ANOTHER PART OF YOUR LIFE IS ABOUT TO
go digital. This time it's data communica-
tion�transferring files, connecting to re-
mote networks, and other "talking" a
computer does on the phone with fellow
computers. By going digital, you don't
need to use error-prone analog modems;
instead you get nearly error-free data
communication at speeds modems can
only dream of. Telephone companies in
the United States, Canada, Japan, and
Western Europe offer digital services that
cost about the same to use per call as high-
speed modems. These services are called
switched digital services because they take
advantage of switched phone lines to keep
costs down. Placing a call switches on a
connection to a phone line, and ending a
call switches off the connection, freeing the
line for another call. You pay only for calls
you make, not for full-time use of a dedi-
cated phone line.
MacWorld started using switched digi-
tal services last summer to transmit
scanned-image files and Aldus PageMaker
files of magazine pages to and from Digi-
tal Pre-Press International (DPI), a San
Francisco prepress service bureau. The
files, which range in size from several
hundred kilobytes to over 100 megabytes,
used to be copied onto disk cartridges and
carried to DPI by bike messenger. In ad-
dition, MacWorld's San Francisco office
keeps stories and artwork on a file server
that sister magazines in other countries can
quickly access using switched digital ser-
vices. Previewing and copying those files
by modem takes forever, and transfers
too often fail due to noisy phone lines.

What You Get

U.S. TELEPHONE COMPANIES OFFER
two types of switched digital services,
switched 56-kbps service and ISDN (In-
tegrated Services Digital Network). With
switched 56 service you get a single
phone line that can transfer digital data
at speeds up to 56,000 bits per second.
You can't use the line to make voice calls
or send Group 3 faxes.

ISDN service gives you more than
one communication channel on a single
line. You get two or more 64,000-bps
channels for digitized voice, digital data,
or Group 4 (high-speed digital) fax calls.
These are called B (Bearer) channels.
You can use B channels independently or
combine the channels for higher data-
transmission rates. For example, you
might talk on one B channel while your
computer communicates on another, or
you might combine two B channels for
video teleconferencing.

The B channels only carry content.
Another channel, called the D (Delta)
channel, carries signaling information
that makes and controls B-channel con-
nections. The D channel operates at
16,000 bps or 64,000 bps depending on
the type of ISDN service. Signaling
doesn't keep the D channel busy, so it can
also be used for data transfer at rates
equivalent to those of a 9600-bps modem
or for X.2 5 packet switching.

In addition to data-transfer capabili-
ties, ISDN can improve the sound qual-
ity of voice calls. ISDN also offers fea-
tures such as the ability to display a
caller's phone number before you answer
a call, add that number to a list of unan-
swered calls, or use it to look up infor-
mation about the caller in your database.
(Access to the caller's number may be re-
stricted or blocked in some states.) Several
newly announced applications can use
ISDN voice features, including Northern
Telecom's Meridien TeleCenter, and
Cvpress Research's PhonePro.

Where It Goes

ALTHROGH SWITCHED 56 AND ISDN
are switched services, you can't use them
to call just anywhere. You can call other
switched digital lines served by the same
central office. Calling beyond your local
telephone company's central office be-
comes more complicated, especially for
ISDN. The transmission rate drops to
56,000 bps because that's the maximum
for the trunk lines that carry calls outside
a central office. Likewise, calls outside
your service area (as depicted in your
telephone book), state, or country run at
56,000 bps.

For example, MacWorld and DPI are
served by the same central office in San
Francisco and can transfer files at 64,000
bps over an ISDN line. But when Mac-
World Japan places an ISDN call to Mac-
World in San Francisco, the two commu-
nicate at 56,000 bps. Still, that transfer
speed is many times faster than a high-
speed modem.

Where It Is

YOU MAY HAVE TROUBLE OBTAINING
ISDN or switched 56 service from your
local phone company. Neither service is
available everywhere. Right now ISDN
service is concentrated in the business
districts of major metropolitan areas (see
"ISDN Lines in the U.S."). Switched 56
service is more widely available but is not
universal. Even in areas where switched
digital services are available, phone com-
pany representatives may not know
ISDN and switched 56 exist, or they may
not know them by those generic names.
Each phone company has its own names
for these services. Pacific Bell, for ex-
ample, has an ISDN service named
Centrex IS and a switched 56 service
named CenPath. Fortunately this picture
should improve during the next year as
telephone companies install more ISDN
and switched 56 services and promote
them more aggressively.

Like names, charges for switched
digital services vary among phone com-
panies. Your local phone company may
compound the cost of getting ISDN or
switched 56 by requiring that you also
have an unrelated service such as
Centrex, which gives business customers
PBX capabilities from the phone-com-
pany central office. Pacific Bell charges
$540 to $650 to establish Centrex service
and install two Centrex lines (its mini-
mum) with a switched digital service on
one of them. In addition to paying the
phone company's installation costs, you
must buy special equipment to attach to
the phone lines (the type of equipment
varies depending on the type of service
the phone company offers). Pacific Bell
charges $30 to $40 monthly per switched
digital line and $15 per conventional
voice Centrex line. You also pay connect
charges at standard voice-call rates.
Lower-cost switched digital services are
available in some parts of the United
States and should become more wide-
spread this year.

