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The type of cable table
Back to the fundamentals. Here’s a quick reference of what type of cable to use in what situation. Most modern networking equipment nowadays have auto MDI/MDIX which automatically detects and switches to cross-over or straight through. However, if one side of the device has the duplex and speed settings hard coded, it could render the auto MDI/MDIX feature useless.
So here’s a table with all the combinations and the type of cable to use:
| Hub | Switch | Router | Workstation / PC | |
| Hub | Crossover | Crossover | Straight | Straight |
| Switch | Crossover | Crossover | Straight | Straight |
| Router | Straight | Straight | Crossover | Crossover |
| Workstation / PC | Straight | Straight | Crossover | Crossover |
In general when you connect two similar devices ie, switch to switch, router to router, workstation to workstation, you will use a cross-over cable, and for all others – use a straight through. That’s a good way to remember what to use.
Layer 1 Basics
In order to understand more complex network technologies it is important to have a strong understanding of the basics.
So here we are Layer 1 – Ethernet, the most commonly used access medium for LAN network. An Ethernet cable consists of a Cat5/Cat5e/Cat6 cable and a RJ45 connector. The cable comprises of 4 twisted copper pairs totaling 8 pins.
The 10Base-T and the 100Base-TX standard both use 2 pairs, one pair to transmit and one pair to receive.
Note that in a straight through setting, the transmit pair occupies pins 1 and 2. The receive pair occupies 3 and 6. Pairs 4,5 and 7,8 are left unused. In order to connect two PCs together, a crossover cable should be used by basically connecting the transmit pair from one side to the receive pair on the other side and vice versa.
The mappings in a crossover cable are Pins 1-3, 2-6, 3-1, and 6-2.
Modern Cisco switches and router interfaces support Auto-MDIX (Auto medium-dependent interface crossover) which automatically detects and swaps the receive and transmit pairs when a wrong cable is used.
RJ-45 Copper Ethernet cables
Copper cables are usually much cheaper compared to fiber and are often used for shorter distance patch cabling. Cat 1 – 4 are being omitted as they are not commonly deployed nowadays as they do not support 100 Mb Base-T.
| Category | 10/100 Mb Fast Ethernet 100BASE-TX | 1 Gb (1000 Mb) Gigabit Ethernet 1000BASE-T | 10 Gb Ethernet 10GBASE-T |
|---|---|---|---|
| Cat5 | 100 meters | Not Supported | Not Supported |
| Cat5E | 100 meters | 100 meters | Not Supported |
| Cat6 | 100 meters | 100 meters | 55 meters |
| Cat6A | 100 meters | 100 meters | 100 meters |
| Cat7 (Shielded) | 100 meters | 100 meters | 100 meters |
