[From the previous episode: we reviewed what a networkA collection of items like computers, printers, phones, and other electronic items that are connected together by switches and routers. A network allows the connected devices to talk to each other electronically. The internet is an example of an extremely large network. Your home network, if you have one, is an example of a small local network. is and how messages might travel across it.]

We continue here with more basics on how electronic communications work. If it feels like this might be complicated stuff, you’re right. In fact, the whole thing is so complicated that technologists have pulled the process apart so that it’s easier to handle in smaller chunks, solving one problem at a time.

Before we look at how this actually works, let’s start with an analogy. If you work for a large, global corporation as one of many people that keep things humming, you have lots of communicating to do with people all over the world. Thankfully, it’s easy to do these days – just bring up an email or instant message, type a few words, perhaps attach a report or memo, and you’re good.

But a mere 30 years or so ago, things were very different. That report? You had to spend a good half hour or so in the copy room making a copy for everyone that might want to read it. (Or for whomever you wanted to see how busy you’ve been.) Then you spent another 15 minutes slipping these copies into “interoffice envelopes” for delivery far and wide.

And then those envelopes went into the “out” pile, and, after some time, they magically showed up at all their destinations. As far as you could tell, someone personally walked (or drove or flew) each one to where it needed to go.

Following a Report

But, of course, that’s not what happened. Let’s follow the path of one particular interoffice envelope that’s going from your desk to a remote office in, say, Cincinatti (assuming you’re not already in Cincinatti — if you are, then pick the city of your choice). What you wouldn’t have seen was something more like this:

• Someone in your office would combine all the outgoing mail from your office or area and send it to the mail room.
• In the mail room, someone would separate the mail by destination. Other interoffice envelopes heading for the same place would be bundled together into a bigger envelope. An address label would be put on that envelope to ensure that it got to the right sorting room.
• Someone from a mail company (might be the Post Office, might be a private company like Federal Express or UPS) would come collect all those packets and take them to a facility where mail comes in from many different companies. All the mail heading to, say, Cincinnati from many different companies would be combined in bins labeled “Cincinnati.”
• They would then load those bins onto trucks and send them on their way until they reached that same kind of sorting center in Cincinnati (or wherever). Along the way, your bin might go through various different towns. It might get shuffled from one truck to another. It might even hop a plane, which would probably fly to some hub and disgorge its bins so that someone could put them all onto different planes for delivery to other major cities like Cincinnati, where they would be loaded back onto trucks. Some logistics person would need to plan and coordinate each of these transfers to make sure that the right bins got to the right sorting centers. In many of these hops, your memo is riding alongside lots of other totally unrelated cargo and passengers.
• Once at the destination sorting center, the bins would be unpacked, separating out the packages for each company destination. Your company’s package would go onto a truck and be delivered to your company’s local mail room.
• In the mail room, the large envelopes would be opened up to separate out all of the individual interoffice envelopes. Those would be delivered to the different departments.
• Once at your department, the administrative folks would separate and deliver the interoffice envelopes to their intended recipients. When the admin of the person to whom you sent the report walks up to the office with the interoffice envelope, as far as the recipient could tell, that admin walked (and drove and flew) all the way from your office to hand-deliver the report.

You can almost picture a tall building, with you at the top, and your package going down to a level below for sorting and then down to the ground floor for actual transport. Well, this is surprisingly similar to how the email you send today gets there. It’s just that it’s happening on wires (or over the air) instead of trucks and planes.

A Visual View

The process is simplistically illustrated in this video. (No, Quentin Tarantino I’m not…)

The dotted lines between your floor and your recipient’s floor are there to indicate the fact that, as far as you can tell, this was the direct route your message took. All that messiness along the way is completely invisible to you thanks to the services of the mail sorters and truck drivers and everyone else involved.

The thing to keep in mind is that each service is independent of any other service.

• The people combining envelopes in the mailroom have no idea which transport company is going to haul this stuff out of the mailroom. They shouldn’t have to know; it has no impact on how they combine envelopes.
• Likewise, the driver of the truck picking up the envelopes has no idea what’s inside them and where the final destination might be. She doesn’t care; she’s simply making her rounds, picking things up and taking them to the central warehouse, where further routing will happen – by someone else.

This separation of tasks into manageable bites is key to understanding the notion of a stackRelated to communications: A way of organizing parts of a complicated process (like communications) so that any task relies on tasks below it and feeds the tasks above it. Related to computing: A place in memory where you store “where was I?” information when you go from, say, one function into another. Before starting a new function, you store where you were in the old one so that, when the new function ends and you’re back in the old one, you can figure out where you were and continue on.. And it’s fundamental to how electronic communications work.