It's been a very long time since the first article in this networking series, and now it's up to me to go into the details of the OSI reference model. I've decided to start from the bottom and work my way up through the seven layers because this is the best way to start troubleshooting a network. Too often, when something goes wrong, people tend to overlook the simplest things, like a loose cable, and than its just hours and hours of headaches. So today I'm going to talk about the first layer of the OSI reference model. If you want to read more about the OSI layers read my previous article. But what was the first layer's name, again? Don't worry, I have a tip for you which will help you remember the order of those layers: just remember that All People Seem To Need Data Processing (Application, Presentation, Session, Transport, Network, Data link, Physical).

The Physical Layer is all about the electrical and mechanical specifications for activating, maintaining and deactivating the physical link between end systems. What is the medium on which the bits travel? How will the bits be encoded? How many volts do I use to represent a "1" bit? These are just a few questions that should come to your mind when you think about Layer 1 issues. But if you want to remember it in as little words as possible, just think Binary Transmission, or Media and Signals. I wouldn't want to go into the intricate details of electrical engineering, so the next part of this article will be a guide through the different kinds of media that are used today's LANs. And I'll start with copper.

The Copper Network

Coaxial Cable

Copper is the most used media in the LANs, mostly because of it's low cost. To better understand why the different kinds of cable (Coaxial, UTP, STP) are built the way they are, we must consider what actually happens along the copper conductor. The bits are encoded as electrical signals. The easiest way to encode the bits would be to use 0V (volts) to represent a logical "zero" and +5V to represent a logical "one". This is called the NRZ (non-return to zero) encoding. However, to better insulate the electrical signal from noise, a more complex method of encoding is used - Manchester encoding. This results in a 1 being encoded as a low-to-high transition of the electrical signal, and a 0 as a high-to-low transition. The electrical noise can have many causes such as EMI (electromagnetic interference) and RFI (radio frequency interference) from nearby electrical devices as common as air-conditioning or computers. This was one of the things that were taken into consideration when the Coaxial Cable (coax) was born.

The coax has a copper conductor, surrounded by a plastic insulator, which in it's turn is covered by a copper shielding to provide protection from EMI and RFI. The shielding is part of the data circuit. All this is placed into an outer jacket to prevent mechanical damage to the wire. The wire is terminated with a BNC connector. Speeds of 10 - 100 Mbps (Megabits per second) can be (theoretically) achieved. Even though the signal is well protected from EMI, coax only has historical significance now, because it has been replaced by better alternatives such as UTP and STP. The TIA/EIA standards specify that for the new LANs coax should not be used any more.