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In the first two columns, I discussed my goals: to fill you in on how to network a home office or small business, cheaply and effectively. I also covered the choices for the physical cabling for your network: 10BASE-T (twisted pair, or UTP) category 5/four-pair, for new installations where you have expert help; for the rest of us, 10BASE-2, Thin Ethernet, coax cabling from computer to computer. You could build the wire up yourself, adding BNC connectors to RG58 A/U coax, plenum or PVC jackets (the latter is cheaper), or buy sections ready-made. Better, do one of the two following: for small, temporary, or casual installations, run wire directly from computer to computer; for more professional installations, and for better appearance and foolproof operation, use the AMP LAN-LINE Thinnet Tap System. Now, let's talk about The Rules for Thin Ethernet. To install Thin Ethernet cable, you must follow a few, simple Rules, specified in a standard written by the IEEE (Institute for Electrical and Electronic Engineers) committee 802. In short, it is referred to as 10BASE-2, for 10 Megabits per Second, Baseband, #2 (which is wiring with Thin Ethernet). Note that we aren't worrying about the other rules concerning Thick Ethernet and twisted pair here—this is just the straight stuff you need to know to get your network working first time, every time. 1) You must wire your network in a bus. That is, there is one continuous run of wire, starting at one end, proceeding from computer to computer, and ending at the other end. Unlike AC power, you can't split off a wire from the middle and run it out somewhere else. This is why it's called a bus. You can run the wire in spirals, Xs, or boxes, but you must be able to electrically trace the wire directly from one connection, to the next, to the next, and so forth. 2) Don't run the wire right next to an electric motor, the ballast of a fluorescent lamp, or anything else that's electrically noisy. Keep it apart. And, don't try to bend the wire tightly—say, more than a circle with a 3" diameter. You might break the center conductor! 3) The total effective length of wire you can use must total up to no more than 185 meters (that's about 608 feet). "Effective" means to add the length of wire to other factors mentioned below. Note that the use of certain network cards, or of boxes called "repeaters", can extend this length. Also, note that the cheapest and easiest way to double this length is to put two Ethernet cards in your fileserver, and run TWO lengths of network, each 608 feet or less. More on that later. 4) You MUST run the cable by the back of each connected computer, and use a BNC T-Connector (or equivalent) to attach it directly to the BNC connector on the back of your network card. You may NOT run a length of cable from the T-Connector to the network card. However, you MAY use the Thinnet Tap System cable, which looks like you are doing that (although you aren't). Remember, the File Server is just another connected computer. 5) At each end of the cable, you must attach a 50 ohm BNC terminator. You can buy these for about $4 each. Note that you should be able to remove the terminator from one end, and measure the resistance between the center and the outside of the BNC jack, and get about 55 ohms (+/- 10 ohms), indicating that everything is installed correctly. If you have both terminators attach, and measure from a T-Connector, you'll get about 27 +/- 5 ohms. 6) At one end, and one end only, the terminator must be grounded. You can buy terminators with grounding straps attached to them, or just use a length of lamp wire. The central screw on most electrical outlets is a good earth ground; the back of a PC (say, a screw in the power supply) is another. You can test an outlet with a cheap device from Radio Shack to make sure it has a good ground. 7) For every break in the cable, such as for a barrel connector or for a T-connector, add 6' to the effective length of the cable. If you are using the Thinnet Tap System, you don't have to do this, but you DO have to add TWICE the length of the drop cables you intend to install. 8) You may have no more than 32 T-connectors attached to computers (or other devices) on this segment of cable. This shouldn't be a problem in the small business or home office application! 9) Each T-connector with a device attached to it must be spaced more than 4' apart. If you follow the above nine rules, and you used the correct type of network cable with properly installed connectors, your network will work hassle-free. Promise! Let's say, for argument's sake, that you are running cable through a drop ceiling. At every wall, you are dropping a loop down through the wall, out a hole, and over to a nearby computer. Each of these loops can take up 25' or more of cable, easily, since the run of cable must go physically by the back of the computer (rule 1). What happens when 608' runs out all too soon? You have three choices. The first is to add a repeater box to the end of a run of cable, and then attach to that box a second run of cable. The second run can be another 608 effective feet in length, and must follow all the rules above. But, we must add a tenth rule: 10) There may be no more than two repeaters, as the cable runs, between any workstation computer, and the file server computer. This rule limits us to five total segments of 608' each, with four repeaters in use—assuming the file server is attached to the middle segment. Repeaters need AC power (you gotta plug them in!), have to be placed somewhere safe (you gonna wedge a cable converter-like box inside a sheet rock wall?) and cost several hundred dollars. This choice probably isn't the best one for