If vou plan to make ISDN or switched
56 calls outside your state or service area,
vou must choose a long-distance company
and pay it for those calls. Switched digi-
tal calls cost the same as ordinary voice
calls on AT&T, MCI, and Sprint.

To get ISDN at your building, you
must be located within about 2 miles of
a phone company central office that has
switching equipment for routing ISDN
calls. This distance will in-
crease to about 3.5 miles as
telephone companies install
new switching equipment
over the next few years. For
switched 56 service vou must
be within about 3.5 miles of a
central office with switched
56 switching equipment. The
geographical areas served by
central offices vary widely;
they are roughly the size of�
but by no means coincide
with � zip code areas. If you
are farther away than the
maximum distance, the phone
company must run a high-
speed dedicated line from a
switched digital central office
to your phone company cen-
tral office or directly to your
building. You pay a monthly
fee for full-time use of that
line and also pay by the call
for calls that must be routed
beyond the central office. A
dedicated line is generally not
economical unless you install at least six
ISDN or switched 56 lines on it.

Inside Your Building

NEITHER BASIC-RATE ISDN (TWO B
channels and one D channel) nor switched
56 service requires special wiring inside
your building. Basic-rate ISDN service,
like ordinary voice service, uses two of the
four wires routed to common RJ- 11
modular jacks. Switched 56 service re-
quires either two wires or four, depending
on the specific type of service your phone
company offers. You can usually replace
an existing voice line with basic-rate
ISDN or switched 56 service without any
wiring change inside the building.

To use an ISDN line, you must install
a terminator, called an NTl, to maintain
signal quality on the phone-company
lines. The NTl terminator usually con-
nects to your RJ-l 1 wall jack. It costs $100
to $300, and you must buy one that has
been made for the switching equipment
installed in your phone company's central
office. As the switching equipment mi-
grates to new standards over the next few
years, you should have a wider choice of
vendors. Switched 56 service does not
need a terminator.

Connecting to a Mac

ALTHOUGH ISDN AND SWITCHED
56 services transfer data digitally, you
can't connect them directly to a com-
puter. You don't need an ordinary mo-
dem to convert digital data from the
computer to analog data (sounds) or the
reverse. Instead you need a device that
prepares the digital data from a com-
puter for transmission on digital phone
lines. Think of it as a digital modem.

The ISDN digital modem is called a
terminal adapter (TA). You can buy a
freestanding TA from Northern Tele-
com, Fujitsu, Hayes, UDS/Motorola,
and other companies for $750 to $2000.
You can also install a TA inside any
Macintosh with a spare NuBus slot.
Apple should already be shipping its
$1099 ISDN NB Card. Apple's NuBus
board and some of the freestanding TAs
convert a second B channel to let you
plug in a standard telephone, answering
machine, or Group 3 fax machine. Alter-
natively, you can buy an ISDN telephone
with an integral TA and data port for
$500 to $1000 from AT&T or Fujitsu,
or for $700 to $1100 from Telrad.

Digital modems for switched 56 go
under two names, depending on the type
of switched 56 service your phone com-
pany offers. One is called a DSU/CSU
(Digital Service Unit/Channel Service
Unit) and the other is called a Data Unit.
AT&T, Northern Telecom, UDS/
Motorola, and others sell them. Some
ISDN terminal adapters can also be used
as switched 56 digital modems.

To further complicate the situation,
digital modems are not all compatible
with each other, nor is every digital mo-
dem compatible with all types of switch-
ing equipment used in phone-company
central offices. You may be unable to call
someone else who has switched digital
service because your digital modems are
incompatible. And if you're not careful,
you could buy a digital modem that
won't work with the switching equip-
ment at your phone-company central of-
fice or with the switching equipment at
your new central office after you move
across town.

Using a Digital Modem

DIGITAL MODELS WITH SWITCHED
digital services can speed up file transfer,
screen sharing, and remote network ac-
cess. For simple file transfer between two
Macs, you can use Microphone II,
WhiteKnight, VersaTerm Pro, or an-
other terminal program. To remotely
operate (and exchange files with) another
Macintosh, you can use screen-sharing
software like Timbuktu/Remote. Prod-
ucts like Farallon's PhoneNet Liaison
and Shiva's TeleBridge let you access
E-mail, file servers, and other network
services from a remote Mac.

When used with a freestanding digi-
tal modem connected to the Mac's mo-
dem or printer port, none of these appli-
cations can take full advantage of
switched digital speed. The Mac serial
ports can run at 57,600 bps asynchro-
nously, but today's system software, which
handles serial communications for appli-
cation programs, can't sustain that rate.
The serial port can usually sustain a
throughput rate of 19,200 bps�the exact
speed depends on factors such as the speed
of your Mac, the communications proto-
col, and the design of the application
program.

