At the February 2015 mHealth Working Group meeting, JSI mHealth Technical Advisor Miquel Sitjar presented an introduction to mobile infrastructure and cellular networks. As mobile approaches to global health are becoming seemingly ubiquitous, it is important to have a working knowledge of the way cellular networks function. This blog post will serve to provide an overview of the components important to the functioning of a cellular network.
First of all, what is a cellular network? A cellular or mobile network is a wireless network distributed over land areas called cells, each served by at least one fixed-location base station. Cell phones work within these cells, and can switch from one cell to another as you travel. The first component of the network is the base transceiver station (BTS), the cell tower we are accustomed to seeing, which sends out a signal at a particular frequency. The maximized area it covers without overlapping with another frequency is usually a hexagon. (Fun fact – the hexagonal shape reminded engineers working on the first network designs of the hexagonal cells in a honeycomb, which is why they were named “cells”).
In a typical cell cluster, BTS’s communicate with each other via a microwave transmission. A “ground zero” BTS receives a signal via the backbone through a subterranean optical fiber trunk, which it then transmits to other BTS’s around it over microwave antennas. Existing cellular networks can be easily expanded by connecting more BTS’s to the microwave network. However, since the bandwidth provided by the BTS connected to the optical fiber is shared by all of the other BTS in the cluster, mobile networks become slower as they move away from the backbone, as bandwidth is approximately halved for every microwave connection.
All of these base stations form a network called the base station subsystem. This subsystem is divided into two completely independent pathways – the circuit switching network and the packet switching network.
The circuit-switched network primarily handles voice communications and SMS. In this network, data from one or more BTSs are sent to a Base Station Controller (BSC), which is the brain behind the BTS. Information is then sent to the Transcoder and Rate Adaptation Unit (TRAU), which adjusts the quality of your voice based on the quality of the signal. From there, the signal goes to the Mobile Switching Center (MSC), which is associated with communications switching functions, such as routing. The MSC interfaces with the Public Switched Telephone Network (PSTN), which connects back to the cellular network.
The packet-switched network primarily handles mobile data, and is similar to any internet or computer network. The information being transferred is divided up into packets of data. The packets are sent to the Packet Control Unit (PCU), which sends them through the Packet Network. The Serving GPRS Support Node (SGSN) is responsible for handling all of the packet-switched data in the network. The Gateway GPRS Support Node provides the data a gateway to the outside world, and connects to the main data network.
The circuit-switched and packet-switched networks are completely independent, with separate components, which is why you can have a perfectly good voice connection on your phone but not be able to access mobile data.
An important characteristic of any cellular network is its ability to handoff, or transfer, your call from one cell to another as you physically move through cells. As you sit in a moving car making a call, you are able to pass through multiple base stations while still maintaining call quality. This is because your phone and the BSC are constantly communicating to keep track of the quality of the signal the phone is receiving from each of the cells around it. As soon as it senses that the quality of the signal of an adjacent cell is stronger, the BSC hands off the call to that cell, ensuring that you always have the highest quality signal available.
In the next presentation in the Cellular Networks 101 series, we will discuss the different networks (1G, 2G, 3G, 4G), and location methods (GPS vs triangulation).
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