Standards & Protocols
A review of GSM and some of its more interesting features, including system
services and components, is presented.
By Mike Rodbell
A
few years back, we took a look at the general architecture of Global System
for Mobile Communications (GSM) (CSD, Premiere Issue, p. 66). Since many of you may
not have had an opportunity to read those original columns, I'll review the general
architecture of GSM as an introduction to a number of columns to follow in the next
several months, where we'll review many of the important areas in the world of GSM,
such as the following:
1. The services. As a digital system (rather than an
analog cellular phone system),
the available services extend well beyond analog voice. Short messages, two-way paging,
fax, and other forms of data are easily added into the mix. Through the use of digital
technology, GSM is also able to address some of the problems and concerns that occur
in the older lines of analog cellular technology. Security services are easily added
to the digital voice and control applications. Voice quality can be improved through
advances in networking and coding techniques that
mask the effects of noise on wireless
analog channels.
2. The general system architecture. GSM is based on the cooperation of many interrelated
components. These components range from the mobile terminals to the switching systems
responsible for coordinating the transfer of information.
3. The protocols and interfaces. With the rich set of services and components,
there is a substantial set of standard interfaces and protocols. Some of these are
derived from previously existing
standards, such as portions of Integrated Services
Digital Networks (ISDN) and Signaling System 7 (SS7). Others deal directly with the
challenges of digital wireless communications.
With each of these areas containing many components, it shouldn't come as a surprise
that there is a huge volume of work defining the GSM system and its interfaces.
Initially developed to support a standardized approach to digital cellular communications
in Europe, the GSM protocols are rapidly being
adopted to the next generation of
wireless telecommunications systems, Personal Communication Systems (PCS), and global
Low Earth Orbit (LEO) satellite communication systems. While some adaptations, such
as operating over different frequencies or completely different physical air interfaces,
are required to operate over PCS or LEOs, the wealth of GSM services still provides
a strong basis for addressing other challenges inherent in wide area wireless networks.
When we last looked at GSM, the US
cellular market was largely analog; since then,
quite a bit has changed. While US cellular systems still include a significant mix
of analog and digital systems, GSM-based cellular capabilities are starting to appear
in many forms, both through traditional cellular as well as in microcell-based PCS.
Formal standardization of the US GSM variant, PCS 1900, has been addressed by the
GSM standardization bodies, and systems in several major markets are now available.
In the US, competing digital cellular
standards include time division multiple access
(TDMA) systems based on the IS-54 standards, and the IS-95 systems that rely on code
division multiple access (CDMA) air interfaces.
GSM and its companion standards DCS1800 (for the UK, where the 900-MHz frequencies
are not available for GSM) and PCS 1900 (in the US), have been developed over the
last decade to allow cellular communications systems to move beyond the limitations
imposed by the older analog systems. Analog system capacities are being
stressed
with more users that can be effectively supported by the available frequency allocations.
Compatibility between types of systems had been limited, if nonexistent. By using
digital encoding techniques, more users can share the same frequencies than had been
available in the analog systems. The acceptance of these systems has grown from approximately
sixty nine networks at the end of 1994 to over 139 networks as of August of 1996.
With the advent of the establishment of the PCS 1900 standards in
1995, the spread
of GSM-based systems will likely continue.
A mobile equivalent to ISDN
As a digital network that derives many of its features from ISDN, GSM offers a
rich set of services that include voice, circuit-switched data, packet data, and
fax. Through leveraging the digital nature of the GSM network, a greater level of
privacy can also be provided. The services provided by GSM are a blend of Advanced
Intelligent Network (AIN), ISDN, and features solely relevant in the
domain of wireless
communications. Before digging too deeply into the areas of GSM standardization,
lets take a look at the types of things one will come to expect from a GSM system.
Probably the most significant aspect of GSM, particularly in light of the features
of previous analog systems, is the ability to provide integrated voice and data services.
Toll quality voice services are provided through advances in voice processing through
the regular pulse excitation ý long-term prediction
voice algorithm that operates
at
13 kbps. Data services include the GSM messaging service, facsimile transmission,
and data communications at rates of up to 9,600 bps full duplex.
Security is effective enough that mobile users no longer need worry about either
the privacy of their conversations or someone surreptitiously making false charges
to their ac-counts. User account information is programmed into subscriber identity
module (SIM) cards. When the SIM card is plugged into a GSM
phone, the phone assumes
the unique identity of the user whose registration information is contained in the
SIM card. Through this information, cryptographic mechanisms are applied to authenticate
users on the network, and conversations are encrypted to prevent anyone from eavesdropping
on GSM calls.