Some programs can achieve full
switched digital speed if they are used
with an ISDN NuBus board. These pro-
grams use the Communications Toolbox
part of system software to transfer data
synchronously at 56,000 bps or 64,000 bps
through the ISDN board. At this writing,
VersaTerm Pro and Timbuktu/Remote
are compatible with the Communications
Toolbox, and compatible versions of the
other products mentioned above are in
progress. (The Communications Toolbox
is built into System 7 and can be added as
an INIT to Systems 6.0.4 through 6.0.8.)

On Your Network

WITH SWITCHED DIGITAL SER-
vices you can go beyond connecting 
Mac to a distant network. You can
interconnect multiple AppleTalk
networks and share network ser-
vices. For instance, MacWorld af-
filiates in different countries can
link networks to access each
other's file servers and exchange
E-mail by placing ISDN phone
calls. After linking networks, a
MacWorld editor uses Timbuktu (Remote Acess Software)to
preview large illustrations (by
opening them on the distant
network's file server) and to trans-
fer files. Timbuktu or a new prod-
uct, Aspects, let several users work
on the same document simulta-
neously from separate Macs.

You can also access a distant
network with the Chooser and
connect to file servers, items
shared with System 7's file shar-
ing, and printers. If you use the
Chooser to mount a distant file
server or shared item on your
desktop, you can copy files to or
from the server with the Finder.
With applications that take advan-
tage of System 7's publish and
subscribe technology, you can in-
clude live copies of material from
a distant file server or shared disk.
If someone changes the original
material, your documents are au-
tomaticaliy updated across the
switched digital network link. Ap-
plications that use System 7's pro-
gram-linking capability to share
services, such as MacProject II 2.5
and Claris Resolve, could be lo-
cated on remote networks.

To set up a switched digital
link between AppleTalk networks,
you connect each network to a
router, each router to a modem,
and each digital modem to a
switched digital phone line. The
router examines packets of data traveling
on the local network and routes packets
headed for the distant network across the
phone line.

Some routers, such as Farallon's

- $395 PhoneNet Liaison, are software
that run on any Macintosh connected to
the network. Hardware routers connect
independently to the LocalTalk cabling.
They include Engage Communication's
$1895 SyncRouter, Shiva's $599 Tele-
Bridge, Solana Electronics' $2195
H-Server, and International Transware's
$899 TransTalk. Although the Phone-
Net Liaison and the TeleBridge routers
cost less, they communicate asynchro-
nously at only 19,200 bps to 38,400 bps.
The other routers communicate syn-
chronouslv at the switched digital
service's full 64,000 bps or 56,000 bps.

Routers made by different companies
are incompatible, and more compatibil-
ity problems exist between routers, digi-
tal modems, and phone-company central-
office switching equipment. In setting
up its international file server, for ex-
ample, MacWorld first tried H-Servers,
found them to be incompatible with
something in Tokyo, switched to Synch-
Routers, and is currently evaluating
TransTalk routers for its enhanced net-
work-security options. Other people in
Tokyo, New York, and Paris, however,
have used H-Servers without any trou-
ble whatsoever.

To make your network more secure,
some routers let vou restrict access bv
users on linked networks to specific zones
in your network. Without this zone-
blocking control someone at MacWorld
could accidentally print a docu-
ment on DPI's imagesetter�a
costly mistake. Some routers offer
other security provisions that help
keep hackers out of your network.

Experienced users say network
performance across a switched
digital link rivals that of a direct
connection to a LocalTalk net-
work with five to ten active users.
Apparently a private 56,000-bps or
64,000-bps connection using a
hardware router approximately
matches an individual's share of a
232,000-bps LocalTalk or Phone-
Net-type connection. Of course
performance declines when several
people actively use an ISDN or
switched 56 network connection
simultaneously. Your choice of
digital modem and router may also
give you lower performance.

Not only are switched digital
services fast, they are also ex-
tremely reliable. You can't transfer
multimegabyte files frequently us-
ing ordinary modems and voice
phone lines without failures due to
line noise. But MacWorld editors
routinely use ISDN to transfer
tens of megabytes across town and
across the Pacific without any fail-
ure. Other ISDN and switched 56
users report the same reliability�
no errors, period.

ISDN Today

SWITCHED 56 AND ISDN PROMISE
a lot and deliver it to some Macin-
tosh users, but don't hang up your
modem just yet. You'll still need it
to connect to commercial informa-
tion services, national E-mail ser-
vices, and electronic bulletin
boards. And if vou sometimes ac-
cess files and E-mail on your office
network or your office Macintosh from
home or a hotel room, you'll want a mo-
dem for that too. Switched digital ser-
vices can't help you with those connec-
tions today.

Right now those most able to take
advantage of ISDN or switched 56 ser-
vices are businesses needing to link loca-
tions and transfer lots of data regularly but
not constantly. The rest of us must anx-
iously wait while switched digital services
come to our telephone prefixes.