Ethernet crossover cable
As a crossover cable or crossover cable is referred to in the computer network technology ( LAN technology ) is a eight-wire twisted pair cable in which one of the two RJ45 connectors certain wires are reversed ( engl. to cross: cross ). While a straight-through ( straight through ) cable connects computers to network switches, you can connect using a crossover cable two computers (or two switches) directly. With the proliferation of Auto-MDI -X crossover cables are not required, since network devices to perform the intersection of the wires automatically when needed electronically. So with Gigabit Ethernet Crossover cables are still provided, but unnecessary because virtually all devices support auto MDI -X.
Construction
The Ethernet media type 10BASE -T and 100BASE -TX using one pair being used for the sending direction, one for the receive direction. Thus, a connection can occur, which wire pairs need to be crossed, so that the transmit pair of one device is connected to the receive pair of the other device. With network components such as repeaters, hubs, bridges or switches the intersection is provided internally to the RJ45 connectors and is referred to as MDI -X. Computer and switch are therefore connected by uncrossed twisted pair cable, where on both sides the same assignment is used (same positions on both plugs are connected to each other ). These are for 10BASE -T and 100BASE -TX on both sides either EIA/TIA-568A or B.
If now two computers are connected directly to a cable, the signal wire pairs must be crossed in the cable. This is achieved by one of the two connector is wired according to different. Without the intersection of pairs is no connection. Instead of a crossover cable, you can also use a straight-through cable with a crossover adapter to make the crossing of wire pairs.
Before Auto-MDI -X was widespread, most hubs and switches possessed an uplink port that had not crossed internally and could be connected by uncrossed cable with a standard MDI - X port of another switch. When using a crossover cable to the uplink port was not necessary and you could two MDI - X ports together.
For comparison, the two male ends of a straight-through patch cable.
A crossover adapter.
Variants
The crossover cable is available in 3 versions:
10BASE -T and 100BASE -TX Ethernet
Here only two pairs of wires are used / crossed (1-2 and 3-6). As these standards are not all pairs are used for transmission, also need not be connected or crossed all pairs.
That is, in the crossed cable of this type, the twisted pairs are 1-2 and 3-6 3-6 and 1-2 crossed with, the assignment of the pairs 4-5 and 7-8 is not important.
100BASE- T4 Ethernet
100BASE- T4 Ethernet is widely used in some U.S. because it can also be used with older CAT-3 cables. But then four pairs of wires are used for transmission, therefore, all four pairs of wires must be crossed.
In principle, there are two fours in this cable bundle, once known by the 10BASE -T and 100BASE -TX four bundles ( 1-2 and 3-6) and in addition the pairs 4-5 and 7-8, which just joined or crossed be.
That is the twisted wire pairs 1-2, 3-6, 4-5 and 7-8 are connected with 3-6, 1-2, 7-8 and 4-5, precisely in this arrangement, and with this twist.
1000BASE -T Ethernet
Here CAT-5 cables are needed at least analogous to 100BASE -T4 are also here all four pairs in operation, therefore, would have a crossover cable, correspond to the 100BASE -T4 type. For 1000BASE- T no more crossover cables all interfaces but are necessary today ( LAN card, switches, etc. ) automatically recognize the situation and turn the pin assignment according to ( Auto MDI -X). So also switch Switch or PC - PC connections with any cables can be manufactured.
The pin assignments are to be found in the article RJ45.
Manufacturing, electrical details
Cross cable can be produced according to the above assignment is relatively easy even.
If you compare the cable according to the 100BASE- TX standard here, so there are (required Cat 5 or better ) a common source of error in addition to the wrong cable quality. Is the twist with ISDN or 10BASE-T or due to the relatively low signal frequencies with almost no influence, so it comes at 100 Mbit / s, a significant importance.
Fast Ethernet uses symmetric differential signals to minimize the electromagnetic radiation and the reduction of common mode noise. Therefore, it is important that the wire pairs are twisted together. This works as follows: the voltage approaches a wire of the pair of wires to a positive voltage, so changes to the other wire simultaneously to an equal negative voltage - the resulting electromagnetic fields cancel each other. In any case, therefore 1 and 2 form a twisted pair cable with 100BASE- TX pin, the same is true for the pair on pins 3 and 6 (for 100BASE- T4 cables also pin 4-5 and 7-8 twisted pairs form ). Further, all wires of a pair should be exactly the same length, the twist may only on a short piece of cable ( max. approx 1.5 cm ) are missing or removed.
This type of error can only track expensive RF cable tester or some appropriate Gigabit Ethernet network interface cards, the simple LED tester, however, working with direct current and therefore do not know which wire pairs are twisted. Logically, this also applies to 10BASE-T cabling, which issued false pairs interfere here far less.