ADSL: Putting A Charge Into Your Copper Cable
By Jeff Newman
Asymmetric Digital Subscriber Line (ADSL) is one of many types of xDSL technologies emerging. These technologies promise an affordable high-bandwidth medium for connecting relatively nearby sites. The research and development of ADSL was spearheaded by the telephone companies with the goal of bringing TV and video-on-demand services to regional customers to compete with the TV broadcast mediums.
The progression of the ADSL design has made it promising not only for digital TV broadcast, but also for other high-speed data applications--including high-speed Inter
net access, corporate information delivery to remote and branch offices, audio/video on demand, and a virtually endless list of other high-speed interactive applications--without interrupting voice services on the copper pair. In ideal conditions and distances, ADSL can transmit data at up to 6 Mbps in one direction, while allowing rates of more than 64 Kbps in the reverse direction (some versions promise as high as 9 Mbps downstream).
Considering that the average reliable transmission speed of traditional analog modems is about 30 Kbps, ADSL is roughly 200 times faster, and it uses the exact same copper medium.
At MCI Developers Lab in Richardson, Texas, Network Computing tested ADSL modems from Amati Communications
Corp. (ATU-C, ATU-R), Aware (Ethernet Access Modems) and Paradyne Corp. (5170/5171 ADSL Modems) to evaluate the strengths and weaknesses of ADSL.
We gave these ADSL units quite a workout and, in the end, found no major problems with the technology: ADSL is technically ready for
deployment. Couple that with the fact that its costs are dropping as the technology is maturing, and you should be hounding your telephone company for it right now.
No Additional Wiring Needed
The main advantage of ADSL is that it operates over the sam
e twisted copper pair already used today. There is no need for expensive switch upgrades or extra line installation or termination--as there is with ISDN; ADSL uses existing telephone termination points. Also, ADSL is a dedicated connection service, unlike ISDN, which operates by switched circuit connections that are tarriffed according to call duration and channel utilization.
To use ADSL technology, place an ADSL modem on the remote end and transmit data signals through the copper pair wire to the local telephone central office, which has its own ADSL modem. The ADSL modems we tested convert the data bits from the source (any computer on your network or other device) to a form suitable to be transmitted over a twisted pair (analog waveform).
In the process, redundant parity bits are coded into each sequence of data to ensure safe delivery until it is error-checked and demodulated on the telephone company side.
However, you don't need the telephone company to use ADSL. If your branch offices are across a campus, for instance, you can use a copper pair between those sites. It is possible to place a "remote" ADSL modem on the receiving end, and a "central" ADSL modem at the transmitting end of the connection with nothing but the copper wire in the middle. With a telephone company providing a backbone link, you can connect offices that are located great distances from each other, but are close to their respective telephone carriers' central offices.
ADSL lets you send several different types of data over different frequencies simultaneously. We were able to transmit separate signals using the best frequency for the particular application (data, voice or video). Depending on the line-coding method used to implement ADSL, frequencies can be aff
ected by distance and interference. Power is not required to operate voice telephone services (as it is with ISDN) while sharing the copper link. However, you need power from an outlet to run ADSL services. If the unit loses power, the power supplied by the phone compa
ny will continue to power voice services, which operate on a different frequency than ADSL. Most ADSL devices use what is known as a plain old telephone service (POTS) splitter to separate the frequency used for voice before being processed by the modem.
|
REPORTS
ANALYTICS
Follow 
