Tuesday, October 21, 2008


TDMA, FDMA, DAMA, PAMA, FAMA, CSMA! What? . Let's just say it's all about sharing. There turns out to be more than one way to multiplex a cat

There are a lot of ways you can cut a little slice of RF. We can start small. Even Stereo FM radio multipexes. Frequency Division Multiplexing (FDM) is a form of signal multiplexing. In FM, a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel. I expalined that once before. So let's consider other multiplexing protocols. I'll start with satillite then get weird.

FDMA for example shares the available radio spectrum by the communications signals that must pass through that spectrum. The terminology for this is “multiple access.” That's the MA in all those acronyms. FDMA is Frequency Division Multiple Access. In that form of multiplexing, individual frequencies are allocated for each communications signal within the band. It requires high-performing filters in the radio hardware.

Contrast that with Time Division Multiple Access or TDMA. With TDMA the signal is divided into frames, each frame into time slots and each user is assigned a slot. A guard period is used to synchronize this data stream so a single receiver can handle the multiple transmitters. This is used in 2G cellular systems for example.

CDMA is Code Division Multiple Access. This also allows multiple transmitters to share a channel. CDMA uses a "spread-spectrum system." This technology is a tad older than the others, first used in communications satellites in the 1970s. CDMA structures the signal with direct sequencing and frequency hopping. Direct sequencing spreads the normally narrow band information over a wider set of frequencies. A FCC rule change in 1981 allowed it's modern use in WiFi and Bluetooth.

So many acronyms today... DAMA assigns communication channels or circuits based on demand hence Demand Assigned Multiple Access. Requests are issued bu users to a network control system. This is usually done with a pair of frequencies- one transmits, one receives. This is not technically a multipel access system for some as the mangement is near-real time not allowing for concurrent use. PAMA is Permanently Assigned Multiple Access which is almost self-explanatory. It's also called FAMA the F standing for Fixed. FAMA/PAMA is a very innefficient sytesm as the channel remains assigned to the single user whether they are using it or not.

All this multiplexing is messy. It sounds brilliant and efficient but like most inventions by mere mortals sometimes it doesn't work. Sometimes things that are supposed to be sharing a data channel try to use it at the same time. It's called a collision. So we come to CSMA, Carrier Sense Multiple Access. It's a contention protocol. This system detects data collisions by monitoring traffic. The Carrier detects a collision and reacts by staging random time intervals to stagger the two colliding users. Multipel collisions will cause greater and greater intervals of waiting. This is happening on the ethernet network that connects to the back of your PC.