AT Interface For Wireless Applications;
X.25 Interface For Hosts 

A RAM Mobile Data Whitepaper
June 1993

RAM Mobile Data
10 Woodbridge Center Drive
Woodbridge, NJ 07095 
(908) 602-5603

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In what seems to many a twinkling of an eye, location-independent
networking has moved from theory to practice. The location-independent
networking foundation is here, now. There are wireless networks connected
to wired networks, and wireless modems linking mobile computers to those
wireless networks. The fundamental paradigm is complete, and subscribers
are exchanging data messages wirelessly while sitting in taxicabs,
restaurants and lobbies.

What exists in the way of applications on the wireless network now is
literally the tip of an iceberg. Many software vendors are working
feverishly to make their communications-oriented applications
"wireless"-ready. Ericssson's announcement of its MOBiDEM AT modem will
make that job much easier, and speed up the application development
process. Research In Motion's RIMGATE technology will make it easier to
connect host systems and services to the wireless network. The combination
of these two technologies, plus RAM Mobile Data's incorporation of them in
its wireless network, represents a breakthrough for location-independent
networking, in general, and wireless networking, in particular.

Dealing with differences

Wireless networks are different from dial-up networks. The former use
packet-switched radio media; the latter use physically-switched wire
connections. Many applications exist already that take advantage of
dial-up networks and wired modems. The defacto standard interface for such
modems is the AT Command interface. These dial-up network applications
typically incorporate that AT interface

Before the advent of MOBiDEM AT, these existing dial-up-oriented
applications could not be used for wireless networking. Nearly all radio
modems had non-AT-like interfaces, and the one that did use AT Commands
lacked a radio-efficient transport protocol and data compression
capability. As a result, applications developers were forced to address
these wireless/dial-up differences in modifying existing applications for
use with wireless networking, or for writing original applications
targeted at the new networking environment. Developers had to gain
expertise both in applications and communications. Naturally, the
development process has been slowed somewhat by these realities.

The MOBiDEM AT

What Ericsson has done is created a wireless modem that works much like its
wired-modem counterparts. It responds to AT commands, and allows most
existing AT-compatible applications to work with the wireless network just
as they do with the dial-up network. The MOBiDEM AT communicates with
other MOBiDEM AT-equipped systems. It does not communicate directly with
wired AT-type modems.

Unlike an earlier AT radio modem attempt, Ericsson made sure its MOBiDEM AT
had all packet and radio protocols built in. Hence, the differences are
successfully hidden from developers, and they can approach development
along familiar AT-oriented applications lines.

A definitive model for open systems interface is the International
Standards Organizations' Open Systems Interconnect (OSI) model, which
breaks up the networking components into seven exclusive layers. The
MOBiDEM AT maps into five of those seven layers, leaving only the
presentation and application layers to be handled in software.
Functionally, the MOBiDEM AT contains a Mobitex packet generator (MPAK),
the Mobile Transport Protocol (MTP/1), the Mobile Compression Protocol
(MCP) and a packet assembler/disassembler or PAD. The MTP/1 transport
protocol is an open specification accepted by the Mobitex Operators
Association specifically to address reliable mobile transport needs. Thus,
within its small enclosure, the MOBiDEM AT has the capability of
generating Mobitex packets, seeing to their reliable radio transport,
compressing and decompressing the data for efficient wireless utilization,
and preparing the packets for transmission or reception.

The MOBiDEM AT is easily attached to virtually any computer that supports a
serial interface. It will work with all platforms including Sharp's
Wizard, DOS PCs, Windows PCs, Apple's PowerBook, Sun's SPARC notebook, the
HP 95 LX and the new HP 100LX palm tops. The MOBiDEM AT is a breakthrough
for applications developers, and through them, for wireless networking
subscribers.

RIMGATE technology

The subscriber side is only one side of the location-independent networking
equation, albeit an important one. The network must not only be able to
connect mobile computer users with one another, but must be able to
connect them to their company host systems or LANs, and to other
host-based public network services. The X.25 wide-area network standard is
a natural choice for connecting wireless network to hosts and public
services. Research In Motion has developed the RIMGATE gateway technology
to link Mobitex wireless networks with X.25. RIMGATE communicates with the
MOBiDEM AT modem on one side, and with X.25 on the other. It also
optionally provides the required compression/decompression functions.

Another RIMGATE feature is its scripting capabilities. A single wireless
packet can invoke a script in RIMGATE that will complete a lengthy log-on
procedure, for example. As a result, wireless network traffic is minimized
even as it triggers sophisticated scripts.

RAM completes the picture

With subscriber and host sides covered, all that remained was the wireless
network itself. RAM Mobile Data is already set up for MOBiDEM AT. Users
with MOBiDEM AT-equipped computers can communicate with one another using
familiar applications operating in familiar ways.

RAM is also incorporating RIMGATE technology in its network. This will
immediately provide MOBiDEM AT to X.25 connectivity, and down the road,
will provide RAM subscribers with off-the-shelf access to X.25 public
network services.

Looking Further

There is no question that MOBiDEM AT will unleash a burst of wireless
application development. Lotus and AT&T have already jumped on the
bandwagon with cc:Mail Remote and Easy Link, respectively. Other
developers who were weighing the advantages against the investment in
developing wireless applications, now see a much easier and lower-cost
pathway, and are committed to bringing wireless applications to market
very soon.

There are, however, some applications that may not be best suited to the AT
interface. The AT interface uses a serial data port and supports only a
single session (connection) per driver. Some applications may need to
support multiple sessions per driver. RAM and Ericsson have a developers
program aimed specifically at such applications and their developers. APIs
are in the offing that will use the same MTP/1 transport services but
support multiple access methods. For their part, Ericsson is making
available discount-priced radio modems, and RAM is offering reduced-rate
network air time for developers. RAM is also offering expert
communications-system consultants to help expert applications developers
meld their applications with location-independent networking.

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