Senin, 05 Juli 2010

Why Are Some Modems Slow?

Slow Modem Cause #1
Unlike the old dial up modems, DSL modems use a different frequency range to the human voice that is transmitted at 4KHZ and below. This 4KHZ band is also known as the base band. DSL modems use the broad band frequencies which are in the 4KHZ to 4MHZ range allowing them to provide higher bandwidth and to work simultaneously with normal telephone voice calls. Interference between these frequencies can cause modem performance problems. A DSL filter(s) can be installed in order to prevent interference between the home telephone(s) and the DSL service.
Slow Modem Cause #2
Another common problem experienced by DSL broadband users is one of attenuation. Attenuation is a term used to describe the reduction in signal strength, as distance increases so does attenuation. The level of attenuation determines the speed that you will ultimately receive from the DSL service.
It is true, however, that attenuation should be reasonably constant given that the distance between your home and the service provider's exchange doesn't change. But some users experience different levels of performance during different time periods throughout the day. So what causes varying degrees slow modem problems?
Slow Modem Cause #3
One explanation is SNR (Signal to Noise) ratios. Noise caused by electromagnetic interference from electrical devices degrades the signal strength. Generally speaking, evenings are the worst time for line noise because many people are home using electrical equipment such as televisions and air conditioners. There is little a user can do about a high SNR, however, the phone company can be contacted for a line test to be done. In many cases components of the line leading into the home can also be replaced to help reduce noise.
Slow Modem Cause #4
Another possible contributor to slow modem response may be backhaul oversubscription by the service provider. Backhaul oversubscription means that the connection between the exchange and the core provider network has insufficient capacity to carry all the DSL subscribers together at full load. This may seem unhelpful at first, but this strategy helps to keep DSL line costs down.
The contention ratio is the definition that service providers use to describe the number of users that will be sharing a backhaul service. The lower the contention the lower the chances that you will share infrastructure with other users. For example a contention of 20:1 is better than 50:1. Some business class DSL services offer a contention as high as 1:1 which means that the line will be guaranteed all the provisioned bandwidth between the modem and the core network.
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Andres Villalva B.IT, AssDip(Eng), CCNP, MasterCNE is a co-founder of and writes articles based on over fifteen years of experience in the Information Technology industry. Find Online Learning including Network Virtualization, IT Cover Letters, Resumes, Interview Tips, Training, Online Learning and more at
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