In my last post we began laying out the foundation for understanding the differences between the technologies that drive today's cellular phone systems in the United States and throughout most of the rest of the world.  The two reigning technologies – CDMA and GSM/TDMA – account for the vast majority of cellular systems but there is one exception to be noted here… in Japan the system provided by NTT DoCoMo – the dominant carrier there – is a proprietary system that is neither purely CDMA or purely TDMA.  It's a horse of a different color but doesn't exist outside of the islands of Japan (with rare, and nominal) exception.

That said, it's time to jump in.  Let's take a look at the two dominant technologies – CDMA (Code Division Multiple Access) and TDMA/GSM (Time Division Multiple Access/Global System for Mobile Communication) and compare them to each other, and to the granddaddy of all the technologies, good old (and I do mean old) analog technology.  Imagine, if you will, that you have been invited to three different cocktail parties in one evening and decide to go and spend a little time at all three. 

The first cocktail party is the Analog party.  When you enter the party there's a room with a closed door and a sign over it that says "Conversations held here."  There's a long line of people – in couples – in front of that door and it's moving very slowly.  When you ultimately enter that room and begin conversing with your companion you realize that until the two of you leave you've got the room locked up, metaphorically speaking, and that nobody else at the party can have a conversation until you have completed yours and have left the room. 

This is fundamentally the way that analog technology works.  It's virtually the same as a landline telephone in that when you make a call the two parties are connected and that line is tied up until such time as you release the line, whereupon someone else can make a call on that line.  It's grossly inefficient and incredibly expensive to operate that way because the operator must have enough "channels" for communications to occur at peak hours in to keep everyone happy and talking.  If not, your "Grade of Service" (GOS, in telecom terms) deteriorates and calls can't be completed into or out of the network, making for unhappy customers who eventually leave to go to another network where they can make and receive calls as they please. 

The next cocktail party is the TDMA/GSM party.  This time when you enter the party there is a similar sign saying "Conversations held here" over a door to a room with a line in front of it, but the line is moving much faster and, when you get into the room, you see that there are six couples having conversations.  The weird thing is, though, that they're all speaking at different times.  The first couple has a slot of time, let's call it Minute #1, that they are allowed to speak.  The second couple has Minute #2, and so on all the way up to the sixth couple who has Minute #6.

The first couple speaks for the duration of their Minute #1 and then stops.  Then the next couple speaks during Minute #2 and they stop (now couple number one and couple number 2 are both silent).  This continues all the way up to the last couple, who have Minute #6, and after they are done the conversation rotates back to the first couple again for them to have a conversation for another minute.  This continues in a round-and-round fashion and, occasionally one of the couples will leave the room and their "slot" is filled by another couple waiting outside the room.  Whichever minute the departed couple leaves is now assigned to the newly entered couple. 

This is, in a nutshell, the way that TDMA – Time Division Multiple Access – works.  It divides the available time up into slots and assigns the various slots of time to people who want to carry on a conversation.  In real terms, we have put all of these people on the same radio channel – in our metaphor the room represents one radio channel – and divided the time into six slots. 

Imagine, though, that instead of having a full minute of time that each couple has 10 seconds.  The first 10-second slot is filled by the first couple and it goes around the room until each couple has spoken for 10 seconds and then it repeats starting with the first couple again.  That's easy enough to grasp, but now let's really let our imaginations go crazy and presume that each couple's time slot was only for a fraction of a second… the conversation rotates around the room so quickly from one couple to the next that your ear really can't hear that you've only been speaking for a fraction of a second and interprets it as if it was one continuous, fluid conversation. 

What we've done here is divided time into tiny slots that are filled by snippets of conversations so that multiple conversations can be held simultaneously – Time Division Multiple Access.  Put this way you can see how it actually makes some sense. 

The CDMA cocktail party is totally different.  We'll cover that in the next post and you'll find out why Code Division Multiple Access is like the United Nations of wireless standards.