BROCADE IP PRIMER

The Physical Layer (Layer 1)One other feature in a lot of Data Link Layer protocols is error correction. Thereare a lot of physical media to choose from (as we will discuss more in the nextsegment). Some media are very reliable. Some are not. A Data Link Layer protocoldoes well to know what the next layer is. In many cases, it will anticipatethe unreliability and employ error correction to compensate.Of all the Data Link Layer protocols there are, clearly Ethernet is the one that isbest known. It is an extremely common protocol, and we'll be discussing it inmore detail later in this chapter. Some other examples include Token Ring,Asynchronous Transfer Mode (ATM), and High-Level Data Link Control (HDLC).The Physical Layer (Layer 1)Well, we made it. After all the work that's been done in the upper six layers, thefinished frame is handed from the Data Link Layer to the Physical Layer. ThePhysical Layer is the layer that does the actual transmitting of data. This is thelayer where the data actually moves. You can think of the upper six layers as“getting the data ready.” When it reaches Layer 1, “it goes.”The Physical Layer takes the frame from the Data Link Layer and converts it tobits. We'll go into this much more in Chapter 2, but for now, all you need toknow is that it's a long stream of 1's and 0's.0110111000101011011100001010111Once converted to bits, the Physical Layer uses its physical media to conveythe data. There are a lot of different ways to do this. Some physical protocolssend electrical signals across copper wire. Some physical protocols send alaser signal across fiber optics. Some protocols generate pulses of microwavesinto the air. Each different method has their advantages and disadvantages,but they all get the job done.Once the Physical Layer has sent its bits, the receiving Physical Layer picksthem up, shapes them into a frame, hands it off to the receiver's Data LinkLayer, and we start the journey back up the stack. The Physical Layer has doneits job. It's the layer that actually sends the data.Examples of Physical Layer protocols include Unshielded Twisted Pair (UTP),Fiber, IEEE 802.11 (wireless networking), and so many other variations. Therehave been people that have demonstrated the ability to network using carrierpigeons. You don't typically get as much bandwidth that way as you would with,say, fiber, but the point is, it doesn't matter. Whatever suits your need. Whetheryou need to beam signals to an orbiting satellite and back to Earth, or whetheryou're using two cans and a string, the Physical Layer sends the data.And that's the OSI Reference Model, folks. Next, I'd like to talk about a DataLink Layer Protocol that anyone reading this book is going to have to get reallycozy with.Brocade IP Primer 19