Layer 1 - Physical: This is the wires and connectors of the network, as well as the electrical signal on the wire. Examples that you can see on your computer right now: The network cable that runs to the wall, switch, or cable modem. Also, there is probably a green or amber light next to that cable which is evidence of physical, Layer 1 connectivity. If your network connection is ethernet, the cable plugs into your computer using an RJ45 jack and the cable is called Category 5 (Cat 5; you may have Cat 6 if you are really up to speed). If you are on dial-up, then the phone cable connected to your computer is using an RJ11 jack and Cat 3 (or, twisted-pair) cabling for Layer 1. If your computer is using wireless 802.11 to connect to the internet, then the antennaes and radio signal are the physical, Layer 1 elements.
The physical layer is usually the most common culprit of network connection problems. Cables have a tendency to get unplugged, cut, pinched, damaged, or just plain go bad. Also, it is possible you have the wrong type of cable, there is electrical interference, or power issues. A wise troubleshooter will always verify physical connectivity twice before proceeding with any troubleshooting; I cannot tell you how frustrating it is to troubleshoot for an hour using every advanced resource available to you and then find out that the cable is unplugged.
Layer 1 is the third largest consideration in designing a network, after considering cost-justification and security. You need to make sure you have the right cables and adapters for the network, and that they will be secure from damage, interference, and maliciousness. You also need to plan for the limitations of your signals; cables can only be so long and can only handle so many users. Radio signals are easier to prevent damage to, but are increasingly running into problems with interference and will never be the most secure solution. Radio signals are also faced with many limitations and will make a larger impact in your budget.
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