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Installing A Computer Network What sort of computer network do you want in your home, school or small business? Should you run coax or twisted pair cable and when do you need a hub? Here's a primer on basic network planning. By BOB DYBALL & GREG SWAIN Getting a new computer network up and running can sometimes be just as challenging as ironing the bugs out of an existing one. However, before implementing a new network, there are a few things you need to consider. To begin with, you need to know the 4  Silicon Chip basics of network wiring so that you can sensibly plan the layout. You also need to think about how the network might need to be expanded in the future. This could involve connecting adjoining buildings, adding additional users or modifying the system to cater for extra network traffic. Many aspects of networking affect each other, so you need to consider them all before going ahead. The wrong choices can break a network and lead to frustration and added expense later on. A computer network is made up of a number of different components. Apart from the PCs, you need network cards (one for each PC), network cable and, depending on the type of network, a hub, router or some other device. Network cabling standards are based on the Open Systems Interconnection model (or OSI model), as released by the Interna­tional Stand- ards Organisation (ISO) in 1984. The OSI model helps separate the different functions of a network into seven “layers”. These layers are shown in Table 1. Although there are some grey areas, most networking proto­cols fit the OSI model. In practice, this means that different networking protocols can successfully coexist on the same net­work. This concept is known as “protocol independence”, which means that a network designer can use the same hardware for different protocols. A simple example of this might involve viewing web pages across an intranet using IPX/SPX instead of, say, TCP/IP. We’ll look more closely at the OSI network layers a little later, when we get to repeaters, switches, bridges and routers. Simple 10Base-2 Network Max. Segment Length = 185 metres Workstation 1 Workstation 30 50 Terminator “T” Connector FIG.1: A 10BASE-2 NETWORK has all the PCs wired along a single line, in a “bus” configuration. Each network interface card (NIC) is fitted with a T-piece and these are connected using lengths of coaxial cable fitted with BNC connectors. A 50Ω coax terminator is fitted to each end of the network. A disadvantage with this type of layout is that a break anywhere in the coax generally brings the whole network down. Ethernet Ethernet is the most widely used LAN technology today and supports virtually all popular network proto­ cols. It operates according to the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) access method. OK, let’s find out what this mouthful of jargon really means. The name might sound complicated but the principle is really quite simple. CSMA/CD allows multiple work­ stations to access a network by “listening” until no signals are detected (Carrier Sense). If a station has traffic to send, it then transmits and checks to see if more than one signal is present (Collision Detect). Each station only attempts to transmit if it detects that the network is free. If a packet of data is transmitted and a collision takes place, the stations transmitting immediately stop and enter a random countdown period before attempting to re-send the data. Planning your network Many small to medium-size networks had humble beginnings. Often, they started “life” as just a couple of PCs networked together in an office, with additional workstations and servers progressively added as required. However, there’s a limit to how far you can go with an ad hoc approach. Keep adding equipment and, sooner or later, you’re going to run into prob­lems. It’s important to realise that there are a number of ground rules for wiring up a network. For example, the maximum distance between work­stations and the number of work­ stations that can be added are directly related to the type of cable used. If you need to add lots of work­ stations or cover large distances, it will be necessary to add repeaters and/or bridges to connect different sections of the network together. In addi­tion, you may have to add switches (or routers) to break up network traffic in areas that are heavily used. Basically, a switch filters unnecessary traffic from individual segments of the network, so that it is faster overall. In addition to the number of users, bandwidth requirement is an impor- tant consideration. Networks operating at 10Mb/s have been the standard in small installations until recently but the new 100Mb/s systems offer substantial performance