Common Control Channel
Three logical sub-channels are:
- Is the paging channel, that the base station uses to announce incoming calls. Each mobile station monitors it continuously to watch for calls it should answer.
- Is the random access channel that allows the users to request a slot on the dedicated control channel. If two requests collide, they are garbled and have to be retried later.
- Is the access grant channel which is the announced assigned slot.
Cellular Networks
A Cellular Network is formed of some cells. The cell covers a geographical region and has a base station analogous to 802.11 AP which helps mobile users attach to the network and there is an air interface of physical and data link layer protocol between mobile and base station. All these base stations are connected to the Mobile Switching Center which connects cells to a wide-area net, manages call setup, and handles mobility.
There is a certain radio spectrum that is allocated to the base station and to a particular region and that now needs to be shared. There are two techniques for sharing mobile-to-base station radio spectrum:
- Combined FDMA/TDMA: It divides the spectrum into frequency channels and divides each channel into time slots.
- Code Division Multiple Access (CDMA): It allows the reuse of the same spectrum over all cells. Net capacity improvement. Two frequency bands are used one of which is for the forwarding channel (cell-site to subscriber) and one for the reverse channel (sub to cell-site).