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An example of the server room rat’s nest.

A badly cabled system can cost even the smallest company thousands of dollars in system downtime, employee overtime, and stress.
Every tech has walked into at least one server room at a job site and seen something like this: a rat’s nest of cables that even the best tech is fearful of breathing on, lest it bring down an important piece of equipment. Ending up with a mess like this is deceptively simple. You start out with a few servers, add a few more, and a few more. Before you know it, cables are everywhere and your system is so complex that even the best tech will have a hard time figuring it out! In today’s Tele-blog I’ll cover the basics of structured cabling and how it can help to prevent down time in your system.

Good Cable Management Means Reliable Connections.

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An example of Structured cabling I installed at a recent job site.

The backbone any system is the cabling connecting its subsystems together and connecting it to other systems.  This cabling carries your services out to your customers, your employees, and allows you to connect with other telecommunications services. If this cabling fails or becomes intermittent, the service carried on that cable will follow suit. With that in mind, it is important to make sure you take good care of your cabling so it can give you many, many years of reliable service. Caring for cabling means, first, understanding how it is constructed.

The Basics of Cable Construction

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Image From http://en.wikipedia.org/wiki/Twisted-pair_cable

Most data communications and voice communications are transported via twisted pair cables. Like the image, these cables use solid 24AWG copper wire twisted together to form a balanced transmission line. These balanced lines are then twisted around each other as well to prevent cross talk and put into a sheath to keep them all together.  These cables can be anywhere from two pairs (CAT3) to 25 pairs (Cable used for RJ-21 wiring). The copper itself is not able to provide a lot of strength to the cable because it is a soft metal. Most of the strength comes from the PVC sheath around the cables.  There are “armored” cables are made to withstand high stress environments such as runs on telephone poles, radio towers, and direct burial. But this cable is quite expensive and not flexible enough to be used in a server room setting.

In order for these cables to work at their peak, the twist of the internal pairs must be keep intact. If there is any distortion or interruption to the twist, this will cause a reduction in the maximum bandwidth the cable can carry.  In order to make sure that your cabling doesn’t have any distortion in its internal twist rates, it is important to make sure there are no stress points.

Don’t let cable rest on a sharp edge or make sharp bends, this can cause distortion to the twist rate and can result in the copper pairs breaking over time.  Make sure cables make gradual bends, no more than 60 degrees, this will help your cable last longer and keep its full bandwidth.

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Notice the cables are tied to the cable tray where they make the transition from vertical to horizontal. This tie down will make sure they don’t slip and cause stress on the cables.

The Transition Point : From Cable to Connector.

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Image from http://en.wikipedia.org/wiki/Modular_connector

The next important part of the cabling equation is the termination, where the cable connects to your equipment or a patch panel. This is where you can lose a lot of bandwidth from sloppy work, and the potential for down time because connection as not secure. There are two main types of terminations to be considered here: plugs and sockets.

70% of the plugs you will see in both the computer network and Telecomm world will be modular plugs of some kind, as shown. These connectors are designed to push through the insulation on the pairs and make contact with the copper inside. The gold contacts are pushed into place via a crimper, which also pushes a plastic retainer up that will hold the PVC sheath in place.  If done correctly, these connections can be quite strong can tolerate the same amount of force that a cable could take, the key here being “If done correctly.”

Below are two images of 8 pin modular connectors in the field: The top one is done correctly, the bottom is not.

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Correctly crimped 8 pin Modular connector.
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Incorrectly crimped 8 pin modular connector

Here you can see that the correctly crimped connector has the jacket pushed as far up into the connector as it will go. This allows the the bottom retainer clip to dig into the PVC sheath on the cable. This keeps the cable in place and prevents stress from being placed on the copper wire in and the gold pins. Keep in mind the PVC jacket can take much more stress than the soft metal of the copper can.

In the image of the incorrectly crimped connector, the PVC jacket is hanging outside of the connector. This means all the stress is place on the copper wires. This connection will fail over time! Either all at once, or intermittently, either way it will be most likely to happen at the worst time, if Murphy has his way with it.

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Image from http://en.wikipedia.org/wiki/RJ21#RJ21″>http://en.wikipedia.org/wiki/RJ21#RJ21

The other common type of connector is the RJ-21, see above. These connectors are normally pre-installed on cables and so we won’t cover them in this post.  Most the time they are used for multiple voice line or signaling line connections. This type of connector is not rated for data communications.

Properly Patched Panels.

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The connection point on the back of a patch panel.

The socket part of the equation is normally in the form of a patch panel. This is a bank of 8 pin modular sockets on the from and 110 punch blocks on the back.

The main thing to keep in mind is that there is no built in method for a 110 punch block to secure its connection. The wires are just mechanically pushed into the connection points, that’s it! Therefore it is vital that you secure the cable as it comes into the patch panel. In the image above you can see the cables are tied to the cable management arm, this prevents stress from being put on the connections, which could cause a failure.

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An example of a poor patch panel termination.

As you can see in the image above there is nothing securing the cable, so all of the stress is on the 110 connections.  This image also shows the another thing to keep in mind, the twist rate must be keep as much as you can up to the point where it connections to the 110 blocks, this will make sure you get the maximum bandwidth out of the connection. The connection in the image above will not have the max bandwidth, in fact this connection is only able to get up to 50Mb/s. This is due to how long the pairs go without the right twist rate, remember when connecting to a 110 block on a patch panel keep the wire length as short as you can and keep as much of the twist as you can.

Next Time on Tele-BLOG!!!

 

In my next post I will talk about cable labeling and cable documentation.  Using the simplest tools to get the job done quickly and keeping your documentation simple and clear.

As always remember Keep it simple!!

 

Jeremiah

Owner: JP Telecom

Structured Cabling: How not to drive yourself insane.

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