A network cable has 8 conductors, twisted into four pairs. This twisted-pair design not only resists some external interference but also reduces internal crosstalk caused by its own signals. The twist pitch (the number of twists per unit length) varies between different pairs, and these pairs complement each other to optimize the cable’s transmission performance.
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The 8 conductors of a network cable are typically color-coded as follows: Orange-White, Orange, Green-White, Green, Blue, Blue-White, Brown-White, Brown.
Their specific functions are listed below:
- Transmit Data (+)
- Transmit Data (-)
- Receive Data (+)
- Reserved for telephone use
- Reserved for telephone use
- Receive Data (-)
- Reserved for telephone use
- Reserved for telephone use
The two most commonly used standard wiring orders for network cables are T568A and T568B. In structured cabling, the T568B standard is usually adopted for installation.
- T568A Standard: From left to right, the wiring order is: 1-Green-White, 2-Green, 3-Orange-White, 4-Blue, 5-Blue-White, 6-Orange, 7-Brown-White, 8-Brown.
- T568B Standard: From left to right, the wiring order is: 1-Orange-White, 2-Orange, 3-Green-White, 4-Blue, 5-Blue-White, 6-Green, 7-Brown-White, 8-Brown.
- Straight-Through Cable (568B/A on one end, 568B/A on the other): Used for connections between a switch and a computer, or a switch and a router.
- Crossover Cable (568A on one end, 568B on the other): Used for direct connections between two computers.
In 100Mbps network transmission, only 4 conductors (1, 2, 3, 6) are generally used. According to standards, these are the four conductors from the orange and green pairs. Specifically:
- Conductors 1 and 3 form one group (for data transmission/reception).
- Conductors 2 and 6 form another group (for data transmission/reception).
- Conductors 4 and 5 serve as backup lines.
- Conductors 7 and 8 can be used for telephone connections.
However, when using conductors 7 and 8 for telephones, the cable length should not be too long, and long-term use is not recommended. This is because if the telephone line pair is in a ringing state, the ringing current may cause crosstalk to the data line pairs.
In Gigabit network transmission, all 8 conductors must be used. Specifically:
- Conductors 1, 2, 3, and 6 are used for transmitting data.
- Conductors 4, 5, 7, and 8 are used for receiving data.
Gigabit networks adopt a "2 transmit, 2 receive" mode: two wire pairs are dedicated to data transmission, and the other two pairs are dedicated to data reception.
Gigabit networks usually follow the T568B wiring standard (the same standard as mentioned earlier): 1-Orange-White, 2-Orange, 3-Green-White, 4-Blue, 5-Blue-White, 6-Green, 7-Brown-White, 8-Brown.
Therefore, Gigabit networks require an 8-conductor cable. A 4-conductor cable can only support either data transmission or reception (but not both simultaneously) in a Gigabit network.
When a network cable is connected to a POE device, it needs to transmit both power and data. There are two standard POE power supply modes: Alternative A and Alternative B.
- Alternative A: Uses conductors 1, 2, 3, and 6 to supply power.
- Alternative B: Uses conductors 4, 5, 7, and 8 to supply power.
Since 100Mbps communication uses only 4 conductors (1, 2, 3, 6) and Gigabit communication uses all 8 conductors, the requirements for cables differ between 100Mbps and Gigabit POE switches:
- For 100Mbps POE switches:
- Under the Alternative A standard: Only conductors 1, 2, 3, and 6 need to be connected.
- Under the Alternative B standard: An 8-conductor cable is required, and all 8 conductors must be connected.
- For Gigabit POE switches:
- Due to the need for full 8-conductor data transmission, both Alternative A and Alternative B require an 8-conductor cable with all 8 conductors connected.
The above covers the functions and application scenarios of the 8 conductors in a network cable.
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