| Internet access speed is a popular bragging topic these days. For example the
following discussion is overheard at a local party:
Bob: "I have a 1.56 megabit DSL line."
Group: "Oooooh!"
Mary: "I have a 384K DSL line."
Group: "Ahhhhhhh!"
Joe: "I'm using a 56K modem."
Group: "What a shame!"
What is being referenced is the rate at which data bits are being sent from
your computer to the Internet or visa versa. A 56K modem can receive data at a
maximum rate of about 56,000 bits per second. A 384K DSL line can receive data
at a maximum rate of 384,000 bits per second. The actual data rate is almost
always less than the advertised maximum - sometimes considerably less. In this
article we will discuss some of the factors that effect the actual data rate.
Traffic Jams
To understand one of the factors that will effect your actual data rate,
think of the information highway as a normal system of roads. Each road has its
capacity and speed limit. At stop signs or traffic signals, you may have to wait
while other traffic passes. And, of course, there may be traffic delays. In the
same way, your data travels over many links. Each link has its capacity and data
rate. Routers are devices that connect the various links and are the equivalent
of stop signs and traffic signals. Your data may have to wait while someone
else's data passes.
Using the highway analogy, there are additional delays (traffic jams) during
peak use hours. You will achieve the faster data rates after 11 p.m. and before
7 a.m. (except Friday or Saturday nights).
Overhead
There is always some loss, and the faster your connection is, the greater
that loss is going to be.
Wrapped around that data is some overhead that can range from 2 percent to 25
percent of the total data sent. This extra information ensures that the data is
transmitted and received correctly at both ends of the connection, and that the
data packets arrive at the right destination.
There is no way to control or discover exactly how much overhead was used,
but generally the percentage is small.
Sharing Bandwidth
While your dial-up modem, DSL, ISDN or other Internet connection may be a
dedicated line, all of an ISP's connections get combined into one or more shared
connections. In most cases, these shared connections have less capacity than the
combined total of all the customer connections they serve. Done judiciously,
this works better than you probably think. Since most Internet users spend more
time reading their e-mail and Web pages than they do downloading them, they are
only using a fraction of their connection's actual capacity. Overbooking allows
an ISP to combine several customer connections into a single link that is
smaller (and less expensive) than the combined total of all the connections they
serve, without reducing the amount of data sent to a customer when they are
downloading data.
The problem is that some ISPs, cable modem companies, and DSL providers take
the overbooking concept too far. They funnel so many users into a small combined
connection that normal customer demand overwhelms the capacity of the combined
connection. This is a particular problem during peak usage hours, when line
speeds can slow to a crawl.
Unfortunately, there is no remedy for this problem, and it is difficult to
detect since web servers will slow down during peak usage as well. As
competition in the high-bandwidth Internet connection business heats up, you
will have more options and your ISP will have more incentive to maintain more
reasonable overbooking ratios. Until that time, however, your only options are
to complain to your ISP or switch to another Internet provider with a better
track record.
Special Factors for 56K Modems
Unfortunately a 56K modem will never be able to use its full bandwidth. In
fact the modems will usually only connect between 40-48K, depending on several
factors.
Interference from radio signals, power lines, and other sources disrupt most
56K modem signals. Phone company line amplifiers that boost a telephone signal
over a long distance, PBX switchboard systems, and other phone equipment alter
the phone signal and introduce just enough noise to prevent high speed
connections.
Also, the FCC does not allow 56K modems to use the full range of signals that
phone company equipment can generate. They are concerned that it will cause
interference with other phone lines. So no 56K modem in the United States ever
connects at the full 56K data rate.
Measure Your Actual Data Rate
Conveniently, there is a web
site which has a test that will help you measure the real world performance
of your Internet connection.
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