Network Cable Math

Introduction

Today I'm going to be discussing ethernet cables and if you've been reading this blog for a while, you know I'm no stranger to ethernet problems. I work with it quite a lot at work so I'm familiar with products like the ws-c2960s-48lps-l cisco catalyst and all the cables that ethernet entails. I am looking at category 5e (cat5e) cabling today. During some routine testing, I was seeing bit errors occurring on 100 meter long Ethernet cables that were operating at 60°C. This prompted me to investigate the effect of temperature on Ethernet bit error rate. Some businesses use a host server such as Hostiserver to minimise any faults and improve their web experiences. I found it surprising how complicated doing design work with these cables can be. In this post, I am going to review some basic cable characteristics and the kind of things that engineers need to look at when designing networks using these cables. There is nothing earth-shattering in this discussion -- things are just more complicated than you might expect. Let's dig in ...

Background

What Limits Cable Reach?

Most people think that the reach of Ethernet is 100 meters -- at true statement for a system operating at 20°C. We usually do not discuss what limits the reach of the cable. For the discussion at hand today, I will assume that signal attenuation limits the reach of the cable. There are other factors that can limit the reach of a system, but for today we will only look at signal attenuation.

The 100 meter reach number is based on some assumptions:

• 90 meters of cat5e cable

  Cat5e cable is composed of 4 pairs of 24 American Wire Gauge (AWG) solid wires (8 wires total). The pairs are twisted to reduce the effects of electromagnetic interface. Unfortunately, the signal attenuates as it travels down the cable. The amount of attenuation is referred to as insertion loss.

• 10 meters of cat5e patch cables

  Cat5e patch cables are composed of 4 pairs of 24 AWG stranded wires (8 wires total). The stranded wire is more flexible and is easier to bend than solid core wire, but it has higher insertion loss.

• 4 connectors

  Connectors introduce additional losses that must be accounted for.

Communication systems are composed of transmitters, channels, and receivers. A minimum requirement for a reliable communication systems is for the transmitters to send a signal that is large enough for the receiver to clearly interpret after being attenuated by the channel.

Cat5e Attenuation Limits

The cat5e cable and connector attenuation limits are set by the industry standard TIA-EIA-568-B.2. I repeat these limits in Equation 1.

Eq. 1

where

• IL_100mCable is the insertion loss introduced by the 100 meters of cat5e cable (solid wire) at 20 °C.
• IL_{100mPatchCable} is the insertion loss introduced by the 100 meters of cat5e patch cable (stranded wire) at 20 °C.
• IL_Connector is the insertion loss introduced by a single connector at 20 °C.
• f is the signal frequency expressed in MHz.

The industry has taken the approach that, as long as a cable has less attenuation than the specification's limit, enough signal from the transmitter will get through the channel to the receiver for reliable reception. Reliable reception is defined as a Bit Error Rate (BER) less than 1E-10.

Analysis

Insertion Loss @ 20 °C

Figure 1 shows a screenshot of the Mathcad worksheet that I am using for my analysis. I have duplicated the insertion loss chart of TIA/EIA-568-B.1.

Figure 1: 100 meter Cat5e Insertion Loss Calculation.

Insertion Loss Increase at Higher Temperature

TIA-EIA-568-B-1 specifies how the insertion loss calculations are to be performed at elevated temperatures. Cable loss is to modeled as a 0.4% per °C increase in temperature. For the exact quote, see Appendix B. Since all the losses are multiplied by 0.4%/°C and we must keep the insertion loss under the 20 °C value at all frequencies, the maximum reach of Ethernet reduces by 0.4%/°C. So if we run our Ethernet over 60 °C cable, the maximum range is reduced by 16 meters. The calculation is shown in Equation 2.

Eq. 2

Conclusion

It appears that the increased temperature I am encountering with this cabling is reducing the maximum length supported by Ethernet. I will need to adjust my test configurations.

Appendix A: Quote on Stranded Cable Insertion Loss Tax

The following quote from TIA-EIA-568-B-1 addresses the modeling approach to be used for cat5e patch cables.

Cat 5e stranded cable table shall meet the values computed by multiplying the horizontal cable insertion loss requirement in clause 4.3.4.7 by a factor of 1.2 (the de-rating factor), for all frequencies from 1 MHz to 100 MHz. The de-rating factor is to allow a 20% increase in insertion loss for stranded construction and design differences.

Appendix B: Quote on Cat5e Insertion Loss Increase with Temperature

The following quote from TIA-EIA-568-B-1 addresses the modeling approach to be used for cat5e patch cables.

Insertion loss is expressed in dB relative to the received signal level. Insertion loss shall be measured for all cable pairs in accordance with ASTM D4566 and 4.3.4.14 at 20 ± 3°C or corrected to a temperature of 20 °C using a 0.4%/°C correction factor for category 5e cables for the measured insertion loss.