BlueSmoke - Guide : Network Crossover Cable

		Date	: Dec 7th, 2000
		Category	: Networking
		Manufacturer	: N/A
		Author	: Jin-Wei Tioh

I remember my maiden foray into networks, way back in 1997. Borrowed two 3Com Etherlink III ISAs (those were expensive back then), and tried to get two Win95 machines to talk to each other. Installation (setting IPs, subnets and such) went without a hitch. Connected both machines using the only ethernet cable I had. Then the moment of truth came. Booted up both machines and tried to ping each other, and the results were less than exciting...

After lots of soul-searching and head-scratching, it finally dawned on me that I needed a crossover cable, not a normal, straight-through cable like the one I was using. I had neither the equipment nor the expertise to go the DIY route, so I bought one. Reconnected the 2 machines, and voila!

This guide will show you how to properly construct a crossover network cable. It is used to directly connect _two_ computers to each other without the use of a hub or switch. Why a crossover cable? A cable is much cheaper than even the cheesiest generic no-name hub, is simple to transport and requires no additional accessories (eg. power brick, etc.)

What is the difference between a straight through and a crossover cable? The ends of a crossover cable are different, whereas a straight through cable has similar ends. The network ports on hubs and switches are usually MDI-X ports, and those on NICs (network interface cards) are usually MDI ports. Hubs and switches act like relays, electronically performing the crossover functions. A crossover cable can also accomplish this, but by physical means, ie. it is hardwired.

Networking using a crossover cable

Networking using a hub / switch

A "straight-through" cable is used when connecting two ports that are not set to the same MDI mode or not set to the same MDI-X mode. When connecting MDI-X ports to ports that are MDI (eg. hub to NIC), a straight-through cable is used. Each internal twisted pair of wires is connected to the same pin number at each end. The twisted-pair wires must be twisted throughout the entire length of the cable and the wiring sequence should conform to AT&T 258A standard.

		Pin #	Signal	AT&T 258A
		1	Transmit+	White/Orange
		2	Transmit-	Orange/White or Orange
		3	Receive+	White/Green
		4	N/A	Blue/White or Blue
		5	N/A	White/Blue
		6	Receive-	Green/White or Green
		7	N/A	White/Brown
		8	N/A	Brown/White or Brown

A "crossover" cable is used when connecting two ports that are set to the same MDI-X mode, or (in this case) the same MDI mode. The specs are identical to a straight through cable, except that the transmit (TX) and receive (RX) pairs must be reversed.

		Pin #	Signal	AT&T 258A
		1	Receive+	White/Orange
		2	Receive-	Orange/White or Orange
		3	Transmit+	White/Green
		4	N/A	Blue/White or Blue
		5	N/A	White/Blue
		6	Transmit-	Green/White or Green
		7	N/A	White/Brown
		8	N/A	Brown/White or Brown

Whew, you might want to take a breather for a moment. You now know the purpose of a crossover cable, its specs and advantages. Let's shift our discussion to cables. For simplicity, I'll only cover twisted pair cables. These are basically multi-strand cables, containing many pairs of wires. The wires in each pair are twisted around each other to reduce unwanted noise. It is the most common form of telecommunications wiring today, and it is provided in two forms. STP (shielded twisted pair), which has a ground shield, and UTP (unshielded twisted pair), which doesn't. Of the two, UTP is the more predominant type.

Twisted pair cables are used for everything from internal telephone wiring to connecting customer houses and buildings to telephone exchanges. When used to implement LANs (local area networks), the two main standards are 10BaseT (10Mbit bandwidth) and 100BaseT (100Mbit bandwidth) Ethernet. For a network cable to properly support 100Mbit data transfers, it must be rated Category 5 (CAT 5). CAT 5 cables supports 10BaseT, 100BaseT and the new 100VG AnyLAN standards. Just as a point of interest, there is also CAT 3 (16Mbit max bandwidth) and CAT 1 (1Mbit max bandwidth) cabling.