GSM's advanced networking services are directly modeled on the types of services
now available with traditional telecommunications systems through AIN. These ýsupplementary
servicesý include
features such as the following:
Call forwarding, where calls can be automatically redirected on any of the following
conditions: always, when the mobile subscriber is busy, when there's no reply, and
when the mobile is out of range.
Call barring, where calls can be barred when they are outgoing, outgoing to
international destinations, outgoing to countries other than home, incoming, and
incoming when roaming abroad.
- Call waiting.
- Call hold.
- Multiparty
service.
- Advice of charge.
- Calling line identity.
- Closed user group.
These services are important features that provide added levels of flexibility geared
towards roaming users.
One of the most important aspects of GSM is its ability to support the needs of
the mobile roaming user. Through the combination of identification information contained
in the user SIM cards and an architecture designed to support user roaming throughout
many
interconnected GSM networks, users can use the network even though they are
no longer in range of their home networks. As we'll see in the discussion of the
GSM architecture, mechanisms are incorporated into the architecture to track users
as they travel through different portions of GSM networks. By tracking these users,
they can be located and reached in the areas to which they've traveled.
GSM system components
As you may have guessed, there are many components involved in GSM
(Figure 1),
and they include the following:
The mobile station (MS), which includes vehicle, portable, and hand-held terminals.
The SIM smart-card provides custom information about users such as the services they've
subscribed to and their identification in the network.
The basestation subsystem (BSS), which is a collection of devices that support
the switched network radio interface. Major components include the base transceiver
station (BTS), which consists of the radio modems and
antenna equipment, and the
basestation controller (BSC), which manages the radio activities of several BTS and
connects to a single NSS. In Open Systems Interconnection (OSI) terms, the BTS provides
the physical interface to the MS, whereas the BSC is responsible for the link-layer
services to the MS. Logically, the transcoding equipment is in the BTS; however,
an additional component, the transcoder/rate adapter unit (TRAU) can also provide
signal transcoding services.
The network and
switching subsystem (NSS), which provides switching between
the GSM subsystem and external networks along with the databases used for additional
subscriber and mobility management. Major components in the NSS include the mobile
services switching center (MSC), home and visiting location registers (HLR, VLR).
The HLR and VLR databases are interconnected through the telecom standard SS7 control
network.
The operation subsystem (OSS), which provides support functions responsible
for the management of
network maintenance and services. Components of the OSS are
responsible for network operation and maintenance, mobile equipment management, and
subscription management and charging.
Mobility management
One of the major features used in all classes of GSM networks (cellular, PCS,
and satellite) is the ability to support roaming users. Through the control signaling
network, the MSCs interact to locate and connect to users throughout the network.
The ýlocation registersý are
included in the MSC databases to assist in the role
of determining how and whether connections are to be made to roaming users. Each
user of a GSM MS is assigned an HLR that is used to contain the user's location and
subscribed services. A separate register, the VLR, is used to track the location
of a user. As the users roam out of the area covered by the HLR, the MS notifies
a new VLR of its location. The VLR in turn uses the control network (which happens
to be based on SS7) to signal the HLR of the MS's
new location. Through this information,
mobile terminated (MT) calls can be routed to the user by the location information
contained in the user's HLR.
Communication management features
The communication management layer provides three primary classes of services:
call control, supplementary services, and the short message service. Call control
services are responsible for routing the calls, determining who is responsible for
the call charges, and the organization that is to
receive payment. Supplementary
services include call forwarding, call barring, and passwords for security. Finally,
the communication management layer includes services to handle short message services
that are more efficiently handled through packet oriented transfers than the traditional
circuit-switched connections supported by the mainstream GSM system.
The interactions between the many components participating in the GSM networks
is based on the general concepts of layered protocols. As you
can quickly observe
in Figure 2, there are several GSM interfaces and protocols with which to concern
oneself. These will be our points of conversation for the next several months.
Mike Rodbell is president of DG Technology, a consulting firm that specializes
in the development and integration of distributed processing and communication systems.
He has developed voice and data communication systems for a wide range of military
and commercial systems. He holds a BSCS from Trinity College
of Hartford, CT, and
an MSEE from Loyola College of Baltimore, MD. He can be reached at mrodbell@dg-tech.com
or http://www.dg-tech.com.
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