CELLULAR NETWORKS AND HOW CELLPHONES COMMUNICATE OVER THE NETWORK. by PROF. DR. ERHAN A. İNCE

Transcription

1 CELLULAR NETWORKS AND HOW CELLPHONES COMMUNICATE OVER THE NETWORK by PROF. DR. ERHAN A. İNCE

2 CELLULAR Cellular refers to communications systems that divide a geographic region into sections, called cells. Hexagonal cell shape is perfect over square, circular or triangular cell shapes because it covers an entire area without overlapping i.e. they can cover the entire geographical region without any gaps. The purpose of this division is to make the most use out of a limited number of transmission frequencies. 2

3 Cellular Structure 3

4 Components of a cell phone network 4

5 Called A handset, terminal, Smartphone etc Has a speaker and a microphone. It has gota CODEC for digitizing speech Has got a MODEM for moving ones and zeros around. Has an ANTENNA A BATTERY A keypad and a screen. Also has SIM (Subscriber Identity Module) 5

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7 Hence SIM is a number that identifies your phone. When you send a message your phone knows which network to use and The operator knows who to bill for the call. Though a SIM contains a bit more. What? 7

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9 The carrier extracts the information from a number on the SIM. This is known as Integrated Circuit Card Identifier (ICCID). 9

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12 When you first turn on your phone will pass your SIM card s ID number along with an AUTHENTICATION KEY to your CELLPHONE provider. 12

13 Your provider than generates a random number and uses the KEY to generate a RESPONSE number 13

14 That random number is then sent to your phone and the Same calculation is done using the KEY to generate an other RESPONSE number. 14

15 If the two numbers match the provider will recognize this and connect your phone to the network. 15

16 The SIM also contains the location area identifier. This information The PHONE learns from the NETWORK. It identifies which BASESTATION the phone has last talked to. 16

17 The phone is connected to the base station over an AIRLINK 17

18 Base station has three main components 1 18

19 Base station transceiver is the radio. It produces energy at gigahertz frequency in the form of electricity. 19

20 The electricity is carried up to the second component the antennas over thick coaxial cables. 2 20

21 The antenna converts the vibrating signal at GHz frequencies into ELECTROMAGNETIC waves. These waves then propagate through space at the speed of light. WIRELESS transmission. 21

22 Towers with antennas that look like a stick are OMNIDIRECTIONAL which radiates energies in all directions. 22

23 More often the antennas are FLAT PANNEL type and emit energy in a wedge shape (in a particular direction) 23

24 DIRECTIONAL Antennas 24

25 Mobile BS Fixed BS 25

26 Antenna Patterns Omnidirectional Antenna Directional Antenna 26

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28 Mobile Switching Center keeps track at all time where the subscribers are. 28

29 The connections between the Mobile Switching Center and the base stations are known as BACKHAUL. If there is FIBER between the two locations and the company operating this base station is an affiliate (partner) of the phone company that owns that fiber then they will use fiber to connect the base station back to the MSC. 29

30 In many other cases fiber is not available and we use microwave dishes to set up the connection. 30

31 MTSO also known of the Mobile Telephone Switching Office keeps track of the location of your phone in a database 31

32 The phone learns to which base station it is talking to from the base station and stores that on the SIM card, the Location Area Identifier When you first turn your phone on, it has been pre-programmed to communicate at particular frequencies and it will transmit on these Frequencies its International Mobile Subscriber Identity which will be received by the mobile switch. Mobile switch looks up to see if that person is one of their customers (uses authentication key) Then the switch will check to see if that customer has paid their bill or not. If all is okay MSC will send a signal to the PHONE over the backhaul and the airlink that it has service. 32

33 MTSO picks a frequency pair (DUPLEX channel) that your phone will use when the CELLULAR NETWORK takes the call. 33

34 When the phoning tower switch on these frequencies the CALL is connected. 34

35 The communication that takes place is FULL Duplex. Meaning you can talk and receive at the same time. 35

36 The LIN provided by phone to the MSC is stored at a LOCATION REGISTER 36

37 If someone calls you from the LAND LINE then the call will go over their loop to the central office and to a toll center and then to a cellular telephone Company and the phone call is terminated on the switch. The base station will check the location register to find the LIN for the Called party. The MSC will page the mobile called and if it gets a reply then will Switch on the pair of frequencies and phone will ring and show the calling number 37

38 How about DUAL SIM card phones? They are useful for example when you go to other country and buy a new SIM card for worth for a short duration. 38

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43 Let us look at something very few people know about. 43

44 How is this possible? 44