Wireless Technology Standards  

Wireless Standards Development This section discusses the
evolution of some of the wireless networking standards and the
types of services they support.The phased evolution of wireless
networking standards are referred to as generations:

* 1G--First generation. 1G refers to the initial category of
mobile wireless networks that used only analog technology and
were developed primarily for voice services. Advanced Mobile
Phone Service (AMPS) is an example of a 1G mobile network
standard.

* 2G--Second generation. 2G refers generically to a category of
mobile wireless networks and services that use digital
technology. 2G wireless networks introduce support for data
services. GSM, TDMA and CDMA are examples of 2G mobile network
standards.

* 2G+--Second generation plus. 2G+ refers generically to a
category of mobile wireless networks that have a packet data
overlay built on top of the circuit-switched voice network to
support higher data rates than 2G mobile networks (2G networks
support data in a circuit-switched model). General Packet Radio
Service (GPRS) is an example of a 2G+ mobile network standard.
There is a similar packet data overlay concept for CDMA called
Packet Data Services Node (PDSN), but this is considered 3G as
part of the CDMA 1x solution.

* 3G--Third generation. 3G refers generically to a category of
next-generation mobile networks which operate at a higher
frequency bandwidth (typically 2.1 GHz and higher) and have a
larger channel bandwidth. This enables 3G networks to support
very high data rates, up to 2 Mbps. With the higher bandwidth,
more data and multimedia services are possible. 3G refers to the
radio network and RF technology, and does not affect the
switching core. The switching infrastructure for 3G is still
based on MSCs and the TDM model. The Universal Mobile Telephone
Service (UMTS), based on the Wideband CDMA (W-CDMA) R-99 and
CDMA 2000, are examples of 3G radio networks that are being
developed to fulfill the requirements in the International
Mobile Telecommunications-2000 (IMT-2000) standard by the
International Telecommunication Union (ITU).

* 3G+--Third generation plus. 3G+ refers to an advanced level of
3G that introduces the concept of an all-IP switching core. An
all-IP switching core means that IP replaces the TDM-based MSC
infrastructure with IP-based transport and IP-based signaling.
IP-based signaling is implemented with new protocols like
Session Initiation Protocol (SIP) and Media Gateway Control
Protocol (MGCP). In 3G+ networks, the traditional MSC
implementation goes away and the various MSC functions are
redistributed to several other elements. A good example of this
evolution in the switching core from TDM to packets is 3GPP's R4
and R5 architecture. 3GPP2 also has adopted a similar trend to
transition to an all-IP network. There are also initiatives
under way to develop and migrate to a true end-to-end, all-IP
mobile wireless network where both the switching core and the
RAN are IP based. This evolution is being loosely referred to as
R6 in 3G terminology. Global Systems for Mobile Communications
In the early 1980s, many countries in Europe witnessed a rapid
expansion of analog cellular telephone systems. However, each
country developed its own system, and interoperability across
borders became a limiting factor. In 1982, the Conference of
European Post and Telecommunications (CEPT), an association of
telephone and telegraph operators in Europe, established a
working group to develop a new public land mobile system to span
the continent. Because their working language was French, the
group was called the Groupe Speciale Mobile (GSM). The GSM group
proposed the following criteria for the new mobile wireless
system: * good speech quality * low cost for terminals and
service * international roaming * handheld terminals * support
for introduction new services * spectral efficiency *
compatibility with Integrated Digital Services Network (ISDN) In
1989, the responsibility for GSM development was transferred to
the European Telecommunications Standards Institute (ETSI), and
phase 1 of the GSM specification was published in 1990. The
first commercial service was launched in 1991. When the official
language of the GSM group changed from French to English, GSM
was changed from Groupe Speciale Mobile to Global System for
Mobile Communications. In 1994, phase 2 data/fax services were
launched, and in 1995, the GSM phase 2 standard was completed.
The first GSM services in the United States were launched. GSM
uses a combination of both the time division multiple access
(TDMA) and frequency division multiple access (FDMA)
technologies. With this combination, more channels of
communications are available, and all channels are digital. The
GSM service is available in four frequency bands: *
450-MHz--Upgrade of older analog cellular systems in Scandinavia
* 900-MHz--Original band used everywhere except North America
and most of South America * 1800-MHz--New band to increase
capacity and competition used everywhere except North America
and most of South America * 1900-MHz--Personal communications
service band used in North America and much of South America.
The higher frequency bands provide additional capacity and
higher subscriber densities. One of the unique benefits of GSM
service is its capability for international roaming because of
the roaming agreements established between the various GSM
operators worldwide.