benefits (at a cost) and are gaining in popularity. Common cable types Most small-to-medium networks are run using either coaxial cable or Cat.5 twisted pair cable fitted with RJ45 connectors (the latter look like American-style miniature telephone connectors). However, there are other choices, includ­ing optical fibre, and these are summarised in Table 2. Note that the cable is at the “Physical Layer” of the OSI model. Table 1: The OSI Model Layer Function Data Type Appli cation Interface between the user's appli cation & the network Messages Presentation Establishes data formats, transl ates data, provides data compression & encoding/decoding functions Packets Session Allows server names to ident wy devices & uses these to establ ish connections between devices Packets Transport Breaks up data from the session layer and reassembl es i t to provide reliabl e connection-ori ented data transmission Datagrams & segments Network Gets the data through the network vi a the most effi cient route, using swi tching, routing & addressing Datagrams Logi cal Li nk Control sub-layer (LLC); maintai ns the link between network devices Data Li nk Physi cal Medi a Access Control sub-l ayer (MAC); handl es physi cal addressing, ensuring onl y one devi ce uses the network at a time Transl ates data into binary format for transmission across physi cal medi a Frames Bits February 1999  5 10Base-T/100Base-TX Network Server Workstation Hub FIG.2: A 10BASE-T NETWORK uses a “star” topology, whereby individual workstations are connected to a central hub using inexpensive twisted pair cable. This type of network is more reliable than the bus network shown in Fig.1, since a broken cable only affects one workstation. THE CABLES FOR A 10BASE-T NETWORK are fitted with RJ45 connectors which plug directly into the network cards in the individual PCs (left). The other ends of the cables are then plugged into the ports on the hub (see above). The hub shown here has eight regular ports, which means that it can accommodate up to eight PCs on the network. It also has an “uplink” port so that additional hubs can be easily added as the network expands. 6  Silicon Chip As mentioned above, the type of cabling you choose depends on your network requirements and on the “topology” of the net­ work. So let’s take a look at the more popular options. (1) 10Base-2: this option is based on thin, screened 50Ω coaxial cable. For this reason, it also known as a thin-Ethernet system, or as “Thinnet”. Its advantages are that it’s inexpen­ sive, simple to use and good in highnoise environments. Fig.1 illustrates a simple 10Base-2 network. Note that all the workstations are wired along a single line, in a “bus” ar­rangement. Each network interface card (NIC) is fitted with a T-piece and these are connected together using lengths of coaxial cable fitted with BNC connectors. A 50Ω coax terminator must be fitted at each end of the network. Up to 30 workstations can be connected in this fashion. The maximum length of the network specified for 10Base-2 is 185 metres (without repeaters) and the workstations must be at least 0.5-metres apart. A disadvantage with this type of layout is that a break anywhere in the coax generally brings the whole network down. In addition, 10Base-2 can only be used in half-duplex mode, the network card either transmitting or receiving at any given time (but not both at once). 10Base-2 is mainly used where relatively few users need to be connected over a long distance (up to 185 metres) and where speed is not an overriding consideration. (2) 10Base-5: also called thick-Ethernet or “Thicknet”, this standard is based on “thick” 50Ω coax. Unlike 10Base-2, the individual network cards are connected to the cable via tran­sceivers and special AUI (application user interface) drop cables fitted with DB15 connector plugs. A 50Ω terminator is fitted to each end of the cable. The advantage of 10Base-5 is that it can accommodate up to 100 stations over a distance of 500 metres without a repeater. However, this standard is not often used these days, since the thickness of the cable makes it difficult to run. It also re­quires network cards fitted with DIX connector sockets and is rather expensive for small to medium networks. (3) 10Base-T & 100Base-TX: per- Table 2: Network Cabling Standards Cabling Standard Topology Minimum Cable Spec. S peed Max. Length Min. Length Between Nodes Max. Segment Length Max. No. Of Segments Max. No. Of Nodes Max. No. Of Nodes/ Segment Arcnet Star or bus RG-62 90/93-ohm 2.5Mb/s 600 m N/A N/A N/A 255 32 Arcnet Plus Star or bus RG-62 90/93-ohm, UTP or optica yibre (FO) 20Mb/s Coax: 600m U T P : 120m FO: 3500m N/A N/A N/A 255 32 10Base-5 Bu s 50-ohm 10Mb/s 2500m 2.5m 500m 5+3 30 0 100 10Base-2 Bu s 50-ohm 10Mb/s 925m 0.5m 185m 