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Networks thrive on attention.

Invest in portable tools.

You can't get away from them: you're surrounded by networks. There's the POTS, the ISDN link to your favorite internet provider, and the LAN to which your computer is connected. These networks also may be connected to high-speed WANs that use fiber-optic cables to achieve sufficient speed and capacity.

Maintaining a network is a significant investment. If you are responsible for the performance of a network, what should you do to ensure that it works as expected? Here is a suggested list of seven practices to include in a strategic network maintenance program:

* Management involvement in network decision making;

* Preparation and planning;

* Problem prevention;

* Early problem detection;

* Quick problem isolation and resolution;

* Investment in tools and training;.

* Quality improvement approach to network management and maintenance.

Many of these items can be grouped under a good-management heading and apply equally well to any project, not just networks. However, all seven practices have two common threads--increased visibility and awareness. You can't determine how healthy your network is unless you can measure it in some way.

Unlike electrical technicians in the past, modem network managers can't sense what's wrong just by touching the wires. They need hand-held and notebook PC-size equipment to help them. Generally, going to a smaller, less expensive instrument forces you to compromise some features, such as flexibility.

For example, many hand-held cable testers on the market today can help certify the installation of large, sophisticated networks. All the relevant tests and pass/fail limits you need for a particular type of cable are built in. However, they can't investigate ringing, reflections, or crosstalk.

Cable test specifications, such as TSB-67 for Category 5 UTP, specified by the TIA, already include operating margins so you shouldn't have to worry about these signal details. The only time you may need to understand a problem more fully is in the case of a marginal pass or fail.

Also, hand-held testers often display graphics on LCDs but are unlikely to have Windows interfaces. You need an instrument the size of a notebook PC to handle this level of man-machine interface.

WHAT TYPES OF TOOLS ARE THERE?

The ISO seven-layer OSI model shown in Table 1 describes the hierarchy of protocols used in modern data communications. Layers 1, 2, 3, and 4 focus on cabling, connections, and the operation of the network itself.
Table 1. ISO Reference Model

Layers Functions

 7 Application -- provides interface with network users

 6 Presentation -- performs format conversion

 5 Session -- manages connections for application programs

 4 Transport -- ensures error-free end-to-end delivery

 3 Network -- handles internetwork addressing routing

 2 Data Link -- performs local addressing and error detection

 1 Physical -- includes physical signaling and interfaces


Cable testers address the wiring and connections within the physical layer. LAN/WAN testers concentrate on Layers 2, 3, and 4, providing comprehensive statistics of errored data frames and packets, lists of top talkers, and maps of network users by address.

Protocol analyzers usually can simulate traffic and examine communications at any level of the ISO model. In addition, some analyzers can deal with two different protocols simultaneously, for example, on two different networks.

Finally, in a larger, PC-based instrument, there is the option of running additional applications, such as a report generator or data base. There is traffic simulation software that closely approximates the real-time burstiness of specific applications.

For example, a new application may greatly improve your efficiency on a certain job but consumes 10 times the bandwidth of the old application. What happens to the network if the new application becomes very popular and many more people start using it?

Network traffic simulation addresses this kind of planning problem. There also are network simulation and planning/mapping software packages that allow you to create a virtual network with virtual users. These kinds of tools help you get the network design correct before you start laying the cables.

CABLE TESTERS

Several types of copper twisted pair cabling are in use around the world. Of them, Category 5 UTP has become very popular because its performance is guaranteed through 100 MHz. This cable suits 100Base-TX Ethernet, for example.

Table 2 lists several of the tests a technician would perform to verify the installation of a network.
Table 2. Category 5 Cable Tests

Transmission Parameter Channel Performance

NEXT @ 100 MHz 27.1 dB
PSNEXT @ 100 MHz NA
Attenuation @ 100 MHz 24.0 dB
ELFEXT @ 100 MHz 17.0 dB(*)
PSELFEXT @ 100 MHz 14.4 dB(*)
Return Loss @ 100 MHz 8.0 dB(*)
Propagation Delay @ 10 MHz 555 ns
Delay Skew 50 ns


(*) -- Recommendations for 1000Base-T

At the most basic level, cable testers must establish that the connection from point A to point B exists. It's not good enough, though, to determine continuity only. High-speed signals will be degraded if wires from different pairs have crossed to create split pairs.

