THE EVOLUTION OF GSM DATA
Kevin Holley, Mobile Systems Design
Manager, BT, United Kingdom
Tim Costello, Cellular Systems Engineer, BT,
Kevin Holley is Chair of the European Telecommunications Standards
Institute's SMG4 Group. ]
GSM data services have launched
a new era of mobile communications. The early analogue cellular
modems were unattractive to the market as they were slow and unreliable.
GSM now offers a high quality 9600 bits per second link which opens
the way for useful data applications and the Short Message Service
provides guaranteed delivery of small data packets even if the phone
is switched off when the message is first sent. Now the market for
data is moving onwards (more bursty) and upwards (more traffic),
and the ETSI standards groups are working towards higher data rates
but more significantly also towards packet data services, initially
with GPRS and multislot data but eventually UMTS will provide much
higher bandwidth.. This will certainly broaden the appeal to end
users because data is routed more efficiently through the network
and hence at lower cost, and also access times are reduced. Yet
there are still very few applications which make the most of mobile
data. Time is wasted through the use of inefficient user protocols
and it is rare for an application to provide fast recovery after
call failure. It is very important for application designers to
consider the capabilities of the lower level bearer and this paper
aims to identify some of the key aspects of this task.
Cellular access to data services has
been around for a long time. Even in the mid 1980s engineers were
busy designing modems which could counter the effects of radio fading.
Yet the data access services were not popular. Connections were
unreliable and slow. Equipment was bulky, with the term "soap
on a rope" in vogue as users struggled with many boxes to connect
between the phone and the data terminal. GSM has turned this around,
with high quality data solutions involving reliable connections
and compact phones with only a single wire between phone and terminal.
At the same time as GSM networks became
popular, the growth of the Internet started to reach out to the
general public. The now-ubiquitous web browser has inspired a generation
of data users and it is possible to find nearly everything somewhere
on "the web". Large companies all over the world have
embraced this technology and made it their own by providing in-house
Intranets, firewalled from the outside world yet accessible to all
in the company.
Figure 1 - Businesses
are very dependent upon corporate LANs
The mobile computer user has also seen
dramatic changes over the last 10 years. The "laptop"
computer is now virtually extinct, with high speed colour PCs now
available. Much smaller handheld devices called Personal Digital
Assistants or PDAs have emerged and become very popular.
With these developments in the role
and presentation of data particularly in businesses the main demand
for data services can be described as open access to public information
on the Internet, or secure access to private information on company
Intranets by a mobile population using handheld PDAs or notebook
PCs. But this is only what the end user wants to see, so how can
we achieve that in reality?
GSM was conceived to be the "mobile
arm" of ISDN, using signalling based on Q.931 and providing
access to all the ISDN data services. The communications path is
digital so the speech is converted to ones and zeros before transmission..
This made it quite easy to introduce data services because there
was no need for any modem at the user end. GSM is a widely adopted
standard with roaming (enabling subscribers from one operator’s
network to communicate via another operator’s network), a key
consideration of all developments. Thus it is possible to use the
same application in many countries without the need to reprogram
the dialled number or any other setup change.
GSM provides two basic data services,
transparent and non-transparent.
The transparent service provides a
consistent delay and a throughput of 9600 bits per second, with
variable error rate according to the propagation conditions.
The non-transparent service provides
a consistent low error rate with a throughput up to 9600 bits per
second and variable delay according to the propagation conditions.
Whichever "bearer type" is
chosen, there are many options for connection between the GSM network
and the far end data network. The most commonly used ones are PSTN
modem and ISDN data service. With either of these, the throughput
is limited to 9600 bits per second and so flow control and/or rate
adaptation are required. The rate adaptation as well as the basic
data conversion from GSM protocols to the fixed network protocols
is performed by an Interworking Function (IWF).
Figure 2 - Basic functions in the IWF
For the mass-market, the most obvious
data service to offer is the non-transparent service, as this provides
built-in error correction. However we have already seen devices
on the market which allow end to end data compression by using the
transparent data service. Claims of 36000 bits per second can be
met and in some cases exceeded, depending on the type of data being
transmitted. In addition GSM has now developed a standard data compression
capability built on V.42bis, which can be applied between the mobile
terminal and the network. This service will allow non-compressed
data from a users terminal to be compressed in the mobile station
and de-compressed in the network, thus
overcoming the limitation that the reduced speed on the air interface
Facsimile devices are very complex.
