|
 |
|
PAPER: General Packet Radio Service (GPRS)
By Peter Rysavy, Rysavy Research, for
GSM Data Today online journal.
September 30, 1998 (Note: see home page for a more recent article on GPRS)
Wireless communications lets people live and work in ways never before
possible. With over two hundred million cellular subscribers worldwide,
users have overwhelmingly embraced the concept of having a telephone that
is always with them. And now business users also want a data connection
with the office wherever they go, so that they can have access to e-mail,
the Internet, their files, faxes and other data wherever and whenever it
is needed, giving them a competitive advantage and more flexible lifestyles.
A number of wireless data services are available today, but none are as
exciting as a forthcoming data service for GSM networks called General
Packet Radio Service (GPRS).
GPRS refers to a high-speed packet data technology, which is expected
to be deployed in the next two years. It is expected to profoundly alter
and improve the end-user experience of mobile data computing, by making
it possible and cost-effective to remain constantly connected, as well
as to send and receive data at much higher speeds than today. Its main
innovations are that it is packet based, that it will increase data transmission
speeds from the current 9.6 Kbps to over 100 Kbps, and that it will extend
the Internet connection all the way to the mobile PC -- the user will no
longer need to dial up a separate ISP. GPRS will complement rather than
replace the current data services available through today’s GSM digital
cellular networks, such as circuit-switched data and Short Message Service.
It will also provide the type of data capabilities planned for "third generation"
cellular networks, but years ahead of them.
In this paper, we take a look at
Why is GPRS Important?
The most important aspects of GPRS are that it allows data
transmission speeds to over 100 Kbps, that it is packet based, and that
it supports the world's leading Internet communications protocols, Internet
Protocol (IP) and X. 25.
The fact that GPRS will operate at much higher speeds
than current networks should provide a huge advantage from a software perspective.
Today, wireless middleware is often required to allow slow speed mobile
clients to work with fast networks for applications such as e-mail, databases,
groupware or Internet access. With GPRS, wireless middleware will often
be unnecessary, and thus it should be easier to deploy wireless solutions
than ever before.
Whereas today’s wireless applications tend to be text
oriented, the high throughput offered by GPRS will finally make multimedia
content, including graphics, voice and video practical. Imagine participating
in a video conference while waiting for your flight at the airport, something
completely out of the question with today’s data networks.
Why is packet data technology important? Because packet
provides a seamless and immediate connection from a mobile PC to the Internet
or corporate intranet allowing all existing Internet applications such
as e-mail and Web browsing to operate smoothly without even needing to
dial into an Internet service provider. The advantage of a packet-based
approach is that GPRS only uses the medium, in this case the precious radio
link, for the duration of time that data is being sent or received. This
means that multiple users can share the same radio channel very efficiently.
In contrast, with current circuit-switched connections, users have dedicated
connections during their entire call, whether or not they are sending data.
Many applications have idle periods during a session. With packet data,
users will only pay for the amount of data they actually communicate, and
not the idle time. In fact, with GPRS, users could be "virtually" connected
for hours at a time and only incur modest connect charges. For detailed
information about how GPRS works, see "For Network Managers" below.
While packet-based communications works well with all
types of communications applications, it is especially well suited for
frequent transmission of small amounts of data, what some call short and
bursty, such as "real time" e-mail and dispatch. But packet is equally
well suited for large batch operations, and other applications involving
large file transfers.
GPRS will support the widely used Internet Protocol (IP)
as well as the X.25 protocol. IP support is becoming increasingly important
as companies are now looking to the Internet as a way for their remote
workers to access corporate intranets. For further discussion about remote
access, see "GPRS and Remote Access" below.