5+3 90 30 10Base-T Star Cat.3 10Mb/s 2.5m 100m 1024 1 10Base-FL Star Optica yibre 10Mb/s N/A N/A 2000m 1024 1 100Base-TX Star Cat.5 UTP 100Mb/s N/A 2.5m 100m 102 4 1024 1 100Base-T4 Star Cat.3-5 UTP 100Mb/s N/A 2.5m 100m 1024 1024 1 100Base-FX Star Opti ca yibre 100Mb/s N/A 2.5m 2000m 102 4 1024 1 Token Ring Star/Ring STO, UTO or opti cal fibre 4Mb/s or 16Mb/s N/A 2.5m U T P : 45m S T P : 101m 33 U T P : 72 S T P : 260 haps now the most popular standard, this uses twisted pair cable to connect individual workstations to a central hub or repeater. This arrangement is known as “star” topology, as shown in Fig.2. A 10Base-T network runs at 10Mb/s, while a 100Base-TX network runs at 100Mb/s. Generally, Cat.5 unshielded twisted pair (UTP) cable is used but shielded twisted pair (STP) cable may be necessary in electrically noisy areas. These cables are fitted with inexpen­ sive RJ45 connectors which plug directly into the hub and into most network cards. Since all workstations in a 10Base-T network are wired in a “star” arrangement, a broken cable only affects “traffic” to and from one workstation. For this reason, 10Base-T networks are more reliable than 10Base-2 networks using bus topology. 10Base-T networks have an edge in speed over 10Base-2 (and 10Base-5) systems too, if the network cards are used in “full duplex” mode. Both UTP and STP cables are available in solid core and stranded core. It is important to use the correct cable in a given situation, as the performance differs between the two types. The maximum distance (segment length) between the hub and a workstation is 100 metres and the rule is 10 metres maximum for stranded-core “patch” cables and 90 metres maximum for solid core “LAN” cables. In a simple 10Base-T network, patch cables are used to connect individual workstations directly to the central hub, as shown in Fig.2. This means that the maximum distance between any two workstations Hub Patch Panel Wall Outlet Workstation Solid core cable; 90m max. Patch cables; 10m max. FIG.3: SOLID CORE CABLE must be used to connect a workstation back to a hub for distances greater than 10 metres. This diagram shows how a mixture of patch cable and solid-core cable can be used to connect a workstation to a hub via a wall outlet and a “patch” panel. (Namlea Data Systems). is 20 metres. If greater distances are required, solid-core LAN cable must be used. Fig.3 shows how a mixture of patch cable and LAN cable can be used to connect a workstation to a hub via a wall outlet and a “patch” panel. Apart from less noise immunity (if using UTP), the main disadvantage of 10Base-2 is the need to buy a “hub” to connect all the workstations together. However, 10Mb/s hubs are now a relatively low-cost item, with typical 8-port units selling for about $135. By contrast, an 8-port dual-speed 10-100Mb/s hub will set you back $500 or more. If you already have a 10Mb/s hub and you are planning a new network, consider buying 10-100Mb/s network cards instead of ordinary 10Mb/s cards (the dual-speed cards are not that much more expensive). In addition, you should buy Cat.5 cabling in­stead of settling for Cat.3 cable. This will allow you to easily upgrade to a 100Mb/s network later on, simply by replacing your existing 10Mb/s hub with a 100Mb/s unit. Although 100Mb/s hubs are still expensive, their prices are rapidly dropping and so this approach offers an easy upgrade path if you need the extra bandwidth later on. (4) Arcnet: an older networking standard than Ethernet but still used February 1999  7 Using Repeaters To Extend A Network Repeater Repeater Repeater Segment 2 Segment 1 Repeater Segment 4 Segment 3 Segment 5 Collision Domain FIG.4: REPEATERS CAN BE USED to extend a 10Base-2 network beyond the basic 185-metre limit. The 5-4-3 rule applies here. This rule states that the network is limited to five segments, four repeaters and three groups of work­stations. (Namlea Data Systems). in some installations. The length of cabling is limited by a maximum propagation delay limit of 31ms. (5) Token Ring: requiring special network cards, this system is usually more expensive than 10Base-2 or 10Base-T Ethernet net­works. It is useful in situations where there is relatively heavy network use, since each workstation is forced into taking its turn for network access. A multistation access unit (MAU) is required to terminate the cables from the work­stations. (6) 10Base-FL & 100Base-FX Optical Fibre: often used where large distances are required and in situations where high levels of electromagnetic interference are present. Fibre optic cabling can be interfaced to Cat.5 twisted pair cabling via converters, transceivers or hubs fitted with fibre optic ports. Generally, fibre optic cabling is used in large profession­ al installations