ADSL speeds not all they're cracked up to be
document 0113 from Network Fusion nwfusion.com
By Tim Greene
Network World, 1/6/97
If you believe everything you hear about Digital Subscriber Line (DSL) technology, you probably think that any day now you'll get Internet access fast enough to melt phone wires.
But the fact is there is a big gap between what is happening in the labs and the real world, carrier trials show.
Vendors talk about Asymmetric DSL (ADSL) supporting 8M bit/sec over residential-grade phone lines, but planned service rollouts will only support speeds from 128K to 1.5M bit/sec.
''We're just beginning to understand what we have to tackle,'' said Flynn Nogiueria, marketing director overseeing GTE Corp.'s DSL trials.
Carrier trials so far yield two key results: the quality and length of actual local loops can limit DSL bandwidth, and carriers need an architecture that not only works, but also is inexpensive to provision and maintain.
While carriers wait for ADSL gear that pushes more bandwidth over longer distances, their initial rollouts rely on lower speed but more mature technologies that plug into existing architectures: DSL based on ISDN at 128K bit/sec and High-speed DSL (HDSL) at up to 768K bit/sec over two wires.
''We just want to get this up and running and out the door,'' said Mark Gallegos, director of DSL product development for Pacific Bell.
Going the distance
It is important to users that distance questions get sorted out. If the user has to be within a limited range of a central office, service availability for, say, a large telecommuting force will be restricted and spotty.
Basic Rate Interface ISDN, the 128K bit/sec dial-up choice for many telecommuters and Internet users, works over 18,000 feet of two- wire copper and that can be doubled with a repeater. But users continue to complain that they cannot get it because they are too far away.
Ameritech's ADSL trials show that ADSL modems can support 1.5M bit/sec over a local loop that is 13,000- to 18,000-feet long. They expect technology developments to push that distance to more than 18,000 feet by the time they roll out service toward the end of the year.
The most optimistic bandwidth/distance result is from MCI Communications Corp. trials in Sergeant Bluff, Iowa. MCI reports that NEC Australia gear is delivering 6M bit/sec over 13,000 feet.
Other carriers are reluctant to reveal numbers, but most say the optimal scenario MCI paints is the exception. The rest are still trying to understand the restrictions imposed by the gauge of the copper wire in the local loop and whether the line is festooned with load coils and bridge taps that are used to enhance lines for voice.
To help deal with the problem, carriers embrace Rate Adaptive DSL (RADSL). It allows a single device to adjust bandwidth based on how good and how long a line is.
In deciding on what speeds to offer, carriers must also anticipate that factors beyond their control will affect the actual speeds Internet users get to their PCs over DSL lines.
''The telco can provide 1.5M bit/sec to the central office. Then, where the customer goes, nobody knows. They may hit a zillion other servers, and they may not be able to handle anywhere near 1.5M bit/sec,'' Nogiueria said.
Carriers are also struggling with how to provision the services. They would like to dump DSL traffic onto their data networks as soon as possible.
Carrier preference is for ATM to ride directly out of the customer premise over ADSL.
Carriers want those types of systems, from the user premises to the central office, before they widely deploy services. And those systems will not be available for general deployment until late this year.
As a result, they predict that next year will be when ADSL gets beyond the experimental user. |
