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.
Installing and maintaining a network are significant investments. “If a LAN has to be upgraded every two or three years, optical cable is an economically feasible alternative,” said Stephen Montgomery, president of ElectroniCast. “The major cost of the network is the actual installation of the cable. Of course, if the business will move in a year or two when its lease is up, then copper twisted pairs may be the better solution.”
If you are responsible for the performance of a network, what should you do to ensure that it works as expected? Fluke suggests a 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.
Investing 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, modern 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. But generally, going to a smaller, less expensive instrument forces you to compromise some features, such as flexibility.
For example, many hand-held cable testers 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 to the same degree that general-purpose signal generators, power meters, and DSOs can. But is there a need to examine each signal?
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.
The current state of small testers was summarized by Dave Ushler, product marketing manager at Digitech Industries: “Simply because a tester or analyzer comes in a smaller package doesn’t necessarily mean that the user interface is easier to deal with. The trend is toward Windows operation and easier menus with better prompts and fewer keystrokes for repetitive tests. The best combination is a more powerful tool with an easier-to-use format in a smaller case.”
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.1 Layers 1, 2, 3, and 4 focus on cabling, connections, and the operation of the network itself. Cable testers, TDRs, and DSOs can verify that the wiring gets from point A to point B and that the signal hasn’t been significantly degraded.
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. Hewlett-Packard and Ganymede Software have developed 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× 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, such as offered by MIL 3, 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.
2
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.
Because crosstalk can be affected by connectors, these measurements generally won’t be the same when the tests are run from the other end of the cable. To ensure meaningful results, TSB-67 specifies that NEXT measurements must be taken at a minimum of 1-MHz intervals from 1 MHz to 100 MHz.
3
“Cable testers primarily are used during a new installation,” said Dave Skingley, a marketing manager at Fluke. “However, network additions, moves, and changes may impact network performance and require that existing cabling be verified or recertified.”
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.
The increased demand for fiber-optic cable networks is prompting cable installers to earn certification to work with both fiber and copper. According to Lisa Schwartz, a product manager at Datacom Technologies, “Installers want to be able to install and certify the whole job. Having a combined tool that does everything with one test system is a big convenience.”
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 more easily 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 troubleshooting.
In describing a typical LAN tester, Mr. Skingley said, “It performs a variety of functions including monitoring network activity and real-time analysis of network parameters, analyzing all major network protocols, testing NICs and Hub/MAU ports, and generating network traffic. The tester identifies stations sending different protocols, consuming bandwidth, or generating errors.”
By comparison, Oded Agam, director of technical services at Radcom Equipment in North America commented, “Solving performance issues requires the capability to conduct real-time analysis on higher layer protocols. You need to have a full decode capability of all layers. As an example, solving a problem of slow TCP performance over a high-speed ATM network requires filtering and decoding the sequence numbers in the TCP layer.”
Because protocol analyzers provide a general solution and LAN/WAN testers supply more specific solutions, their use implies different levels of training and experience. The capabilities of the two types of instruments also imply a two-tier troubleshooting approach.
Mr. Skingley of Fluke added, “When a technician is dispatched to solve a problem, he takes with him the appropriate tools. If he cannot solve the problem, he escalates it to a more experienced staff member. With tools that fit specific job junctions and expertise levels, network maintenance efficiency is maximized.”
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. For example, Radcom has set up a free web site, www.protocols.com, where you can acquire information and specifications for more than 300 protocols.
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.
References
1. Jacob, G., “Protocol Analyzers Smooth Traffic Flow on the Information Highway,” EE-Evaluation Engineering, October 1994, pp. 50-56.
2. “Backgrounder: Testing Cable Installations to New Standards,” Fluke Corp., September 1998, #98-25b.
3. Koeman, H., “TIA Technical Service Bulletin #67 (TSB-67) Explained,” Fluke, September 1995.
LAN/WAN Products
Windows 95-Based LAN Analyzer
The LAN900 is a 2-lb, portable, PC-based LAN protocol analyzer. The battery-powered unit has a TFT color display and interfaces to Ethernet, Fast Ethernet (10/100M) and token ring networks. Features include English language seven-layer decodes of TCP/IP, Novell NetWare, AppleTalk, DECNet, Banyan VINES, XNS, IBM SNA, SNMP, NetBIOS, ISO, Sun NFS, and routing protocols. Capture to disk; the ability to filter inputs at every layer, byte, or bit; and built-in expert solution recommendations also are provided. 10M Ethernet: $8,700. Digitech Industries, (800) 821-2265.