where performance considerations outweigh the cost. Repeaters and the 5-4-3 rule Often, it will be necessary to extend a network further than the basic recommended distance. In that case, you may need to add a repeater, to overcome signal losses in the cable. A repeater is one of the simplest devices you can use to extend a network. It can be considered as a “black box” that amplifies the signals coming into it, before passing them on to other devices on the network. Repeaters cannot change packet or protocol types; nor can they “segment” a network to reduce traffic congestion. There are “rules” defining how many repeaters you can use in a network, since too many would cause timing problems and data collisions. With Ethernet technology, the number of repeaters is limited by the “5-4-3” rule. This rule states that the network is limited to five segments, four repeaters and three groups of work­ stations (ie, only three segments can be connected to work­stations). Fig.4 Adding Hubs To A 10Base-T Network 100m Hub 1 100m Hub 2 Hub 3 100m Hub 4 100m Collision Domain FIG.5: EXTRA HUBS CAN BE ADDED to increase the number of ports as the network grows. A 10Base-T network can have up to four cascaded hubs, each spaced up to 100 metres apart using Cat.5 cable. A 100Base-TX network is limited to two hubs spaced no more than five metres apart but this can be increased using a bridging port. (Namlea Data Systems). 8  Silicon Chip shows the basic scheme. You can also use repeaters to connect networks in two different buildings together and to link networks using different types of cable. Some companies, such as Black Box, stock many specialised converters and interface options to patch different types of networks together and/or to extend them over large distances (eg, via fibre optic cable). This equipment can dramatically extend the maximum distance covered by a given network. Low-Cost Network Starter Kit Hubs Hubs are basically multi-port repeaters and are used in 10Base-T (and 100Base-TX) networks to connect servers and work­stations together in a star configuration. A passive hub doesn’t do much more than provide a way to connect the various parts of the network. By contrast, an active hub can extend the coverage of a network just like a dedicated repeater. As the network grows, additional hubs can be added to in­crease the number of available ports. In practice, this involves cascading the hubs together, as shown in Fig.5. The maximum distance between hubs is 100 metres for 10Base-T and 5 metres for 100Base-TX. If you wish to cascade 100Base-TX further than five metres, a bridging port must be used. As well as their regular ports, many hubs also come with an uplink port. When two hubs are cascaded together, the uplink port on the first is connected to one of the regular ports (it doesn’t matter which one) on the second. The uplink port on the second hub can then be used to cascade a third hub, and so on. Fig.6 shows how this is done. Provided you use an uplink port to connect to the next hub, regular Cat.5 patch cable can be used. Alternatively, hubs that don’t have uplink ports can be cascaded by connecting two regular ports together via a crossover cable. You don’t use a crossover cable if you connect to an uplink port, because the pins connec­ tions are already crossed over in the socket. As an alternative to cascading, some hubs can also be “stacked” to create one logical hub. This involves using a special cable to connect the hubs together via their “stack” ports. This facility is particularly important in Fast Ethernet environments where IDEAL FOR USE AT HOME or in a small business, this 10Base-T “Network Starter Kit” from Nam­lea Data Systems contains all the parts you need to create a local area network (LAN). It comes with an 8-port hub, three network cards, three 5-metre Cat.5 cables and a plug­pack power supply. As supplied, you can network up to three PCs. Up to eight PCs can be connected by adding extra network cards and cables as required. Two versions are available: (1) Cat. 39NSK0803I with ISA cards; and (2) Cat. 39NSK0803P with PCI cards. A 100Base-TX fast Ethernet starter kit is also available. This version contains a 100Mbs 4-port only two repeater counts are allowed. Hubs are usually non-intelligent devices and will simply pass everything to all workstations. Don’t forget to apply the 5-4-3 rule when hub, two PCI cards and two 5-metre cables (Cat. 39NSK0402F). Namlea Data Systems (NDS) is a company that specialises in networking equipment, including switches, hubs, print servers, routers, patch panels, cables and a wide range of connectors and cables. For further information, contact Namlea Data Systems, 22 Cleg St, Artarmon, NSW 2064. Phone (02) 9439 6966; fax (02) 9439 6965. www.ndsonline.com.au