6.4.3 Describe packet switching.
Teaching Notes:
Students need to be aware that when a message is dissembled into packets, the packets may take different paths and pass through different nodes to arrive at the same destination, and that packets can be discarded. Virtual circuits are not required.
JSR Notes:
Once again, lots of good stuff in the text, but I’ll focus you a bit here.
First the analogy. You're in O2 stadium after a concert, with 99 of your friends (...yes, 99. Think Facebook.) There are two ways you can try to make it to the meeting point outside. Option 1: All hold hands, and go together (imagining that many other large groups are doing the same thing...) "Nope, no letting go of hands. Just wait for that group to pass..." etc.
Option 2: Everyone just go as quickly as they can, whatever route they choose, squeezing in and out of people along the way, and re-assemble back at the meeting point. (And the sad part of this analogy: "Anyone who doesn't make it there in 5 minutes, we're leaving without you!")
(Actually, analogies of traffic from one part of town to another is more accurate, or even the way a "peloton" of cyclists in a race splits up some times. But the analogy above is a bit more fun.)
Certainly you should be aware of the various things that need to be in each packet and why. But the big thing here is what the teaching note focuses in on: the “switching” in packet switching. Yes, in packet switching the data is grouped into small packets which are sent. But the reason that they are divided up into smaller groups in the first place is that those packets can then switch the route that they are taking depending on traffic in given routes. In fact, theoretically, even with a very large file being downloaded to your computer the packet switching way could have each and every packet following a different route.
And in terms of the possible routes a packet can take, it’s almost infinite. The analogy of flying from New York to LA is a good one; there is not just one direct way to do that trip; you can literally come up with tens of thousands of different routes through major airports and minor along the way, but always be going in a Westerly direction. Doing a “traceroute” in the Network utility of the Mac OS is an interesting visual way to see just how many “hops” there can be from node to node as a packet finds its way (or not) to its destination.
And that’s the other teaching note not to forget: Packets can indeed get “lost”; that’s why the “time to live” information is decremented as it makes its way around.