CIS-325 Data Communications1 Dr. L. G. Williams, Instructor

CIS-325 Data Communications2 Chapter Eleven Wireless Networks

CIS-325 Data Communications3 Advanced Mobile Phone Service §First generation of cell phones §3 basic devices l mobile l base transceiver l mobile telephone switching office (MTSO) §Voice and Control Channels

CIS-325 Data Communications4 Operations §Outgoing from mobile l input phone number and press send l mobile links to base xcvr via control channel l base to MTSO to POTS l MTSO routes connection back to mobile via voice channel l mobile shifts from control to voice

CIS-325 Data Communications5 Operations §Incoming to mobile l call goes from POTS to MTSO l on control channel, MTSO searches for mobile by PAGING every active mobile l if found, MTSO rings it and establishes voice channel connection l uses xcvr with strongest signal from mobile

CIS-325 Data Communications6 Cells §In a city, there might be one MTSO §There will be many xcvrs, each in its own area or CELL §Cell is hexagonal, with dia. of 6 miles or less §See fig 11.1, page 318

CIS-325 Data Communications7 Channels §Every mobile in a cell who is talking must have its own channel §Otherwise, there will be interference §More channels = more users §Rule of thumb - one channel can support 20 users

CIS-325 Data Communications8 Spectral Allocation §In U.S., we use two 25mHz bands l one band from mobile to xcvr l one band from xcvr to mobile §Each channel uses 30 kHz §So - about 832 channels for a city §But - channels split between two companies §21 for control, 395 for voice §So - can support about 7900 users

CIS-325 Data Communications9 Frequency Reuse §Also called Spatial Allocation §Cells are small, so signals can be low power §Can use same freq. in a cell that is far away, but not one nearby l at least 7 cells must intervene §So - each cell can use about 57 channels §So - each cell can support 1140 users

CIS-325 Data Communications10 Hand-offs §Mobile phones move from cell to cell §As signal fades, mobile asks MTSO for a new base xcvr §MTSO polls all base xcvrs to see which has strongest signal with mobile §Mobile shifts to new channel coming from new base

CIS-325 Data Communications11 Roaming §Mobile has moved to a different city or subscriber area §MTSO will check with your provider, via land-line §Then will allow you access §Keeps track of billing data

CIS-325 Data Communications12 Global System for Mobile Communications (GMS) §Second-generation system §Widely used in Europe §Most of the same concepts as AMPS §System is digital, not analog §Uses fewer channels, but they are multiplexed

CIS-325 Data Communications13 Subscriber Identity Card §Plug this into any mobile and it becomes ‘yours’ §Allows very flexible use of system and components

CIS-325 Data Communications14 Mobile Services Switching Center (MSSC) §Replaces MTSO, but has same functions §Controls many Base Station Controllers §Each Controller may control one or more base transceivers §MSSC connects system to POTS

CIS-325 Data Communications15 Multiple Access §Use spectrum more efficiently by multiplexing channels §Frequency-division multiplexing §Time-division multiplexing §Space-division multiplexing l essentially frequency re-use

CIS-325 Data Communications16 Code-division Multiplexing §Move message to higher bandwidth l more room §Encode each signal l one bit may now have a 6-bit code §Combine several of these channels into same signal, then un-mix at destination §can put up to 100 signals on one channel

CIS-325 Data Communications17 Third Generation Wireless Communications §Universal Personal Communications §Identify yourself to any system, anywhere §PCS - Personal Communications Services §Global standards for terminals, cell phones §Smaller or micro-cells §Much greater bandwidth (150 mHz)

CIS-325 Data Communications18 Satellite Communications §Geosynchronous l 22,300 miles above l Simple to track and find l Needs strong signal to cover distance l Slight delay in signal due to distance

CIS-325 Data Communications19 Satellite Communications §Highly Elliptic Orbiting Satellites (HEOS) l GEO can’t reach poles very well due to oblique angles l Russian’s use HEOS l 65 degrees off equater l Elliptic orbit close at South, far/high in North l Need 3 satellites for coverage

CIS-325 Data Communications20 Satellite Communications §Low Earth Orbiting Satellites (LEOS) l Constellation of small, “inexpensive” satellites l miles high l Need >60 for 24 hour coverage l Sophisticated hand-off between satellites l Tables 11-1 and 11-2, pg. 333 lists proposed systems

CIS-325 Data Communications21 Iridium §A proposed LEOS system §Motorola §Voice, data, fax, paging §15-20 ground stations §Will not replace cellular system §Satellites at 420 nm §Phone call at $3/minute