1. COMPUTER NETWORKING 2 LECTURE : CELLULAR NETWORKS

2. Introduction 2  Wireless  Communications system in which electromagnetic waves carry a signal through atmospheric space rather than along a wire  Most systems use radio frequency (RF, which ranges from 3 kHz to 300 GHz) or infrared (IR, which ranges from 3 THz to 430 THz) waves

3. Evolution to cellular networks – communication anytime, anywhere  radio communication was invented by Nokola Tesla and Guglielmo Marconi: in 1893, Nikola Tesla made the first public demonstration of wireless (radio) telegraphy; Guglielmo Marconi conducted long ditance (over see) telegraphy 1897  in 1979 the first commercial cellular phone service was launched by the Nordic Mobile Telephone (in Finland, Sweden, Norway, Denmark).

4. Timeline of Major Developments 4  Mobile Telephone System (MTS)  Introduced in 1946  Simplex (one-way transmission) and manual operation  Improved Mobile Telephone System (IMTS)  Introduced in 1969 using a 450 MHz band  Advanced Mobile Phone Service (AMPS)  Introduced in 1983  First system to employ a “cellular” concept

5. Frequency reuse  is a method used by service providers to improve the efficiency of a cellular network and to serve millions of subscribers using a limited radio spectrum  is based on the fact that after a distance a radio wave gets attenuated and the signal falls bellow a point where it can no longer be used or cause any interference  a specific frequency range will have only a limited coverage area  beyond this coverage area, that frequency can be reused by another transmitter

6. Network Cells  the entire network coverage area is divided into cells based on the principle of frequency reuse  a cell = basic geographical unit of a cellular network; is the area around an antenna where a specific frequency range is used  when a subscriber moves to another cell, the antenna of the new cell takes over the signal transmission  a cluster is a group of adiacent cells, usually 7 cells; no frequency reuse is done within a cluster  the frequency spectrum is divided into subbands and each subband is used within one cell of the cluster

7. Frequency Reuse Patterns

8. Network cells

9. Cells

10. Cellular network components

11.  BTS (Base Transceiver Station) – main component of a cell and it connects the subscribers to the cellular network  BSC (Basic Station Controller) – it is an interface between BTSs and it is linked to BTSs by cable or microwave links; it routes calls between BTSs; it is also connected to the MSC  MSC (Mobile Switching Center) – the coordinator of a cellular network, it is connected to several BSCs, it routes calls between BSCs; links the cellular network with other networks through fiber optics, microwave or copper cable

12. Components of a cellular phone (MSU – Mobile Subscriber Unit)  radio transceiver – low power radio transmitter and receiver  antenna, usually located inside the phone  control circuitry – formats the data sent to and from the BTS; controls signal transmission and reception  man-machine interface – consists from a keypad and a display; is managed by the control circuitry  Subscriber Identity Module (SIM) – integrated circuit card that stores the identity information of subscriber  battery, usually Li-ion, the power unit of the phone

13. Cellular Call Completion  Components of a signal:  Mobile Identification Number (MIN) - an enclosed representation of the mobile telephone’s 10-digit telephone number.  Electronic Serial Number (ESN) - a fixed number assigned to the telephone by the manufacturer.  System Identification Number (SID) - a number assigned to the particular wireless carrier to which the telephone’s user has subscribed.

14. Analog Access 14  Analog Cellular Systems  First generation system  Based on FDMA (Frequency Division Multiple Access), where frequency band is divided into a number of channels. Each channel carries only one voice conversation at a time.  AMPS operates on 800 MHz or 1800 MHz  Advantages: Widest coverage  Limitations: Inadequate to satisfy the increasing demand Poor security Not optimized for data

15. FDMA 15

16. Digital Access 16  TDMA (Time Division Multiple Access)  CDMA (Code Division Multiple Access) Digital wireless technologies provide greater system capacity.

17. TDMA 17  TDMA  Second generation system  Enables users to access the whole channel bandwidth for a fraction of the time, called slot, on a periodic basis  Has applications in satellite communications  Advantages Improved capacity

18. TDMA 18

19. CDMA 19  CDMA  Third generation system  Separates users by assigning them digital codes within a broad range of the radio frequency  Advantage Improved capacity, coverage, voice quality, and immunity from interference

20. An Overview of Cellular Technologies 20

21. Global System for Mobile Communications (GSM)  A version of time division multiple access (TDMA) technology, because it divides frequency bands into channels and assigns signals time slots within each channel.  Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common in the United States.  Makes use of silences in a phone call to increase its signal compression, leaving more open time slots in the channel.

22. GSM characteristics  previous standard in cellular communication  GSM – global digital standard for cellular phones that offered roaming facility  GSM operate in frequency bands: 900MHz, 1800 MHz, 1900 MHz  GSM provides voice and data services

23. Subscriber Identity Module (SIM) card  SIM – a memory card (integrated circuit) holding identity information, phone book etc.  GSM system support SIM cards  other systems, like CDMA do not support SIM cards, but have something similar called Re-Usable Identification Module (RUIM)

24. Channels  Control channels  Setting up and maintaining calls  Establish relationship between mobile unit and nearest BS  Traffic channels  Carry voice and data

25. Typical Call in Single MTSO Area (1)  Mobile unit initialization  Scan and select strongest set up control channel  Automatically selected BS antenna of cell Usually but not always nearest (propagation anomalies)  Handshake to identify user and register location  Mobile unit monitors for pages (see below)  Mobile originated call  Check set up channel is free Monitor forward channel (from BS) and wait for idle  Send number on pre-selected channel  Paging  MTSO attempts to connect to mobile unit  Paging message sent to BSs depending on called mobile number  Paging signal transmitted on set up channel

26. Typical Call in Single MTSO Area (2)  Call accepted  Mobile unit recognizes number on set up channel  Responds to BS which sends response to MTSO  MTSO sets up circuit between calling and called BSs  MTSO selects available traffic channel within cells and notifies BSs  BSs notify mobile unit of channel  Ongoing call  Voice/data exchanged through respective BSs and MTSO  Handoff  Mobile unit moves out of range of cell into range of another cell  Traffic channel changes to one assigned to new BS Without interruption of service to user

27. Call Stages

28. Cellular Call Completion

29. Call Sequence 1. Subscriber initiates call by keying in number and presses send 2. MTSO validates telephone number and checks user authorized to place call Some service providers require a PIN to counter theft 3. MTSO issues message to user's phone indicating traffic channels to use 4. MTSO sends ringing signal to called party All operations, 2 through 4, occur within 10 s of initiating call 5. When called party answers, MTSO establishes circuit and initiates billing information 6. When one party hangs up MTSO releases circuit, frees radio channels, and completes billing information