Realistic ADSL speeds

ADSL service is capable of transferring massive amounts of data in a short time over normal telephone wires. How do you know that you are getting the fastest connection possible? There are a few limiting factors that apply to all ADSL services, namely: TCP/IP Packet overhead, the distance from your physical location from the Central Office (CO), and of course, how much bandwidth is available from the remote sites that you are visiting/browsing. In general, most ADSL connections will be recognized as running between 80 and 87 percent of the purchased speed. Here's why:

TCP/IP packet overhead

The first and most important reason is the way that the Internet was built. Information zooms from one place to the other on the Internet in a "packet" format. Essentially, one computer sends its information in small, bite sized, pieces of information to another computer, which then acknowledges packet reception and waits for the next packet to arrive.

Each TCP/IP packet contains some of the data being transmitted as well as some control information to make sure that packets go where they are supposed to, and in the correct order. This information "overhead" contains additional information for ADSL connections. PPP over ATM packet header information gets added by the upstream bandwidth provider, this adds about 10% overhead to each packet. PPP over Ethernet also adds a few bits of information, which is about 3% of total packet size.

Considering packet overhead, the theoretical data transfer limit (without overhead information) is 87% of the purchased speed. For example, if you purchased a 960/120 package, maximum data rate would be 835/104 (or 104K/sec downstream, 13K/sec upstream).

Physical location

The ADSL signal travels across the upper frequencies of the telephone line that runs from the Bell Central Office (CO). Unfortunately, High Frequencies suffer from what is called Attenuation, which is cumulative signal degradation the further it has to travel along a the wire.

ADSL signals can operate on the local telephone loop anywhere up to 18000 feet from the Central Office. Connection speed is unfortunately affected by signal attenuation, with signal degradation becoming a factor past 10000 feet from the CO. Corrosion and a thin telephone wire gauge add to the signal attenuation, so older telephone wires can't carry as much data as newer, thicker wires.

Remote available bandwidth

In the days of modem connections, most websites had more than enough available bandwidth to satisfy all of its visitors, being able to serve hundreds of simultaneous connections. With the advent of broadband connections, users are requesting a larger chunk of the available site bandwidth, which sometimes leads to reduced speeds.

To illustrate, think of a water pipe with a multitude of attached faucets. If you open facet after faucet, the water coming out of the pipe will be coming out at a slower and slower rate. If you open bigger faucets, water pressure is reduced to a greater degree. If most of the faucets are opened, water will have enough pressure to only trickle out. To maintain the same pressure for all faucets, one would have to increase the water flow.

This same principle applies to available site bandwidth. To maintain consistant speed for all its users, a website needs to upgrade its "pipe" to the Internet. Some sites upgrade their connections, while others don't; those that don't start slowing down. The top speed of most sites, during prime time hours, rarely peaks over 80K/sec per user. The old "World Wide Wait" still applies today (albeit to a lesser degree)

To sum up...

Even if many factors, like packet overhead, physical location and remote bandwidth, limit the upper speed of an ADSL connection, it is still a viable alternative to a modem connection, achieving 20x the speed of a 56k modem. We just need to keep in mind the realities of ADSL technology and not expect the impossible from your connection.

Related Links

• DSL Reports
• xDSL FAQ