modular plug pin positioning Most Ethernet cables are wired
straight-through (pin 1 to pin 1, pin 2 to pin 2, and so on). In some instances, the
crossover form (receive to transmit and transmit to receive) may still be required. A cable for Ethernet may be wired to either the
T568A or T568B termination standard at both ends of the cable. Since these standards differ only in that they swap the positions of pairs 2 and 3the only pairs used by the formerly common 10BASE-T and 100BASE-TXa cable with T568A wiring at one end and T568B at the other functions as a crossover cable for the older, two-pair standards. A 10BASE-T or 100BASE-TX host normally uses connector wiring called
medium-dependent interface (
MDI), transmitting on pins 1 and 2 and receiving on pins 3 and 6. An infrastructure node (such as a
hub or a
switch) normally uses the complementary wiring arrangement, called
MDI-X, the
X standing for
-crossover. MDI-X simply reverses the pairs, transmitting on pins 3 and 6 and receiving on pins 1 and 2. These ports are connected using a
straight-through cable so each transmitter talks to the receiver on the other end of the cable. (Modern twisted-pair Ethernet uses all four pairs differently, and the MDI–MDI-X distinction does not apply.) Later equipment often can automatically switch between MDI and MDI-X arrangements as needed, obviating crossover cables and manual selection, but in the conventional arrangement, when two nodes having the same (fixed) type of port need to be connected, a crossover cable is required. If both devices being connected support 1000BASE-T, they will connect regardless of whether a straight-through or crossover cable is used. A 10BASE-T transmitter sends two
differential voltages, +2.5 V or −2.5 V. A 100BASE-TX transmitter sends three differential voltages, +1 V, 0 V, or −1 V. Unlike earlier Ethernet standards using
broadband and
coaxial cable, such as
10BASE5 (thicknet) and
10BASE2 (thinnet), 10BASE-T does not specify the exact type of wiring to be used but instead specifies certain characteristics that a cable must meet. This was done in anticipation of using 10BASE-T in existing twisted-pair wiring systems that did not conform to any specified wiring standard. Some of the specified characteristics are
attenuation,
characteristic impedance,
propagation delay, and several types of
crosstalk. Cable testers are widely available to check these parameters to determine if a cable can be used with 10BASE-T. These characteristics are expected to be met by 100 meters of 24-
gauge unshielded twisted-pair cable. However, with high-quality cabling, reliable cable runs of 150 meters or longer are often achievable and are considered viable by technicians familiar with the 10BASE-T specification. 100BASE-TX follows the same wiring patterns as 10BASE-T, but is more sensitive to wire quality and length, due to the higher
bit rates. 1000BASE-T uses all four pairs bi-directionally using
hybrid circuits and
cancellers. Data is encoded using 4D-PAM5; four dimensions using
pulse-amplitude modulation (PAM) with five
voltages, −2 V, −1 V, 0 V, +1 V, and +2 V. While +2 V to −2 V may appear at the pins of the line driver, the voltage on the cable is nominally +1 V, +0.5 V, 0 V, −0.5 V and −1 V. 100BASE-TX and 1000BASE-T were both designed to require a minimum of
Category 5 cable and also specify a maximum cable length of .
Shared cable 10BASE-T and 100BASE-TX require only two pairs (pins 1–2, 3–6) to operate. Since common Category 5 cable has four pairs, it is possible to use the spare pairs (pins 4–5, 7–8) in 10- and 100-Mbit/s configurations for other purposes. The spare pairs may be used for
power over Ethernet (PoE), for two
plain old telephone service (POTS) lines, or for a second 10BASE-T or 100BASE-TX connection. In practice, great care must be taken to separate these pairs as 10/100-Mbit/s Ethernet equipment
electrically terminates the unused pins ("Bob Smith Termination"). Shared cable is not an option for Gigabit Ethernet as 1000BASE-T requires all four pairs to operate.
Single-pair In addition to the more computer-oriented two and four-pair variants, the
10BASE-T1,
100BASE-T1 and
1000BASE-T1 single-pair Ethernet (
SPE) physical layers are intended for
automotive,
IoT, and
M2M applications or as optional data channels in other interconnect applications. The distances that single pair operates at full duplex depends on the speed: 1000 m (1 km) with 802.3cg-2019 10BASE-T1L; with 100BASE-T1 (link segment type A); up to using 1000BASE-T1 link segment type B with up to four in-line connectors. Both physical layers require a balanced twisted pair with an
impedance of 100 Ω. The cable must be capable of transmitting 600 MHz for 1000BASE-T1 and 66 MHz for 100BASE-T1. 2.5 Gbit/s, 5 Gbit/s, and 10 Gbit/s over a 15 m single pair is standardized in 802.3ch-2020. In June 2023, 802.3cy added 25 Gbit/s speeds at lengths up to 11 m. Similar to PoE,
Power over Data Lines (PoDL) can provide up to 50 W to a device. == Connectors ==