Electrical specifications for category 3, 5e, and 6 UTP, plus 150 W STPA appear in TIA/EIA568B and are summarized in Table 8.1. Please note that these are cable specifications, not overall channel specifications. Channel specifications (which include connectors, jumper cables, work area cables, equipment cables, and other factors) are always worse (see note in Section 7.2, "SNR Budgeting").
Compared to category 3 cabling, categories 5e and 6 higher have progressively tighter twists and better plastic insulation with less dielectric loss at high frequencies. The resulting cables pick up less noise and have a superior frequency response (Table 8.1). Category 3 cabling is specified only up to 16 MHz, and therefore useful only up to about 25 Mbaud. Category 5e, 6, and 150 W STPA carry the attenuation specifications up to higher frequencies.
Category 3 cabling, while no longer, widely available in the market, is included because many buildings still have this wire installed. Category 5e is a better choice for new buildings because it performs better and costs no more.
Compared to any of the UTP categories, 150 W STPA specifies larger conductors (22 AWG versus 24 AWG) and higher characteristic impedance (150 W versus 100 W ). As a result, the 150 W STPA bandwidth is much higher than either UTP category, and its performance is specified up to a higher frequency. Balancing these advantages are some severe practical disadvantages, discussed in Chapter 9. Although at the time of writing 150 W STPA is recognized by TIA/EIA 568B.2, it is not recommended for new cabling installations and may be dropped from future versions of that standard.
Table 8.1. TIA/EIA568B UTP and 150 W STPA Electrical Specifications
Item 
Cat3 
Cat5e 
Cat6 
150 W STPA 
Notes 

H( w ) (dB) 

0.064 MHz 
0.9 
” 
” 
” 
Maximum allowable attenuation in dB per 100 m (328 ft) at 20 °C 
0.256 MHz 
1.3 
” 
” 
” 

0.512 MHz 
1.8 
” 
” 
” 

0.772 MHz 
2.2 
1.8 
1.8 
” 

1.0 MHz 
2.6 
2.0 
2.0 
” 

4.0 MHz 
5.6 
4.1 
3.8 
2.2 

8.0 MHz 
8.5 
5.8 
5.3 
3.1 

10.0 MHz 
9.7 
6.5 
6.0 
3.6 

16.0 MHz 
13.1 
8.2 
7.6 
4.4 

20.0 MHz 
” 
9.3 
8.5 
4.9 

25.0 MHz 
” 
10.4 
9.5 
6.2 

31.25 MHz 
” 
11.7 
10.7 
6.9 

62.5 MHz 
” 
17.0 
15.4 
9.8 

100.0 MHz 
” 
22.0 
19.8 
12.3 

200 MHz 
” 
” 
29.0 
” 

250 MHz 
” 
” 
32.8 
” 

300.0 MHz 
” 
” 
” 
21.4 

Z C 
Characteristic impedance (ohms) as measured on a 100 m length 

Min 
85 
90 
90 
135 

Max 
115 
110 [1] 
110 [1] 
165 

Max, at 10 MHz 
5.45 
5.45 
5.45 
5.45 
Permitted group delay (ns/m) varies slightly as a function of frequency. 
[1] NOTE (1) ”As implied by return loss specification.
POINT TO REMEMBER
8.1.1 UTP Modeling
What follows is a discussion of how to compute the model parameters for UTP cables (see Section 3.1, "Signal Propagation Model"). Since 150 W STPA is so similar to UTP, this section also develops the 150 W STPA parameters at the same time.
For 24gauge or similar cabling, parameter w should be set to 10 MHz ( w =2 p ·10 7 rad/s). Parameters Z and v appear directly in the specifications for UTP and 150 W STPA cables. [65]
[65] TIA/EIA 568B presents a formula for the worstcase cable delay at 10 MHz from which you may calculate v = 0.6116 c , which is rounded down to 0.6 c for all categories in Table 8.3.
R DC requires that you know the conductivity of annealed copper at 20 °C. The standards specify worstcase cable behavior at 20 °C. If your application operates at a significantly different temperature, increase the resistance by 0.39% per degree Centigrade. The formula for the DC resistance, per meter, of a UTP cable counts the resistance of both conductors (outbound and return). Hence, the factor of 2 appears on the lefthand side of this expression.
Equation 8.1
where 
s is the conductivity of the conductors, S/m, and 
d is the conductor diameter, m. 
Sometimes the total DC resistance of a cable (outbound plus return path included) is specified directly on the datasheet, in which case you may use that value instead of calculating it from the conductor diameter. Datasheet values must always be used for cables with complex conductor construction, such as coppercoated steel conductors, stranded conductors, or tinplated conductors.
Example: Calculation of DC resistance for Category 3 UTP
For annealed copper as prepared for ordinary cables at 20 °C, s = 5.80 ·10 7 S/m.
For AWG 24 copper conductors, the diameter d = 0.508 mm.
The worstcase DC resistance for this grade of cabling is specified as 0.1876 W /m, suggesting that conductors with a diameter 5% smaller than nominal size AWG 24 are permitted under the standard.
To compute the skineffect resistance R , you first need to compute the skin depth at frequency w . The following equation shows the calculation of skin depth as it ordinarily appears [69] .
Equation 8.2
where 
d is the skin depth, in meters , 
w is the frequency at which the skin depth is specified, rad/sec, 

