10BASE-T
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10BASE-T is an implementation of Ethernet which allows stations to be attached via twisted pair cable. The name 10BASE-T is derived from several aspects of the physical medium. The 10 refers to the transmission speed of 10 Megabits per second (Mb/s). The BASE is short for baseband. This means only one Ethernet signal is present on the send and/or receive pair. In other words there is no multiplexing as with broadband transmissions. The T comes from twisted pair, which is the type of cable that is used. Contrary to popular belief, the 802.3i 10baseT specification does not indicate a maximum length of cable distance between 2 nodes or between a node and a repeater, but instead specifies certain "characteristics" which a cable must meet (see below). This is quite different from later specifications (like 100baseTX) which specify a maximum cable length of 100 meters.
Unlike earlier Ethernet standards such as 10BASE5 and 10BASE2, 10BASE-T does not specify the exact type of wiring to be used. 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.
10BASE-T uses RJ-45 jacks wired to one of TIA/EIA-568-B's termination assignments, T568A or T568B. Only the second and third pairs are used (orange and green); though these are wired opposite in the two standards - T568A puts pair two (orange) on pins 3 and 6, pair three (green) on pins 1 and 2; T568B is the reverse.
A 10BASE-T node (such as a PC) transmits on pins 1 and 2 and receives on pins 3 and 6. 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 (the 10BASE-T standard therefore refers to the hub or switch as having an “internal crossover” or “embedded crossover” function). Hub and switch ports with such an internal crossover function are to be labelled with an “X” (for example, 3Com labels their ports 1X, 2X, and so on). To connect two PCs directly together (without a switch), a 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 automatically enable and disable their crossover (some vendors call this “Auto MDI”), so all switch ports can be connected to both PCs and also to other switches, using straight-through cables.
The termination standards are as follows:
| Pin | Pair | Wire | Color |
|---|---|---|---|
| 1 | 3 | 1 |  white/green |
| 2 | 3 | 2 |  green |
| 3 | 2 | 1 |  white/orange |
| 4 | 1 | 2 |  blue |
| 5 | 1 | 1 |  white/blue |
| 6 | 2 | 2 |  orange |
| 7 | 4 | 1 |  white/brown |
| 8 | 4 | 2 |  brown |
| Pin | Pair | Wire | Color |
|---|---|---|---|
| 1 | 2 | 1 | white/orange |
| 2 | 2 | 2 | orange |
| 3 | 3 | 1 | white/green |
| 4 | 1 | 2 | blue |
| 5 | 1 | 1 | white/blue |
| 6 | 3 | 2 | green |
| 7 | 4 | 1 | white/brown |
| 8 | 4 | 2 | brown |
10BASE-T was the first vendor-independent standard implementation of Ethernet on twisted pair wiring. However, it was in fact 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 the link-beat.
In the OSI model, 10BASE-T is at the physical layer. Ethernet encompasses both addressing at the data link layer and a number of physical-layer implementations. In this model, 10BASE-T is one of the possible physical layer standards for Ethernet-- some others include 10BASE2, 10BASE5, Fast Ethernet, and Gigabit Ethernet. Network layer protocols, such as IP, do not generally need to know whether they are being hosted on 10BASE-T or not, provided they know that they are being hosted on Ethernet.
