An imperfect DSL world

Real-world ISP tests show even the littlest thing can disrupt DSL transmissions; AM radio is an unexpected culprit.

By Tim Greene
Network World Fusion, 3/9/98

San Jose, Calif. - Digital subscriber line (DSL) modems are not as fast or asstable as equipment makers would have you believe, according to one of the first real-world studies of the high-speed devices.

The study by Internet service provider Avalon Networks, Inc. shows that behind the promise of using regular copper telephone lines to support 7M bit/sec Internet access lies the reality that DSL is an immature technology susceptible to failure when faced with real network conditions.

Avalon, which is preparing a DSL-based Internet access service in Sioux City, Iowa, found that line length, circuit quality, electrical interference and even AM radio signals can reduce the top speed of asymmetric DSL (ADSL). Some of these factors can even cause connections to fail, according to the ISP.

Avalon last week shared its results with Network World and this week will reveal the results publicly at the DSLcon '98 conference here. Full results will be posted at dsl.avalon.net later this week.

This is the first time a service provider has revealed details of its technology trials, though many of DSL's problems have been acknowledged in the past by modem makers and other service providers that have run tests.

"The [regional Bell operating companies] have this information, they just aren't releasing it," said Randy Carlson, an analyst with The Yankee Group, in Boston.

Avalon last November began testing modems from seven vendors on 17 regular copper phone lines leased from US WEST, Inc. The goal was to find out whether modems at either end of the lines could connect, and if so, at what speeds. Avalon found that speeds declined as line length increased and eventually the modems couldn't connect at all.

"We haven't hit nirvana on modems yet," said David Lacey, Avalon's president.

In general, Avalon found that products based on the standard-based DSL technology, known as discrete multitone (DMT), were more susceptible to speed drops due to low-quality circuits and long lines than were devices using the alternative, carrierless amplitude modulation (CAP) technology.

But DMT seems to be maturing rapidly. Lacey said a prototype DMT-based modem from Pairgain Technologies, Inc. worked as well as the best performing CAP modem, Westell Technologies, Inc.'s FlexCap2.

Avalon also wanted to know whether modems would drop connections over periods ranging from 24 to 72 hours, so the company measured how long it took to re-establish dropped connections. That period ranged from 12 seconds for Pairgain modems to 35 seconds for modems from Amati Communications Corp., now part of Texas Instruments, Inc.

"All these vendors are claiming they have the best product. Well, a lot of them have some problems yet," Lacey said.

For example, in Avalon's tests, Amati modems failed to establish connections on lines that posed no problem for other vendors' modems. Also, Aware, Inc.'s X200 modem frequently negotiated connection speeds that were too high to maintain, and as a result the connections frequently dropped, Lacey said.

With many DMT devices, the amount of traffic flowing toward the customer modem can affect the maximum speed of traffic flowing away, Avalon found. When the upload stream is pushed to its limits, the downstream bandwidth plummets, Lacey said.

That could affect TCP sessions, which require a back channel bandwidth 10% the size of the forward channel bandwidth to keep the session alive. The problem did not appear with CAP modems, Lacey said.

Other factors can hurt modem performance, too. A T-1 line inside the same network cable as a DSL line can cause crosstalk that reduces the maximum bandwidth the DSL line can carry. However, Avalon tests found negligible interference from ISDN lines and from other DSL lines.

But the company did find that a Sioux City, Iowa AM radio station interfered with DSL lines. The station's radio signal invaded frequencies in the wire that would otherwise have been used by the DSL modems. This interference reduced the speed at which the DSL modems could connect, but not critically, Lacey said.

Customers might think speed is everything, but Avalon did not look at it that way. Web browsers cannot handle more than about 1.5M bit/sec worth of data anyway, so giving customers more bandwidth on the access link would not improve performance, Lacey said.

Avalon was more interested in modems being able to establish connections over 10,000-foot lines and being able to maintain those connections without a technicians tinkering, he said.