Cellular Networks — Inside Out

Cellular technology, especially GSM (Global System for Mobile communication), is incredibly advanced. Behind the very simple task of calling up on a mobile phone lies some amazing technology, and this article will delve into how cellular networks function. To simplify things, we’ll approach the problem in a top-down manner. 

Base stations



Your mobile phone at its very heart is a little more than a cordless phone that you use at home. Basically, it’s a radio transmitter that is used to send and receive information (voice, text messages, fax, data, etc) from its base station. In the case of cordless phones, the base usually sits inside the house and the cordless unit is able to function if it’s within the reach of its radio signal. 

But if a mobile phone is similar to a cordless, where is its base station? The answer, as you probably know, are those tall towers that you see every couple of kilometers on the top of buildings. The base station serves as a ‘cell’ having a range spanning many kilometers in diameter. From here, another interesting question comes up. What happens when you move out of the range of one base station with a cellphone and enter another? This is where a very important capability of these networks comes into play. At any given point of time, the cellular network is constantly polling the phone to see which base station is best suited for it to be on. If the network sees that a phone is moving out of the range of one base station and consequently into the range of another, it automatically instructs it to start using the new base station, which by then is ready to accept its connection as well. This process is known as ‘hand-over’ and occurs within 400 ms, so that the subscriber is hardly aware of a break. 

Now let’s see how cellphones make calls to other cellphones and landline numbers.

Mobile switching centers



Base stations are often organized in clusters that are collectively managed by Mobile Switching Centers (MSC). Each MSC of a cluster is connected to MSCs of other clusters and a PSTN (Public Switched Telephone Network). So when a cellphone makes a call, there’s a process of passing it to the nearest base station, which further sends it to the nearest MSC, which in turn contacts either other MSCs (if the call is to another mobile phone) or the PSTN (if call is to a landline). The process works in exactly the reverse way for incoming calls.

Arrangement of cells and frequency reuse



When a cellular network is being planned and designed, cells are usually denoted in the shape of hexagons. This is because while circles depict the range of a base station more accurately, hexagons can be fitted together in a jigsaw like fashion which eases planning. A network can be envisaged as a mesh of hexagonal cells, each with a base station at its center. 

Cellular operators are assigned frequencies by the government and are not allowed to use anything outside that. To squeeze all the millions of subscribers into this (usually) tiny spectrum, frequencies must be reused in different areas of the network. This is done by ensuring that no two cellphones (or base stations) within the range of each other function at the same frequency. A common way of doing this is by using 7-cell clusters. In the figure, different letters show different frequencies. Depending on the network, 4, 12 or 21 cell clusters can also be used.

Anuj Jain