GSM Technology Differentiator One of the advantages of GSM is
that it offers a subscriber identity module (SIM), also known as
a smart card. The smart card contains a computer chip and some
non-volatile memory and is inserted into a slot in the base of
the mobile handset. The memory on the smart card holds
information about the subscriber that enables a wireless network
to provide subscriber services. The information includes: * The
subscriber's identity number * The telephone number * The
original network to which the subscriber is subscribed A smart
card can be moved from one handset to another. A handset reads
the information off the smart card and transmits it to the
network.

GSM Network Elements A GSM network consists of the following
network components: * Mobile station (MS) * Base transceiver
station (BTS) * Base station controller (BSC) * Base station
subsystem (BSS) * Mobile switching center (MSC) * Authentication
center (AuC) * Home location register (HLR) * Visitor location
register (VLR) Mobile Station The mobile station (MS) is the
starting point of a mobile wireless network. The MS can contain
the following components: * Mobile terminal (MT)--GSM cellular
handset * Terminal equipment (TE)--PC or personal digital
assistant (PDA) The MS can be two interconnected physical
devices (MT and TE) with a point-to-point interface or a single
device with both functions integrated. Base Transceiver Station
When a subscriber uses the MS to make a call in the network, the
MS transmits the call request to the base transceiver station
(BTS). The BTS includes all the radio equipment (i.e., antennas,
signal processing devices, and amplifiers) necessary for radio
transmission within a geographical area called a cell. The BTS
is responsible for establishing the link to the MS and for
modulating and demodulating radio signals between the MS and the
BTS. Base Station Controller The base station controller (BSC)
is the controlling component of the radio network, and it
manages the BTSs. The BSC reserves radio frequencies for
communications and handles the handoff between BTSs when an MS
roams from one cell to another. The BSC is responsible for
paging the MS for incoming calls. Base Station Subsystem A GSM
network is comprised of many base station subsystems (BSSs),
each controlled by a BSC. The BSS performs the necessary
functions for monitoring radio connections to the MS, coding and
decoding voice, and rate adaptation to and from the wireless
network. A BSS can contain several BTSs. Mobile Switching Center
The mobile switching center (MSC) is a digital ISDN switch that
sets up connections to other MSCs and to the BSCs. The MSCs form
the wired (fixed) backbone of a GSM network and can switch calls
to the public switched telecommunications network (PSTN). An MSC
can connect to a large number of BSCs. Equipment Identity
Register The equipment identity register (EIR) is a database
that stores the international mobile equipment identities
(IMEIs) of all the mobile stations in the network. The IMEI is
an equipment identifier assigned by the manufacturer of the
mobile station. The EIR provides security features such as
blocking calls from handsets that have been stolen. Home
Location Register The home location register (HLR) is the
central database for all users to register to the GSM network.
It stores static information about the subscribers such as the
international mobile subscriber identity (IMSI), subscribed
services, and a key for authenticating the subscriber. The HLR
also stores dynamic subscriber information (i.e., the current
location of the mobile subscriber). Authentication Center
Associated with the HLR is the authentication center (AuC); this
database contains the algorithms for authenticating subscribers
and the necessary keys for encryption to safeguard the user
input for authentication.

