CSCI 465 D ata Communications and Networks Lecture 18 Martin van Bommel CSCI 465 Data Communications & Networks 1

Wireless Phone Networks Prior to cellular was Mobile radio telephone service – Provided by one high-power transmitter/receiver – Support 25 channels – Effective radius of 80 km Move to Cellular radio network – Lower-power systems with shorter radius – Numerous transmitters/receivers CSCI 465 Data Communications & Networks 2

Cellular Network Organization Area is divided into cells – uses tiling pattern to provide full coverage – each with own antenna (max 100 Watts) – each with own range of frequencies – served by base station consisting of transmitter, receiver, and control unit – adjacent cells use different frequencies to avoid crosstalk cells sufficiently distant can use same frequency band CSCI 465 Data Communications & Networks 3

Cellular Geometry Divide area into cells – Square cells easy, but distances not equal – Hexagonal pattern permits equidistant antennas – Distance between cell centers – In reality, slight variation due to topographical limitations, local signal propagation, and limits on antennae CSCI 465 Data Communications & Networks 4

Frequency Reuse Objective – Share nearby (but not adjacent) cell frequencies without interfering with each other Power of base transceiver controlled – Permit communications within cell on given frequency – Limit escaping power to adjacent cells – Allow multiple simultaneous conversations in cell Generally 10 to 50 frequencies assigned per cell CSCI 465 Data Communications & Networks 5

Frequency Reuse Patterns Pattern of N cells K frequencies Cell has K/N freqs. AMPS – Advance Mobile Phone System K = 395, N = 7 < 57 freqs/cell 6

Increasing Cellular Capacity Add new channels – Add more frequencies to the system Frequency borrowing – Move frequencies into the congested cells from less used adjacent cells – Dynamically assign frequencies Cell splitting – Non-uniform topography and traffic distribution – Use smaller cells in high use areas less power but more handoffs CSCI 465 Data Communications & Networks 7

Operation of Cellular System CSCI 465 Data Communications & Networks 8

Cellular System Channels Two types of channels between mobile unit and base station (BS) Traffic channels – Carry voice and data Control channels – Set up and maintain calls – Establish relationship between mobile unit and nearest base station (BS) CSCI 465 Data Communications & Networks 9

Typical Scenario Mobile unit initialization (power on) – Scan and select strongest control channel Repeated periodically to account for mobility – Handshake between unit and MTSO via BS Identify user and register location Mobile-originated call – Unit checks if preselected channel is free – Unit sends number on channel – BS sends request to MTSO CSCI 465 Data Communications & Networks 10

Typical Scenario (2) Paging – MTSO attempts to complete connection by sending paging message to certain BSs – BS sends paging signal on setup channel Call accepted – Called unit recognizes its number – Responds to BS – passed on to MTSO – MTSO sets up circuit by assigning channels CSCI 465 Data Communications & Networks 11

Typical Scenario (3) Ongoing call – Two mobile units exchange voice or data by going through BS and the MTSO Handoff – If mobile unit moves out of range of one cell, finds new BS and establishes channel for it, switching call to new BS Hangup – Either unit hanging up sends signal to MTSO which instructs BS channels to be released CSCI 465 Data Communications & Networks 12

First Generation: Analog Original cellular telephone networks – Analog traffic channels Advanced Mobile Phone Service (AMPS) – Early 1980s in North America – Also common in South America, Australia, China Replaced by later generation systems – Still in use in some areas CSCI 465 Data Communications & Networks 13

AMPS Parameters CSCI 465 Data Communications & Networks 14

AMPS Operation AMPS-capable phone has read-only memory with numeric assignment module (NAM) – NAM contains Telephone number of phone – assigned by provider Serial number of phone – assigned by manufacturer – When power on, transmit both to MTSO MTSO has database of stolen units - blocked MTSO uses phone number for billing CSCI 465 Data Communications & Networks 15

