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Hack 8 CDPD, 1xRTT, and GPRS: Cellular Data Networks

If you can't roll your own
wireless, you might try one of these mobile phone carrier
networks.

When it comes to data rates, most
people are in agreement that faster is better. But current
communications technology always involves a trade-off between speed,
power, and range. 54 Mbps may be great if you can get it, but on a
large scale, this can be difficult to maintain. The 802.11 protocols
compensate for increased range by scaling back the data rate, but
these devices simply aren't designed to serve
hundreds of people scattered over many miles.

There are times when any data to
the Internet is better than none at all, no matter how slow it might
be. For example, you might need to log in to a remote machine or send
a quick email while traveling, when Wi-Fi or even wired network
access just isn't available. Or maybe you want to
have an alternate communications channel into a wireless node in a
remote place (say, on a mountaintop or deep in the woods) where
telephone lines aren't even available. For these
situations, you might consider exploiting the biggest advantage of
the commercial mobile data networks: their ubiquity.

Mobile networks maybe be slow and relatively expensive, but you
can't beat their coverage compared to current
Wi-Fi networks. They can give you an IP
address just about anywhere, but be warned that most mobile data
services are not cheap. Most charge by the byte, and all charge for
airtime while you are using it.

The type of data service you can use depends on the underlying
wireless technology. Obviously, before choosing a technology,
determine the coverage area of the mobile network in the place you
intend to use it. The three leading mobile data services are
described next, in decreasing order of availability in the U.S.

CDPD on TDMA

CDPD
stands for Cellular Digital
Packet Data. It works over the enormously popular
Time Division Multiple Access (TDMA) mobile
network, which is easily the most widely deployed mobile network in
the U.S. CDPD
"modems"
typically use a serial port or PCMCIA slot and offer speeds of up to
19.2 Kbps (real world is typically closer to 9,600 bps).

It looks like TDMA operators are generally migrating to GSM, so it is
probably unlikely that TDMA data services will ever be upgraded. In
some areas, TDMA is being phased out altogether, making it difficult
to obtain a CDPD account. But despite the relatively slow
speed of CDPD, you can't
beat its coverage. Virtually all of the populated regions of the U.S.
are covered by TDMA.

1xRTT on CDMA

CDMA stands
for
Code Division Multiple Access: it is the second
most popular mobile technology in the U.S. The original CDMA data
services offered speeds of 9600 bps to 14.4 Kbps. A new upgrade
called 1xRTT boasts speeds of up to 144 Kbps, but by many reports,
real-world throughput is somewhere between 60 and 80 Kbps,
occasionally bursting to 144 Kbps if you get lucky. If you think the
802.11 protocol names aren't confusing enough, you
should really try following mobile phone technology. 1xRTT is also
known in various circles as CDMA2000 Phase 1, or simply 95-C.

1xRTT is just the first phase of the CDMA2000 plan. A few communities
are now trying the experimental 1xEV-DO technology, which can
theoretically achieve 2 Mbps from fixed locations over CDMA. This
technology hasn't yet been widely deployed. Also, we
are told to expect 1xRTT Release A by the end of 2003. This is a
software upgrade that promises 144 Kbps uploads and downloads of up
to 300 Kbps.

GPRS on GSM

GPRS stands for General
Packet Radio Service, and is the data service available on
Global System for Mobile communications (GSM)
networks. The original GSM data services offered only 9,600 bps
throughput, but GPRS allows real-world speeds of 20 to 30 kbps. GPRS
is a packet-based protocol, meaning that the GPRS radio transmits
only when it actually has data to send. This can save on battery
usage, and theoretically makes more efficient use of the network. A
number of nifty gadgets such as the HipTop by Danger (http://www.danger.com/) use GPRS for
connectivity.

Eventually, GPRS may be replaced by technologies like
Enhanced Data for Global
Evolution
(EDGEyou have to ask yourself how they
can use these acronyms with a straight face), which offers
theoretical speeds of up to 384 Kbps over GSM. EDGE is still
experimental, and hasn't yet been widely deployed.
As of this writing, GSM coverage is increasing rapidly in the U.S.
but still isn't as ubiquitous as CDMA or TDMA. Much
of the rest of the world has a more thoroughly deployed GSM network.

If you find that you need simple wireless connectivity beyond what
you can hope to provide with 802.11 technologies, commercial data
networks are a viable alternative. They don't come
cheap, but can be perfect for many low bandwidth
applications.