The IP protocol is ubiquitous and familiar, but what is
X.25, and why is support for it important? X.25 defines a set of communications
protocols that prior to the Internet constituted the basis of the world’s
largest packet data networks. These X.25 networks are still widely used,
especially in Europe, and so wireless access to these networks will benefit
many organizations. But what does this really mean? Quite simply it means
that any existing IP or X.25 application will now be able to operate over
a GSM cellular connection. You can think of cellular networks with GPRS
service as wireless extensions of the Internet and existing X.25 networks,
as shown in figure one. |
Figure one: GPRS as an extension of other packet
networks.
|
The User Experience
We now look more closely at how the user takes advantage
of GPRS. We have already emphasized the packet nature of GPRS, which makes
a GPRS connection similar in many ways to a local area network (LAN) connection.
Just as with a LAN connection, once a GPRS mobile station registers with
the network, it is ready to send and receive packets. A user with a laptop
computer could be working on a document without even thinking about being
connected, and then automatically receive new e-mail. The user could decide
to continue working on their document, then half an hour later read the
e-mail message and reply to it. All this time the user has had a network
connection and not once had to dial in, as he or she must today with circuit-switched
connections. Furthermore, GPRS allows for simultaneous voice and data communication,
so the user can still receive incoming calls or make outgoing calls while
in the midst of a data session.
Since there is almost no delay before sending data, GPRS
is ideally suited for applications such as extended communications sessions,
e-mail communications, database queries, dispatch, and stock updates to
name just a few. In addition, the high throughput of GPRS will remove many
of the obstacles from the use of multimedia, graphical web-based applications.
For example, mobile users will be able to easily use graphically intensive
web-based map application to get directions while traveling. Furthermore,
with almost no transmission delay and high throughout, it will be more
practical to use enterprise applications such as SAP* wirelessly and remotely.
Platforms and GPRS
Because GPRS supports standard networking protocols, configuring
computers to work with GPRS will be very straightforward. In the case of
IP communications, you will be able to use existing TCP/IP protocol stacks,
such as the stack that comes with Windows 95 or Windows 98, Windows CE
and Windows NT. TCP/IP stacks are readily available for most other platforms
as well. With all the developments in the handheld computer area, you can
expect a multitude of hardware platforms to take advantage of GPRS:
-
Laptops or handheld computers connected
to GPRS-capable cellphones or external modems
-
Laptops or handhelds with GPRS-capable
PC Card modems
-
Smart phones that have full screen capability
(e.g. Nokia* 9000)
-
Cellphones employing microbrowsers using
the Wireless Application Protocol
-
Dedicated equipment with integrated GPRS
capability, e.g. mobile credit-card swipers
GPRS coincides with another important technology development:
the replacement of a cable connection to a cellphone by a short radio link.
Intel, Ericsson, Nokia, IBM, Toshiba and others are already working on
such wireless connections in an initiative called "Bluetooth".
As we discuss next, GPRS is also complementary with an
important industry trend associated with remote access: the transition
from dial-up remote access to Internet-based remote access.
GPRS and Remote Access
Traditionally companies have provided remote access for their
workers using dial-up modem connections into corporate modem pools. But
as companies have established high speed connections to the Internet, and
as remote workers have an increasing number of options for connecting to
the Internet, companies are now looking to the Internet as a way for their
remote workers to access corporate intranets. This is especially effective
because most communications applications today work over IP networks --
including many originally designed for modem dial-up connections. It can
be highly cost effective to use the Internet instead of making long distance
phone calls, and in the case of international connections, much more reliable.
But this Internet-based technique does raise an issue of privacy from hackers.
A new technology is emerging which addresses privacy and authentication
concerns, referred to as a virtual private network (VPN).
A VPN is a method of having private communications across
public networks. It adds additional software at each end of the connection
-- in our case the mobile computer and the corporate network. This software
establishes what are called "tunnels". Within this tunnel, information
is encrypted and additional information is added to each packet to prevent
tampering. Various standards are available or being finalized to define
interoperability between VPN products, including the Point to Point Tunneling
Protocol (PPTP), Layer Two Tunneling Protocol (L2TP), SOCKS and IPsec (Secure
IP). A wide range of companies already offer VPN solutions today, including
router vendors, network software providers, firewall suppliers and companies
specializing in this area. Since most VPN solutions are quite flexible
in their feature set, corporate IT can choose the level and type of protection
desired, such as 56 bit encryption or 128 bit encryption.
