although ethernet networks originally used thick or thin coaxial cable, most installations currently use unshielded twisted pair (utp) cabling. the utp cable contains eight conductors, arranged in four twisted pairs, and terminated with an rj45 type connector. a normal straight-through utp ethernet cable follows the eia568b standard wiring and pinout as described in table 8-2.

   

    uplink switches, crossover cables, and mdi/mdix switching

    in the wiring table above, the concept of transmit and receive are from the perspective of the pc, which is wired as media dependant interface (mdi). in this wiring, the pc transmits on pins 1 and 2. at the hub, the perspective is reversed, and the hub receives on pins 1 and 2. this wiring is referred to as media dependant interface - crossover (mdi-x).

    when connecting a pc to a pc, or a hub port to another hub port, the transmit pair must be exchanged with the receive pair. this exchange is done by one of two mechanisms. most hubs provide an uplink switch which will exchange the pairs on one port, allowing that port to be connected to another hub using a normal ethernet cable. the second method is to use a crossover cable, which is a special cable in which the transmit and receive pairs are exchanged at one of the two cable connectors. crossover cables are often unmarked as such, and must be identified by comparing the two connectors. since the cable connectors are clear plastic, it is easy to place them side by side and view the order of the wire colors on each. on a straight-through cable, the color order will be the same on both connectors. on a crossover cable, the orange and blue pairs will be exchanged from one connector to the other.

    the fvm318 firewall incorporates auto uplinktm technology (also called mdi/mdix). each local ethernet port will automatically sense whether the ethernet cable plugged into the port should have a normal connection (e.g. connecting to a pc) or an uplink connection (e.g. connecting to a router, switch, or hub). that port will then configure itself to the correct configuration. this feature also eliminates the need to worry about crossover cables, as auto uplinktm will accommodate either type of cable to make the right connection.

    cable quality

    a twisted pair ethernet network operating at 10 mbits/second (10base-t) will often tolerate low quality cables, but at 100 mbits/second (10base-tx) the cable must be rated as category 5, or cat 5 or cat v, by the electronic industry association (eia). this rating will be printed on the cable jacket. a category 5 cable will meet specified requirements regarding loss and crosstalk. in addition, there are restrictions on maximum cable length for both 10 and 100 mbits/second networks.