Linux Unwired [Electronic resources] نسخه متنی

9.1 Cellular Data

There are several types of cellular data networks. The most
popular are General Packet Radio Services (GPRS) and
1x Radio
Transmission Technology (1xRTT). At the time of this writing,
slightly faster Enhanced Data rates for
GSM Evolution (EDGE) and
1x Evolution
Data Only (1xEV-DO) networks are emerging.

9.1.1 CSD

You use Circuit Switched Data (CSD) when you use
your cellular phone as a dial-up modem. When you do this, you use
your voice plan. Generally, this is not the best option: CSD calls
typically don't receive the full throughput
that's available to a data connection. However,
there is a high-speed variant called High Speed CSD (HSCSD) that can
provide you with better speeds.

Unless you need to dial into a private network using a modem, we
suggest that you use a packet-switched protocol, such as GPRS, EDGE,
1xRTT, or 1xEV-DO, to make your data connection. With these
technologies, you're not dialing a bank of modems;
rather, you're effectively using your cellular
carrier as your ISP and your phone as a network adapter.
Additionally, CSD calls are billed by the minute; with the exception
of one plan offering from Verizon Wireless (Express Network
NationalAccess) that we're aware of, packet-switched
data connections are billed by the amount of data used, rather than
the amount of time you spend online (unless you have an unlimited
plan, in which case you are paying a flat rate).

If your cellular carrier and GSM device supports it, you can make an
HSCSD at speeds up to 40 kbps. To enable this capability, you must
issue the command
AT+CBST=speed,0,1,
where speed is a value supported by your
phone (you can enumerate the supported values by issuing the
AT+CBST=? command). For example, request 14.4 kbps
with AT+CBST=14,0,1.

The isdn4linux FAQ has some information on using
HSCSD with ISDN: http://www.mhessler.de/i4lfaq/i4lfaq-6l#config_gsmv110.
The following sites have information on HSCD commands, although
support varies from device to device, and some providers do not
support HSCD at all (contact your cellular provider if you are
unsure):

http://www.gcrsoft.com/datalhttp://www.nc9210.de/9210/tipps/at_hscsdhttp://www.zelaskowski.de/pda/hscsdl

9.1.2 GPRS and EDGE

GPRS sits on top of
Global
System for Mobile communications (GSM), a cellular networking
protocol that breaks a channel into timeslots so that up to eight
users can share the same channel; at any given time, a channel is
dedicated to one user only. A channel is 200 kHz of bandwidth within
the 850, 900, 1,800, or 1,900 MHz bands. GSM is the most widespread
digital cellular technology with 970 million users at the end of
2003. It's available in the U.S., much of Asia,
Europe, and many other places.

Although a given timeslot supports a slow data rate (typically
between 9.6 and 13.4 kbps), one timeslot is sufficient for each side
of a voice conversation. GPRS phones and PC Cards combine multiple
GSM timeslots (up to eight in theory, but the equipment
we've seen maxes out at four) and typically support
a downstream data rate of 40 kbps (we have found that this translates
to a peak of 30 kbps for HTTP transfers). Upstream data rates are
typically less, as low as 9.6 kbps, but this is governed by the
number of timeslots your device supports for upstream data, as well
as by the number of timeslots your cellular carrier makes available
for this purpose.

EDGE is an improvement over GSM in that it increases the data rate
per timeslot significantly. Instead of 9.6 kbps to 13.4 kbps, EDGE
supports between 48 and 70 kbps per timeslot. However, to take
advantage of EDGE speeds, you need a handset or PC Card that supports
EDGE, such as the Nokia 6200 cell phone or the
Sony Ericsson GC82 PC Card Modem.
EDGE
devices are backward-compatible with GSM and GPRS, so if
you're in a location without EDGE coverage, you can
still connect at the slower GPRS speeds.

GSM devices require the installation of a Subscriber Identity Module (SIM).
You (or the salesperson) insert this card when you first get the
phone. If you have more than one phone, you can swap SIMs and use the
phone that is currently holding the SIM. However, most cellular
providers lock the device to their network, so you can use the phone
onlwith them. So, if you buy a phone from AT&T Wireless and
insert the SIM you bought from T-Mobile, you'll
receive an error message. However, there are many ways to remove this
lock. Some carriers will do it for you if you contact their customer
support and ask; this is usually done for customers who have been
with the carrier for a while, have an account in good standing, and
are planning to use the phone overseas (you can save money by buying
a pay-as-you-go SIM from a local cellular
provider and swapping SIMs while you are abroad). Figure 9-1 shows a Nokia 6200 that is being inserted with
an AT&T Wireless SIM card (that's the battery
next to it, which we had to remove to get at the SIM).

Figure 9-1. Inserting a SIM card into a Nokia 6200

9.1.3 1xRTT and 1xEV-DO

1xRTT is an improvement to CDMAone,
the first version of Code Division Multiple Access
(CDMA), a digital cellular protocol that supported data rates up to
14.4 kbps. 1xRTT cranks it up to 144 kbps upstream and downstream.
Instead of slicing up cellular channels by timeslots, CDMA uses
spread-spectrum technology to support multiple users in each 1.25
MH-wide CDMA channel within the 800 and 1900 MHz bands. Each user
within a given CDMA channel is associated with a code, and their
signals (tagged with the associated code) are spread across the
channel. although CDMA is not as widespread as GSM, there are still
many users (188 million at the end of 2003). It's
available in the U.S., parts of Asia, Latin America, and Europe.

1xEV-DO improves on 1xRTT by supporting
burst speeds up to 2.4 Mbps while still keeping channels only 1.25
MHz wide. At the time of this writing, Verizon Wireless has begun
1xEV-DO trials in San Diego and Washington, D.C. (priced the same as
its 1xRTT offering). Initial reports indicate that 300-500 kbps are
the likely real-world speeds.

CDMA phones do
not use SIM cards. As a result, you can't move your
account to a new phone as easily as you can with GSM phones. You must
contact your cellular provider, deactivate the old phone, and
activate the new one. (Your carrier may also allow you to do this
through its customer support web site).