Ethernet over twisted pair
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There are several standards for Ethernet over twisted pair or copper-based computer networking physical connectivity methods. The currently most widely used of these are 10BASE-T, 100BASE-TX, and 1000BASE-T, running at 10 Mbit/s, 100 Mbit/s, and 1000 Mbit/s (1 Gbit/s) respectively. These three standards all use the same connectors. Higher speed implementations nearly always support the lower speeds as well, so that in most cases different generations of equipment can be freely mixed. They use 8 position modular connectors, usually (but incorrectly) called RJ45 in the context of Ethernet over twisted pair. The cables usually used are four-pair Category 5 or above twisted pair cable. Each of the three standards support both full duplex and half-duplex communication. According to the standards, they all operate over distances of 'up to 100 meters'.
The common names of the standards are derived from several aspects of the physical media. The number refers to the theoretical maximum transmission speed in Megabits per second (Mbit/s). The BASE is short for baseband, meaning that there is no frequency division multiplexing (FDM) or other frequency shifting modulation in use; each signal has full control of wire, on a single frequency. The T designates twisted pair cable, where the pairs of wires are twisted together for purposes of reducing crosstalk (FEXT and NEXT) when the pulsing direct current goes across the wires and creates electromagnetic induction effects. Where there are several standards for the same transmission speed, they are distinguished by a letter or digit following the T, such as TX. 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 of these is 100BASE-TX, which uses two pairs of a Category 5 cable, one in each direction (thus allowing for two links over a standard four pair category 5 cable if appropriate wiring adaptors are used at the ends). 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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Twisted-pair Ethernet standards are such that the majority of cables can be wired 'straight-through' (pin 1 to pin 1, pin 2 to pin 2 and so on), but others may need to be wired in the 'crossover' form (receive to transmit and transmit to receive).
10BASE-T and 100BASE-TX only require two pairs to operate, pins 1 and 2 (transmit or TX), and pins 3 and 6 (receive or RX). Since 10BASE-T and 100BASE-TX need only two pairs and Category 5 cable has four pairs, it is possible, but not standard, to run two network connections (or a network connection and two phone lines) over a cat 5 cable by using the normally unused pairs in these 10 and 100 Mbit/s configurations. This is not possible with 1000BASE-T since it requires all four pairs to operate, pins 1 and 2, 3 and 6 — as well as 4 and 5, 7 and 8.
It is conventional to wire cables for 10 or 100 Mbit/s Ethernet to either the T568A or T568B standards. Since these standards only differ in that they swap the positions of the two pairs used for transmitting and receiving (TX/RX), a cable with TIA-568A wiring at one end and TIA-568B wiring at the other will be a crossover cable. The terms used in the explanations of the 568 standards, tip and ring, refer to older communication technologies, and equate to the positive and negative parts of the connections.
A 10BASE-T node (such as a PC) that transmits on pins 1/2 and receives on pins 3/6 to a network device is most often on a "straight-through" cable in the "MDI" wiring pattern where RX goes to RX and TX goes to TX. A straight-through cable is usually used to connect a node to its network device. In order for two network devices or two nodes to communicate with each other (such as a switch to another switch or computer to computer) a crossover cable is often required at speeds of 10 or 100. If available, connections can be made with a straight-through cable by means of an "MDI-X" port, also known as an “internal crossover” or “embedded crossover” connection. Hub and switch ports with such internal crossovers are usually labelled as such, with "uplink" or “X”. For example, 3Com usually labels their ports 1X, 2X, and so on.
To connect two PCs directly together without a switch, an Ethernet crossover cable is often used. Although many modern Ethernet host adapters can automatically detect another PC connected with a straight-through cable and then automatically introduce the required crossover, if needed; if one or neither of the PC does not, then a crossover cable is required. If both devices being connected support 1000BASE-T according to the standards, they will connect regardless of the cable being used or how it is wired.
To connect two hubs or switches directly together, a crossover cable can be used, but some hubs and switches have an “uplink” port used to connect network devices together, or have a way to manually select MDI or MDI-X on a single port so that a straight-through cable can connect that port to another switch or hub. Most newer switches have automatic crossover ("auto MDI-X" or "auto-uplink") on all ports, eliminating the uplink port and the MDI/MDI-X switch, and allowing all connections to be made with straight-through cables.
100BASE-TX follows the same wiring patterns as 10BASE-T but is more sensitive to wire quality and length, due to the higher bit rates.
1000BASE-T uses all four pairs bi-directionally and the standard includes auto MDI-X, however implementation is optional. With the way that 1000BASE-T implements signaling, how the cable is wired is immaterial in actual usage. The standard on copper twisted pair is IEEE 802.3ab for Cat 5e UTP, or 4D-PAM5; 4 Directions using PAM (pulse amplitude modulation) with 5 voltages, -2, -1, 0, +1, and +2 [1]
Unlike earlier Ethernet standards using broadband and coaxial cable, such as 10BASE5 (thicknet) and 10BASE2 (thinnet), 10BASE-T does not specify the exact type of wiring to be used but instead specifies certain "characteristics" which a cable must meet. This was done in anticipation of using 10BASE-T in existing twisted pair wiring systems that may not conform to any specified wiring standard. Some of the specified characteristics are 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, and 100 meters is the stated maximum length for baseband signal runs. However, with high quality cabling, cable runs of 150 meters or longer are often obtained and are considered viable by most technicians familiar with the 10baseT specification, though -- as with all CSMA/CD network environments — the absolute limit on run length is determined by the size of the collision domain and cable quality. In reality, what meets the standards may not work, and those that don't meet the standards might work.
100BASE-TX and 1000BASE-T both require a minimum of Category 5 cable (5e or 6 with 1000) and also specify a maximum cable length of 100 meters. Furthermore while 10BASE-T is more tolerant of poor wiring such as split pairs, poor terminations and even use of short sections of flat cable, 100BASE-T is not as much so, and 1000BASE-T is 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
Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, ...) exist for Ethernet over twisted pair, and most network adapters are capable of different modes of operations. In 1995, a standard was released for allowing two network adapters connected to each other to negotiate the best possible shared mode of operation. The autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation.
When two linked interfaces are set to different duplex modes, the effect of this duplex mismatch is a network that functions much slower than its nominal speed. The primary rule for avoiding this is to avoid setting one end of a connection to full duplex and the other end to autonegotiation.
Duplex mismatch may be inadvertently caused when an administrator configures an interface to a fixed mode (e.g 100 Mbit/s full duplex) and fails to configure the remote interface, leaving it set to autonegotiate. Then, when the autonegotiation process fails, half duplex is assumed by the autonegotiating side of the link.
The resulting duplex mismatch results in a dramatically slow network, in which many collisions, and especially late collisions occur on the interface set to half-duplex, and FCS errors are seen on the full-duplex side. [2]
Gigabit Ethernet standards require autonegotiation to be on in order to operate.
[edit] See also
- Twisted pair
- IEEE 802.3
- Ethernet physical layer
- 25-pair color code
- Computer network
- Ethernet
- Ethernet extender
- Power over Ethernet (PoE)
- RJ-45
- Wireless local loop (WLL)
