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New Member

Difference Betn Cat5 4pair and Cat5 2pair cable?


As we all know Cat5 cable has 4 pair of wire but only 2 pairs are used for Tx and Rx, Rest 2 are not used.I want to know that why 4 pairs are kept in wire if not in use. And can we use 2pair Cat5 cable in place of Cat5 4pair cable.Will there be any effect on performance of LAN and if so,then how it will effect.



Network Engineer


Re: Difference Betn Cat5 4pair and Cat5 2pair cable?

4 pairs are in Cat5 cable so that any or all pairs can be used for something.

* 10BASE-T Ethernet uses only 2 pairs (1/2 and 3/6).

* 100BASE-TX Fast Ethernet uses only 2 pairs (same ones as above).

* 10/100 ports use only 2 pairs (same ones as above).

* IEEE 802.3af Power over Ethernet can use the unused 2 pairs (4/5 and 7/8) to deliver power over the data cable to 802.3af-compatible devices like Ethernet or IP Phones, Wireless Access Points, etc.

* 1000BASE-T Gigabit Ethernet uses all 4 pairs (1/2, 3/6, 4/5, 7/8).

Strictly speaking, 2-pair Cat5 cable is NOT Cat5 cable. True Cat5 cable always has 4 pairs. You can have cable with 2 pairs and similar transmission characteristics to Cat5, that works with 10, 100, or 10/100 devices only. But you can't call it a Cat5 cable run because it won't deliver PoE or 1000BASE-T. If you put a cable scanner on there and run a Cat5 certification test, it will fail the test because it's missing 2 pairs.

If it is 2-pair-only cable, and has an equal or greater number of twists per foot (English) or meter (metric) than true Cat5 cable, then it probably won't perform any different than true Cat5 cable for 10, 100, and 10/100 connections.

If it's 4-pair Cat5 cable, and you run 2-pair for a normal 10, 100, or 10/100 connection and also punch down the unused pairs in the same cable for another 10, 100, or 10/100 data outlet, that may work but it is bad cabling practice. Also, you can't claim that the cable run is Cat5, because the standards require all 4 pairs to be terminated on the same data port. Effects on performance: you may run into crosstalk problems, where data is corrupted as it travels on the wire. The link's performance can be degraded because the devices on each end may have to send frames/packets multiple times to get them through once.

Hope this helps.

New Member

Re: Difference Betn Cat5 4pair and Cat5 2pair cable?

Thanks Dear.

It helps me a lot.




Re: Difference Betn Cat5 4pair and Cat5 2pair cable?

If the cabling obtains a Category rating, it meets a basic specification. Many higher-quality cables are well beyond the specification ... giveing additional headroom for sub-optimal conditions (noise, tight bends, etc).

Structured cabling is used for much more than data networking. There are adapters and BALUNs for almost any application, from baseband video to baseband RF, serial (RS232, RS485), terminal connections (3270, 5250, async), phones ... nearly anything that cable can carry.

Each of the other (non-Ethernet data) applications use pairs other than pair 2 & 3 (pins 1&2, 3&6).

For example, a T1 uses pairs 1 and 3 (pins 1&2, 4&5), 56K DDS uses pairs 3 and 4 (pins 1&2, 7&8), ATM over copper uses pairs 3 & 4 (pins 1&2, 7&8).

The list goes on, and is fairly long. Structured Cabling Systems are for much more than just data, but even within the data utilizations, there are many pair combinations in use.