• communicating in remote areas or at sea where there is no mobile phone signal
• after natural disasters where the mobile phone network may be unavailable
• communication by people who want to avoid their calls being 'tapped' - satellite-to-satellite communications do not go through the PSTN.
  

• Low Earth orbit (LEO) satellites such as Iridium. Iridium has 66 satellites that orbit the Earth at altitudes averaging 900km. These satellites orbit the Earth approximately twice every hour and so global coverage is always available. Obstacles, such as mountains, can be overcome as the next satellite moves into the line of sight.
• Geosynchronous Earth orbit (GEO) satellites such as Thuraya. Thuraya has 3 satellites that orbit the Earth at altitudes of 35,000km. This means that there is a latency experienced in calls. The satellites remain in a fixed position above the Earth, so obstacles, such as mountains, can prevent calls from being made at certain locations. The Thuraya satellite system covers Europe, Asia, the Middle East and Australia.

The actions that take place when a satellite phone call is made include:

1. A number is dialled from a satellite phone.
2. The request is transmitted by microwaves to the satellite in space.
3. The request if either passed to another satellite or sent to a satellite receiver on the Earth using a different frequency.
4. The call is connected through the PSTN.
5. If the call is to another satellite phone, then the signal is sent back to a satellite in space and then down to the receiving satellite phone.
   

• making and receiving  phone calls, including video calls
• sending and receiving SMS and MMS
• browsing the World Wide Web
• sending and receiving emails.

Some mobile phones, known as 'smart' phones, can be used for other applications. Some of these include:

• contact list, task list and calendar
• music player
• FM radio
• navigation software with built-in GPS
• currency converter
• camera.

• Mobile stations.
• Cells that have base station transceivers - often known as a mobile phone mast that gives signal coverage to a cell.
• Mobile switching centre - this controls all the calls taking place on the mobile telephone network.
• Base station controller - this manages the communication between a set of BSTs and the MSC.
• Public switched telephone network - when a call is being made from one mobile phone company to another mobile phone company or to a landline, then the call needs to go through the PSTN.

When a phone call is placed fro a mobile phone to a landline phone, a number of actions take place, which include the following:

1. The caller dials the landline number from the mobile phone.
2. A call request is sent by microwaves to the BST for the cell that the mobile phone is currently connected to.
3. This request is passed to the MSC.
4. The MSC checks the caller's status, including whether: there is enough credit to make the call; the dialled number is allowed.
5. The MSC then connects the call through the PSTN and allocates a frequency to the MS.

When a phone call is made to a mobile phone, the following actions take place:

1. The MSC receives a request for a call to a mobile phone number.
2. The MSC identifies within its active database which BSC the mobile phone is connected to.
3. The request is passed to the BSC.
4. The request to dial the mobile phone number is then broadcast through all the BSTs under the control of the BSC.
5. The mobile phone being called receives the request and acknowledges this to the BST.
6. The call is then connected by the MSC using a frequency allocaed to voice calls.

If a mobile phone user is moving, then the mobile phone system needs to keep track of where the mobile phone is. This is done by:

• A database stores all the current cell locations of mobile phones.
• When a mobile phone moves into another cell, it registers with the cell and the database is updated.
• As the mobile phone moves from one cell to another, the user does not notice this change unless the signal is weak at the border of the cells.
• If a mobile phone is in an area that is covered by more than one cell, then it will seek the strongest signal and registers with that cell.