In many cellular systems (GSM or otherwise) there is a frequency duplex between the uplink and downlink frequency band. If you use a single antenna to serve a BTS, then somehow you need to split the frequency band between the Rx and Tx side by means of a Duplexer.
The most common technology for this is the so-called Cavity Duplexer. I've used those devices (and seen them in use) for a long time, but never really opened one so far. The problem is that they are finely tuned, and each mechanical change can severely impact performance. As I had to repair a broken SMA socket on one of them recently, I took the chance to take a picture
In the first picture you can see the bottom side. This consists of a milled aluminum block, with a series of circular cavities. The Tx output of the BTS is connected to the SMA socket on the bottom right, the antenna to the SMA socket on the top side, and the Rx port to the SMA socket on the bottom left of the picture:
The small cylindrical objects in the center of the cavities are not milled from the same part, but they are separate pieces mounted by screws from the bottom of the unit.
The second picture shows the top section of the duplexer:
You can see a ~ 4mm aluminum plate with lots of (now empty) holes which are for the ~ 117 screws with which the top plate is screwed against the bottom part shown in the first picture.
The important part, however, are the screws that you can see sticking out of the top part. Those are used for tuning and present "obstacles" in the path of the waves as they pass through the cavities.
The big miracle for me is not that there are some resonances which build up a filter, but that you can actually transfer as much as 100W of RF power from the Tx input through to the antenna output.