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Cell site, mobile phone transmitter. This is what your phone "talks" to.
How do mobile phones work?
Well, let's get one misconception out of the way first — it's nothing to do with satellites, unless you're talking about specialist (and expensive) satellite phones used at sea.
Conventional mobile phones, or "cellphones" to give them their correct technical name, work using a series of land-based transmitters called "cell sites," hence the name PLMN — Public Land Mobile Network — the technical name for the networks run by the likes of Vodafone, O2 etc.
Cell sites are built in various locations a few miles apart so that the coverage they provide forms a mesh of cells. As long as your phone is within a cell it is communicating with the cell site. Cell sites may be on masts, or on top of tall buildings. They're linked to telephone exchanges using either fibre optic cabling or microwave radio links. Mobile phone operators have their own network of exchanges just like fixed line operators, and the hardware equipment is identical — the only difference is in the software controlling the exchange.
The software for a mobile network has to cope with the fact that your phone moves about from one cell site to another, and thus from one exchange to another, unlike fixed lines which are always found wired to a particular port on a particular exchange. To do this, each mobile network has a central database of all its subscribers called a Home Location Register (HLR). The HLR is shared between all the exchanges and contains the current location of your phone — the cell site and exchange serving it — so that any exchange on the network can route a call through to whichever exchange is handling your phone at that time. The exchanges are called Mobile Switching Centres (MSC), which is just a fancy term for a telephone exchange.
Let's take each element of the network one at a time.
The Cell & Cell Site
Cells are particular geographic areas. They're typically shown on diagrams as hexagonal, so that they tessellate nicely. In reality they're circular, with a slight overlap at the edges as the signal from the cell site at its centre spreads out and weakens. Adjacent cells use different frequencies of radio signal so that where signals overlap at the edges there's no interference between cells. Mobile phones can be traced to within a particular area by a process called triangulation — measuring the signal strength between the phone and three cell sites to determine the approximate distance from each, based on signal strength. Phones and cell sites do this continually, so that as your phone moves around it can be handed off from one cell to the next to get the best signal. The equipment controlling the cell site is called the Basestation Controller (BSC). A BSC typically controls several cell sites, or "basestations," and also provides the interface between the cell sites and the MSC (telephone exchange).
In reality, cells vary in size — the geographical footprint they cover. Built-up areas with a high concentration of users have smaller cells close together; more rural areas with fewer users have larger cells. This is because each cell site has a finite limit to the number of simultaneous users it can support, so putting small cells close together in busy areas allows more users to be served.
Cell sites, connected to a Basestation Controller (BSC). Each BSC is connected to a Mobile Switching Centre (MSC).
Cells mesh together to cover a geographical area — adjacent cells use different frequency ranges to avoid interfering with each other. In reality they're more circular, with a strong signal at the centre radiating outward, getting weaker as it travels further from the cell site.
1G, 2G, 2.5G…
The first generation of cellphones, retrospectively referred to as "1G," used analogue radio signals between the phone and cell site. Later digital systems introduced in the '90s, referred to as 2G, used digital radio links.
Early mobile phone networks were often incompatible with each other, so in 1982 the European telecoms standards body CEPT (Conférence Européenne des Postes et des Télécommunications, later the European Telecommunications Standards Institute, ETSI) set up a committee called Groupe Spécial Mobile (GSM) to design a standard international 2nd-generation cellular system.
With GSM, a system called Time Division Multiple Access (TDMA) shares a radio band between multiple subscribers by allocating a time slot for each one. The timeslot is 0.5769 milliseconds, giving each subscriber exclusive use of the radio channel for that time, using PCM to digitally transmit the voice. Original 2G GSM systems could carry data over the circuit-switched phone network in a similar fashion to how dial-up internet works on fixed lines. Because of the small time slot length and the number of bits used for each PCM sample in GSM, though, the maximum data rate is 14.4kbit/sec — excruciatingly slow for today's needs. A technique called High Speed Circuit Switched Data allows multiple time slots to be bonded together to achieve speeds comparable to fixed-line dial-up, but still too slow for modern internet access.
To overcome these limitations, a system dubbed 2.5G was introduced, letting the cell site split off data traffic onto a separate packet-switched network. This is called General Packet Radio Service (GPRS), and can achieve speeds of up to 171kbit/sec. A later evolution of GPRS called EDGE (Enhanced Data rates for GSM Evolution) used more efficient modulation to achieve slightly better rates of 384kbit/sec. Calls are still passed over the circuit-switched network using GSM timeslots.
3G
In the early '90s, work began on standards for a 3rd-generation system capable of even faster data speeds. The Universal Mobile Telecommunications System (UMTS) was finally standardised in 2000, providing data rates of 128kbit/sec up to 2Mbit/sec.
UMTS uses 1885–2025MHz for the uplink (phone to cell site) and 2110–2200MHz for the downlink (cell site to phone), split into channels 5MHz wide. A technique called Code Division Multiple Access (CDMA) allows multiple subscribers to use the same frequencies simultaneously, unlike the TDMA system in GSM where each subscriber is allocated a time slot. CDMA allocates each subscriber a code, combined with the data to be transmitted to produce a combined radio signal; a "correlator" at the other end uses the same code to extract the actual data.
