Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Uživatelský manuál stáhnout pdf (Strana 75)

100

s can

exchange

up to twice the

data

in half the time

Class 5

modem

Herb Friedman

COMPUTERS

[JTIME!...PAR FICULARLY

WHERE

concerns

personal computing,

log-

arithmic.

Each new item

of hardware

and

software takes

less time to develop,

debug, and

get into common

use than

its predecessors.

Particularly

where it

concerns

modernized communications,

there

is little time

lost between

the

thought

and the application.

I must

have plugged along

for some

15 years

with a modem that

ran at 110

baud,

which was

fast as anyone

needed

because most of us used

a TTY

(teletypewriter)

for mainframe and per-

sonal computer

I /O; and 110

words -per-

minute,

which

what 110 baud

works

out to be,

was

fast as the TTY could

operate.

But then

got the electronic

termi-

nals

and TTYs that

could

operate

at 300

wpm;

and it

seems

that almost over-

night most

computerists

were

convert-

ing to 300 -baud modems so that they

could apply the full potential

the new

TTYs

and

on -line matrix printing.

don't think 1 used

a 300 -baud

modem for more

than a couple of years

before

the

-line information services

were

pushing 1200 -baud operation, be-

cause personal

computers could

store

information for later

transmission and

retrieval; it was no longer

necessary to

print in step with

the data- transmission

rate.

Not

Fast

Enough

But

even 1200 baud wasn't

fast

enough for business

use, and

about

year or so ago 2400 baud really caught

fire

as the preferred modem

speed

for

asymmetrical communications on con-

ventional

subscriber

telephone lines -

what is

called the switched network.

With

telephone costs

skyrocketing

because of

what

is euphemistically

termed

"telephone deregulation,"

heavy users of modem communications

pushed the capability of the con-

ventional

dial -up telephone system

its limit. But just

as a

marathon

runner

has "the wall"

which

endurance runs

Class

modems

don't

look much

different

than the usual

full-

feature

modem. You

almost

have

read

the

fine

discover

that model 224E from MultiTech

Systems

features

error

correction, speed correction

for fixed

-speed computer

ports.

and

data compression.

out, so,

too,

does the switched network

have

wall.

For conventional two -wire subscrib-

er lines, it's 2400 baud; which

for all

practical purposes means

a maximum

of 2400

bps

(bits

per second). (While

there has been

some success on the

switched network with

higher baud

rates, it

usually means half -duplex oper-

ation, and /or synchronous modems -

which

tend to

get very

expensive, and

are not all that reliable. The higher

baud -rate modems

usually have auto-

matic fallback

to a

slower

baud

rate

when

the going gets rough.)

Now one

of the facts of life is that if

you

can't blow up a

wall,

you can only

get past it by going over,

under, or

around the sides. In

the case of the 2400

baud "wall," depending how you look

it, we

either tunnel under,

or get a

boost and go over in order

to exceed the

2400 bps (bit

per

second) limitation.

The

way

it's done is

with

something

called the

Class 5

Protocol, which is

based on Microcom, Inc.'s

Microcom

Network Protocol

(almost

always short-

ened to

simply

MNP).

Software

to Hardware

l nlike

a conventional

modem that

depends on

the communications soft-

ware

to provide

error correction, a Class

modem

contains both error -correction

and

data -compression firmware (mean-

ing built -in software). You

probably all

understand error correction, whereby

the originate

and answer computers or

modems software -handshake each

block of data. If the handshakes don't

match, the originating station keeps re-

peating the block until it receives a

handshake that indicates data has

been

received properly.

Data compression

is something

dif-

ferent. Basically.

the originating

modem

strips out unneeded

bits of data.

It samples

repeated characters,

and

when

three

more appear

in succession,

it compresses

the three characters

into

enough data so that

the three can

(Continued

on page

95)

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