[Tutor] OT: need computer advice from wise Tutors

ALAN GAULD alan.gauld at btinternet.com
Wed Jun 30 09:06:35 CEST 2010

> > If its a typical ADSL line it will be conneced to a DSLAM at 
> > the centeral office(by the telco) and  that will be shareed. 
> > ADSL also loses bandwidth the further you are from the office 

> process has to be diversified for each customer, because it
> all travels at the speed of light. 

It is diversified on the individual line leading to your house 
but it is shared from wherever it gets converted to a digital
multiplex. That could be at the roadside cabinet or at the 
central office. At that point the data is mixed together 
using a round-robin type algorithm. The more active signals 
that are present the lower the proportion of the total bandwidth 
available to each.

The speed of transmission is not really relevant (it only affects 
transit time not bandwidth) although it is as you say the 
speed of light (but not the speed of light in a vacuum, much 
lower than that) within the bearer. However within the electronics 
the signal travels more slowly - around the speed of sound - but 
fortunately for very small distances.

> sometimes had a weak signal depending on the distance
> from the consumer. This signal could be modified from the cable
> company, so the box amplified the 'weak', but not further away,
> signal.

Its not a matter of signal strength but of signal smearing as it 
travels along the line. The digital data is sent as pulses (a great 
simplification!) but those pulses lose their shape as they travel 
along the line until eventually the electronics cannot distinguish 
one from another. The only way to improve is to send the 
pulses more widely spaced, which reduces the bandwidth, 
or to have repeater boxes which regenerate the pulses at 
regular intervals. On long distance routes, between cities, the 
telco will install repeaters but for domestic use they progressively 
reduce the bandwidth.

> So this would also be a matter of what was being sent/received 
> being modified, depending on the priority (given by the ISP provider) 
> of the current DSL (or whatever connection) consumer? 

No, it doesn't matter what priority the comnsumer has it is down 
to the physical characteristics of the line. Telcos typically use 
copper or aluminium conductors in the local lines with paper 
or PTFE insulation and either crimped, twisted or soldered joints. 
All of these affect the transmission characteristics but the biggest
factor is the length of the line. Its not economical to install 
repeaters in every consumer line so the bandwidth must be 
adjusted to match what the line is capable of. (This is done 
dynamically so you may even find your bandwidth varies slightly 
depending on the weather!) In practice it should be fairly stable
and the biggest variations will be due to contention at the 


Alan G.
(Who works for a telco! :-)

More information about the Tutor mailing list