CDMA has the advantage of being less susceptible to interference and able to operate in noisy environments — increasingly important as more devices make use of the wireless spectrum. For CDMA to work correctly, each phone needs to transmit so the signal at the cell site is received with equal power; normally, phones nearer the cell site produce stronger signals than those far away, so a CDMA cell site commands each phone to reduce its power if needed, so phones with weaker signals aren't drowned out.
Rollout of 3G technology was delayed by the high cost of the equipment, coupled with the high licence fees demanded by various governments for access to the spectrum.
4G
The latest set of standards to be deployed are referred to as 4G, based on two different types of wireless technology depending on the network and location:
- LTE (Long Term Evolution), based on GSM/EDGE & UMTS technology, and the most common system worldwide (including the UK)
- WiMax, based on the IEEE 802.16 standard
Neither of these technologies meets all the original definitions of "4G" that the International Telecommunications Union (ITU) originally defined for a 4G system, but nevertheless they've been marketed as 4G because they offer a significant upgrade over 3G systems. Despite this, some purists insist on referring to the two systems as 3.9G, preferring to reserve the "4G" name for later developments.
LTE networks provide downstream speeds of up to 299.6Mbit/sec and upstream speeds of up to 75.4Mbit/sec, and the first LTE network was launched in 2009, in Sweden.
The original goal of LTE was that voice calls would no longer be carried over circuit-switched networks, but over packet-switched networks using Voice over IP (VoIP). VoIP doesn't imply "the Internet" — VoIP simply means any IP network, of which the Internet is one. For carrier-class voice networks (cellular and fixed line), transitioning to VoIP means using their own private IP network, not piggybacking on the Internet, in order to maintain quality of service.
Deployment costs and call quality issues with swapping out circuit-switched networks for VoIP have caused many carriers to look for stop-gap measures, though, and so LTE does provide for voice calls to be handled in the traditional circuit-switched way — currently what many operators use.
The Mobile Switching Centre (MSC), Visitor Location Register & Home Location Register
The Mobile Switching Centre is a digital telephone exchange, built using identical equipment to the systems used for fixed lines. In many parts of the world the Ericsson AXE system has become the dominant system used in mobile and fixed line networks. Digital telephone switches such as the AXE consist of a control system running the control software, and a switching fabric built using Time and Space Switches.
On exchanges handling fixed lines, a part of the exchange called a Subscriber Concentrator (Subscriber Switching Stage, in AXE terminology) provides the physical interface to each line via a number of ports, and concentrates the lines down into the core of the exchange, so the core only needs enough equipment to handle the anticipated traffic, not capacity for every subscriber to use the system at once — because in reality that never happens. The Basestation Controller (BSC) in a cell network is the equivalent.
On a fixed line exchange, a subscriber can always be found wired to a particular port on a particular exchange, and the software on that exchange holds a record for each subscriber detailing its phone number, port number, status, types of calls it can make and so forth — often called the "Class of Service" record, in phone engineer speak. The software on an MSC has to account for the fact that a subscriber may at any time be connected to any BSC, and different BSCs are connected to different MSCs. So the Class of Service record has to keep track of the BSC/cell site location of each subscriber as they move around, and when they move to a BSC handled by a different MSC, the entire Class of Service record has to be moved from one MSC's control system to another.
To allow this, Class of Service records are held centrally in a database called a Home Location Register, accessible by any MSC on the network. When a subscriber moves into the jurisdiction of a particular MSC, the Class of Service record is temporarily transferred from the Home Location Register to that MSC. The database on a particular MSC containing all the Class of Service records assigned to it at any given time is called a Visitor Location Register, and the Home Location Register is updated to show which MSC the subscriber is currently assigned to, so incoming calls can be routed to the right one.
If your phone moves from one MSC to another during a call, the old path is released and a new one set up through the new MSC instantaneously, without you noticing — hence true mobile networks weren't possible until the advent of solid-state switching, which can do this at the necessary speed.
More information on how digital exchanges work and their history can be found on our Digital Switching page.
Roaming
Roaming is the facility which allows your phone to work in other countries. Roaming agreements exist between many network operators, so customers of one network can make use of another network in countries not covered by their "home" network. The Home Location & Visitor Location Register system described earlier makes this possible — the HLR in your home network directs all traffic to the MSC currently handling your phone, which just happens to be on someone else's network. Some network operators in the UK make use of this facility to expand coverage domestically too, allowing roaming between otherwise competing UK networks when out of range of a cell site on your home network.
Virtual Mobile Network Operators
Some companies (particularly supermarkets) have jumped on the mobile bandwagon by reselling service on an existing "real" network, letting them offer mobile service under their own brand without the cost of building and running their own physical infrastructure. These are referred to as Virtual Mobile Network Operators (VMNOs).