Twisted-pair cables - nomenclature, standards, parameters

Original text (in Polish) by Lukasz Kopciuch
Twisted-pair (balanced) cables are used to transmit signals in telecommunication systems and computer networks. Currently they are mainly used in analog telephony and Ethernet networks. Twisted-pair cables may consist of one or more pairs. The twisting strongly reduces the influence of external electromagnetic interference and mutual interference between the pairs, called crosstalk.
Types and markings
Acronyms for balanced cables are standardized by ISO/IEC 11801:2002 (Annex E). The information contained in the cable description should have the following syntax: xx/yyTP, where the letters yy characterize individual wires/pairs and the letters xx describe the overall construction of the cable. The letters TP are the abbreviation of the balanced element, i.e. Twisted Pair.
The xx and yy markings can be as follows:
  • U – unshielded
  • F – foiled (screened with foil)
  • S – shielded (screened with braid)
  • SF – shielded and foiled
Typical balanced cables
  • U/UTP – unscreened cable
  • F/UTP – foil screened cable with unscreened balanced elements
  • U/FTP – unscreened cable with foil screened balanced elements
  • F/FTP – foil screened cable with foil screened balanced elements
  • SF/UTP – overall braid and foil screened cable with unscreened balanced elements
  • S/FTP – overall braid screened cable with foil screened balanced elements
  • SF/FTP – overall braid and foil screened cable with foil screened balanced elements
Standards and guidelines for the deployment of structural cabling
Country/Region Poland Europe USA World
Standard PN-EN 50173 EN 50173 TIA/EIA 568A ISO/IEC 11801

Categories/classes of twisted-pair cables
The categories and classes according to TIA/EIA 568A and EN 50173:
Standard TIA/EIA 568A ISO 11801  EN50173 Connectors Application Band
cat. 1 Class A    Telephone services
up to 100 kHz
cat. 2 Class B   Voice services and older terminal systems up to 1 MHz
cat. 3 Class C RJ11
10BASE-T Ethernet, telephone cables
up to 16 MHz
cat. 4 - RJ45 16 Mbps Token Ring up to 20 MHz
cat. 5/5e Class D RJ45 FastEthernet 100Base-TX, GigabitEthernet 1000Base-T up to 100 MHz
cat.6 Class E RJ45 ATM622, GigabitEthernet 1000Base-T up to 250 MHz
cat. 6A Class EA RJ45 GigabitEthernet, 10-GigabitEthernet 10GBase-T up to 500 MHz
cat. 7 F GG45,
10GBase-T, CCTV, 3-play services
up to 600 MHz
cat. 7A FA GG45,
10GBase-T, CATV up to 862 MHz, 3-play services, ready for 40G up to 1 GHz


Pin/pair assignments for eight-conductor 100-ohm balanced twisted pair cabling (8P8C/RJ-45)
T568A and T568B termination (pinout) for 100Base-T networks:
  • T568B (more popular)
  • T568A
If the two sides of the cable are terminated in the same way, it will be a straight-through cable, often called a patch cable. Otherwise, it will be a crossover cable.
Straight-through cables are used to connect computers/peripherals to networking devices such as hubs and switches, while crossover cables are usually used to create a direct connection between two computers.
T568A pinout on both ends - straight-through cable

T568B pinout on both ends - straight-through cable

T568B and T568A pinout - crossover cable
Parameters of twisted pair cables
Attenuation (insertion loss - IL)
Traveling along a cable, the signal loses some of its power. The attenuation is measured as the ratio of input power to output power, in decibels per unit length.
The attenuation of a twisted-pair transmission line depends on the following factors:
  • Cable length
  • Frequency - the attenuation increases with frequency of the signal
  • Construction and quality of of the cable, including resistance to aging that impairs its performance and increases attenuation
  • Humidity
Near-End Crosstalk (NEXT)
Near-end crosstalk measures the ability of a cable to reject crosstalk between pairs of wire at the near end of the circuit. Crosstalk is an undesirable condition of mutual interference between signals traveling through adjacent pairs of wires. The NEXT value for a given cable type is usually expressed in decibels per a length of the cable and varies with the frequency of transmission. It is measured as the ratio of the test voltage applied to a pair to the voltage induced in an adjacent pair.
Power Sum NEXT (PS NEXT)
PS NEXT measures the total amount of crosstalk between one pair of wires and the neighboring three pairs in the same cable. This parameter is especially important for cabling used in high-speed networks.
Far-End Crosstalk (FEXT)
FEXT is a measurement of how the far end of one wire pair affects the near end of another pair. The value of the parameter depends on the length of the transmission line.
ELFEXT is derived by subtracting the IL of the disturbing pair from the FEXT this pair induces in an adjacent pair. This operation normalizes the results for length.
Power Sum Equal Level Far End Cross Talk (PSELFEXT)
Power Sum ELFEXT (PSELFEXT) is the sum of FEXT values from 3 wire pairs as they affect the other wire pair, minus the insertion loss of the channel. A signal source is located at the opposite end of the line with respect to the measuring point.
Attenuation-to-Crosstalk ratio (ACR)
ACR is the difference between insertion loss and near-end crosstalk (NEXT). The ACR parameter indicates how much stronger is the attenuated signal compared with the crosstalk at the destination (receiving) end of a communications link.
Return Loss (RL)
RL measured in dB is the ratio of the input signal to the amount of signal that is reflected back towards the transmitter. The reflection of the signal is caused by the variations of impedance in the connectors and cable.
Delay Skew
The Delay Skew indicates the difference in propagation delay between the "fastest" and "slowest" pairs. The differences between propagation delays between pairs are caused by different lengths of the twisted wires. This parameter is critical for systems using all pairs for simultaneous transmission.