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Fundamentals of Cat 5e

August 31st, 2010 Leave a comment Go to comments

This week, I have had to return to some networking fundamentals to emphasize a few points regarding how much an IT department can and should do themselves when it comes to network cabling.

At first glance, pulling your own network cables seems like a cost effective way to save money for a company. Many might make their own patch cables, after ordering the cable in bulk. While the fundamentals of the “do-it-yourself” network cabling project are straight forward, there are a few considerations that go into choosing the right cable for the job which often get overlooked.  This may help you when you are staring at that box of network cable, wondering how it compares to the box right next to it.

It can also be used as material to present to management if you change the title to “Why organizations should outsource network cabling projects to people who know what they are doing.”

Patch Cables are not for Wall Cabling

There is a difference in patch cabling versus normal wall cabling.  Don’t buy one for the other.  It is as fundamental as the reason an electrician use different wire in the wall than you do for an extension cord.

Wall cabling is made of solid wire.  Just like the electrical wire in your home, it is expected that this wire will sit unmoved as it connects the connection in the wall back to the source or patch panel.

Just like the extension cords in your home or on your appliances, Patch Cabling is made of stranded wire.  Many small wires wrapped together make continuous movement of the cable possible; reducing the likely hood it will break.  This is the wire that goes from your computer, back to the wall, or from the patch panel, back to the network switch.

The physical differences are enough that you don’t want to change the roles of these two types of cables.  The cabling has an entire industry of connectors relying on the wire to be solid or stranded in order to successfully connect and stay connected.

Pre-Decision on UTP Cat5e

There are different grades of cable available when you approach your networking project.  I will fast forward the conversation as most people find themselves looking at Cat 5e UTP cable as a standard.  The UTP is Unshielded Twisted Pair, and the category number represents the performance of the cable as it was tested and approved.  Yes, there is category 6, but the release of category 5 enhanced (that’s where the 5e comes from) has been approved for gigabit traffic and has such an arguably close tolerance to category 6, that most people find themselves at category 5e.

UTP Shielding Options

Here is one of the more overlooked options that seem to be overlooked.  While the internals of our Cat 5e cable defines the quality of the data that runs over it, there are different jacketing materials and classifications for the outside of the cable itself which can be important.   Most of these derived from general wiring requirements for fire code.

All bulk format UTP network cable comes with a different rating in order to meet different UL-NEC requirements.  This is the rating of the National Electric Code, which is published by the National Fire Protection Association.  The majority of what you find available will be labeled CM, CMG, CMR, or CMP, while you might stumble upon a CMH or a CMX.

The good news is that you can put them in a ranking order, with CMP at the top, but when you consider cabling an entire facility and you don’t need CMP, then the cost is significant enough to recognize the alternatives.

The ‘P’ in CMP is for Plenum.  In HVAC world if any part of your air flow that would go through your heating or air flow system is referred to as plenum.  This would include inside of the ducts themselves or in areas where return air is cycled above ceiling tiles.   These cables are meant to have a low-smoke and are less toxic when the jacket material burns.

The ‘R’ in CMR is for ‘Riser’.  Riser cables are designed to prevent the spread of fire from floor to floor and are setup for vertical shaft applications. If you are running cable from one floor to another, CMR would the minimum.  They may be overkill in a residential application, as they were designed to separate fire zones.

CMG and CM are the general cables to be used everywhere else.  They pass the vertical flame tests, and most likely the cables you will install in a residential application.

US & Canada Fire certifications (Source – Wikipedia)

Class            Acronym                        Standards

CMP            Plenum                        CSA FT7 [11] or NFPA 262 [11](UL 910)

CMR            Riser                                    UL 1666

CMG            General purpose            CSA FT4

CM                                                   UL 1685 (UL 1581, Sec. 1160) Vertical-Tray

CMX            Residential                        UL 1581, Sec. 1080 (VW-1)

CMH                                                  CSA FT1

Testing 123

While most departments have a cable tester able to light up a fancy LED light when their cable is successfully connected, few possess the equipment to truly diagnosis network cabling.  Unshielded Twisted Pair cable is designed to have different twists of cabling in order to prevent noise or attenuation in the cable.  While a blinky LED light will tell you if you have the right cable hooked up at both ends, it falls short on telling you that you have a connection capable of transmitting data across.  After you invest the money in the material, the time, and the equipment to make your cabling project come true, you might want to consider outsourcing it next time to a company specialized in network cabling.

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