They change modem speed during the call, have training to determine
the acceptable data rate, cannot be flow controlled and sometimes
even operate to a non standard protocol. This makes it very difficult
to insert a digital bearer in the middle of a fax call. To get facsimile
to work with GSM, the standards-makers had to terminate the fax
protocol and re-create it, both at the terminal side and in the
interworking function. Despite the complexity, GSM provides a very
good facsimile group 3 capability.
The GSM standard incorporated a two-way
paging function. Called Short Message Service, or SMS, this allows
160 characters of text to be sent to or from a GSM handset. Messages
are sent to a Short Message Service Centre (SMSC), which stores
SMS until it can be successfully delivered to the destination. Whilst
in many cases the first attempt is successful, it may be that the
destination handset is switched off or out of coverage, so the SMSC
makes many attempts to send the message, possibly over several days.
SMS has two distinct service classes: Mobile Terminated (messages
sent to the mobile) and Mobile Originated (messages sent from the
Figure 3 - General access to SMS-SC and onward connection
There is another short text-based messaging
service called Cell Broadcast or CBS, which has the ability to broadcast
messages in a given geographical area, making it suitable for information
type applications available to all subscribers in a given area.
All of these data facilities have been
implemented for some years by the manufacturers. We have seen the
emergence of the GSM data card, a PC card which looks like a modem
to a PC, but instead of converting the PC data to modem tones it
converts the PC data to a proprietary protocol which is then in
turn converted by the handset into standard GSM data protocols.
The GSM standards-makers, however, always envisaged that GSM handsets
would provide a direct data connector, and this has been achieved
by some manufacturers already. Some rely on additional PC software
drivers but others effectively implement the entire GSM data stack
in the mobile phone.
These PC interconnect solutions provide
for data and fax calls to and from the mobile user. The Short Message
Service / Mobile Terminated is now implemented in all mobile phones,
and quite a few also have the Mobile Originated capability too.
However mobile phone keypads are limited in terms of the speed of
data entry and a PC interconnect solution for SMS makes the business
of entering and sending messages much easier.
The PDA market is not served as well
by the GSM handset makers. PDAs have difficulty in supporting a
PC card slot because the power requirements are significant. They
are also limited in terms of memory and CPU power and so complex
communications drivers are a non-starter. The best solutions for
PDAs are provided by manufactures who have a phone with direct data
connectors (physical or other e.g. optical).
Some manufacturers have taken the step
of integrating PDAs with phones. These are attractive because everything
comes in one box and all services work together seamlessly. However,
the offerings to date have been quite limited because they are large
for a phone and do not have all the capability which people expect
from a PDA. In the future we expect much more from this area.
The general trend is for data applications
to generate increasingly bursty data streams, this makes for inefficient
use of a circuit switched connection. With present data traffic
levels accounting for only a small part of the mobile network usage,
there seems little need to worry about efficiency. However, fixed
networks have seen an enormous growth in data traffic, not least
because of the rise and rise of Internet access demand, and there
is no reason to suppose that this will not spread to the mobile
networks as technology and customer expectations move on.
The existing GSM network is optimised
for circuit switched voice calls and therefore is inefficient at
setting up and carrying very short data bursts.. Figure 4 shows
a comparison made of the number of radio timeslots required to carry
data traffic generated as the user base increases. It shows that
with only a few subscribers the same number of channels are required
for circuit and packet switching. As the user base (and traffic
volume) increase then the number of channels required increase faster
if circuit switching is used. Packet switching is more efficient
at carrying bursty data, and mobile networks should employ packet
switching right up to the end terminal if they are to cope with
the expected demand for future data services.
The network and radio capacity required
to support a large amount of bursty data would make it uneconomic
or impossible (ie limitations on the number of physical radio channels
available) to implement.
A packet switched mobile network opens
the way for a range of new applications. IP interconnectivity and
Point-to-Multipoint (PTM) transfers become a reality with GPRS,
applications currently restricted to fixed line connections will
migrate to the mobile world.
Figure 4 - Radio Timeslots Required
to Support Growing Data
The very robust 9600 GSM data service
works with good quality of service throughout existing GSM cells.
With any design for high speed mobile data there is a trade-off
between data rate and coverage area, and GSM has provided a good
data speed with coverage wherever there is voice coverage. A strong
requirement for a new faster service has emerged with a more relaxed
requirement for total coverage. Thus the GSM standard will soon
include a 14.4k bits/s data service as a basic enhancement to the
existing bearer service classes. This will be achieved by reducing
the error protection mechanism and therefore it will not be possible
to use 14.4k bits/s over the whole of the GSM service area, instead
requiring the user to stay in good to moderate coverage.
As well as increasing the rate for
a normal call, the GSM standards will offer timeslot concatenation.
