The OSI Model: Lower Layers

According to Networking for Dummies, the OSI Model is a description of the various parts of a computer network. There are seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Today, I will discuss the first three layers, which are commonly referred to as the lower layers. The lower layers deal only with the mechanics of getting data from Point A to Point B.

1. Physical Layer: the actual hardware used to connect multiple computers on a network. In this layer, the standards for connection cables, electrical signals, et cetera, are defined. Any device that merely facilitates the transport of data without checking or modifying it in any way is a Physical Layer device.
   
2. Data Link Layer: the lowest-level portion of the network that assigns meaning to data. Data Link Layer devices break data into chunks (packets) and works out how to send the packets to the intended recipient. To do this, each piece of hardware is given a Media Access Control (MAC) address. The Logical Link Control (LLC) sublayer then acts as an interface between the MAC sublayer and the Network Layer. Additionally, the Data Link Layer provides some error correction at the packet level.
   
3. Network Layer: the layer that deals with transporting "stuff" -- that's a technical term -- from one computer to another. The Network Layer performs logical addressing: the assignment of IP addresses to computers on the network. Routers are classified as Network Layer hardware. They take data from one computer and forward it on to its destination.

Quote of the Week (Second Edition!):
Vizzini: Inconceivable!
Inigo Montoya: You keep using that word. I do not think it means what you think it means.
--"The Princess Bride"