Making Things Talk

50 MAKING THINGS TALK

Layers of Agreement

Before you can get things to talk to each other, you have to lay some ground rules for

the communication between them. These agreements can be broken down into five

layers, each of which builds on the previous ones:

•Physical

How are the physical inputs and outputs of each device

connected to the other? How many connections between

the two devices do you need to get messages across?

•Electrical

What voltage levels will you send to represent the bits of

your data?

•Logical

Does an increase in voltage level represent a zero or a

one? This is one of the most common sources of problems

in the projects that follow.

•Data

What’s the timing of the bits? Are the bits read in groups

of 8, 9, or 10 bits? More? Are there bits at the beginning or

end of each group to punctuate the groups?

•Application

How are the groups of bits arranged into messages? What

is the order in which messages have to be exchanged in

order to get something done?

This is a simplified version of a common model for

thinking about networking called the Open Systems Inter-

connect (OSI) model. Networking issues are never really

this neatly separated, but if you keep these elements

distinct in your mind, troubleshooting any connection

will be much easier. Thinking in layers like this gives you

somewhere to start looking for the problem, and a way to

eliminate parts of the system that ...