Chapter 11 CIS 325 data communications

Advanced Mobile Phone ServiceFirst generation of cell phones 3 basic devices mobile base transceiver mobile telephone switching office MTSO Voice and Control Channels... Outgoing from mo

Data Communications

Dr L G Williams, Instructor

Chapter Eleven

Wireless Networks

Advanced Mobile Phone Service

First generation of cell phones

3 basic devices

mobile

base transceiver

mobile telephone switching office (MTSO)

Voice and Control Channels

Outgoing from mobile

input phone number and press send

mobile links to base xcvr via control channel base to MTSO to POTS

MTSO routes connection back to mobile via voice channel

mobile shifts from control to voice

Incoming to mobile

call goes from POTS to MTSO

on control channel, MTSO searches for mobile

by PAGING every active mobile

if found, MTSO rings it and establishes voice channel connection

uses xcvr with strongest signal from mobile

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

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

Spectral Allocation

In U.S., we use two 25mHz bands

one band from mobile to xcvr

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

Frequency Reuse

Also called Spatial Allocation

Cells are small, so signals can be low powerCan use same freq in a cell that is far away, but not one nearby

at least 7 cells must intervene

So - each cell can use about 57 channels

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

Then will allow you access

Keeps track of billing data

Global System for Mobile Communications (GMS)

Second-generation system

Widely used in Europe

Most of the same concepts as AMPSSystem is digital, not analog

Uses fewer channels, but they are multiplexed

Subscriber Identity Card

Plug this into any mobile and it becomes

‘yours’

Allows very flexible use of system and components

Mobile Services Switching

Center (MSSC)

Replaces MTSO, but has same functionsControls many Base Station ControllersEach Controller may control one or more base transceivers

MSSC connects system to POTS

Code-division Multiplexing

Move message to higher bandwidth

more room

Encode each signal

one bit may now have a 6-bit code

Combine several of these channels into same signal, then un-mix at destinationcan put up to 100 signals on one channel

Third Generation Wireless

Communications

Universal Personal Communications

Identify yourself to any system, anywherePCS - Personal Communications ServicesGlobal standards for terminals, cell phonesSmaller or micro-cells

Much greater bandwidth (150 mHz)

Satellite Communications

Geosynchronous

22,300 miles above

Simple to track and find

Needs strong signal to cover distance Slight delay in signal due to distance

Satellite Communications

Highly Elliptic Orbiting Satellites (HEOS)

GEO can’t reach poles very well due to oblique angles

Russian’s use HEOS

65 degrees off equater

Elliptic orbit

• close at South, far/high in North

Satellite Communications

Low Earth Orbiting Satellites (LEOS)

Constellation of small, “inexpensive” satellites 200-700 miles high

Need >60 for 24 hour coverage

Sophisticated hand-off between satellites

Tables 11-1 and 11-2, pg 333 lists proposed systems