m is the magnetic permeability of the conductor in H/m, and 

s is the conductivity of the conductor, S/m. 
Next use the skin depth to calculate the AC resistance of one of the signal conductors at frequency w . If the current distribution were uniform around the periphery of each conductor, the resistance would simply be 1/( p d d ( w ) s ). Unfortunately, life is not so simple.
The proximity effect distorts the pattern of current flow on the surface of the conductors, increasing the effective AC resistance by a fixed constant k p dependent on the conductor geometry (see Section 2.10.1, "Proximity Factor"). The proximity factors listed in Table 8.2 take into account the existence of two conductors (thus the values are each bigger than 2).
Equation 8.3
Expand d ( w ) according to [8.2].
Equation 8.4
Table 8.2. Proximity Factors for TwistedPair Cabling
Cable type 
Proximity factor k p 

cat3 
2.3 
cat5e 
2.3 
cat6 
2.3 
150 W STPA 
2.06 
Example: Calculation of Nominal SkinEffect Resistance for Category3 UTP
Assume w = 2 p ·10 7 rad/sec (10 7 Hz).
Copper being a nonmagnetic conductor, the magnetic permeability m = 4 p ·10 “7 H/m.
For annealed copper as prepared for ordinary cables at 20 °C, s = 5.80 ·10 7 S/m.
The proximity factor k p = 2.3 (Table 8.2).
For AWG 24 copper conductors the nominal diameter d = 0.508 mm.
The bestfit value in Table 8.3 for the worstcase specifications indicates a skineffect resistance of 1.452 W /m, suggesting that the worstcase specifications permit a combination of a diameter smaller than nominal size AWG 24, plating or other impurities in the surface layer of the copper, surface roughness effects, and measurement error combining to increase the nominal skineffect resistance by 22%.
Note about dielectric loss : For PVC used below 40 °C, assume a nominal effective dielectric loss of q = 0.02 (Table 8.3 shows a bestfit, worstcase value of 0.01578 for category 3 UTP). Above 40 °C the PVC dielectric material typically used in category 3 cables exhibits a strong temperature dependence. Do not use PVC cable at temperatures greater than 40 °C, or 104 °F, a temperature easily attained in an enclosed attic. If you need to work at elevated temperatures , specify a less temperaturedependent cable, such as an FEP, PTFE, or PFA plenumrated cable (see Section 8.6 "Category3 UTP at Elevated Temperature.").
8.1.2 Adapting the MetallicTransmission Model
As you might expect, the many possible combinations of surface plating, types of shielding, and dielectric make it difficult to accurately predict the performance of all twistedpair cables from the basic information provided on a datasheet. Fortunately, the copperbased cable propagation model is highly adaptable. By only adjusting two parameters, you can easily produce a model that mimics the performance of just about any twistedpair cable.
To adapt a model, start with [8.1] and [8.4] for DC resistance R DC and AC resistance R . That usually gets you pretty close to the worstcase frequencydomain specifications in the vicinity of w . Then tweak R , which scales the attenuation at all frequencies above the onset of the skin effect, and q , which increases the curvature of the attenuation graph at the very high end, until you get the best match to the worstcase specifications. That's how all the models in Table 8.3 were built. The "best match" criteria was a leastsquares fit of the attenuation magnitude in dB. The models optimized for performance all the way down to DC match the worstcase specification to within 0.16 dB per 100 m at all specified frequencies.
