A medium-dependent interface (MDI) describes the interface (both physical and electrical/optical) in a

computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

from a physical-layer implementation to the physical medium used to carry the transmission.

Ethernet over twisted pair Ethernet over twisted-pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. They are a subset of all Ethernet physical layers. Early Ethernet used various grades of coaxial cable, but in 1984, Star ...

also defines a medium-dependent interfacecrossover (MDI-X) interface. Auto–MDI-X ports on newer network interfaces detect if the connection would require a crossover and automatically choose the MDI or MDI-X configuration to complement the other end of the link.

Ethernet

The popular

Ethernet Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

family defines common medium-dependent interfaces. For

10BASE5 10BASE5 (also known as thick Ethernet or thicknet) was the first commercially available variant of Ethernet. The technology was standardized in 1982 as IEEE 802.3. 10BASE5 uses a thick and stiff coaxial cable up to in length. Up to 100 stat ...

, connection to the coaxial cable was made with either a

vampire tap A vampire is a mythical creature that subsists by feeding on the Vitalism, vital essence (generally in the form of blood) of the living. In European folklore, vampires are undead, undead humanoid creatures that often visited loved ones and c ...

or a pair of

N connector The N connector (also, type-N connector) is a threaded, weatherproof, medium-size RF connector used to join coaxial cables. It was one of the first connectors capable of carrying microwave-frequency signals, and was invented in the 1940s by P ...

s. For

10BASE2 10BASE2 (also known as cheapernet, thin Ethernet, thinnet, and thinwire) is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors to build a local area network. During the mid to late 1980s, this was the dominant ...

, the connection to the coaxial cable was typically made with a single

BNC connector The BNC connector is a miniature quick-connect/disconnect RF connector, radio-frequency connector for coaxial cable. It was introduced on military radio equipment in the 1940s, and has since become widely used in radio systems and as a common t ...

to which a T-piece was attached. For twisted-pair cabling

8P8C A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such as in computer networking, telecommunication equipment, and audio headsets. Modular connectors were originally developed for ...

, modular connectors are used (often called ''RJ45'' in this context). For fiber, a variety of connectors are used depending on manufacturer and physical space availability. With 10BASE-T and 100BASE-TX, separate twisted pairs are used for the two directions of communication. Since twisted pair cables are conventionally wired pin to pin (straight-through) there are two different pinouts used for the medium-dependent interface. These are referred to as MDI and MDI-X (medium-dependent interface crossover). When connecting an MDI port to an MDI-X port, a straight-through cable is used, while to connect two MDI ports or two MDI-X ports, a crossover cable must be used. Conventionally, MDI is used on end devices and routers while MDI-X is used on hubs and

switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

. Some hubs and switches have an MDI ''uplink'' port (often switchable) to connect to other hubs or switches without a

crossover cable A crossover cable connects two devices of the same type, for example Data terminal equipment, DTE-DTE or Data circuit-terminating equipment, DCE-DCE, usually connected asymmetrically (DTE-DCE), by a modified electrical cable, cable called a cros ...

MDI vs. MDI-X

The terminology generally refers to variants of the

technology that use a female 8P8C port connection on a computer, or other network device. The X refers to the fact that transmit wires on an MDI device must be connected to receive wires on an MDI-X device. Straight-through cables connect pins 1 and 2 (transmit) on an MDI device to pins 1 and 2 (receive) on an MDI-X device. Similarly, pins 3 and 6 are receive pins on an MDI device and transmit pins on an MDI-X device. The general convention is for

network hub In network science, a hub is a node with a number of links that greatly exceeds the average. Emergence of hubs is a consequence of a scale-free property of networks. While hubs cannot be observed in a random network, they are expected to emerge in ...

s, bridges and switches to use the MDI-X configuration, while all other nodes such as

personal computer A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...

workstation A workstation is a special computer designed for technical or computational science, scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating syste ...