To explain this it is necessary to go into a little more detail
about the GSM radio interface. A single radio carrier on GSM is
200kHz wide and provides up to 8 simultaneous channels. Each logical
channel is given one eighth of the time on the physical carrier.
But a single call could potentially be allocated more than one of
these logical channels and hence the aggregate data rate could be
higher. The GSM multislot standard will combine up to 6 channels
to give the user a rate up to 64k bits/s. There is no benefit in
increasing the data rate beyond this because the switch network
is based on narrowband ISDN 64kbits/s circuits, so the rate limitation
starts to be the core network rather than the access.
Figure 6 - Multiple routes provide higher capacity
But the biggest change is that of providing
packet radio access. The new Generalised Packet Radio Service (GPRS)
will do just that, it will offer operators the ability to charge
by the packet, support data transfer across a high speed network
and up to 8 timeslot radio interface capacity.
GPRS introduces two new nodes into
the GSM network - the Serving GPRS Support Node (SGSN) and Gateway
GPRS Support Node (GGSN). The job of the SGSN is to keep track of
the location of the mobile within its service area and to send and
receive packets from the mobile, passing them on, or receiving them
from the GGSN. The GGSN then converts the GSM packets into other
packet protocols (IP or X.25 for example) and sends them out into
Figure 5 - GPRS Network Architecture
With GPRS, a terminal logs on to the
network separately for GPRS. It then establishes a Packet Data Protocol
(PDP) context which is a logical connection between the mobile and
a Gateway GPRS Support Node. Having done this it is now visible
to the outside fixed network, e.g. the Internet and can send and
There are four radio channel coding
schemes defined for GPRS, to allow the data rate to be increased
when coverage is good, and ensure packet loss is never too great
when coverage is not so good. These coding schemes allow between
9.05kbit/s and 21.4kbit/s per GSM timeslot in use. Thus the maximum
data rate for GPRS is 8 times 21.4kbit/s or approximately 164kbit/s.
data service performance
The performance of all GSM data services
will vary according to radio conditions. However it is possible
to categorise GSM data services according to a few key elements:
* access time - how long it takes to
get into a ready-to-transfer-data state
* throughput - users data transfer
rate through the GSM system
* packet size - Size of data that is
managed as one entity through the GSM system
Switched Non Transparent
Shown graphically, this gives a much
better idea of how the limitations of narrowband ISDN switching
impact on the maximum throughput when comparing GPRS with multislot
data (see figure 6).
Figure 6 - GSM Data
Service Maximum Throughput
to fit an application
All data services can be broken down
into a few key components as follows:
All of the GSM data services can be
used with any of the interconnect options in the centre column,
however clearly the bit rate will be very important to Video in
particular. The main element in choosing the GSM service is the
size of "packet". For very small packets sent unicast
or small-scale multicast, then SMS would provide an ideal transfer
mechanism over GSM, whatever the end interconnect requirements.
For larger packets then GPRS becomes the most attractive bearer,
but this will not be available for a while so circuit switched data
may have to be used. The terminal in use will also be important,
not only from its size but also how much data is required to be
stored. A despatch service would only need a few messages, whereas
an email application would require more storage.
Whilst this way of studying the requirements
is very useful in terms of defining the service to use, it is of
paramount importance to consider the service required by the user
rather than looking at the technology available and making mobile
access possible. The following examples show what a difference this
can make to the user perception:
World Wide Web access
The user wants to be able to view
web pages and hyperlink from one page to another. One way to
provide this is to set up a stack consisting of HTTP, PPP, TCP,
IP and a bearer connection based on GSM with its own inbuilt
error control. However all of this is rather inefficient for
the transfer of HTML text and graphics. A really big gain could
be made by sending the page requested as a Mobile Originated
packet and then assembling the required data at an intermediate
point before sending an alert to the mobile which then dials
up and downloads the information all in one go. This allows
pre-compression of the data and removes any Internet delay from
the user’s airtime. If graphics are not required then in
many cases the Short Message Service alone can provide a transport
mechanism! Solutions along these lines are being developed one
called MASE (Mobile Application Service Environment), which
is an API lying between the application and the communications
system. Another is Mowgli which is a server positioned between
two networks which assists in the communications between two
users by modifying the data to be suitable/optimised for that
The user wants to be able to send
and receive email. This can be achieved with POP3/SMTP on top
of PPP, TCP, IP and a GSM bearer. But again this is inefficient
for text transfer, which can be achieved using compression and
file transfer techniques like zmodem, rather than an open Internet
The difficulty with moving forwards
from the present position is that there is a need for more than
the traditional systems integration tasks. As well as providing
user software and drivers, the systems integrators need to get additional
functionality built into the Office Automation, Email and Internet
Service Provider business so that online times can be reduced. There
is some scope for additional value added service businesses in taking
the Internet data and converting to a more mobile-friendly format
before passing on to the mobile user, however for the Corporate
customer this is unlikely to be popular in the long term as concerns
over the security of intranet data are very high.
