Ethernet over twisted pair
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There are several standards for Ethernet over twisted pair cable. By far the most widely used are 10BASE-T, 100BASE-TX, and 1000BASE-T, running at 10 Mbit/s, 100 Mbit/s, and 1000 Mbit/s respectively. These three standards all use the same connectors and implementations of one nearly always support the lower speeds as well (the main exception being some 100 megabit hubs) so that in most cases different generations of equipment can be freely mixed. They use 8 position modular connectors (usually but incorrectly referred to as RJ-45) and are usually wired with four-pair Category 5 or above twisted pair cable, though some very old 10BASE-T networks may still be operating on lower grades of cable. All support full duplex communication. Most operate over distances of up to 100 metres.
The common names of the standards are derived from several aspects of the physical media. The number refers to the transmission speed in megabits per second (Mbit/s). The BASE is short for baseband, meaning that there is no frequency division multiplexing or frequency shifting modulation in use. The T designates twisted pair, the type of cable that is used. Where there are several standards for the same transmission speed, they may be distinguished by a letter or digit following the T. Some higher-speed standards use twin-axial cable, designated by CX.
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[edit] Standards
10BASE-T was the first vendor-independent standard implementation of Ethernet on twisted pair wiring. It was an evolutionary development from AT&T StarLAN which had both 1 Mbit/s and 10 Mbit/s versions. 10BASE-T is essentially StarLAN-10 with the addition of a link status pulse. The link status pulse prevents the use of half-connected links on which one side would be unable to avoid or detect collisions.
There are several standards for Fast Ethernet (100 Mbit/s) over twisted pair, collectively called 100BASE-T. The most commonly used is 100BASE-TX, which uses two pairs of a Category 5 cable, one in each direction. There was also 100BASE-T4 (half-duplex, 100 megabit over four pairs of Category 3 or above) and 100BASE-T2 (full-duplex, 100 megabit over two pairs of Category 3 or above but too late to make much of an impact).
The most commonly used standard of Gigabit Ethernet (1000 Mbit/s) over twisted pair is 1000BASE-T, which uses four pairs of Category 5e. There is also 1000BASE-TX, which was cheaper to implement but required Category 6 cable and was therefore a commercial failure and 1000BASE-CX which was available earlier but relied on specialist balanced shielded twisted pair cable and was therefore only used in high end products.
The first standard for 10 gigabit Ethernet over twisted pair cable was 10GBASE-CX4 which uses four pairs of twin-axial cable (coax with the center core replaced by a twisted pair) with a connector based on that used for Infiniband. This was followed by 10GBASE-T which uses Category 6a or Category 7 cable. Neither is yet in common use.
[edit] Cabling
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All versions of twisted-pair Ethernet are designed so that the majority of cables can be wired straight through but a small number of cables may need to be wired in a special crossover form. 10BASE-T and 100BASE-TX require two pairs, one of which must be on pins one and two and the other on pins three and six. 1000BASE-T requires two additional pairs one on pins 4 and 5 and one on pins 7 and 8. Since 10BASE-T and 100BASE-TX need only two pairs and standard Category 5 cable has four pairs it is possible to run two network connections or a network connection and two phone lines down one Category 5 cable.
It is conventional to wire cables for Ethernet to either the T568A or T568B standards. Since these standards only differ in that they swap the positions of the two pairs used by 10BASE-T and 100BASE-TX, a cable with TIA568A wiring at one end and TIA568B wiring at the other will be a crossover cable for those standards.
A 10BASE-T node (such as a PC) transmits on pins 1 and 2 and receives on pins 3 and 6. This is known as the "MDI" wiring pattern. So that a “straight-through” cable can be used to connect a PC to its hub or switch, a 10BASE-T hub or switch receives on pins 1 and 2 and transmits on pins 3 and 6. This is referred to as "MDI-X", “internal crossover” or “embedded crossover”. Hub and switch ports with such an internal crossover function are supposed to be labelled with an “X” (for example, 3Com labels their ports 1X, 2X, and so on) but not all manufacturers follow this. To connect two PCs directly together (without a switch), an Ethernet crossover cable must be used.
To connect two switches directly together, a crossover cable can be used. Or some switches have an “uplink” port (which is wired the same as a PC — it transmits on pins 1 and 2, and receives on pins 3 and 6), so a straight-through cable can be used. Some switches and network cards automatically enable and disable their crossover (known as “Auto MDI-X”), so all can be connected to both PCs and also to other switches, using straight-through cables.
100BASE-TX follows the same wiring patterns as 10BASE-T. 1000BASE-T has different crossover requirements coming from its use of all four pairs bidirectionally but the vast majority of 1000BASE-T equipment supports auto MDI-X anyway so it is rarely necessary to use such cables in practice.
Unlike earlier Ethernet standards such as 10BASE5 and 10BASE2, 10BASE-T does not specify the exact type of wiring to be used or a maximum length but instead specifies certain "characteristics" which a cable must meet (see below). This was done in anticipation of using 10BASE-T in existing twisted pair wiring systems that may not conform to any specified wiring standard. Instead, 10BASE-T wiring is specified using a set of characteristics that a 10BASE-T link segment must conform to. These include attenuation, characteristic impedance, timing jitter, propagation delay, and several types of noise. Cable testers are widely available to check these parameters to determine if a cable can be used with 10BASE-T. These characteristics are expected to be met by 100 meters of 24 gauge unshielded twisted-pair cable. However, with high quality cabling that is available, cable runs of 150 meters or longer are often obtained and are considered viable by most technicians familiar with the 10baseT specification.
In contrast 100BASE-TX and 1000BASE-T both require a minimum of Category 5 cable and specify a maximum cable length of 100 meters. Furthermore while 10BASE-T is extremely tolerant of poor wiring such as split pairs, poor terminations and even use of short sections of flat cable, 100BASE-T is less tolerant and 1000BASE-T less tolerant still. Since testing of cable is often limited to checking if it works with Ethernet, running faster speeds over existing cable is often problematic. This problem is made worse by the fact that Ethernet's autonegotiation takes account only of the capabilities of the end equipment not of the cable in between.
[edit] Autonegotiation and duplex mismatch
The autonegotiation standard does not allow autodetection to detect duplex setting if the other computer is not set to autonegotation. When two interfaces are connected and set to different "duplex" mode, the effect of the duplex mismatch is a network that works but at much slower than its nominal speed. The primary rule for avoiding this is that you must not set one end of a connection to a forced full duplex setting and the other end to autonegotiation.
Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, ...) exist for Ethernet over twisted pair cable using 8P8C modular connectors (not to be confused with FCC's RJ45), and most devices are capable of different modes of operations. In 1995, a standard was released for allowing two network interfaces connected to each other to autonegotiate the best possible shared mode of operation. This works well for the case of every device being set to autonegotiate. The autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation.
Interoperability problems leads network administrators to manually set the mode of operations of interfaces on network devices. What would happen is that some device would fail to autonegotiate and it had to be set into one setting or another. This often leads to duplex setting mismatches: in particular, when two interfaces are connected to each other with one set to autonegotiation and one set to full duplex mode, a duplex mismatch results because the autonegotiation process fails and half duplex is assumed — the interface in full fullduplex mode then transmits at the same time as receiving, and the interface in half duplex mode then gives up on transmitting a packet. The interface in half duplex mode is not ready to receive a packet, so it signals a clash, and transmissions are halted, for amounts of time based on backoff (random wait times) algorithms.
[edit] See also
- IEEE 802.3
- Ethernet physical layer
- 25-pair color code
- Computer network
- Ethernet
- Ethernet extender
- RJ-45
- Wireless local loop (WLL)
