Page 239 - Anatomy of a Robot

09_200256_CH09/Bergren 4/17/03 11:24 AM Page 224
224 CHAPTER NINE
OSI Seven-Layer Model
Some years ago, a group got together in an attempt to define a model for the way com-
munications should be structured, which was known as the Open Systems
Interconnection (OSI) seven-layer model (www.scit.wlv.ac.uk/ jphb/comms/std.7layer
.html). Nobody really followed the model from top to bottom, but Transmission Control
Protocol/Internet Protocol (TCP/IP) network communication comes the closest; how-
ever, the model is useful at the very least as a checklist for the types of things we might
want in a communications system. Given that it’s also worth learning just for network
communications, let’s delve into it.

LAYER 1: PHYSICAL LAYER

The data layer is the lowest layer and defines the physical and electrical characteristics.
It is the layer dealing with sending bits over the physical medium. All communications
have a physical layer of some sort. In some systems, it may be the only layer. Baseband
communications, modulation, demodulation, and transmission through the channels are
all topics that loosely belong in this layer.

LAYER 2: DATA LINK LAYER

This layer deals with blocks of data on the physical media. It controls the sharing of the
communication path, frames, flow control, and some low-level error checking. This is
the multiple access (MAC) layer in network communications. Many strategies exist for
sharing access to a transmission channel. Access and error-checking techniques are top-
ics we can cover that belong to this layer.

LAYER 3: NETWORK LAYER
This layer is responsible for routing, making, maintaining, and breaking connections.
This is the IP layer in network communications.

LAYER 4: TRANSPORT LAYER

This layer is responsible for the error-free transmission of data from one machine to
another. This is the TCP layer in network communications.