Almost all VPN technologies operate independently of the
communications link, meaning the same VPN technology will work with a dial-up
modem connection, Ethernet connections, ISDN connections and most importantly
for us, wireless connections. See figure two. |
Figure two: tunneling with VPN technology
|
Companies using VPNs will be able to smoothly migrate
from existing wireless technologies to GPRS. Today their users can make
circuit-switched connections to an Internet service provider, and then
establish a VPN connection. Once GPRS becomes available, the Internet connection
will extend to the mobile computer and the user will no longer need to
dial a separate ISP. The net result is wireless connectivity that works
hand in hand with VPN technologies to let remote workers easily access
corporate resources and to stay in touch with their work teams.
The Road Map
According to Kevin Holley, the chair for the GSM SMG4 committee,
which develops GPRS standards, the first version of the GPRS standard is
complete, while a next version of the standard that adds advanced features
such as point-to-multipoint communications is in development. Most GSM
vendors such as Alcatel, Ericsson, Lucent, Motorola, Nokia, Nortel, and
Siemens have been active in the standards process and many are developing
the necessary infrastructure elements. Field trials are expected in 1999
and deployment will begin in the year 2000. Though the GPRS standard specifies
support for both X.25 and IP, it is likely that vendors and operators will
emphasize IP service. It is also likely that GPRS will first roll out in
European countries. As of late 1998, no operators have announced when they
will deploy GPRS service, but it is very likely that many will do so, especially
since the infrastructure cost of deploying the service is relatively modest.
At this time no cellphones or modems that support GPRS have been announced
but it is possible that eventually all new GSM phones will support GPRS.
Where does GPRS fit in with other GSM data developments,
as well as data capabilities for other wireless networks? The first improvement
with GSM data is increasing existing circuit-switched data speeds from
9600 bps to 14.4 kbps. The addition of V.42 bis compression over the airlink
will further increase throughput by about a factor of two. After that,
and before GPRS is available, some carriers will begin offering high speed
circuit-switched data (HSCSD) which like GPRS combines multiple voice channels
to offer higher data rates. SingTel in Singapore announced in May of 1998
that using HSCSD technology it will soon be offering data rates of up to
38.4 kbps.
Another development is referred to by some as "direct
IP access". The user makes a circuit-switched data call, but rather than
switching the call into the public switched telephone network, the carrier
terminates it at a router that is connected to the Internet. From the user
perspective, the carrier appears like an Internet service provider offering
dial-up service. This hybrid circuit/packet type of service is a good stepping-stone
to GPRS and will also work with HSCSD.
And GSM standards bodies are already defining data networking
technologies that will build upon GPRS. One such technology is called Enhanced
Data rate for GSM Evolution (EDGE) which will offer a maximum theoretical
rate of 384 kbps, though normal operating speeds will be about half this
rate. Beyond EDGE, third generation cellular systems will eventually offer
data rates to 2 Mbps. The table below summarizes all these developments.
