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

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

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, Sta ...

also defines a medium dependent interface crossover (MDI-X) interface. Auto MDI-X ports on newer network interfaces detect if the connection would require a crossover, and automatically chooses the MDI or MDI-X configuration to properly match 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 1 ...

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 stati ...

, connection to the coaxial cable was made with either a

vampire tap A vampire tap (also called a piercing tap) was a device for physically connecting a station, typically a computer, to a network that used 10BASE5 cabling. This device clamped onto and "bit" into the cable (hence the name "vampire"), inserting a ...

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 ...

s. For 10BASE2, the connection to the coaxial cable was typically made with a single

BNC connector The BNC connector (initialism of "Bayonet Neill–Concelman") is a miniature quick connect/disconnect radio frequency connector used for coaxial cable. It is designed to maintain the same characteristic impedance of the cable, with 50 ohm and 7 ...

to which a T-piece was attached. For twisted-pair cabling 8P8C, modular connectors are used (often incorrectly 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 of ...

. 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 DTE-DTE or DCE-DCE, usually connected asymmetrically (DTE-DCE), by a modified cable called a crosslink. Such a distinction between devices was introduced by IBM. The cross ...

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 (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...

workstation A workstation is a special computer designed for technical or 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 systems. The term ''workstat ...

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, 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.

Auto 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 cross over the transmit and receive signals in the cable, so that they are matched at the connector level. The confusion of needing two different kinds of cables for anything but hierarchical star network topologies prompted a more automatic solution. Auto MDI-X (aka "auto crossover") automatically detects the required cable connection type and configures the connection appropriately, removing the need for crossover cables to interconnect switches or connect PCs 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 on the interface and duplex setting must be set to "auto". Auto MDI-X was developed by Hewlett-Packard engineers Daniel Joseph Dove and Bruce W. Melvin. A pseudo-random number generator decides whether or not a network port will attach its transmitter, or its receiver to each of the twisted pairs used to auto-negotiate 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 where 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 ...

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 auto MDI-X to MDI-X (hub or switch), especially when autonegotiation 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 continues to work over crossover cables as well, even if the pairs are unusually swapped, crossed, or if the polarity of a pair is unexpectedly inverted.IEEE 802.3-2012 ''40.1.4 Signaling''

References

External links

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

Ethernet

MDI vs. MDI-X

Auto MDI-X

See also

References

External links