AMPS Call Sequence Subscriber initiates call keying in number MTSO validates number and checks if user authorized to call MTSO issues message indicating traffic channels to use MTSO sends ringing signal to called party When answered, MTSO – establishes circuit – initiates billing information When one party hangs up, MTSO – releases circuit, – frees radio channels, and – completes billing information CSCI 465 Data Communications & Networks 16

AMPS Control Channels 21 full-duplex 30-kHz control channels – Transmit digital data using FSK – Data transmitted in frames Control information can be transmitted over voice channel during conversation – Mobile unit or base station inserts burst of data Turn off voice FM transmission for about 100 ms Replace it with an FSK-encoded message – Used to exchange urgent messages Change power level Handoff to another base station CSCI 465 Data Communications & Networks 17

2 nd Generation: GSM Global System for Mobile Communication First appeared in 1991 in Europe Similar to working of AMPS Designed to support phone, data, and image Rates up to 9.6 kbps GSM transmission is encrypted using secret keys CSCI 465 Data Communications & Networks 18

GSM SIM Subscriber Identity Module Smart card or plug-in module to activate unit stores – subscriber’s identification number – networks subscriber is authorized to use – encryption keys Can use any unit anywhere with your SIM CSCI 465 Data Communications & Networks 19

Second Generation: Digital Provide higher quality signals, higher data rates for support of digital services, with overall greater capacity Key differences – Digital traffic channels – data or digitized voice – Encryption – easy to encrypt digital traffic – Error detection and correction – Channel access – channels are shared in CDMA Time division multiple access (TDMA) Code division multiple access (CDMA) CSCI 465 Data Communications & Networks 20

CDMA Each cell allocated frequency bandwidth which is split in two – Half for reverse, half for forward – Uses direct-sequence spread spectrum (DS-SS) Each bit encoded using pre-assigned code which represents it using several bits Orthogonal codes permit multiple access CSCI 465 Data Communications & Networks 21

CDMA Advantages Frequency diversity – Spread over larger bandwidth – Noise bursts and fading have less effect Multipath resistance – Signal delay and echo does not interfere Privacy – Unique coding implies privacy Graceful degradation – More users means more noise and more errors – Leads to slow signal degradation until unacceptable CSCI 465 Data Communications & Networks 22

CDMA Disadvantages Self-jamming – Some cross correlation between users – Arriving signals not perfectly aligned Near-far problems – Signals closer to receiver have less attenuation – Transmissions from remote units more likely to fail CSCI 465 Data Communications & Networks 23

Third Generation: 3G high-speed wireless communications to support multimedia, data, and video in addition to voice 3G capabilities: voice quality comparable to PSTN 144 kbps available to users over large areas 384 kbps available to pedestrians over small areas support for Mbps for office use symmetrical and asymmetrical data rates packet-switched and circuit-switched services adaptive interface to Internet more efficient use of available spectrum support for variety of mobile equipment allow introduction of new services and technologies CSCI 465 Data Communications & Networks 24

3-G Driving Forces trend toward universal personal telecommunications access – GSM (Global System for Mobile communications) telephony with subscriber identity module – personal communications services (PCSs) – personal communication networks (PCNs) technology is digital – time division multiple access (TDMA) or – code-division multiple access (CDMA) PCS handsets low power, small, and lightweight CSCI 465 Data Communications & Networks 25

Fourth Generation: 4G Rapid increase in data traffic on wireless – More terminals can access Internet – Permanent connections to – Multimedia services – Support for real-time services Instant messaging Two standards – LTE – Long Term Evolution – WiMax 4G – IEEE committee standard CSCI 465 Data Communications & Networks 26

4G - OFDM Both standards based on OFDM – Orthogonal Frequency Division Multiplexing Uses multiple carrier signals at different frequencies – Sends some bits on each channel – All subcarriers dedicated to single data source Extends symbol period on each signal – Intersymbol interference higher at higher bit rates – Each subcarrier carries lower bit rate CSCI 465 Data Communications & Networks 27