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Fast Ethernet - are you selling the right pipes?

Fast Ethernet - are you selling the right pipes?

Just when you thought it was safe to tackle megabit Ethernet, along comes Gigabit Ethernet. But, as the song says, don't worry, be happy. It will be a few more months before standards are in place for the gigabit technology.

Meanwhile, 100 megabit (Fast) Ethernet technology has settled down nicely, to the point where most resellers can be reasonably comfortable with configuring and installing such networks.

But there are still a few wrinkles that can cause problems.

The first can occur with the cabling, the second with the connectors and the third with incompatible components on the network. "Incompatible?" we hear you ask. But all these elements conform to standards, right? Yes, but there are variations within the standards that can cause problems.

Take the cabling first. While the move to fast Ethernet is gaining momentum and many end-user organisations can see its benefits, there are still many network resellers that have questions about which cables to use.

Fast Ethernet can operate at 10 times the speed of Ethernet and, in many cases, the existing twisted pair cable can be used in the migration to Fast Ethernet.

It is important to note that with cabling, the 100Base-T standard refers to the way the cable is used rather than the type of cable.

With Fast Ethernet there are three cabling systems - two for twisted pair copper and one for fibre. The 100Base-TX method calls for two pairs of shielded or unshielded twisted pair high-quality copper cabling - Category 5 cabling.

The 100Base-T4 requires four pairs of ordinary-quality copper cabling. One pair carries transmission data and one pair is dedicated to reception, while the other two pairs are bi-directional. Spreading the 100Mbit/sec signal across three pairs of cables allows for its use with lower grade Category 3 cabling.

Adaptec, a major player in the Ethernet market in the US, recommends that 100Base-T4 is best suited to workstations because of its lower price. In a technical paper it has published on the issue Adaptec said: "100Base-TX is more desirable in network servers that can take advantage of full duplex fast Ethernet."

As for the fibre option - 100Base-FX - this is best applied in the interconnection of repeaters to form a fibre optic backbone network. In a typical installation, this will see fast Ethernet repeaters on each floor of the building, or in each department, supporting 100Base-TX or T4 workstations. The repeaters will be interconnected using 100Base-FX links.

Having got the cabling right, it is important to make sure the right connections are used at each end.

"We have seen Category 5 cabling that did not have Category 5 connectors - making it useless for fast Ethernet," said Andrew Mitchell, man-aging director of the network products division of HarTec.

"We have an ongoing education program for installers and network designers to make sure this does not happen."

Also crucial is that connectors do not cause electromagnetic interference (EMU). Higher data rates with Fast Ethernet mean that poor connections are a potential source of severe interference to other electronic devices. The Spectrum Management Agency (SMA) has established a framework that introduces technical limits for emissions from electrical, electronic and communications services. The electromagnetic compatibility (EMC) framework is designed to protect Australia's radiofrequency spectrum.

"Although mandatory, the EMC framework is based on industry self-regulation," said David Brumfield of the Radiocommunications Standards Team at SMA.

"However, the SMA will back this up with a program of random audits," Brumfield said. It will also investigate complaints of EMU.

Fast Ethernet is now a fully ratified IEEE standard, having passed a full review in June 1995. The standard has been assigned the name 802.3u.

"Any Category 5 cabling will do 100 megabit Ethernet," said Barney Tomasich, market development manager for structured cabling systems at Anixter Australia. "The only proviso is you have to be careful in mixing and matching cable, connectors and patch panels," he said.

"It's not like 'Sussan' where 'this goes with this, goes with that'. If each component of the cabling infrastructure comes from a different manufac-turer, each will meet the IEEE spec but each is also likely to vary from the others by a small amount allowed in the standard. However, if all the components come from the same manufacturer, they are all likely to be exactly matched to have the same impedance - say, 105ohms, for example," said Tomasich, who is also on the ISO committee for overseeing fast Ethernet standards.

"In auditing cabling networks, we have found that about 25 per cent could not operate ATM (asynchronous transfer mode) mode due to poor installation practice and poor intermixing ofcomponents.

"Our advice to resellers is to look for a supplier that will offer an end-to-end solution," Tomasich said.

Echoing this advice was Rafik Razzouk, national marketing manager for networking distributor GEC Alsthom. "One of the major issues is managing the network," he said.

"Choosing all the components from a single vendor ensures complete compatibility but also means managing the network when it is installed and operational is also easier." vDown and dirty in the ductWhen moving a customer up to fast Ethernet, you could find yourself on your hands and knees pulling twisted pair cable from a duct.

Use the best cable you can afford or that the customer is willing to pay for. Remember that the most expensive part of the exercise will be the labour associated with installation.

If the user can afford Category 5 cable then invest in it now even if the plan is to use 100Base-T4 (that could use Category 3 cable).

Pull at least four pairs of cable to each desk. If you planned to use just two pairs, there could be a time when the extra pairs will be useful later. Remember the marginally higher cost will far outweigh having to call back next year and re-cable.

Plenum-rated cable costs more and is meant for use in the walls and ceilings. In a fire such cable produces less noxious gas than PVC-coated cable.

That's good news for firefighters. Check the building code for the local authority in which you are working. The code may require the use of such cabling anyway. Twisted pair premises cabling should be the solid conductor type that has better electrical properties. Stranded cable is suitable only for patch panels where flexibility is required.


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