Services like multislot data and GRPS
are very useful in moving the base technology forwards, but if the
same goals can be achieved with the existing data services, we should
be looking at prototyping services now on the current networks.
We need to produce reliable user solutions which can cope with dropped
calls without restarting a session, perhaps automatically re-establishing
should this be needed. We should avoid using inefficient protocols
for mobile just because they are existing and in common usage. People
need access to common services, not necessarily using the same network
protocols as are used in the higher bit-rate, lower error-rate fixed
networks. The time is now right to look at producing a standardised
mobile access mechanism for fixed network services, focusing on
increasing the effective throughput and immunity to dropped calls
and thus reducing the needed airtime.
do we go from here?
There are many goals yet to be achieved
with wireless data. The whole data industry is moving away from
X.25 based services towards TCP/IP based services. Data transport
is becoming cheaper and cheaper as economies of scale drive down
the cost of routing equipment. User devices are becoming more sophisticated
and smaller. The idea of email and web browsing in your pocket can
be achieved today yet it is expensive and in some cases awkward
to use. But the mass market for data devices is the key to future
success. Defining that mass market and the most useful services
is the key. We must make the best use of the available data services
to avoid excess cost.
The long-term goal is the UMTS vision,
quoted from the UMTS Forum Regulatory report as:
"The Universal Mobile Telecommunications
System, UMTS, will take the personal communications user into the
Information Society of the 21st century. It will deliver advanced
information directly to people and provide them with access to new
and innovative services. It will offer mobile personalised communications
to the mass market regardless of location, network or terminal used."
Basically the implications of this
are that all telecommunications networks are integrated and users
really can get fast data rates of 2Mb/s in some areas. But 2Mb/s
is only useful for the right applications and those applications
need some market experience before they can be well defined. There
are bound to be some winners and some losers but at least some of
the battles must be fought with the existing second generation cellular
capability of up to around 64kbits/s. The future expectation of
growth in the mobile market is driven by data and multi-media. This
expectation is now being realised on fixed networks, mobile networks
through the enhancement on GSM and introduction of new capabilities
such as GPRS will start to build that market. UMTS is the "utopia"
for meeting the market expectation, however, it needs to be technologically
advanced sufficiently to meet that need.
GSM data provides a wealth of opportunity
for transferring a variety of information to the user on the move.
From simple email to video and multimedia, almost anything is possible.
The future will bring even higher bandwidths, larger markets and
with them lower costs. However in order to achieve the goal of mass
market data use the applications must be developed with mobile in
mind. Users will not want to bother with redialling just because
of a dropped call, and they will not be happy to pay for two timeslots
when one would suffice if only the data was pre-compressed and used
an efficient transfer protocol. We need to look at the system as
a whole, starting with the users needs and ending with the data
Terminals will be very important to
the success of mobile data. Whilst there will be roles for all types
of data terminal, the most successful ones will provide unified
mailboxes, not one button for fax, another for internet email, yet
another for X.400 and a fourth for SMS.
The future is bright for data, if we
get it right!
- Global System for Mobile communications
- what’s in store?, K A Holley, BT Technology Journal Vol.
14 No. 3 July 1996
- Mobile Data Services, C J Fenton,
W Johnston and J D Gilliland, BT Technology Journal Vol. 14 No.
3 July 1996
- GPRS standards
GSM 02.60 GPRS Service Description:
GSM 03.60 GPRS Service Description: Stage 2
- Mowgli Developments - http://www.cs.helsinki.fi/research/mowgli/
- 5. A Regulatory Framework for UMTS,
June 1997, UMTS Forum
The authors would like to acknowledge
the support provided by colleagues at BT Laboratories, in particular
Alan Clapton and Chris Fenton.
Kevin Holley is currently working as
a Mobile Systems Design Manager with BT. He has been involved in
the development of the GSM standard since 1988, particularly on
the data services and Short Message Service. He also works closely
with the UK operator Cellnet in defining their GSM services. He
is now chairman of SMG4, the ETSI data development group for GSM
Tim Costello is currently working as
a Cellular Systems Engineer with BT. He is working on future mobile
data applications and particularly GPRS.