Level II Cable Tester
The hand-held DSP-4000 Digital Cable Analyzer™ performs tests up to 350 MHz in preparation for Category 5e and 6 specifications. A new digital design supports Time Domain Crosstalk (TDX™) analysis which is used to locate defects causing crosstalk. The new design also provides diagnostic capabilities and high speed testing while maintaining Level II accuracy. Options include manufacturer-specific link interface adapters, the DSP-FTA410S Fiber Test Adapter, the DSP-FTK fiber-optic test kit, and CableManager™ data-base software. DSP-4000 including main and remote units: $5,995; DSP-FTA410S: $3,995; DSP-FTK: $995; CableManager: $295. Fluke, (888) 498-9360.
Multi-Protocol Analyzer
The HP Internet Advisor is a protocol analyzer with a selection of interfaces and modules to support capture and analysis of data from popular LAN/WAN and ATM network technologies. Features include drill-down capability, complete seven-layer protocol decodes, active stimulus/response tests, and network performance statistics.
Expert diagnosis and network health reporting also are provided. The built-in Pentium PC with 32 MB of memory and up to a 3 GB hard drive can run optional software such as IP Switching Analysis Software, SQL Server or Sybase Commentator, and HP Internet Reporter. From $13,500. Hewlett-Packard, (800) 452-4844.
Application-Specific Testers
The Cable Verifier Series of pocket-size cable test instruments comprises more than 20 products. Twelve CVxx models cover copper LAN, telecommunications, and CATV applications. CV22 handles only Thinnet 50-W BNC coaxial networks; CV52 includes Ethernet 10Base-T, T568, and token ring as well as Thinnet. All cable testers use LEDs to verify correct connections, miswired pairs, or open wires. The FV43 fiber-optic tester has a LED bar graph that indicates power loss in -2-dBm increments from -22 to -40 dBm. CV22: $99.95; CV52: $795; FV43: $195. Wavetek Wandel & Goltermann, (800) 854-2708.
Communicating Cable Testers
Category 5, enhanced CAT 5e, and proposed CAT 6 cable standards are supported by the LANcat System 6 Cable Tester. The modular construction of the main and remote instruments allows you to replace the 100-MHz C5 Performance Modules with corresponding 250-MHz C5e modules, for example. PSNEXT, PSACR, propagation delay, delay skew, and CAT 5 headroom margin are measured to better than Level II accuracy. LinkTalk™ automatically finds wires suitable for two-way voice communications. DualView™ provides the remote unit with the same graphic display of cable faults as shown on the main unit. The FIBERcat Performance Module adapts the LANcat to fiber-optic testing. LANcat: $5,500 per pair. Datacom Technologies, (800) 468-5557.
Two-Channel Analyzer
The RC-100 WL WAN/LAN Analyzer interfaces to and fits beneath a notebook PC. Internetwork testing is supported by simultaneous two-channel monitoring and simulation. All popular LAN/WAN protocols are supported. Software is available for analysis and simulation of frame relay, X.25, and ISDN protocols. Analysis software also is available for SMDS, FUNI, and encapsulation protocols. Optional interfaces include ISDN/BRI (S/T and U); E1/FE1; T1/FT1; Ethernet 10Base-2, -5, and -T; token ring STP and UTP; and V-series RS-232, RS-449, RS-530, V.35, and X.21/V.11. $12,000. Radcom Equipment, (800) 723-2664.
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 and routing
2
Data Link—performs local addressing and error detection
1
Physical—includes physical signaling and interfaces
Table 1
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
Table 2
Glossary
ATM
DSO
ELFEXT
FEXT
ISDN
ISO
LAN
NEXT
OSI
OTDR
POTS
PSELFEXT
PSNEXT
TDR
TIA
UTP
WAN
—wide area network—unshielded twisted pair—Telecommunications Industry Association—time-domain reflectometer—power sum near-end crosstalk—power sum equal level far-end crosstalk—plain old telephone service—optical time domain reflectometer—open systems interconnection—near-end crosstalk—local area network—International Organization for Standardization—integrated services digital network—far-end crosstalk—equal level far-end crosstalk—digital storage oscilloscope—asynchronous transfer mode
Copyright 1999 Nelson Publishing Inc.
March 1999
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