Ethernet Over Twisted Pair
Early Ethernet cabling had generally been based on various grades of coaxial cable, but in 1984, StarLAN showed the potential of simple unshielded twisted pair by using Cat3 cable—the same simple cable used for telephone systems. This led to the development of 10BASE-T and its successors 100BASE-TX and 1000BASE-T, supporting speeds of 10, 100 and 1000 Mbit/s respectively. Often the higher-speed implementations support the lower-speed standards making it possible to mix different generations of equipment; with the inclusive capability designated 10/100 or 10/100/1000 for connections that support such combinations.:123
All these three standards define both full-duplex and half-duplex communication. However, half-duplex operation for gigabit speed isn’t supported by any existing hardware. The higher speed 10GBASE-T running at 10 Gbit/s, consequently defines only full duplex point-to-point links which are generally connected by network switches, and doesn’t support the traditional shared-medium CSMA/CD operation.
All these standards use 8P8C connectors, and the cables from Cat3 to Cat7 have four pairs of wires; though 10BASE-T and 100BASE-TX only require two of the pairs.
A 40GBASE-T standard, transporting 40 Gbit/s over up to 30 m Cat.8 cable is being defined as P802.3bq.
The Institute of Electrical and Electronics Engineers (IEEE) standards association ratified several versions of the technology. The first two early designs were StarLAN, standardized in 1986, at one megabit per second, and LattisNet, developed in January 1987, at 10 megabit per second. Both were developed before the 10BASE-T standard (published in 1990 as IEEE 802.3i) and used different signalling, so they were not directly compatible with it.
In 1988 AT&T released StarLAN 10, named for working at 10 Mbit/s. The StarLAN 10 signalling was used as the basis of 10BASE-T, with the addition of “link beat” to quickly indicate connection status. (A number of network interface cards at the time could work with either StarLAN 10 or 10BASE-T, by switching link beat on or off.)
Using twisted pair cabling, in a star topology, for Ethernet addressed several weaknesses of the previous standards:
- Twisted pair cables could be used more generally and were already present in many office buildings, lowering overall cost.
- The centralized star topology was a more common approach to cabling than the bus in earlier standards and easier to manage.
- Using point-to-point links instead of a shared bus greatly simplified troubleshooting and was less prone to failure.
- Exchanging cheap repeater hubs for more advanced switching hubs provided a viable upgrade path.
- Mixing different speeds in a single network became possible with the arrival of Fast Ethernet.
The common names for the standards derive from aspects of the physical media. The leading number (10 in 10BASE-T) refers to the transmission speed in Mbit/s. BASE denotes that baseband transmission is used. The T designates twisted pair cable, where the pair of wires for each signal is twisted together to reduce radio frequency interference and crosstalk between pairs. Where there are several standards for the same transmission speed, they are distinguished by a letter or digit following the T, such as TX.
Twisted-pair Ethernet standards are such that the majority of cables can be wired “straight-through” (pin 1 to pin 1, pin 2 to pin 2 and so on), but others may need to be wired in the “crossover” form (receive to transmit and transmit to receive).
It is conventional to wire cables for 10- or 100-Mbit/s Ethernet to either the T568A or T568B standards. Since these standards differ only in that they swap the positions of the two pairs used for transmitting and receiving (TX/RX), a cable with T568A wiring at one end and T568B wiring at the other is referred to as a crossover cable. The terms used in the explanations of the 568 standards, tip and ring, refer to older communication technologies, and equate to the positive and negative parts of the connections.
A 10BASE-T or 100BASE-TX node such as a PC uses a connector wiring called medium dependent interfaces (MDI), transmitting on pin 1 and 2 and receiving on pin 3 and 6 to a network device. An infrastructure node (a hub, switch, or router) accordingly uses a connector wiring called MDI-X, transmitting on pin 3 and 6 and receiving on pin 1 and 2. These ports are connected using a “straight-through” cable, so each transmitter talks to the receiver on the other side.
Nodes can have two types of ports, MDI (Uplink port) or MDI-X (Regular Port, ‘X’ for internal crossover). Hubs and switches have regular ports. Routers, servers and end hosts (e.g. Personal Computer) have uplink ports. When two nodes having the same type of ports need to be connected, a crossover cable is often required at speeds of 10 or 100 Mbit/s, else connecting nodes having different type of ports (i.e. MDI to MDI-X and vice verse) requires straight-through cable. Thus connecting an end host to a hub or switch requires a straight-through cable. On switches/hubs sometimes a button is provided to allow a port to act as either a normal (regular) or an uplink port, i.e. using MDI-X or MDI pinout respectively.