most of us folk worrying about a small network. The second choice is to add another network card to the file server computer. Thin Ethernet cards with BNC connections are dirt cheap. Each card in a server may have a separate run of up to 608' in length attached to it. Given enough slots (and interrupts!) you can keep throwing more cards into the server to increase your length and capacity. This is also a smart idea, in order to improve performance. If you have (say) 20 users, and can split them into two groups of ten, each group using a different cable attached to a different Ethernet card in the server, then the server will keep each group from seeing the other group's traffic, thus allowing much more efficient work. Your network will work faster and better. Finally, if (heaven forbid!) a cable is physically broken, the other cable (and thus the other group) will keep on working just fine while you find the break and fix it. If you can arrange to place your file server computer somewhere convenient, this is a great solution. By the way—each of those two segments of cable may in turn be extended by a repeater, too, subject to Rule 10 above. The third choice is both easier and more difficult. Without spending one extra cent, you can increase the effective length of your single segment of cable to 300 meters (that's just over 1000'). This trick is called "Extended Segments." Some Ethernet adapter cards for the PC have this capability, such as many made by SMC/Western Digital, Thomas-Conrad, Intel, and 3Com. Even a few of the low-cost far-east clones will do this. Note that very few people you talk to will know what you're asking, though. The cards that allow extended segments will either auto-detect and auto-switch to that mode, or you will have to set that mode by a jumper on the card (or with a setup program). The manual usually mentions this fact. You may NOT use a repeater; thus, rule #10 above is out. But, for this case only, you must add a rule #11: all Ethernet cards and accessories attached to an extended Ethernet segment (more than 608 effective feet in length but less than 1025 feet) must be compatible with extended segments and properly set up for extended segments. Remember, that means that you'll have a hard time buying Ethernet-based print servers, laptop computer adapters, CD-ROM servers, and other goodies you would normally attach to an Ethernet cable. If you are prepared for this, have got to save every dollar, and/or can't physically place the file server computer at a location that would let you use the multiple-Ethernet card solution above, then you should consider this choice. What do you do when something goes wrong? Get out your little multimeter and test the cable. From any T-connector, you should read 27 +/- 5 ohms; from any end of the cable, remove one of the terminators, and you should read 55 +/- 10 ohms. Remove BOTH the terminators, and you should read open circuit—infinity ohms! If you get these readings, your cable is probably A-OK. If not, track it down. Take the terminator off one end. Trace the wire away from that end to the next T-connector. Disconnect the wire from the T-connector (the end going toward the terminator you removed). Put the terminator on here. Now, go to the other end of the cable and repeat the tests. Failed? Move that terminator forward to the next T-connector. Passed? That last segment of cable that you just disconnected is defective—replace it! (The BNC connectors at the ends are often the reason why.) Note that if you are using a Thinnet Tap System installation, you can still do this, by using a Test Cable assembly which they sell. That breaks the run of cable into two segments, by bringing out two BNC connectors, at the location you plug the Test Cable into. By testing both sub-segments, you can narrow your search in half. AMP also sells a testing kit that has a simple go/no-go box you just plug into a jack. However, since the coax cable is permanently installed in the wall, you run into problems much less—nobody is fooling with it (except the mice). Now that I've covered all the rules, the whys and the wherefores, let me make it simple. Here's how you wire a small network, with three workstation computers and one fileserver computer. Buy three ready-made 50' section of Thin Ethernet (or whatever length will do). Buy two BNC 50-ohm terminators, one with a grounding strap. Buy four BNC T-connectors. Put a T on the BNC connector on the back of each computer. Run a Thin Ethernet wire from computer #1's T to computer #2; from #2 to #3; and from #3 to the file server computer #4. Put a terminator on the unused end of the T connector on computer #1 and #4; attach the grounding strap from the terminator on #4 (only!) to a screw on the back of the computer. (You might want to tape the connections on each T to keep anyone from pulling them loose, or just risk it!) There—it's all done, and it will all work the first time. If you need a more permanent, nicer looking, or more foolproof installation, use the Thinnet Tap System, which will raise your costs by $40 or so per computer. If you need to troubleshoot a problem, or design a larger installation—well, you now know ALL the Rules involved in doing the job. Following the rules makes life pleasant and simple, as well as making you look real smart. Just don't start calling your terminators "Arnold"! So far, we've seen how to physically install and troubleshoot the wiring needed for any simple, small network. In the next issue, I'll start the discussion of what to do about the network cards—how to select them, how to install them, and how to make the mess you once called your PC to work again. See you then!
Six Easy Lessons
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