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Cell site, mobile phone transmitter

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 - its 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 like the one on the left 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 like the one in the photo or ontop of tall buildings. The cell sites are 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. In order to do this each mobile phone 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 that cell site) such 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.

Lets take each element of the network one at a time...

The Cell & Cell Site

Cells are particular geographic areas. They are typically shown on diagrams as hexagonal so that they tesselate nicely. In reality they are 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 signals so that where the signals overlap at the edges of the cell there is no interference between cells. Mobile phones can be traced to within a paricular 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. The 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 which controls the cell site is called the Basestation Controller (BSC). A BSC typically controls several cell sites or 'basestations' and the BSC also provides the interface between the cell sites and the MSC (telephone exchange).

In reality cells vary in size, i.e the geographical footprint they cover. Built up areas containing a high concentration of users will have smaller cells close together. More rural area with fewer users will 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 and BSC

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

Cells mesh together to cover a geographical area. Adjacent cells use different frequency ranges to avoid interfering with each other

In reality the cells are more circular with a strong signal at the centre radiating 						outward getting weaker as it travels further from the cell site

In reality the cells are 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, retrospectivly referred to as '1G' used analogue radio signals between the phone and cell site. Later digital systems introduced in the 90's, and referred to as 2G, used digital radio links.

Early mobile phone networks were often incompatible with each other and so in 1982 the European Telecoms Standards Body CEPT (Conference Europeene Postes et des Telecommunications) (later the European Telecoms Standards Institute ETSI) set up a comittee called Groupe Special Mobile (GSM) to set about designing 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 such that each subscriber has 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 however the maximum data rate is 14.4kbits/sec, excrutiatingly slow for todays needs. A technique called High Speed Circuit Switched Data allows multiple time slots to be bonded together to achieve speeds comparable with that used on fixed line dial up but still too slow for modern internet access.

To overcome these limitations a system dubbed 2.5G was introduced which allowed the cellsite to split off data traffic onto a separate packet switched network. This is called General Packet Radio Service (GPRS) and can achieve speeds of upto 171kbits/sec. A later evolution of GPRS called EDGE (Enhanced Data rates for GSM Evolution) used more efficient modulation to achieve slightly better data rates of 384kbits/sec. Calls are still passed over the circuit switched network using GSM timeslots

iPhone connected to EDGE data network (indicated by the E) iPhone connected to GPRS data network iPhone connected to GPRS data network

iPhone connected to EDGE data network (indicated by the E)

A few seconds later the EDGE signal has dropped out and it has reverted to GPRS

In some areas no 2.5G data signal (GPRS or EDGE) can be found and it reverts to phone calls only (2G)

3G

In the early 90's work began on designing standards for a 3rd Generation system capabable of even faster data speeds. The Universal Mobile Telecommunications System (UMTS) was finally standardised in 2000 providing data rates of 128kbits/sec upto 2MBits/sec

UMTS uses 1885MHz to 2025MHz for the uplink (phone to cell site) and 2110MHz to 2200MHz for the downlink (cell site to phone) and it is split into channels 5MHz wide. A technique called Code Division Multiple Access (CDMA) allows mutliple subscribers to use the same frequencies simultaneously, unlike the TDMA system in GSM where each subsciber is allocated a time slot. CDMA allocates each subscriber a code which is combined with the data to be transmitted to produce a combined radio signal. A system called a correlator at the other end uses the same code to extract the actual data.

CDMA has the advantage that it is less susceptible to interference and can operate in noisy environments which is important as more and more devices make use of the wireless spectrum. In order for CDMA to work correctly each phone needs to transmit so that the signal at the cellsite is received with equal power. Normally phones nearer to the cellsite will produce stronger signals than ones far away. To overcome this a CDMA cell site commands each phone to reduce its power if needed so that phones with weaker signals are not drowned out.

Rollout of 3G technology was delayed by the high costs 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 and are based on two different types of wireless technology depending on the network and location. These are:

  • LTE (Long Term Evolution) based on GSM/EDGE & UMTS technology and the most common system worldwide (including the UK)
  • WiMax based on the IEEE802.16 standard

Neither of these technologies meet all the original definitions of '4G' that the International Telecommunications Union (ITU) originally defined for a 4G system but nevertheless they have 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 moniker for later developments.LTE networks provide downstream speeds of up to 299.6 Mbits/sec and upstream speeds of up to 75.4 Mbits/sec and the first LTE network was launched in 2009 in Sweden.

The original goal of LTE was that voice calls are no longer carried over circuit switched networks but over packet switched networks using Voice over IP (VoIP). A later eMuseum article will look at VoIP but for now we will point out that VoIP does not imply 'The Internet' - VoIP simply means any IP network of which the Internet is one. In the case of carrier class voice networks (cellular and fixed line) transitioning to VoIP requires 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 with VoIP have caused many carriers to look for stop gap measures however and so LTE does provide for voice calls to be handled in the traditional circuit-switched way and currently this is what many operators are using.

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 conentrates the lines down into the core of the exchange so that the core of the exchange 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 paricular 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, this is often called the 'Class of Service' record in phone engineer speak. The software on an MSC has to take into account the fact that a subscriber may at any time be connected to any BSC and moreover, different BSCs are connected to different MSCs. Thus the Class of Service record has to keep track of the BSC/cellsite 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 control system to another.

To allow this the Class of Service records are held centrally in a database called a Home Location Register which is accessible by any MSC on the network. When a subscriber moves into the jurdistiction of a particular MSC the Class of Service record is temporarily transferred from the Home Location Register to the MSC. The database on that particular MSC containing all the Class of Service records on the MSC at any given time is referred to as a Visitor Location Register. and the Home Location Register is updated to show which MSC the subscriber is currently assigned to so that incoming calls can be routed to the appropriate 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 were not 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 that customers of one network can make use of another network in countries not covered by your '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 elses network. Some network operators in the UK make use of this facility to expand coverage in the UK by 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 exisiting 'real' network allowing them to 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).