Many modern Ethernet host adapters can automatically detect another computer connected with a straight-through cable and then automatically introduce the required crossover, if needed; if neither of the adapters has this capability, then a crossover cable is required. Most newer switches have automatic crossover (“auto MDI-X” or “auto-uplink”) on all ports, eliminating the uplink port and the MDI/MDI-X switch, and allowing all connections to be made with straight-through cables. If both devices being connected support 1000BASE-T according to the standards, they will connect regardless of the cable being used or how it is wired.
A 10BASE-T transmitter sends two differential voltages, +2.5 V or ?2.5 V.
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.
A 100BASE-TX transmitter sends three differential voltages, +1 V, 0 V, or ?1 V.
1000BASE-T uses all four pairs bi-directionally and the standard includes auto MDI-X; however, implementation is optional. With the way that 1000BASE-T implements signaling, how the cable is wired is immaterial in actual usage. The standard on copper twisted pair is IEEE 802.3ab for Cat 5e UTP, or 4D-PAM5; four dimensions using PAM (pulse amplitude modulation) with five voltages, ?2 V, ?1 V, 0 V, +1 V, and +2 V. While +2 V to ?2 V voltage 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 100 meters. Category 5 cable has since been deprecated and new installations use Category 5e.
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 may not conform to any specified wiring standard. Some of the specified characteristics are attenuation, characteristic impedance, timing jitter, propagation delay, and several types of noise. 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, cable runs of 150 meters or longer are often obtained and are considered viable by most technicians familiar with the 10BASE-T specification.
Main article: Category 5 cable § Shared cable
10BASE-T and 100BASE-TX only require two pairs (pins 1–2, 3–6) to operate. Since Category 5 cable has four pairs, it is possible, but not necessarily standards compliant, to use the spare pairs (pins 4–5, 7–8) in 10- and 100-Mbit/s configurations. The spare pairs may be used for Power over Ethernet (PoE); or two phone lines; or a second 10BASE-T or 100BASE-TX connection. In practice, great care must be taken to separate these pairs as most 10/100-Mbit/s hubs, switches, and PCs electrically terminate the unused pins. Moreover, 1000BASE-T requires all four pairs to operate.
Autonegotiation and duplex mismatch
Main articles: Autonegotiation and Duplex mismatch
Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, …) exist for Ethernet over twisted pair, and most network adapters are capable of different modes of operation. 1000BASE-T requires autonegotiation to be on in order to operate.
When two linked interfaces are set to different duplex modes, the effect of this duplex mismatch is a network that functions much more slowly than its nominal speed. Duplex mismatch may be inadvertently caused when an administrator configures an interface to a fixed mode (e.g. 100 Mbit/s full duplex) and fails to configure the remote interface, leaving it set to autonegotiate. Then, when the autonegotiation process fails, half duplex is assumed by the autonegotiating side of the link.
|Speed [Mbit/s]||Distance [m]||Name||Standard / Year||Description|
|1||100 (nominally)||StarLAN||802.3e 1986||Runs over four wires (two twisted pairs) on telephone twisted pair or Category 3 cable. An active hub sits in the middle and has a port for each node. Manchester coded signaling.|
|10||100 (nominally)||LattisNet||(pre) 802.3i 1987||Runs over AT&T Premises Distribution System (PDS) wiring or four wires (two twisted pairs) on telephone twisted pair or Category 3 cable.|
|10||100 (nominally)||10BASE-T||802.3i 1990||Runs over four wires (two twisted pairs) on a Category 3 or Category 5 cable. Star topology with an active hub or switch sits in the middle and has a port for each node. This is also the configuration used for 100BASE-T and gigabit Ethernet. Manchester coded signaling.|
|100||100||100BASE-TX||802.3u 1995||4B5B MLT-3 coded signaling, Category 5 cable copper cabling with two twisted pairs.|
|1000||100||1000BASE?T||802.3ab 1999||PAM-5 coded signaling. At least Category 5 cable with four twisted pairs copper cabling. Category 5 cable has since been deprecated and new installations use Category 5e. Each pair is used in both directions simultaneously.|
|10 000||100||10GBASE?T||802.3an 2006||THP PAM-16 coding. Uses category 6a cable.|
|40 000||?30||40GBASE-T||802.3bq||under development, uses encoding from 10GBASE-T on proposed Cat 8.1/8.2 shielded cable|