1
\$\begingroup\$

Category 5 cable on Wikipedia:

The bandwidth of category 5 and 5e is the same (100 MHz) and the physical cable construction is the same, and the reality is that most Cat 5 cables meet Cat 5e specifications, though it is not tested or certified as such.

So what does cable bandwidth mean? is it correct to say that cat 5 cable frequency response is almost flat in a region with 100Mhz width and 3-dB attenuation? Something like this with B=100Mhz:

                         Cable frequency response

If correct, what is f0 frequency for cat 5 cable?

And how the cat5 max speed, i.e. 100Mbps, is related to 100MHz bandwidth?

If we want to send digital signal with some simple line code such as NRZ, what will be the duration of a bit (bit time) in milliseconds?

\$\endgroup\$
  • 5
    \$\begingroup\$ Google image search: "cat5 insertion loss vs frequency". Cat5 cables are typically about 20dB/100m loss @ 100MHz, and the loss is very much length and frequency dependent. That quote is badly written by someone with little understanding of what they are talking about. \$\endgroup\$ – Tom Carpenter Dec 28 '16 at 22:05
  • \$\begingroup\$ Also worth pointing out that 100Mbit Ethernet does not mean 100MHz. Gigabit, 10G, and 40G can be run over copper, but do not require 1GHz/10GHz/40GHz cable bandwidths. Read up on MLT and PAM encoding. \$\endgroup\$ – uint128_t Dec 28 '16 at 22:34
4
\$\begingroup\$

Basically the quote you have shown was written by someone who either didn't understand what they were talking about, or oversimplified it.

The bandwidth of the cable is a result of the resistance of the cable and the fact that it is capacitive. These act like an R-C low pass filter limiting the bandwidth. Additionally the distributed inductance and capacitance of the cable are frequency dependent so have a more complex affect on the bandwidth.

The "insertion loss" which is a measure of the gain of the cable is dependant on both frequency, but also on length. The longer the cable, the more lossy it is.

Here is one example of the insertion loss of a CAT5e cable, this for a 100m length:

Insertion Loss vs Frequency
Image Source

Here we see that at 100MHz, the loss is actually more than 20dB - a lot more than the 3dB (power gain) or 6dB (voltage gain) point that would be used to specify the bandwidth of a first order low-pass filter as the quote implies.

Further up the Wikipedia page more accurately explains where the 100MHz figure comes from:

The specification for category 5 cable was defined in ANSI/TIA/EIA-568-A, with clarification in TSB-95. These documents specify performance characteristics and test requirements for frequencies up to 100 MHz

Basically, the requirements in terms of insertion loss that must be met for a cable to be classified as Cat5 are only specified up to 100MHz. Beyond this point the specifications of the cable are undefined by the standard - though manufacturers may well provide data at the higher frequencies.


In terms of how the Ethernet speed relates to frequency, that is a touch more complex. The raw data is encoded using various schemes which are described in brief in the wiki page here. For 100BASE-TX, several encoding schemes are used which result in a maximum fundamental frequency of 31.25MHz through the cable itself.

\$\endgroup\$
  • \$\begingroup\$ How 31.25MHz is obtained ? i read somewhere that for a reliable transmission of square wave signal, at least the third harmonic must be transmitted. So by having 100MHz bandwidth, frequency of square wave signal can be at maximum (100/3)MHz (approximately 33MHz). Is it the same 31.25MHz frequency? \$\endgroup\$ – Mehran Torki Dec 29 '16 at 8:27
  • \$\begingroup\$ And is it correct to say? : With 33MHz fundamental frequency, a simple line coding such as NRZ, will give us at max 33Mbps but in 100BASE-TX, each four bit is transmitted by one sample, so we have 33*4=132Mbps but it also uses 4B5B encoding which turns 132Mbps to 100Mbps. Sorry for asking any trivial question. \$\endgroup\$ – Mehran Torki Dec 29 '16 at 8:41
  • \$\begingroup\$ @MehranTorki Not 33MHz, 31.25MHz! (31.25MSps * 4bpS * (4/5)) = 100Mbps. \$\endgroup\$ – Tom Carpenter Dec 29 '16 at 10:52
  • \$\begingroup\$ Great, but how 31.25MHz is determined? not related to third harmonic of square waves? \$\endgroup\$ – Mehran Torki Dec 29 '16 at 10:58
  • 1
    \$\begingroup\$ High-speed Ethernet signals are NOT square waves anyway. \$\endgroup\$ – Dave Tweed Dec 29 '16 at 16:38
0
\$\begingroup\$

The difference between Cat5 and Cat5e is in the way the cables pairs are twisted. The twist in a Cat5e is tighter and this is done to reduce crosstalk.

Fast ethernet (100Mbps) uses only 2 pairs (one for TX and one for RX). Gigabit ethernet uses all 4 pairs for RX and TX at the time (using echo cancellation) which makes it very sensitive to crosstalk.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.