Medium dependent interface
Encyclopedia
A medium dependent interface (MDI) describes the interface (both physical and electrical) in a computer network
from a physical layer implementation to the physical medium used to carry the transmission.
Ethernet over twisted pair
also defines a medium dependent interface crossover (MDIX) interface. Auto-MDIX ports on newer network interfaces detect if the connection would require a crossover, and automatically chooses the MDI or MDIX configuration to properly match the other end of the link.
family defines common medium dependent interfaces.
For 10BASE5
connection to the coaxial cable was made with either a vampire tap
or a pair of N connector
s. For 10BASE2
the connection to the coaxial cable was typically made with a single BNC connector to which a T-piece was attached. For twisted pair cabling 8P8C 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 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 a 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 while MDI-X is used on hubs
and switches. Some network
hubs or switches
have an MDI port (often switchable) in order to connect to other hubs or switches without a crossover cable
.
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 MDIX device.
Straight through cables connect pins 1 and 2 (transmit) on an MDI device to pins 1 and 2 (receive) on an MDIX device. Similarly pins 3 and 6 are receive on an MDI device and transmit on an MDIX device. The general convention was for network hubs and switches to use the MDIX configuration, while all other nodes such as personal computer
s, workstation
s, servers and routers used an MDI interface. Some routers and other devices had an uplink/normal switch to go back and forth between MDI and MDIX on a specific port.
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-MDIX automatically detects the required cable connection type and configures the connection appropriately, removing the need for crossover cables to interconnect switches or connecting 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-MDIX to operate correctly, the data rate on the interface and duplex setting must be set to "auto". Auto-MDIX 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-MDIX 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 1021) of a loop where each end keeps switching.
Subsequently, Dove promoted auto-MDIX within the 1000BASE-T standard and also developed patented algorithms for "forced mode auto-MDIX" 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-MDIX to MDIX (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-MDIX to automatically switch to the proper configuration once a cable is connected. The other four wires are used but are not crossed since auto-MDIX is mandatory at the higher data rates (1000BASE-T transmits and receives on all pairs simultaneously without any dedicated send/transmit pairs).
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
from a physical layer implementation to the physical medium used to carry the transmission.
Ethernet over twisted pair
Ethernet over twisted pair
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
also defines a medium dependent interface crossover (MDIX) interface. Auto-MDIX ports on newer network interfaces detect if the connection would require a crossover, and automatically chooses the MDI or MDIX configuration to properly match the other end of the link.
Ethernet
The popular EthernetEthernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
family defines common medium dependent interfaces.
For 10BASE5
10BASE5
10BASE5 was the original commercially available variant of Ethernet.For its physical layer it used cable similar to RG-8/U coaxial cable but with extra braided shielding. This is a stiff, diameter cable with an impedance of 50 ohms , a solid center conductor, a foam insulating filler, a shielding...
connection to the coaxial cable was made with either a vampire tap
Vampire tap
A vampire tap is a device for physically connecting a station to a network that uses 10BASE5 cabling...
or a pair of N connector
N connector
The N connector is a threaded 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 Paul Neill of Bell Labs, after whom the connector is named.-Design:Originally, the connector was designed to...
s. For 10BASE2
10BASE2
10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors...
the connection to the coaxial cable was typically made with a single BNC connector to which a T-piece was attached. For twisted pair cabling 8P8C 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 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 a 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 while MDI-X is used on hubs
Ethernet hub
An Ethernet hub, active hub, network hub, repeater hub or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. A hub works at the physical layer of the OSI model. The device is a form of multiport repeater...
and switches. Some network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
hubs or switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
have an MDI port (often switchable) in order to connect to other hubs or switches without a crossover cable
Crossover cable
A crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically , by a modified cable called a crosslink...
.
MDI vs. MDIX
The terminology generally refers to variants of the Ethernet over twisted pairEthernet over twisted pair
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
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 MDIX device.
Straight through cables connect pins 1 and 2 (transmit) on an MDI device to pins 1 and 2 (receive) on an MDIX device. Similarly pins 3 and 6 are receive on an MDI device and transmit on an MDIX device. The general convention was for network hubs and switches to use the MDIX configuration, while all other nodes such as personal computer
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
s, workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
s, servers and routers used an MDI interface. Some routers and other devices had an uplink/normal switch to go back and forth between MDI and MDIX on a specific port.
Auto-MDIX
To connect two ports of the same configuration (MDI to MDI or MDIX to MDIX), an ethernet crossover cableEthernet crossover cable
An Ethernet crossover cable is a type of Ethernet cable used to connect computing devices together directly. Normal straight through or patch cables were used to connect from a host network interface controller to a network switch, hub or router.A cable with connections that "cross over" was used...
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
Star network
Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages...
topologies prompted a more automatic solution.
Auto-MDIX automatically detects the required cable connection type and configures the connection appropriately, removing the need for crossover cables to interconnect switches or connecting 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-MDIX to operate correctly, the data rate on the interface and duplex setting must be set to "auto". Auto-MDIX was developed by Hewlett-Packard
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
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-MDIX 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 1021) of a loop where each end keeps switching.
Subsequently, Dove promoted auto-MDIX within the 1000BASE-T standard and also developed patented algorithms for "forced mode auto-MDIX" 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-MDIX to MDIX (hub or switch), especially when autonegotiation
Autonegotiation
Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance...
is deactivated.
Newer routers, hubs and switches (including some 10/100, and all 1 Gigabit or 10 Gigabit devices in practice) use auto-MDIX to automatically switch to the proper configuration once a cable is connected. The other four wires are used but are not crossed since auto-MDIX is mandatory at the higher data rates (1000BASE-T transmits and receives on all pairs simultaneously without any dedicated send/transmit pairs).