Betting on WAN Access Technology
Telephone companies have an advantage over cable companies in that they do not have to upgrade switches or replace wiring to make ADSL work. All they need to do is place back-end modems at the local central office (CO) and remote-end modems at each customer's premises. Unlike ISDN (ADSL's sibling), no additional provisioning, equipment configuration or copper lines are necessary; like cable, implementation is simple, requiring little or no technical support to set up and operate.
Depending on the type of line-coding used to implement the ADSL network and the distance between the customer site and the CO, data transfers can reach speeds of 640 Kbps upstream and up to 9 Mbps downstream. The best line-coding method is a topic of debate in the ADSL industry. The most popular methods are Di
screte Multitone (DMT) and Carrierless Amplitude Phase (CAP) modulation. Although IEEE chose DMT as the standard, many vendors continue to develop for CAP and other line-codi
ng techniques, such as 2B1Q. Until recently, the idea was for carriers to choose a technology and for customers to provide compatible equipment. Now, the trend is shifting toward giving customers a choice of technology, with which the carriers must comply. As a result, customers will find that understanding the differences between line-coding techniques will become increasingly important (line speed and error correction depend on them).
ADSL's topology is similar to that of a star network, which usually doesn't share available bandwidth among everyone at the local CO (see "ADSL Network Access," on page 50). This makes an ADSL network more secure to use than a cable network, which is accessible to everyone on a network segment. ADSL has physical limitations like distance. However, potential problems may crop up between the customer sit
e and the CO and quality and condition of copper wiring that can affect performance.
ADSL operates over a distance of up to 18,000 feet of 24-gauge copper pair wire in optimal conditions. According to specification, ADSL implemented over short distances will allow for full 9-Mbps speed depending on the line-coding. However, as the distance increases, line attenuation naturally degrades the signal, and less bandwidth is available for transmission. This effect is compounded when copper lines that have been in place for long periods of time are exposed to moisture and other environmental elements that can damage the lines. The telephone companies that have mixed very old and new technology in their build-outs over the years to accommodate a growing customer base will always be battling this ongoing problem.
Therefore, no site can be sure that it will get a predictable line speed appropriate to its distance from the CO until service is actually installed. Our tests of ADSL at MCI's labs (see "ADSL: Putting
a Charge Into Your Copper Cable," May 1, page 139), however, show ADSL to be a die-hard technology that works well even with significant adversity from the combined elements of
interference and distance, though potential problems may crop up as carriers begin to pair ADSL with other high-speed lines in large-scale carrier networks. ADSL may introduce interference on other lines carried in the same bundle, for instance. More than likely, however, the problem with ADSL will lie not in the technology, but in regulatory politics and business strategies of the telephone companies.
Like cable companies, local exchange carriers will have to struggle to provide a reliable data network infrastructure, as well as adequate management to handle increasing amounts of data and the rising number of subscribers. But unlike the cable industry, telcos must also compete against their own other product lines, such as ISDN and T1.
The fact is, local exchange carriers have spent plenty of money on ISDN, only to find it dif
ficult, expensive and hard on the network. ADSL doesn't tax local switches the way ISDN does because it allows telcos to put voice traffic on their voice switches and move data traffic away from them. However, the local exchange carriers don't want to shell out more money to introduce and implement yet another technology. Nor do they want to introduce a service that will detract from their abundant crop of T1 subscribers.
This is what makes the new ADSL technology a crapshoot. Network designers trying to plan ahead have no way of knowing how the telcos are going to deploy or tariff the technology. The only reason telcos might ever kill T1 would be to drive demand for T3 services on the back end. As it turns out, they may be forced to provide inexpensive ADSL, or inexpensive T1, through competition from aggressive carriers that will soon invade their territory.
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