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PHY, the Street Level of Network Communications

©2008 Waggledance, Inc

The "street level" of electronic communications is called the PHY (physical) Layer.

The PHY Layer appears as "Phil" and "Phyllis" in this homespun account of networking. Phil and Phyllis are working right where the computer meets the wire, the bottom of seven levels of software/hardware organizaton. In the lower three levels, the work is usually done in hardware, or a mix of software and hardware.

The story begins, however, at the top.


The Seven-Story Mountain

We imagine the computer (or the pod, or the cellphone) as a seven-story building. On the roof of the building, Meggie is chatting with Peggie electronically. Peggie lives on the roof of a similar building in another town. They are each served by a butler in the top floor of their respective buildings. This is really software, but we'll personify it as Butler Ap, for application layer.

Meggie types "I'm like, 'Whoa!'" and hits RETURN. Or hits ENTER. Or clicks on a 'Send' button.

Butler Ap puts "I'm like, 'Whoa!'" into an envelope, writes the name of Meggie's application program and the chat hosting server building's address on the envelope. Then he carries it down a floor, real fast, where Butler Pre puts the envelope into a bigger one, labels that envelope similarly and rushes down with it to Butler Sesh on the floor below.

A chain of further butlers keeps on passing it down, each adding an addressed envelope around the previous one: Sesh runs it down to Butler Tran, Tran trots it down in a bigger envelope to Ned Workahaul, Ned gives it to Link, and Link to Doorman Phil, who puts one last envelope around the nest of envelopes.

Of course, this is not mere bureaucratic bloat. The butlers in the chain also do various checking, encrypting, route figuring, and other fussing besides stuffing and addressing envelopes. Each floor has its own Butler's Guild, and each Guild has its own name for the particular envelope they add: segment, packet, frame, ... to enhance the Guild's status with butler mystique.

Now Doorman Phil has to find a passing bicycle messenger, taxicab, Baker Street Irregular or carrier pigeon to hand it to. (His street has one or two favorites.)

His wife Phyllis, meanwhile, is accepting similar envelopes from workers on the street, discarding the outer wrapper and sending them up a floor. Butler Ned Workahaul, three floors up, reads his envelope and decides whether they belong to this building. (Phyllis, busy with her work at the door, has no time for that. "Not my job, man.").

Now, the chat hosting server computer is a few hundred miles, kilometers, versts or leagues away, so "I'm like, 'Whoa!'" goes hopping between more computers, each time going up the lower three floors where others in Ned Workahaul's guild say "Not for here. We'll send it... ahhh, let's see... that-a-way!" re-addressing it and sending it downstairs.

At the chat hosting server building, the resident Ned Workahaul allows "I'm like, 'Whoa!'" the rest of the way up, where Butler Ap hands it to the chat hosting software, who says "Aha! This is from Meggie to Peggie. I know where Peggie is."

Butler Ap duly wraps "I'm like, 'Whoa!'" in a new envelope, addresses it as instructed, and down it goes again.

More versts and network hops away, in the chat window at Peggie's computer, "(Meggie:) I'm like, 'Whoa!'" appears. Peggy starts to consider a reply.


The OSI Reference Model

The whole world uses a seven-layer schema to organize network thinking and design, the Open Systems Interconnect reference model. Two diagrams of the OSI model are particularly well done:

OSI model with envelopes schematized

OSI model with terminology

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©2008 Waggledance, Inc