What causes data loss on Network Cables?

Network Cables

You might be tempted to think that a cable is a cable is a cable, but then you look around and notice that there are lots of different types on sale, and that data rates seem to be much greater than they used to be. Why only a few years ago everyone had 10M Ethernet and now that’s old hat and everybody has at least 100M and increasingly 1000Base T (Gigabit) Ethernet. Something must be different.

Well the reality is that all cables are not equal, and increasing the data rate brings a lot of new factors into play. A cable that operates happily at 10M is a different beast at 1000M and as you start to use all four pairs in the cabling (as you have to do to get the faster rates) you introduce a whole new set of problems that need to be considered when testing the cabling.

What causes network slow downs?

An electrical signal is composed of electrons moving along the wire. As the speed/frequency/Data rate increases  a number of new phenomena appear, and these need to be taken into account when installing and testing the cabling. As a consequence the standards have had to change to take these effects into account.

There are three main problems that cause issues:

• Signal loss/degradation
• Signal Noise
• Delays

Signal Loss/Degradation

We are all used to the fact that in nature as a signal travels further from its source it weakens. The same is true of electrical signals in wires. Beyond a limit (different for each type of cable) the signal is too weak or distorted to be recognisable. Cable length is the major factor for signal loss.

Higher Temperatures increase signal loss too, about 0.4% per degree Celsius for Cat5e cabling.

Signal loss  is measured in decibels (dB). For some reason, signal loss is measured in negative numbers, e.g. -3dB and the negative sign is usually ignored, so the example would read as 3db of loss. Consequently lower number are better. So 2dB is better than 4dB.

Resistance is a function of the cross sectional area of the conductor. Resistance in the wire limits the signal and dissipates the energy as (a small amount of) increased heat. The longer or thinner the wires the greater the resistance.

The insulation covering the individual wires in the cable inevitably absorbs some of the signal. Since many wires are placed very close together they store this energy, acting in electrical terms, like a capacitor. High Density Polyethylene (HDPE) is commonly used because its electrical properties at high frequencies helps to minimise the losses. Cables that are designed for lower frequency applications may perform poorly at higher frequencies.

Electrically impedance is a combination of resistance, capacitance, and inductance expressed in Ohms. Typical cables are rated at around 100 Ohms. A so called Return Loss occurs when a signal hits a high impedance, for example an incorrect connector or a cable fault, and is bounced back. Potential bounced signals can cause problems on high speed networks, the higher the network speed the more pronounced the problem. Poorly fitted or wrongly specified connectors are a major cause.

Signal Noise

Any electrical signal on the wire not part of the sender’s original signal is classed as noise. Noise is generated both internally and externally.

Twisted pair cables produce no interference, the twists cancel each other out, in theory that is. In real life any variation in the thickness of the wire, in the cable insulation, and in the capacitance of wires or insulation will cause a mismatch and consequently noise. Good quality cables minimise the noise but cannot remove it altogether.

Electrical interference can come from many sources. Cables should always be installed in separate conduits away from mains cables. In industrial applications electric motors (in lifts/elevators) fluorescent lights and air conditioners, are major sources of interference.

In areas of electrical noise it is common to shield cables or to use other technologies, such as optical fibre to avoid interference.

Crosstalk is likely to be much greater than any other noise effect. When a signal travels down a conductor, an electric field is created, which interferes with any wires close by. This is Crosstalk and gets larger at higher frequencies and the more parallel the wires. The twists in the pairs should (in theory) cancel this effect. For good signal cancellation it’s important that the twists are symmetrical and that adjacent pairs have different twists.

Crosstalk is measured in decibels (dB).

Signal to Noise Ratio (SNR)

As the name implies this compares the signal strength to the amount of noise on the pair. A more accurate term for a similar thing is Attenuation to Crosstalk Ratio (ACR). Both terms are also slightly misleading. ACR isn’t a ratio, it’s just the difference between signal strength and NEXT, and SNR includes internal as well as external noise (which for most practical purposes makes no difference). You will hear both terms used to mean the same thing. Only in areas of extreme electrical interference will ACR and SNR be significantly different.

Obviously the difference between the level of noise and the level of signal is important. You are aiming to get a good signal much stronger than any noise. Attenuation greater as frequency increases and NEXT gets lower. The difference for any cable is the ACR. Theoretical bandwidth limits are always higher than the rates used in practical cabling.

Delays

Electrical signals travel very fast, but not infinitely fast. Typical twisted pair cables run at 60% to 90% of the velocity of light. The time taken for a signal to travel down the pair is the Propagation Delay. The propagation delay in itself is not normally an issue, the recommended cable lengths will already have taken this into account. However Delay Skew, described below is a significant factor.

Skew

Think back to the fact that the wires are twisted and therefore the length of the pairs that make up the cable are not equal. Signals sent at the same time will therefore arrive at slightly different times. This difference in arrival times is the Delay Skew, and must be within 50 nanoseconds for Cat5, Cat5e and Cat6 cables.

If Delay Skew is excessive then network devices will have trouble communicating, resulting in very slow or totally non-functioning networks.

How to Prevent Data Loss

• Maintain a minimum Bend Radius of 4 times the cable diameter, and never create a 90 degree bend in the cable.  That will damage the cable.
• Apply cable ties loosely, and at random intervals.  If you overtighten the cable ties, it can pinch or crush the wire.
• Try to minimize the amount of jacket twisting.
• Use Cable Ties, Cable Support Bars, Wire Management Panels, and/or releasable velcro straps to secure cables.
• When creating the cable ends, make sure that the twisted pair cables maintain it’s pair twisting to winin a half inch or less.  Any great will adversley affect the performance of the cable.
• Plan out your installation so that you do not run cable further than 90m, or 295 ft.
• Never run data cable in the same conduit as electrical wiring
• Where possible, home run the data cables.  In otherwords, run from the source, directly to the device.  Do not use splits, extenders, etc.
• Use Cat5e, or cat6 cable, as needed.
• Use a maximum patch cable length of 6m (20 ft) in the wiring close.  If this isn’t possible, decrease your cable run length to compensate.
• Use a maximum patch cable length of 3m (10 ft) at the workstation outlet.  If this isn’t possible, decrease your cable run length to compensate.
• When pulling cable, avoid kinking and tugging.  Constant tension should be used when pulling cable into place, and do not exceed a 25 pound maximum for pulling tension.
• If you are only using a single pair, do not run voice over the 2nd pair.  This will cause interference.
• When routing cables through walls and ceilings, keep as far way as possible from EMI and RFI sources, such as fluorescent lights, electric panels, light dimmers, and electrical motors.