cascading hubs. This means that you have to ensure that you have no more than four ports between any two “nodes” or points on a network. As well as the usual RJ45 sockets, February 1999  9 Cascading Hubs Via The Uplink Port Uplink 8 7 6 5 4 3 2 1 6 5 4 3 2 1 Hub 3 Uplink 8 7 Hub 2 Uplink 8 7 6 5 4 3 2 1 some hubs are also fitted with a BNC connector to allow cascading via 50Ω (10Base-2) coaxial cable. By using coax, the hubs can be up to 185 metres apart – a useful increase on the 100-metre limit imposed by Cat.5 UTP cable. As before, each connector is fitted with a T-piece, the coax run between the T-pieces, and the open ends fitted with 50Ω terminators. This simple feature can save on the cost of buying a re­peater. For example, let’s say that you have two hubs 160 metres apart, each connected to a 10Base-T network. Provided the two hubs are fitted with BNC connectors, you can easily connect these two 10Base-T networks together using 10Base-2 coaxial cable. If the distance between the hubs was 1.5km, you could add two repeat- Hub 1 FIG.6: HUBS ARE CASCADED together by connecting the “uplink” port of the first hub to a regular port on the second hub and so on. Hubs that don’t have uplink ports are cascaded by connecting two of their regular port together via a special crossover cable. ers and connect everything together using three 500-metre segments of 10Base-5 coax. However, this would require hubs fitted with 15-pin AUI ports to accept the thick coax. Altern­ ative­ly, you could use one segment of optical fibre cabling. Bridges Bridges are mainly used to connect two similar Ethernet networks together. In addition, they can also be used to “segment” a busy network to decrease data collisions and boost performance. Bridges work at the Data Link Layer of the OSI model. To get the best from a bridge, it’s important to break the network into segments by grouping workstations and servers that work together – see Fig.7. This is done to minimise traffic between different segments. Often, in a business situation, this is simply done on a departmental basis (eg, the accounts department’s server and workstations on one side of a bridge and the shipping depart­ment’s server and workstations on the other side). Just as with repeaters, there are some specialised bridges to connect networks that use different network media (eg, to convert between Token Ring and Ethernet). Ethernet switches Although hubs can be used to increase the size of a net­work, too much traffic can slow things down. When this happens, switches, bridges and routers can be used to increase the performance by partitioning the network and by filtering network traffic. Linking Two Networks Via A Bridge Server FIG.7: BRIDGES ARE USED TO CONNECT two similar Ethernet networks together or to segment a busy network to decreases data collisions and boost performance. Workstations Bridge Workstations 10  Silicon Chip Server Switches are basically multi-port bridges. They not only partition a large network into smaller “domains” but also filter unnecessary traffic from individual segments of the network. These two steps markedly reduce the incidence of data collisions, making the network faster and more efficient. If your network is getting a little tired, with too many users wanting too much bandwidth, replacing an ordinary hub with a switch can give a worthwhile increase in performance. Networking Gear From MicroGram* NETWORK STARTER KIT IF YOU WANT your first network to be fast, this kit can deliver the goods. It contains all the hardware components required to build a 100Mb/s network for two PCs, including a 4-port hub, two 10/100Mb/s PCI network cards and two Cat.5 cables. Up to four PCs can be supported by purchasing additional network cards and cables. Cat. 11900. Routers Routers work within the network layer of the OSI model. As the name suggests, they find the best “route” for data in large, complicated networks. Routers are more “intelligent” than switch­es or bridges, as they use either MAC (media access control) addresses, IP addresses or other common addresses to determine the best path for data to travel. For example, an IP router can divide a network into various “subnets” so that only traffic destined for particular IP ad­dresses can pass between segments. Routers do not pass “non-routable” network protocols, such as the popular NetBEUI protocol. What’s more, they are not for the fainthearted, since setting them up can be a little tricky. As with a bridge, a router slows down network traffic as it filters the data to determine the route. However, this filtering “overhead” is relatively insignificant compared with the vast im­provements overall that a router can bring to a large network. A special version of a router, known as a “Brouter”, can