Here's a military viewpoint to add to
sgoldgaber's excellent wu, as the world of signals intelligence (
SIGINT) and
electronic warfare (EW) is another important arena where traffic analysis (TA) is employed.
In a tactical environment, knowing as much about the enemy as possible is often the key to winning or losing. How an enemy uses their radios, how many and what kind of radios there are, and the order they are used can be combined with radio direction-finding RDF and other EW techniques to learn about the foe.
For example, the act of communication itself tells the analyst of the organizational structure of the enemy. Who talks to who and in what sequence tells the trained listener a great deal. Without even understanding what is being said, using RDF to determine the location of the radios and TA to plot the order and volume of communications will expose the command structure of the target group. If one has the ability to determine transmitter strength, the task is even easier, as vehicle-mounted radios are more powerful than carried devices, and radios powered from an external generator more powerful than those powered by a vehicle.
The number of radios is also an important piece of information on the battlefield. A cluster of radios usually designates either a command post, a group of vehicles, or a unit of elite troops, as even the equipment-heavy US has yet to outfit every single foot soldier with a radio. This information combined with RDF and TA will usually determine the type of unit, as movement direction and speed are also important puzzle pieces to be incorporated into the big picture. This tactic can be extended even further in some cases into considering the number of radios equipped with scramblers, as many forces do not have the money needed to outfit every radio-carrying unit with encrypted comms.