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.
MDI vs. MDIX
The terminology generally refers to variants of the Ethernet over twisted pair 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 computers, workstations, 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.
To connect two ports of the same configuration (MDI to MDI or MDIX to MDIX), an ethernet crossover cable was 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.[2] 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.[3][4]
When 2 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.[5]
Subsequently, Dove promoted auto-MDIX within the 1000BASE-T standard[5] and also develop patented algorithms for "forced mode auto-MDIX" which allows a link to be automatically established even if the port does not auto-negotiate.[6] 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.
Auto-MDI/MDIX Feature
For RJ-45 interfaces on the ASA 5500 series adaptive security appliance, the default auto-negotiation setting also includes the Auto-MDI/MDIX feature. Auto-MDI/MDIX eliminates the need for crossover cabling by performing an internal crossover when a straight cable is detected during the auto-negotiation phase. Either the speed or duplex must be set to auto-negotiate to enable Auto-MDI/MDIX for the interface. If you explicitly set both the speed and duplex to a fixed value, thus disabling auto-negotiation for both settings, then Auto-MDI/MDIX is also disabled. For Gigabit Ethernet, when the speed and duplex are set to 1000 and full, then the interface always auto-negotiates; therefore Auto-MDI/MDIX is always enabled and you cannot disable it.
Ref: http://en.wikipedia.org/wiki/Medium_dependent_interface
No comments:
Post a Comment