The models optimized only over the frequency range above 1 MHz match the worstcase specification a little better, to within 0.03 dB at all specified frequencies. Modeling always works that way ”the more limited the domain of a model, the better it can match any arbitrary specification. The main difference between the copperpropagation model and the TIA/EIA model (other than that the copperpropagation model includes the correct phase information and is therefore useful for timedomain simulation) has to do with the treatment of the transition from the skineffect mode to the LC (constantloss) mode. TIA/EIA models accomplish this transition over the limited range of frequencies near 1 MHz by incorporating a frequencyresponse term proportional to . The TIA/EIA approach, while enjoying some theoretical basis as an approximation in the range near 1 MHz, is unrealizable in a physical cable and poorly mimics cable behavior below 1 MHz.
To force the metallic propagation model to conform to the unnatural behavior of the TIA/EIA model near 1 MHz, parameter R DC has been increased and R lowered . This adjustment better matches the worstcase TIA/EIA specifications at frequencies above 1 MHz by sacrificing the correct value of DC resistance.
Figure 8.2 compares the signalpropagation model of Chapter 3 to the TIA/EIA568B specifications.
Figure 8.2. Comparison of attenuation computed by metallic transmission model (solid lines) to EIA568B specifications (points).
A perfect match is impossible to achieve. Standards often incorporate mysterious wobbles and bumps in their specifications, bumps that can't be matched with any rational modeling technique. No doubt these bumps are vestigial results of longforgotten standards battles over compatibility with products that existed at the time. Also, remember that specifications represent a worstcase conglomeration of many effects. Each individual effect may cause the performance of a cable in the field to touch the specification limit at one particular frequency, but it may be impossible to construct a single cable that hugs the limit precisely at all frequencies. Lastly, keep in mind that cable installers rarely if ever calibrate their test equipment, and some of the tests aren't that accurate anyway. It is quite possible that you will see cables in the field certified for operation that don't quite meet the specifications.
Table 8.3. WorstCase Transmission Line Parameters for TIA/EIA568B Cables
Cable type 
Z W 
v /c 
R DC W /m 
R W /m 
q rad 
w MHz 
Useful range MHz 
Max. error dB/100 m 

Models optimized to include DC performance 

cat3 
85 
0.6 
.1876 
1.452 
.01578 
10 
0 “16 
.012 
cat5e 
85 
0.6 
.1876 
1.253 
.001153 
10 
0 “100 
.032 
cat6 
90 
0.6 
.1876 
1.257 
.000447 
10 
0 “250 
.161 
150 W STPA 
135 
0.6 
.1142 
1.134 
.000658 
10 
0 “300 
.164 
Models optimized only from 1 MHz up 

cat6 
90 
0.6 
.2902 
1.208 
.000965 
10 
1 “250 
.028 
150 W STPA 
135 
0.6 
.2855 
1.061 
.001136 
10 
1 “300 
.013 
Such is life. To respond to these problems, you should construct a rigorous cable model for system testing that either adds another 2 dB of fixed, flat loss to the standard or extends the simulated maximum cable length by another 10% to 20%. Transceiver designs that pass such testing will likely work well in the field.
Calculated worstcase (slowest) system stepresponse waveforms for 100 m of category 3, category 5e, category 6, and 150 W STPA cabling appear in Figure 8.3. Longer or shorter cables scale the step response generally in proportion to the square of cable length. The stepresponse waveforms have been shifted horizontally to fit on the display.
Figure 8.3. Comparison of stepresponse waveforms for 100 m of cat3, cat5e, cat6, and 150 W STPA cabling (computed).
POINT TO REMEMBER
Fundamentals
Transmission Line Parameters
Performance Regions
FrequencyDomain Modeling
Pcb (printedcircuit board) Traces
Differential Signaling
Generic BuildingCabling Standards
100Ohm Balanced TwistedPair Cabling
150Ohm STPA Cabling
Coaxial Cabling
FiberOptic Cabling
Clock Distribution
TimeDomain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. TwoPort Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )
Notes