Alien Crosstalk | Tele-Tech Services Blog

Alien Crosstalk: When CAT6 Isn’t Helping

The most common physical medium for wired local area networks in the last 20 years has been “Unshielded Twisted Pair”  (or UTP).  This is copper cabling with 8 individual wires inside.  The wires are twisted in a special way to promote speed and noise resistance.  While there has always been some form of copper cabling supporting local area network over the decades, the industry settled on UTP in the mid-1990s. It’s true that Fiber Optic cable is a faster alternative.  However, the balance of price, ruggedness, and ease of installation makes UTP the most popular solution for local area networks.

The “unshielded” part of the UTP name does suggest one weakness; its susceptibility to cross-talk (electro-magnetic interference with in the cable) and alien crosstalk (interference between two different cables).  Crosstalk, in either form, slows down the UTP cable’s ability to carry network information at high speed.  If you imagine the difference between carrying on a conversation in a quiet room versus trying to talk to someone in crowed stadium then you’ll have an idea of why crosstalk is a bad thing.

The CAT’s Meow

The telecom industry classifies the various grades of UTP cable with “categories”.  The category is usually abbreviated as “CAT” as in “CAT3, CAT5, or CAT6, and so on.  As the number gets higher, so does the cable’s performance regarding speed and ability to reject noise.  This feature of “noise rejection” is another way of stating the cable’s ability to resist interference from crosstalk.

Until recently, the most popular category of UTP is “CAt5E”. (The “E” stands for “Enhanced”).  While there is a CAT6, CAT6A, and even CAT7, their high costs have kept them from main stream adoption.  But, the tide is turning and network managers are now beginning to install Cat6 cable for Local Area Networks.

Promises Promises

So why would you want CAT5, CAT6 or even CAT7?  It’s because they guarantee a level of speed.   In the midd-1990’s CAT5 was guaranteed to carry network data at 100Mbps.  Later, Cat5 was redesigned and upgraded to CAT5E which could handle 1Gbps.  And most recently, CAT6 is supposed to transmit data at 10Gbps (over short distances).  All of this is assumes that the network components such as switches and network cards can transmit and receive at those speeds.

Also, the higher grade UTP categories come with promises of greater noise resistance; or the ability to resist crosstalk – both internal and external.  This resistance is an important factor in being able to maintain the high network speeds.

Playing the Victim 

Within the context of cabling lingo, the term “victim” is used to describe a cable which is being affected by outside interference.  When network cables are installed they are bundled tightly in the last few feet as they enter the network room and patch panel.   This tight bundling may have as many as dozens of cables pressed closely together running in parallel.  When this happens the UTP cable are at the greatest risk of “victimizing” each other with crosstalk.

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Cat6 cables will victimize each other with alien crosstalk.  However, the inherent design of Cat6 cable includes a resistance to alien crosstalk to keep noise interference at an acceptable level.  But, what network managers may be forgetting is that when adding new cat6 cables to a network with existing cat5e, they are creating alien crosstalk conditions that will victimize the cat5e at levels it is not designed to resist.  In essence while trying to add faster cabling to their network, they may actually be degrading the performance of the cat5e cabling that is already in place.

Save your Pennies

When you consider that the retail price for Category 6 cabling is about 50%-100% more than category 5E, this can be a significant dollar amount if you’re adding hundreds of cables to your network.  This author recommends saving your IT budget for other innovations in networking such as 802.11 AC -  a 1Gbps WiFi standard.

If you simply must add CAT6 cables to an existing CAT5E network, consider keeping the wire runs separated and using separate patch panels.  Also, there is such a thing as “Shield Twisted Pair” CAT6 but it is another 50% more expensive than regular CAT6.  And, the installation of it is more tedious.

For existing cat5e networks, continue to augment those with more cat5e.  Even though the intention may be to “future-proof” your network infrastructure, consider that 10Gbps speeds to the desktop will probably not be necessary for another 5-7 years (if ever!).   And, most of the aforementioned advice assumes we don’t end up going all wireless very soon anyway!


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