NEXT determines the degree to which a signal on one twisted pair affects the signal at the same end of another pair. FEXT is similar but relates to the disturbance caused by one pair on another when measured at the far end.

Additional testing is required before a different type of service can be run over existing cabling. In 1000Base-T or Gigabit Ethernet systems, all four pairs of wires in a Category 5 cable are used simultaneously. In this case, it's not just one pair that can affect another but three that may crosstalk onto the fourth one. This reasoning gives rise to terms such as PSNEXT, where the effect of three pairs upon the fourth pair is determined at the sending end.

To improve performance of 1000Base-T networks, the TIA has proposed an Enhanced Category 5e specification. Even higher-performance Category 6 cable will be specified in the future with a guaranteed 250-MHz usable frequency. Several cable testers available today claim to meet these proposed standards.

Fiber-optic cables don't suffer from crosstalk, but tests for attenuation, return loss, and propagation delay all apply to both fiber and copper media. Several brands of hand-held cable testers include separate models that test fiber-optic installations, or they provide options for their copper cable testers.

Many testers have a TDR feature that can pinpoint the location of breaks and high-resistance areas, such as poor connections. For fiber-optical cables, this capability becomes an OTDR.

A practical feature having nothing to do with actual cable specifications is two-way voice communications between a pair of testers. Cabling often runs between floors of buildings, and installers work with a partner--at the other end of the cable. It saves a lot of time if these installers can communicate, especially on sites where the use of walkie-talkies is prohibited.

Certain cable tests can be accomplished easier if the slave tester at the far end can loop the signal back to the master unit. In fiber-optic cable testers that automatically communicate with each other, the master can determine the attenuation of a cable to both directions of data flow. A light source is used at each end, and results measured by the slave unit are relayed to the master.

LAN/WAN TESTERS AND PROTOCOL ANALYZERS

LAN/WAN testers can be distinguished by their focus on specific types of networks and types of problems. For example, a hand-held LAN tester may not be able to create a special mask to filter only certain message headers, but this capability is provided by most protocol analyzers. A hand-held LAN tester will be dedicated to Ethernet networks or ATM, for example. A protocol analyzer may have different interface adapters that allow it to operate with many types of networks.

On the other hand, dedicated LAN testers can provide capabilities that protocol analyzers don't. Ease of use and simple, graphical displays of network connections are strong points of hand-held units intended for maintenance and trouble-shooting.

While it's true that many hand-held testers are becoming easier to use, LAN/WAN testing is not static. Network speeds continue to increase, so new hardware interfaces are required. Protocols continue to proliferate.

With all the capabilities of modern, hand-held tools, there is no question that networks can be maintained correctly. The challenge is to encourage a quality-improvement approach to network management and maintenance. The proper tools and training, together with routine network performance monitoring, are all parts of this process.
Glossary

ATM asynchronous transfer mode
ELFEXT equal level far-end crosstalk
FEXT far-end crosstalk
ISDN integrated services digital network
ISO International Organization for Standardization
LAN local area network
NEXT near-end crosstalk
OSI open systems interconnection
OTDR optical time domain reflectometer
POTS plain old telephone service
PSELFEXT power sum equal level far-end crosstalk
PSNEXT power sum near-end crosstalk
TDR time-domain reflectometer
TIA Telecommunications Industry Association
UTP unshielded twisted pair
WAN wide area network
COPYRIGHT 2000 Nelson Publishing
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Copyright 2000 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:Technology Information
Comment:A guide to network management tools and resources is presented.
Author:Lecklider, Tom
Publication:Communications News
Geographic Code:1USA
Date:Mar 1, 2000
Words:1446
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