handle both routable (eg, TCP/IP) and non-routable (eg, NetBEUI) protocols. Network troubleshooting If a network or part of a network doesn’t work correctly, try to analyse the problem. Confronted with a problem, many people rush in and swap network cards about or fiddle with cables and protocol settings without really thinking about the problem. First, make sure that the problem isn’t simply due to user error. If it isn’t and you’re convinced that it’s either a hard­ware fault or a software fault, try starting with a basic network consisting of just a few machines. If the network was functioning but a problem suddenly develops, check INTERNAL 5-PORT HUB CARD THIS 100Mb/s 5-PORT HUB card mounts on the backplane of a PC (typically the server) but does not plug into a slot – it only connects to the power supply. The companion display unit (below) mounts in a spare 3.5-inch drive slot. Cat. 11294. *MicroGram Computers, Unit 1, 14 Bon Mace Close, Berkeley Vale, NSW 2261. Phone (02) 4389 8444; fax (02) 4389 8388. Web site: www.mgram.com.au 5-PORT HUB & LAN CARD IDEAL FOR SOHO (small office/home office) users, this single unit combines a network card and a 5-port hub into one. It plugs into a spare PCI slot on the mother­board (no external power supply needed) and can auto-sense either 10Mb/s or 100Mb/s operation. Four RJ45 ports on the backplane connector allow up to four more PCs to be networked to the main unit. Cat. 11295. 8-PORT HUB DUAL-SPEED HUB THIS 8-PORT DUAL-SPEED HUB features automatic internal switching, to allow communications between ports running at 10Mb/s and ports running at 100MBps. It supports stacking (up to four units can be stacked to form one logical hub) and includes a switched uplink port (port 8). Cat. 11299. February 1999  11 Common Networking Terminology Hubs A hub is the central point of a 10Base-T network and provides a means of connecting the various elements of the network together in star configuration. Hubs come in various sizes, ranging from 4 ports up to 24 ports or more. Additional hubs can be cascaded or stacked to increase the number of available ports as the network grows. Uplink Port This is a port that's used to connect directly to a regular port on another hub, so that the two hubs can be cascaded. The uplink port has its pins configured to allow regular patch cable to be used. If connecting to see if it is reproducible. A simple reboot can often clear up this sort of problem. Don’t overlook the obvious. Before replacing network cards, check your plugs and cables for loose connections. If one machine in a 10Base-T network fails to work, for example, try changing the patch cable to that machine. Most hubs, switches and other network gadgets used for 10Base-T or Token Ring networks have lights to indicate that the cable is connected and all is well. As mentioned earlier, a break anywhere in the cable of a 10Base-2 (bus) network will usually bring the whole network down. You can quickly track a regular port to another hub without an uplink port, a crossover cable must be used. Print Server A print server is a device with one or more parallel ports and is used to connect a printer (or several printers) to a network. Print servers are intelligent devices, which have their own network addresses and simple setup software. Cascading & Stacking Cascading involves connecting two hubs together to increase the number of available ports. When you cascade two hubs, you connect them via RJ45 (Cat.5) cable. Hubs down where the break is by progressively disconnecting the work­stations from one end, transferring the 50Ω terminator to the free end as you go. If you have more than about 10 machines, it may be quicker to split the network into two halves, so that you can identify which half has the problem. There are a number of excellent tools for network diagnos­ tics but don’t forget your DMM. It can easily check for shorts or open circuits on a simple coax network. To test a coax installation, first disconnect the termina­ t or at one end, then check the resistance of the terminator and the resistance across Fig.8: if you have Windows NT, you can use the Event Viewer and Windows NT Diagnostics utilities to help track down networking problems (assuming that the hardware is OK). Alternatively, try using a dedicated diagnostics package. 