Silicon Valley Computer SHUGART 706 User Manual download pdf (Page 49)

MOBILE CELLULAR

/ \

825 835

845 851

FIXED

CELLULAR

(CELL

SITESI

890 896

870

880

NONWIRELINE

(AI

SERVICE

WIRELINE

(BI

SERVICE

RESERVED

FOR

EXPANSION

ALL FREQUENCIES

IN MHz

FIG. 3 -THE 666 -CHANNEL CELLULAR

PHONE

RF spectrum

divided

into

bands for

fixed

and

mobile

equipment

(cell

sites

and phones), and for non -wireline (A) and wireline (B) service.

also reduces the risk of interfering

with

communications in a nearby cell.

NAM's

Every cellular phone contains an

"identification"

PROM or EPROM. In

cellular

terminology,

this is

called a

NAM

(Numeric Assignment Module). A

phone's NAM is programmed

at the time

the phone is purchased; it contains such

information

as:

The telephone number, or ESN (Elec-

tronic

Service Number), assigned to the

phone.

The serial number given to the phone at

the time of its manufacture.

Personal codes that

can be used to lock

and unlock the phone electronically, to

prevent its

unauthorized use.

NAM information is

more useful than

you might

at first imagine. For one thing,

is the job of the NAM to identify the

phone containing it to the cellular systems

it uses.

When

a cellular

phone is turned

on, it makes

an announcement over a con-

trol channel that says, "Here I am." The

cell site responds, "And exactly

who

are

you ?" The reply from the phone consists

of information

contained in its NAM.

That information tells the cellular sys-

tem several things. First, of course, is that

that

particular

phone is now on

the

air

and

ready to receive calls placed to its

number.

The cell site is connected to a

computer at the MTSO (Mobile Tele-

phone

Switching

Office), which is the link

between the cellular system and the con-

ventional

landline phone system, and

which

recognizes all the cellular phones

registered in the calling area it is responsi-

ble for. If the phone is a local one, the

process is more or less

complete at the

point of recognition.

Because they are mobile, cellular

phones may frequently be used outside of

the area in

which

they are permanently

registered. That is called

roaming,

and is

one of the outstanding features of cellular

telephony. You can take your cellular

phone almost anywhere in the country

where

there is service, turn it on, and use

to call

anywhere in

the

world.

In some areas you can roam and use a

foreign

system

without

advance

notifica-

tion.

Other

cellular systems require that

you let them know ahead of

time

that you

are coming. In either case, the NAM in-

formation

transmitted

to the system al-

lows you to log on to it, and tells that

system what

to do

about

your billing.

NOT JUST FOR

CARS,

cellular phones come in

portable models, like this one from GE, that

keep

you constantly

touch.

Cellular

phones have a ROAM indicator,

which

lights

when

you have left your local

area and are

the

operating

area of an-

other system. (The phone realizes that it

has

entered

a system other

than

its own,

and lets you know that.)

The serial number contained in the

NAM, incidentally,

can

serve

second

purpose.

Should a phone be

stolen

and

reported so, it is possible for a system to

recognize that phone

when

it is next

used.

While

tracking down the phone

would

rather difficult, it is easy to cut off service

to that number

automatically, avoiding

the

possibility of your being charged for calls

you never

made.

A/B switching

When

it established the cellular phone

service,

the FCC provided for two cellular

carriers in

each

region.

One,

the wireline

service,

would

be operated

by a phone

company engaged in conventional tele-

phony, frequently the one that already

provided landline service

to the area. The

other, known as the non -wireline, service

would

be operated by a company that

was

engaged in other forms of mobile commu-

nications- perhaps paging, or private

two

-way

radio

services.

Sometimes a re-

gion of cellular service has both types of

carriers,

and

sometimes

only one, at least

when

service is inaugurated. Each service

is assigned a separate set of

frequencies.

See Fig. 3.

Regardless

where in

the country you

are, the non -wireline service is referred to

as the A service, and the

wireline

one

the B service.

Normally

you

subscribe to

only one service or the other (provided

your area offers you a choice), but you

may at times have

occasion to use the

other type -when you

are

roaming, for

example.

To provide for

that, cellular

phones

have A/B switches

to allow

you

to go from

one

type of service (band of frequencies)

to the other. Those switches are generally

not mechanical

devices, but are program-

mable

from

a phone's keypad. Some of

the switches are more flexible in their ca-

pabilities than are others, and the more

sophisticated

of them offer at least the

following

modes of operation:

A (or B) service only -the other is

locked

out.

Give priority to one type of service over

the other.

Automatic

selection

of the one active

service in

an area.

Again, it is

the

intelligence

a phone

applies to the information coming in over

its control channel that makes it possible

for it

select the appropriate A or B

setting.

When

you are roaming, the phone lets

you know

you

are outside of your normal

area of use by illuminating its ROAM

indicator.

Some

phones can apply their

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