Cable This depends on the network speed that you intend to use. If you want to run a 10Base-T, you can get by with CAT 3 cable. However, this severely limits your network's expandability, as you cannot run 100Base-T and 100VG-AnyLAN in the future. Moreover, CAT 5 cable is just slightly more expensive than CAT 3, and supports all the previously mentioned networking standards. Regardless of what cable type you choose, always double check that the cable rating, which is usually stated clearly on the jacket of the cable. That aside, you can use a pre-made straight-through cable, replacing one of the ends with a new one wired with the "crossover" configuration. In this article though, I will be making the cable from scratch.

Connectors

UTP cables are all terminated with RJ-45 modular plugs. They look like the modular plugs on telephone cables (RJ-11) on steroids. Nothing much in this respect, just make sure that you buy RJ-45 plugs with the appropriate rating.

Crimper A crimper is the popular name for a modular crimping tool. You can either purchase a crimper separately or as part of a networking toolkit. It would not be wise to skimp on this piece of equipment, because this is one of the occasions where you really get what you pay for. Sure, you could get some generic cheap crimper for $10 to $20, but after a couple of crimps, it would most likely break. Something in the mid-range would be fine. As a bonus, crimpers in the mid price range and above can usually crimp more than one type of modular plug, like the one used in this article.

Stripper

A stripper is also required. You can hire, er... I mean buy one either separately or as part of a networking toolkit. This is a special tool used to strip the ends off wires. You can improvise with razor blade, pliers, or scissors but don't forget to take precautions so as not to damage the copper wires.

Cutters

You need a pair of wire cutters that is able through (relatively) big strands of copper. The one used in this article is a stripper / cutter combo. As we will see later, it is important that the trimmed wires all have the same length. This means that, no, you shouldn't just make do with a pair of scissors.

Cable Tester This piece of equipment is optional. It basically tests all four pairs of wires in a network cable. Again, you can either buy it separately or get it packaged as part of a networking toolkit. Generally, the higher-priced a cable tester is, the more features / indicators it has. The one used in this article is a mid-range tester packaged in a networking toolkit.

That about wraps up the checklist. Let's get down and dirty.

Step 1	:	First thing to do is cutting a suitable length of cable. Measure what you need, and add a couple of inches for both ends. The industrial failure rate is 1 in 10, so that means there is roughly a 10% chance that something will go wrong. If you don't leave a margin of safety in the cable length, and you foul up, the cable would be shorter than required and you would have a start over with a new length of cable.
Step 2	:	Strip off 2 inches of the plastic jacket off the end of the cable, taking care not to damage the twisted wires inside. Check, double check and triple check the thin wires for any nicks. Do not hesitate cut the whole end off and start over if the wires are damaged.
Step 3	:	Separate the 4 pairs whilst pressing firmly against the base of the jacket. Strictly speaking, CAT 5 cables must only have 1/2 an inch of untwisted wire at each end. Otherwise, it would become out of spec, becoming a potential source of headaches later on.
Step 4	:	At this point, things might get a little hairy. If you are modifying a pre-made straight through cable, you would have already cut off one of the ends. Since the existing end is already wired with the "straight-through" configuration, you only have to install the crossover end. If you are making the cable from scratch as in this article, then you would have to install both the straight-through and crossover ends. Straight through configuration Crossover configuration Untwist the exposed twisted pair wires on your cable. Again, firmly press down on the base of the jacket to avoid untwisting the wires inside the jacket. Once done, arrange the wires as in the diagrams above. Using your cutters, make a clean, straight cut 1/2 an inch above the jacket.
Step 5	:	Take a RJ-45 modular plug, and begin to slide the arranged untwisted in. This might take considerable patience, since if the wires slip, you have to rearrange them all over again. Slide it as far in as it will go. Once you're done, you should end up with something similar to this.
Step 6	:	Finally, here is the fun part. Now is your chance to release all that pent up frustration you accumulated earlier. Grab your crimper, insert the RJ-45 jack and begin to compress the crimper. Note that the crimper used here might differ from yours. Most likely, there will a clicking sound. Keep going until it stops moving. Now the crimper should release, leaving you with a crimped jack. Give it a quick lookover. Part of the jacket towards the back of the RJ-45 jack should be crimped, holding the jacket in the jack. If you didn't do that good a job, cut off the end and start over. Better to have a surefire network cable than to spend hours troubleshooting your network later on. If you are making the cable form scratch, go back to Step 2 and repeat the process for the other end.