ROAD MAP OF DATA SERVICES FOR GSM
(as forecast by Rysavy Research)
| |
Timeframe |
Capabilities |
Notes |
| 9.6 kbps
service |
Available today |
Circuit-switched
data and fax |
Service available
from most GSM operators today. |
| 14.4 kbps
service |
Available over
next 12 months |
Higher speed
circuit-switched data and fax |
Should work
identically to 9.6 kbps service only at higher speed. V.42 bis compression
will further increase throughput by about 200%. |
| Direct IP
Access |
Available from
some carriers today |
Circuit-switched
connection directly to Internet |
Reduces call
set-up time and provides a stepping stone to packet data. Will also be
available for high-speed circuit-switched data services. |
| High-speed
circuit-switched data service (HSCSD) |
Available within
12 months |
High speed
rates to 56 kbps |
A software-only
upgrade for carriers not requiring expensive infrastructure. Operators
will need to decide whether to offer this service or GPRS or both. |
| GPRS |
Available within
two years |
High speed
packet data with transmission speeds over 100 Kbps, with most user devices
offering about 56 kbps |
Extremely capable
and flexible mobile communications. |
| EDGE |
Available within
three years |
High speed
packet data which will triple the rates available with GPRS |
Final high-speed
data technology for existing GSM networks. Will also be used with IS-136
TDMA networks. |
| Third generation
cellular |
Available within
three to five years |
High speed
packet data to 2 Mbps |
Completely
new airlink. |
Data services similar to those for GSM are also being
developed for IS-136 TDMA and CDMA networks. But as a more mature digital
technology, GSM has a strong head start. When GPRS is deployed, no other
wireless data technology will be able to match its capabilities. But it
is also important to remember that GSM already offers excellent data and
fax capabilities that provide more than sufficient capability for many
types of applications. As technologies like GPRS become available, the
scope of data applications that are practical for wireless connectivity
will only increase.
GPRS Details
for Network Managers
To better understand GPRS, we take a quick tour beginning
with the mobile PC and traversing through the network. First, we have a
notebook computer connected to a GPRS-capable cellphone or modem, either
through a serial cable or other type of connection such as Universal Serial
Bus (USB) or local wireless link. Or perhaps the connection device is in
the form of a PC Card. The GPRS phone or modem communicates with GSM base
stations, but unlike circuit-switched data calls which are connected to
voice networks by the mobile switching center, GPRS packets are sent from
the base station to what is called a Serving GPRS Support Node (SGSN).
The SGSN is the node within the GSM infrastructure that sends and receives
data to and from the mobile stations. It also keeps track of the mobiles
within its service area. The SGSN communicates with what is called the
Gateway GPRS Support Node (GGSN), a system that maintains connections with
other networks such as the Internet, X.25 networks or private networks.
See figure two. A GPRS network can use multiple serving nodes, but requires
only one gateway node for connecting to an external network such as the
Internet.
|
Figure three: GPRS system.
|
When the mobile station sends packets of data, it is via
the SGSN to the GGSN, which converts them for transmission over the desired
network, which could be the Internet, X.25 networks or private networks.
IP packets from the Internet addressed for the mobile station are received
by the GGSN, forwarded to the SGSN and then transmitted to the mobile station.
To forward IP or X.25 packets between each other, the
SGSN and GGSN encapsulate these packets using a specialized protocol called
the GPRS tunnel protocol (GTP) which operates over the top of standard
TCP/IP protocols. But the details of the SGSN and GGSN are both invisible
and irrelevant to the user who simply experiences a straightforward IP
or X.25 connection that just happens to be wireless.
An interesting aspect of GPRS is how it achieves its high
speeds to over 100 kbps when circuit-switched data today is limited to
9600 or 14.4 kbps. GPRS uses the same radio channel as voice calls, a channel
that is 200 kHz wide. This radio channel carries a raw digital radio stream
of 271 kbps which for voice calls is divided into 8 separate data streams,
each carrying about 34 kbps. After protocol and error correction overhead,
13 kbps is left for each voice connection or about 14 kbps for data. Circuit-switched
data today uses one voice channel. GPRS can combine up to 8 of these channels,
and since each of these can deliver up to 14 kbps of data throughput, the
net result is that users will be able to enjoy rates over 100 Kbps. But
not all eight-voice channels have to be used. In fact, the most economical
phones will be ones that are limited to 56 kbps. The GPRS standard defines
a mechanism by which a mobile station can request the amount of bandwidth
it desires at the time it establishes a data session.
Peter Rysavy is the president of Rysavy Research,
a consulting firm that works with companies developing new communications
technologies and those adopting them.
Copyright Rysavy Research 1998. All rights reserved.
* Other trademarks are the property
of their respective owners.
Rysavy Research home
page and more articles |
|
|
|
|
|
