Ethernet over twisted pair
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Ethernet over twisted pair refers to the use of a pair of copper cables, twisted around each other, for the physical layer of an Ethernet network (that is, a network in which the Ethernet protocol is used in the data link layer). There are several different standards for a copper-based physical medium. The most widely used are 10BASE-T, 100BASE-TX, and 1000BASE-T (Gigabit Ethernet), 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 called RJ45 in the context of Ethernet over twisted pair. The cables usually used are four-pair 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 coaxial cable, designated by CX.
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[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 differ only 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 is referred to as 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 and 2 and receives on pins 3 and 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 Mbit/s. 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 computers directly together without a switch, an Ethernet crossover cable is often used. Although many modern Ethernet host adapters can automatically detect another computer connected with a straight-through cable and then automatically introduce the required crossover, if needed; if one or neither of the computers 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.
A 10BASE-T transmitter sends 2 differential voltages, +2.5 V or -2.5 V.
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.
A 100BASE-TX transmitter sends 3 differential voltages, +1 V, 0 V, or -1 V[1].
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 dimensions using PAM (pulse amplitude modulation) with 5 voltages, −2, −1, 0, +1, and +2 [2] While +2 V to -2 V voltage may appear at the pins of the line driver, the voltage on the cable is nominally +1 V, +0.5 V, 0 V, -0.5 V and -1 V[3].
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 10BASE-T 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 do not meet the standards might work.
100BASE-TX and 1000BASE-T both require a minimum of Category 5 cable (5e or 6 with 1000 Mbit/s) 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 more slowly than its nominal speed.
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. [4]
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
- Fast Ethernet
- Ethernet extender
- Power over Ethernet (PoE)
- RJ-45
- Wireless local loop (WLL)
[edit] References
- ^ http://books.google.com/books?id=392CdZHdUDEC&pg=PA240&lpg=PA240&dq=%22100BASE-TX%22+2V+voltage&source=web&ots=Jtyqtz0KE6&sig=AnR2pHk04YgKKlYAvTiEheQAlHU&hl=en&sa=X&oi=book_result&resnum=8&ct=result
- ^ http://grouper.ieee.org/groups/802/3/minutes/july98/E2_0798.pdf
- ^ "Voltage-mode line drivers save on power " 2004
- ^ Ethernet Autonegotiation Best Practices