12  Silicon Chip can also be cascaded via a BNC or AUI port (if fitted), to avoid wasting a normal port. AUI ports require special transceivers to connect them to the network. Stacking also increases the number of available ports and involves connecting the hubs via a special cable. The hubs must have special connectors to allow this. Unlike cascading, stacking creates a single logical hub and doesn’t add a repeater count to the network. Media Converters Media converters are devices that allow different cable types to be connected together (eg, 10Base-2 to 10Base-T). the cable connector. In both cases, you should get a reading of about 50Ω. That’s because, when you measure across the connector, the DMM should measure the resist­ance of the terminator at the far end of the cable. If the cable is short circuit, you will get a low reading across the connector. A high reading indicates that the cable has gone open circuit. Alternatively, an incorrect reading across the connector could indicate a dud terminator at the far end, so remove it and check it independently before condemning the cable. There’s not much that can go wrong with a terminator, however; it simply consists of a 50Ω resistor wired across a BNC plug. Don’t forget to check the T-pieces if one or more worksta­tions fails to come up on the network. To do this, reconnect the terminators to both ends of the cable, then disconnect the T-piece from its network card and measure the resistance across it. You should get a reading of about 25Ω (ie, half the resistance of one terminator), since the two terminators act as parallel resis­tors. UTP and STP cables, as used for 10Base-T, are usually wired straight through. They can be easily tested for shorts or open circuits using a DMM. Crossover cables are slightly trickier to check, since you have to know which pins are crossed over. If you have a CRO, you can use it to test for attenuation, either due to long cable runs, poor connectors or kinks in the cable. If a cable has been kinked, or bent at too sharp an angle, this can cause severe attenuation. This is something which can be detected on a CRO, but which cannot be picked up by a DMM. Software sleuthing If the connectors and cables are OK but the network still refuses to function, some software sleuthing may help. For exam­ple, if you have Windows NT, you can use the Event Viewer (click Start, Programs, Administrative Tools, Event Viewer) to track errors. You should also check the various tabs under Windows NT Diagnostics (especially the Network tab) to see if there are any problems. Alternatively, you could try monitoring the network using a dedicated commercial package; eg, the Netmon utility included with SMS. Another good hardware and software package is Black Box’s “Ether­tester”. Networking Test Gear From NDS* UTP/STP PAIRS TESTER THIS ENHANCED NETWORK CABLE TESTER detects shorts and open circuits in UTP/STP cables terminated with RJ45, RJ12 and RJ11 modular plugs. The main unit (Cat. 35RJTST6) is all that’s necessary for testing patch leads, while the “Network Cable Terminator” must also be included for remote testing (Cat. 35RJTST7 for both units). Similar units are also available for checking thin Ethernet (10Base-2) cables and for testing Ethernet ports (eg, on a hub or network card). ADVANCED CABLE TESTER DUBBED THE PENTASCANNER, this handy device can measure crosstalk, attenuation, resistance, impedance, cable length, capacitance and the attenuation-to-crosstalk ratio. It can be used to print easy-to-read certification reports, features customised “auto-testing” and can capture data and upload it to your PC for later analysis using specialised software. Cat. 35RJPS. Specialised test gear There’s also a vast range of specialised network test gear that’s mainly used by professional installers. Included in this range are dedicated cable testers, signal tracers, proto­col analysers and time domain reflect­ ometers (TDRs). You can even get all these functions combined into one dedicated unit! TDRs can determine where a break has occurred in a cable. They do this by measuring the time it takes for a signal to travel down the cable and be reflected back, to give the distance to the break. This makes the TDR an invaluable tool for quickly locating any cabling or socket wiring problems. Advanced cable testers can typically measure crosstalk, attenuation, resistance, impedance, cable length, capacitance and the attenuation-to-crosstalk ratio. Some can even look at such things as CRC (cyclic redundancy checking) errors, protocol and network statistics, collision errors, and overall network utili­sation. Acknowledgement Our thanks to Peter Elderton of Namlea Data Systems (phone 02 9439 6966) for their assistance in the preparation of this article and for permission to reproduce material from their catalog. *Namlea Data Systems, 22 Cleg St, Artarmon, NSW 2064. Phone (02) 9439 6966; fax (02) 9439 6965. www.ndsonline.com.au CAT.5 CABLE TESTER THE MICROSCANNER is designed to check continuity and wiring configuration in Cat.5 cables and can also measure cable length. A tone function allows cables to be traced. Cat. 35RJMS. WIREMAP SCREEN LENGTH SCREEN 10Base-T cable (2-pair, 4 wires) 70-metre cable February 1999  13