Visitor Location Register The visitor location register (VLR) is
a distributed database that temporarily stores information about
the mobile stations that are active in the geographic area for
which the VLR is responsible. A VLR is associated with each MSC
in the network. When a new subscriber roams into a location
area, the VLR is responsible for copying subscriber information
from the HLR to its local database. This relationship between
the VLR and HLR avoids frequent HLR database updates and long
distance signaling of the user information, allowing faster
access to subscriber information. The HLR, VLR, and AuC comprise
the management databases that support roaming (including
international roaming) in the GSM network. These databases
authenticate calls while GSM subscribers roam between the
private network and the public land mobile network (PLMN). The
types of information they store include subscriber identities,
current location area, and subscription levels. Network and
Switching Subsystem The network and switching subsystem (NSS) is
the heart of the GSM system. It connects the wireless network to
the standard wired network. It is responsible for the handoff of
calls from one BSS to another and performs services such as
charging, accounting, and roaming. Figure 2-1 shows a GSM
network and the network elements it contains.

GSM Interfaces The GSM uses various interfaces for communication
among its network elements. Figure 2-2 shows these interfaces.
Mobile wireless communication occurs over the interfaces between
the network elements in a sequential manner. In Figure 2-2, the
MS transmits to the BTS, the BTS to the BSC, and the BSC to the
MSC. Communications also occur over the interfaces to the
management databases (HLR, VLR, AuC, and EIR). Communications
might traverse multiple MSCs but ultimately must reach the
gateway MSC (GMSC). The GMSC provides the gateway to the public
switched telephone network (PSTN). A separate interface exists
between each pair of elements, and each interface requires its
own set of protocols.In the BSS block, mobile communication
occurs over the air interface to the BTS using the ISDN Link
Access Procedure-D mobile (LAP-Dm).

Figure 2-2 GSM Interfaces

This traffic channel carries speech and data. In this example,
voice operates at full-rate 13 kbps (supported by LAP-Dm), and
data operates at full-rate 9.6 kbps. The BTS communicates to the
BSC over the Abis interface using the ISDN LAP-D signaling
protocol. The BSC communicates to the GMSC via the transcoder
rate adapter unit (TRAU), which translates between 16 kbps on
the BTS side to 64 kbps on the GMSC side. This interface uses
the signaling system 7 (SS7) protocol, which defines call set-up
and call services across the interface. At the NSS, the GMSC is
the central node. Link-level traffic and signaling control
occurs over the interface between the GMSC and MSC and the
interface to the external network (PSTN, ISDN or PDN). Different
signaling protocols are used on the interfaces. Some NSS
interfaces involve only control signaling protocols with no
traffic. For example, no traffic is generated on the interfaces
between the GMSC, HLR, and VLR. Instead, these interfaces carry
only signaling using the Mobile Application Part (MAP) of the
SS7 protocol. The MAP is specified in IS-41 and defines the
application layer, signaling protocols, and procedures for
registering mobile users and handling handoffs between cellular
systems. The GMSC establishes call traffic (at 64 kbps) onto the
PSTN via the ISDN user part (ISUP), which is an SS7-based
protocol. The GMSC and MSC exchange traffic (over LAP-D at 64
kbps) and use SS7 (MAP and ISUP) control. GSM Data Services GSM
networks handle both voice and data traffic requirements of the
mobile communication by providing two modes of operation: *
Circuit switched (high-speed circuit switched data) * Packet
switched (GPRS) Circuit switching provides the customer with a
dedicated channel all the way to the destination. The customer
has exclusive use of the circuit for the duration of the call,
and is charged for the duration of the call. With packet
switching, the operator assigns one or more dedicated channels
specifically for shared use. These channels are up and running
24 hours a day, and when you need to transfer data, you access a
channel and transmit your data. Packet switching is more
efficient than circuit switching. The standard data rate of a
GSM channel is 22.8 kbps.