Crossed and Straight cables - when to use them

Crossed and Straight cables - when to use them

The following diagram shows the Normal use of Crossed and Straight cables (see also the notes below).

Notes:

We show Straight cables as BLUE and Crossed as RED. That is our convention the cable color can be anything you choose or more likely the vendor decides.

To avoid the need for Crossed cables many vendors provide UPLINK ports on Hubs or Switches - these are specially designed to allow the use of a STRAIGHT cable when connecting back-to-back Hubs or Switches. Read the manufacturers documentation carefully.

Category 5(e) (UTP) colour coding table

The following table shows the normal colour coding for category 5 cables (4 pair) based on the two standards supported by TIA/EIA (see also our primer on this topic)

100base-T Straight Cable (PC to HUB/SWITCH)

Straight cables are used to connect PCs or other equipment to a HUB or Switch. If your connection is PC to PC or HUB to HUB you MUST use a Crossed cable.

The following cable description is for the wiring of BOTH ends (RJ45 Male connectors) with your category 5 wiring colors (TIA/EIA 568A or 568B though the example uses 568B colors).

We use BLUE for 100baseT straight cables.

NOTES:

1. Wires marked ** are ABSOLUTELY NECESSARY for 100Base-T4 networks (using any combination of category 3/4/5 cables and MAY be for Power-over-Ethernet see below)) but are not required for 100Base-TX (using cat 5/5e ONLY cables) and CAN be used for other purposes e.g. telephony but, .. beware .. read this FAQ and our LAN plus Telephony article before you wire your entire neighbourhood for surround sound.
2. The Power-over-Ethernet spec (802.3af) allows three schemes where power may be supplied. Two of these schemes use pairs 4,5 and 7,8 (marked ** in above table) for power (called Endpoint PSE, Alternative B and Midspan PSE, Alternative B or Mode B), one scheme uses ONLY pairs 1,2 and 3,6 (Endpoint PSE, Alternative A or Mode A) for both signals and power. Depending on which scheme you use pairs 4 and 7,8 may be required.
3. Gigabit Ethernet requires all 4 pairs (8 conductors).
4. All our wiring is now done to the 100baseT spec which you can use with 10baseT networks - but NOT the other way around.

100base-T Crossed cable (PC to PC or HUB to HUB)

Crossed cables are used to connect PCs to one other PC or to connect a HUB to a HUB. Crossed cable are sometimes called Crossover, Patch or Jumper cables. If your connection is PC to HUB you MUST use a Straight cable.

The following description shows the wiring at both ends (male RJ45 connectors) of the crossed cable. Note: The diagrams below shows crossing of all 4 pairs and allows for the use of cat3/4 cables. Pairs 4 and 7,8 do not NEED to be crossed in 100base-TX wiring. See notes below.

We use RED for crossed cables (or more commonly now a red heat-shrink collar at each end).

NOTES:

1. All our crossed wiring is now done to the 100base-T4 spec which you can use with 10base-T networks - but NOT necessarily the other way around.
2. Most commercial cables these days seem not to cross pairs 4 and 7,8. If there is no cat3/4 wiring in the network this perfectly acceptable.
3. Gigabit Ethernet uses all 4 pairs so requires the full 4 pair (8 conductor) cross configuration (shown above).
4. If you are using Power-over-Ethernet (802.3af) then Mode A or Alternative A uses pairs 1,2 and 3,6 for both signals and power. Mode B or alternative B uses 4 and 7,8 to carry power. In all cases the spec calls for polarity insensitive implementation (using a diode bridge) and therefore crossing or not crossing pairs 4 and 7,8 will have no effect.