Circuits and Packets

Circuit Switching

Remember the old movies where one would use a telephone by instructing a switchboard operator? All the phone lines in an area would converge on a single building, where they arrived at sockets on a switch board. You would crank a handle to ring a bell at the exchange, then speak to an operator. “Hello, operator? Please connect me to Alice on Milton 159”. The operator would take a short “patch cable” and plug one end into the socket for your phone line, and the other end into the socket for your recipient’s phone line.

Both lines were then connected together for the durtion of your call, and unable to be used for anything else. We call the pair of unbroken wires that carry electrical impulses from your telephone handset, across town to Alice’s handset and back again a Circuit.

That 1920s telephone network is the kind of communications network that we call a circuit switched network. The problem is that it’s not very efficent. There are only so many unmarried young women in the world who can be enticed to work as switchboard operators. Very quickly after the invention of the telephone, engineers started wondering how to make the wires do more.

Packet Switching

Now, imagine for me if you please, the postal service. The roads are our communication lines. We will communicate by sending postcards. The postie on their motorbike comes by every few seconds, so we just jot a few words on a postcard, drop it in the letterbox, and then write our next sentence on another card. The fact that I am having a correspondence with Alice three blocks away doesn’t preclude Bob from sending alice a quick note asking her to pick up

If I want to communicate with Carmen in San Diego, I can address my postcard to her and give it to the postie, who will will take it down to the local post office where they’ll sort the mail into local and otherwise, then send the otherwise to a sorting centre where it goes into the bin for america. Then it gets loaded onto a plane and lands somewhere in the US, where the reverse happens, it goes into the bucket for california, then gets sent to the san diego sorting centre, and thence into some postie’s satchel.

We call the postcards each carrying a sentence or two packets and this is an example of a Packet Switched Network.

Almost all modern communications networks are packet switched rather than circuit switched. Even the telephone network now chops up our words into chunks and sends them as packets. That’s why we say “I’m going through a tunnel, you’re breaking up”, because some of the packets get lost when we’re holding our phone inside the oven trying to get out of a tiresome phone conversation.