The hidden cost of higher-speed networks - Technology Information

Migrating from 10Base-T to faster signaling can create unexpected problems.

The most common computer networking protocol now in use is 10Base-T Ethernet, operating at the once-impressive rate of 10 Mbps. Thanks to the foresight of telecommunications industry leaders, Ethernet transmission at this speed requires only two of the four pairs of wires found in the widely installed UTP data cable. Not merely spare wires, the extra pairs were designed into the network scheme, anticipating the day when 10Base-T would be replaced by something even faster.

That day has come. 100Base-T, Fast Ethernet, sends signals 10 times quicker than the older 10Base-T. Many existing network installations and almost all new ones are upgrading to Fast Ethernet. And now 1000Base-T (Gigabit Ethernet) signals at a blistering one billion bits per second.

As computer networks switch from 10Base-T to 100Base-T to 1000Base-T, network owners will begin to experience more efficient network processing. Higher bit rates equal more bandwidth, which, in turn, equals more efficient throughputs and, ultimately, happiness for everyone. But this huge increase in network speed will not be gained without significant cost. Besides the obvious dollar costs, there is a significant hidden cost to both existing and new networks.

The hidden cost is the added vulnerability of all networks to failure caused by lost operating margin. No one involved in the development of higher-speed installations will be immune. The impact will be felt by LAN system designers, component manufacturers, distributors, installation contractors, field-tester manufacturers, and ultimately--and most significantly--the end user or client.

10-15% OF CAT 5-RATED NODES FAIL

There is an old joke in the industry that, unless something extremely bad was done to a premise network, Ethernet will run on wet string. That's an exaggeration, of course, but the point is valid: A typical 10Base-T network has a huge safety margin. Components, connections, cabling, and installation can each be off spec, and the network will still work. Most of us have seen the occasional cable run with a splice in the middle. Despite the flaw, the link will pass testing and 10Base-T will function just fine.

This convenient fact is changing as network speeds increase. Many network managers now switching to Fast Ethernet are seeing 10%-15% of their Category 5-rated nodes fail to operate at the anticipated higher speeds. Worse, some nodes don't work at all.

One can guess what will happen when Gigabit Ethernet starts up at the desktop. But both Fast and Gigabit Ethernet are supposed to run on the installed base of Category 5 networks. What has gone wrong?

MARGIN IS CRITICAL

At the higher speeds of these new protocols, performance margins begin to shrink dramatically. The reasons are highly technical, but they can be stated simply:

* Bidirectional signaling with four pairs adds new network complexity; and

* These higher-speed signals are weaker, and the noise accompanying them is relatively strong.

The signal-to-noise ratio of these new high-speed protocols is so low that very exotic and expensive processing techniques are being designed into the network interface cards and other network gear. These technical problems are being solved by chip and system designers, but chip design doesn't directly guarantee premise hardware margin. How can we be assured that the networks have more than enough safety margin to survive the transition to Gigabit and keep the customer happy?

First, contractors and installers must be trained to make a first-class installation. Installing cabling and hardware for these higher-speed networks is not only different from installing power and control circuits, it now becomes a critical skill. Pulling tensions, bend diameters, fill ratios, separation from power circuits, grounding, termination techniques, and many other skills must be studied, practiced, and mastered.

Installers can get good instruction by attending industry or BICSI-endorsed training classes or by aligning with a premise hardware manufacturer. Many of these manufacturers sell their equipment as a system with a warranty backed by trained installers and field testing.

Second, each installation will have more margin if the very best hardware, connectors, and cabling are specified and installed. Systems are only as good as their weakest part. If you have a great installation, but the parts are deficient, the network will have problems at higher speeds. You have probably heard that additional test requirements are being added to the specification for Category 5. This is due to the complexity of both Fast and Gigabit signaling. Also, an enhanced Category 5 (or Se) system will soon be specified. In about a year, Category 6 will be born.

Although these new standards have not been issued in final form, some suppliers are beginning to market components and systems that meet the draft requirements. If the performance values of your components meet these newest standards, by definition the network will have more safety margin.

A third way to overcome the hidden cost of higher-speed networks is to talk to manufacturers of handheld testers. The newest testers are truly amazing. These devices will do tests, such as return loss (RL), that were previously limited to $40,000 network analyzers. This new data will tell you volumes about the quality and margin of a network link or channel.

UNDERSTAND RETURN LOSS

Last, it is vital to understand the data, especially return loss. RL is a summation of all the reflected signal energy coming back to the end where the signal originated. It is like an echo and is not to be confused with crosstalk. RL limits have recently been defined for both CAT 5 and 5e channels. Analyze actual channel RL performance across the frequency range and note the margin that separates it from the limit lines. RL wasn't previously specified because it has no effect on 10Base-T signaling. For the new, higher-speed protocols, it is a critical measurement. It is a strong indication of an installation's performance margin.

As an example, Figure 1 shows the RL measured for a 100-meter channel. The hardware and patch cords are rated CAT 5, and the horizontal cable meets the draft requirements for CAT 6. The orange line is the draft RL limit for Category Se. Although all channel components interact, the horizontal cable dominates the lower frequencies (below 10 MHz). The high frequencies (above 100 MHz) are dominated by the connectors. The midrange is driven by patch-cable quality. Although this channel meets the CAT 5e return loss requirement, the upward "bump" in the data in the critical middle frequencies can be improved. This important frequency range is shaded in the figures.

[Figure 1 ILLUSTRATION OMITTED]

Figure 2 shows the improvement gained by simply changing to better patch cables. All other components remain the same. Notice that the channel RL is improved by 4-6 decibels in the frequency range that transmits most signaling energy. More margin in this range provides more network stability, fewer bit errors, and stronger signals. This finding is often confirmed by field experience. Nodes that fail when a network is upgraded to Fast Ethernet often "heal" with the substitution of better cords. We have heard many end users comment that "I don't know why, but our network seems to run better with these cords." The new generation of handheld testers will give you the ability to measure this RL improvement on your network.

[Figure 2 ILLUSTRATION OMITTED]

The computer network industry is facing new challenges and everyone is affected. Network managers and specifiers must protect themselves and their customers from potential problems generated by the migration to next-generation protocols. The ones who succeed will have both upgraded skills and an understanding of the hidden costs of higher-speed networks.

Russell is vice president of technical marketing, Quabbin Wire & Cable Co., Inc., Ware, Mass. More return-loss data is available at www.quabbin.com/tech_briefs/whatisrl.html

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