by the profile built on the DSLAM) and will have a low occupancy percentage and a high noise margin, due to the lower quality tones not being used. However, as the overall signal-to-noise ratio decreases (think of the length of the loop increasing), then more tones will fall below the 6db level and not be used. At some length, there will be a 100% occupancy (all qualified tones in use), a 6db signal-to-noise ratio on the worst quality tone, and full up and down bandwidth. As the length of the circuit is increased beyond this point (or external noise sources become more prevalent), more of the tones will be dropped. This causes the available bandwidth to decrease. The occupancy and signal-to-noise ratio will continue to be around 100% and 6db. At some point, the downstream rate will fall below 256 kilobits per second or the upstream rate will fall below 128 kilobits per second (the 256 kbps downstream requirement is normally the limiting factor). When this happens, the equipment will no longer synchronize.

During the normal operation of the connection, the signal-to-noise ratio will change, and the equipment is constantly monitoring the connection quality. Either end (DSLAM or end-user modem) can request a re-train. With the profile that we use the DSLAM will force a retrain when the SNR Margin reaches 0db in either the upstream or downstream direction.

In summary, the quality of the information carrying capability of the ADSL physical transport will change as conditions change, and they are constantly changing. It is normal, particularly on long loops, to have a speed that is below maximum. Also, when the speed is below maximum, it is normal to have close to a 6db signal-to-noise ratio and close to 100% occupancy.