s, servers and routers use an MDI interface. Some routers and other devices had an uplink/normal switch to go back and forth between MDI and MDI-X on a specific port. The requirement of connecting the transmitter of one side to the receiver on the other side and vice versa makes it necessary to always have an ''odd number of crossovers'' between two devices, with an MDI-X port containing an internal crossover. Thus, connecting MDI to MDI-X requires a straight-through cable (one crossover in total). Connecting MDI to MDI (no crossover) or MDI-X to MDI-X (two crossovers) requires a(nother) crossover in the cable to get an odd number. When using more complicated setups through multiple

patch panel A patch panel is a device or unit featuring a number of jacks, usually of the same or similar type, for the use of connecting and routing circuits for monitoring, interconnecting, and testing circuits in a convenient, flexible manner. Patch ...

s in

structured cabling In telecommunications, structured cabling is building or campus cabling infrastructure that consists of a number of standardized smaller elements (hence structured) called subsystems. Structured cabling components include twisted pair and opt ...

, the connection can use multiple patch and building cable segments. It is a good idea to have all necessary crossovers on one side, i.e. either on the central hub/switch or on each secondary hub/switch.

Automatic MDI-X

To connect two ports of the same configuration (MDI to MDI, or MDI-X to MDI-X) with a 10 or 100 Mbit/s connection (10BASE-T or 100BASE-TX), an

Ethernet crossover cable An Ethernet crossover cable is a crossover cable for Ethernet used to connect computing devices together directly. It is most often used to connect two devices of the same type, e.g. two computers (via their network interface controllers) or two ...

is needed to connect the pair that each interface transmits on to the ''receive'' conductors of the other interface. The confusion of needing two different kinds of cables for anything but hierarchical

star network A star network is an implementation of a spoke–hub distribution paradigm in computer networks. In a star network, every host is connected to a central hub. In its simplest form, one central hub acts as a conduit to transmit messages. The ...

topologies prompted a more automatic solution. A twisted-pair Ethernet port with ''automatic MDI-X'', or ''auto MDI-X'' or ''auto-crossover'', automatically detects the required configuration, MDI or MDI-X, eliminating the need for crossover cables to interconnect switches or to connect network nodes peer-to-peer. As long as it is enabled on either end of a link, either type of cable can be used. For auto MDI-X to operate correctly, the data rate and duplex setting on the interface must be set to ''auto''. Auto MDI-X was developed by

Hewlett-Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company. It was founded by Bill Hewlett and David Packard in 1939 in a one-car garage in Palo Alto, California ...

engineers Daniel Joseph Dove and Bruce W. Melvin. A pseudo-random–number generator determines whether a network port will start in MDI or MDI-X configuration to begin auto-negotiating the link. When two auto–MDI-X ports are connected together, which is normal for modern products, the algorithm resolution time is typically < 500 ms. However, a ~1.4 second asynchronous timer is used to resolve the extremely rare case (with a probability of less than 1 in 5×10²¹) of a loop in which each end keeps switching. Subsequently, Dove promoted auto MDI-X within the

1000BASE-T In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use i ...

standard and also developed patented algorithms for ''forced-mode auto MDI-X'' which allow a link to be automatically established even if the port does not auto-negotiate. This may or may not be implemented on a given device, so occasionally a crossover cable may still be necessary when connecting an auto-MDI-X to an MDI-X (hub or switch) port, especially when

autonegotiation Autonegotiation is a signaling mechanism and procedure used by Ethernet over twisted pair by which two connected devices choose common transmission parameters, such as speed, Duplex_(telecommunications), duplex mode, and Flow_control_(data), flow ...

is deactivated. Newer routers, hubs and switches (including some 10/100, and all 1-gigabit or 10-gigabit devices in practice) use auto MDI-X for 10/100 Mbit connections to automatically switch to the proper configuration once a cable is connected. Gigabit and faster Ethernet links over twisted pair cable use all four cable pairs for simultaneous transmission in both directions. For this reason, there are no dedicated transmit and receive pairs, and consequently, crossover cables are never required for 1000BASE-T communication. The ''physical medium attachment'' sublayer (PMA) provides identification of each pair and usually works over crossover cables as well, and even if the pairs are unusually swapped, or if the polarity of a pair is unexpectedly inverted.IEEE 802.3-2012 ''40.1.4 Signaling''

References

External links

*
Diagram of non-automatic connections between MDI and MDIX devices
{{Ethernet Network architecture Network protocols Ethernet

Ethernet

MDI vs. MDI-X

Automatic MDI-X

See also

References

External links