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GPRS - General
Packet Radio Service
General Packet Radio Service (GPRS) is
a mobile data service available to users of GSM
mobile phones. It is often described as "2.5G",
that is, a technology between the second (2G)
and third (3G) generations of mobile telephony.
It provides moderate speed data transfer, by
using unused TDMA channels in the GSM network.
Originally there was some thought to extend GPRS
to cover other standards, but instead those
networks are being converted to use the GSM
standard, so that is the only kind of network
where GPRS is in use. GPRS is integrated into
GSM standards releases starting with Release 97
and onwards, first it was standardised by ETSI
but now that effort has been handed onto the
3GPP.
GPRS service
GPRS is different from the older
Circuit Switched Data (or CSD) connection
included in GSM standards releases before
Release 97 (from 1997, the year the standard was
feature frozen) using a GSM phone, in that in
the older system, a data connection establishes
a circuit, and reserves the full bandwidth of
that circuit during the lifetime of the
connection. GPRS is packet-switched which means
that multiple users share the same transmission
channel, only transmitting when they have data
to send. This means that the total available
bandwidth can be immediately dedicated to those
users who are actually sending at any given
moment, providing higher utilisation where users
only send or receive data intermittently. Web
browsing, receiving e-mails as they arrive and
instant messaging are examples of uses that
require intermittent data transfers, which
benefit from sharing the available bandwidth.
Usually, GPRS data is billed per kilobytes of
information transceived while circuit-switched
data connections are billed per second. The
latter is to reflect the fact that even during
times when no data is being transferred, the
bandwidth is unavailable to other potential
users.
GPRS originally supported (in theory) IP, PPP
and X.25 connections. The latter has been
typically used for applications like wireless
payment terminals although it has been removed
as a requirement from the standard. X.25 can
still be supported over PPP, or even over IP,
but doing this requires either a router to do
encapsulation or intelligence built into the end
terminal.
GPRS speeds
and profiles
Packet-switched data under GPRS is
achieved by allocating unused cell bandwidth to
transmit data. As dedicated voice (or data)
channels are setup by phones, the bandwidth
available for packet switched data shrinks. A
consequence of this is that packet switched data
has a poor bit rate in busy cells. The
theoretical limit for packet switched data is
approx. 170 kbit/s. A realistic bit rate is
30-70 kbit/s. A change to the radio part of GPRS
called EDGE allows higher bit rates of between
20 and 200 kbit/s. The maximum data rates are
achieved only by allocation of more than one
time slot in the TDMA frame. Also, the higher
the data rate, the lower the error correction
capability. Generally, the connection speed
drops logarithmically with distance from the
base station. This is not an issue in heavily
populated areas with high cell density, but may
become an issue in sparsely populated/rural
areas.
GPRS class 8 is also known as 4+1. This means
that 4 slots are allocated to downloading and 1
slot to uploading. This profile is appropriate
for applications where data is mostly
downloaded, such as web browsing. If the user
reads more e-mail than he or she sends, this is
also an appropriate profile. Class 8 is usually
selected by default on mobile devices that
support GPRS.
GPRS class 10 is also known as 4+2. This means
that 4 slots are allocated to downloading and 2
slots to uploading, but no more than 5 slots may
be used at the same time. This profile is
appropriate for applications where data is sent
back-and-forth in roughly equal amount, such as
instant messaging.
Other classes exists, including GPRS class 6
(3+2) and GPRS class 4 (3+1) used in older
devices. Some rare devices can do as much as 4+4
(up to 4 slots in both upload and upload, but
maximum 5 total). Those are for industrial use
only, though, as more than 2 upload slots are
considered a health hazard for nearby user.
The transfer speed depends also on the channel
encoding used. The best encoding scheme (CS-4)
is available near the Base Transceiver Station
(BTS) while the worst encoding scheme (CS-1) is
used when the Mobile Station (MS) is further
away from the BTS.
Using the CS-4 it is possible to achieve a speed
of 21.4 kbit/s per time slot. However by using
this scheme the cell coverage is 25% from the
normal. CS-1 can achieve a speed of 9.05 kbit/s
per time slot and has 98% of the normal
coverage.
Each slot can reach a maximum of 14.4 kilobit
per second.
download
upload
GPRS 4+1
57.6 kbit/s 14.4
kbit/s (class 8 & 10)
GPRS 3+2
43.2 kbit/s 28.8
kbit/s (class 10)
CSD
9.6 kbit/s
9.6 kbit/s
HSCSD 28.8 kbit/s
14.4 kbit/s (2+1)
HSCSD 43.2 kbit/s
14.4 kbit/s (3+1)
Note; Like CSD, HSCSD establishes a circuit and
is usually billed per second. For an application
such as downloading, HSCSD may be preferred,
since circuit-switched data is usually given
priority over packet-switched data on a mobile
network, and there are few seconds when no data
is being transferred.
GPRS packet switched data is packet based. When
TCP/IP is used, each phone can have one (or
more?) IP addresses allocated. GPRS will store
and forward the IP packets to the phone during
cell handover (when you move from one cell to
another). A radio noise induced pause can be
interpreted by TCP as packet loss, and cause a
temporary throttling in transmission speed.
GPRS Services
GPRS upgrades GSM data services
providing:
Point-to-point (PTP) service: internetworking
with the Internet (IP protocols) and X.25
networks.
Point-to-multipoint (PT2MP) service:
point-to-multipoint multicast and
point-to-multipoint group calls.
Short Message Service (SMS): bearer for SMS.
Anonymous service: anonymous access to
predefined services.
Future enhancements: flexible to add new
functions, such as more capacity, more users,
new accesses, new protocols, new radio networks.
GPRS in
practice
Telephone operators have priced GPRS
relatively cheaply (compared to older GSM data
transfer, CSD and HSCSD) in many areas, such as
Finland. Most mobile phone operators don't offer
flat rate access to the Internet (with the
notable exception of T-Mobile in the United
States), instead basing their tariffs on data
transferred, usually rounded off per 100
kilobyte.
Typical rates vary wildly, ranging from EUR €1
per megabyte to over €20 per megabyte. In the
US, T-Mobile offers US$20 per month unlimited
GPRS. Other carriers such as AT&T Wireless also
offer flat-rate plans. Orange (UK) offers a 1
Gigabyte package for £88 a month.
GPRS Data on pre-paid packages is usually
expensive, and limited to WAP and MMS.
The maximum speed of a GPRS connection (as
offered in 2003) is the same as modem connection
in an analog wire telephone network, about 4-5
kB/s (depending on the phone used). Latency is
very high; a round-trip ping being typically
about 600-700 ms and often reaching 1 second
round trip time. GPRS is typically prioritised
lower than speech, and thus the quality of
connection varies greatly.
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