GSM
has enabled users to send short messaging services (SMS) to any mobile network
at any time. SMS is a cheap and easy way to send a message to anyone, as
opposed to a voice call or conference. This technology benefits both network
operators and end customers.2G technology is more efficient, 2G network holds
sufficient security for both the sender & the receiver, All text messages
are digitally encrypted, This digital encryption allows for the transfer of
data in such a way that only the intended receiver can receive & read it,
These digital signals consume less battery power, so, it helps in saving the
battery of mobiles.
Network
Standards
The second generation of wireless networks, which
replace analog circuit-switched solutions with digital circuit-switched
solutions. Global System for Mobile Communications (GSM), which is based on
European Telecommunications Standards Institute (ETSI) standards, time division
multiple access (TDMA) IS-136, and code division multiple access (CDMA) IS-136
are the three main 2G technological standards (CDMA). IS-136 was identical to
the Japanese personal digital cellular (PDC) standard. GSM and IS-136
are TDMA technologies. In certain IS-95B deployments, 2G services typically enable
data rates of 9.6 Kbps, 14.4 Kbps, and up to 64 Kbps.
GSM
GSM (Global System for Mobile Communication)
is a digital mobile network widely utilized in Europe and other areas of the
world by mobile phone users. The most frequently used of the three digital
wireless telephony technologies: TDMA, GSM, and code-division multiple access
(CDMA), GSM employs a version of time division multiple access (TDMA) (CDMA).
GSM digitizes and compresses data before sending it along a channel with two
other user data streams, each with its own time slot. It uses the 900 megahertz
(MHz) or 1,800 megahertz (MHz) frequency bands.
This
standard aided to the goal of 2G which was to remove any incompatibility among
systems and facilitating more development towards global roaming. Telephone
services such as emergency calling and facsimile that supported videotext and
teletext, increased data rates from 300bps to 9.6kbps, call diversion, caller
identification and closed user group like supplementary ISDN services were some
of the key services introduced in GSM. The early GSM systems used a 25MHz
frequency spectrum in a 900MHz band which was subdivided into 124 carrier
frequencies of 200 KHz each. And a single 200 KHz channel was shared among
eight users by TDMA which allocates a unique time slot to each one of users.
GMS uses a variety of
cryptographic security algorithms, including stream ciphers for encrypting plaintext
numbers. Three stream ciphers, A5/1, A5/2, and A5/3, ensure that a user's
conversation remains secret. However, both A5/1 and A5/2 algorithms have been
broken and disclosed, making them vulnerable to plaintext assaults.
• Interference with electronic devices -
Because GSM uses a pulse-transmission technique, it is known to cause problems
with electronic devices such as hearing aids. Because of this electromagnetic
interference, mobile phones must be turned off in particular locations such as
airports, gas stations, and hospitals.
• Bandwidth lag - Multiple users share the
same bandwidth when using GSM technologies, resulting in significant latency as
additional users join the network.
• Data transfer rate is limited - GSM has a
data transfer rate that is slightly limited. To achieve faster data rates, a
user needs upgrade to a handset that supports more modern GSM technologies.
• Repeaters - To expand coverage, GSM
technologies require carriers to construct repeaters.
Digital Advanced Mobile Phone System
The Digital Advanced Mobile Phone System (D-AMPS) is a
digital variant of the original analog cellular phone standard, Advanced Mobile
Phone Systems (AMPS). Since the first commercial cellular network was built in
1993, D-AMPS has been widely used in the United States and Canada. D-AMPS employs
both time division multiple access (TDMA) and frequency division multiple
access (FDMA) techniques (FDMA). It uses TDMA to get three channels per AMPS
channel, allowing for a threefold increase in the number of calls per channel.
D-AMPS, like AMPS, employs frequencies between 800 and 900 MHz for
transmission. The first half of the spectrum is used to send signals, while the
second half is utilized to receive them. The frequency band is divided into
30KHz sub-bands, termed channels, by FDMA
Technical
specifications:
- 48.6 kbit/s channel bit rate
- 1.62 bit/s/Hz
- 40 ms frame duration which is divided into 6.67 ms slots
- Each 6.67 ms slot contains 324bits and 260 user data
Personal
Digital Cellular
A 2G standard which is specifically used in
Japan and operated between 800 MHz and 1500 MHz frequencies. The services of
this standard include voice (full and half-rate), and supplementary services
such as call waiting, voice mail, three-way calling, call forwarding, and etc.
PDC offered a data rate up to 9.6 kbit/s for CSD, and 28.8 kbit/s for
packet-switched wireless data (PDC-P). PDC's
lower broadcast strength, compared to GSM, allows compact, portable phones with
light batteries at the tradeoff of poor voice quality and connection problems,
especially in confined environments like elevators.
Access Technology
Multiple access techniques assign dedicated
channels to multiple users through bandwidth division. Methods to divide the
spectrum include frequency-division (FDMA), time-division (TDMA), code-division
(CDMA), and combinations of these methods. In TDMA time is divided into
nonoverlapping time slots that are allocated to different users. Different
users share time and bandwidth in CDMA, which is modulated by orthogonal or
semi-orthogonal spreading codes. The receiver can segregate the signal of
interest from the other CDMA users using orthogonal spreading codes, resulting
in no residual interference between users. For every given signal bandwidth,
however, there are only a finite number of orthogonal spreading codes.
In these multiple access methods is hard versus
soft system capacity. TDMA and FDMA place a hard limit on the number of users
sharing a given bandwidth, since the channel is divided into orthogonal time or
frequency slots, each of which can only support one user. Orthogonal CDMA also
has this hard limit. Semi-orthogonal CDMA, on the other hand,
provides the benefit of soft capacity: there is no absolute restriction on the
number of users.
GSM is based on a hybrid Time Division Multiple
Access (TDMA)/Frequency Division Multiple Access (FDMA) method.
BSS – The base station subsystem element of the GSM network utilized the radio access technology to enable a number of users to access the system concurrently. One channel could handle up to eight users, and by allowing a base station to have many channels, each base station could support a huge number of subscribers. Base stations are carefully located by the network provider to enable complete coverage of an area. A cell is a term used to describe the region covered by a base station.
NSS – is a GSM system component that handles call out and mobility management for mobile phones roaming on the network of base stations. Mobile phone companies own and implement it, and it allows mobile devices to connect with one other and with telephones on the public switched telephone network (PSTN). Because the phones are not fixed in one area, the architecture includes particular characteristics and functions.
• Mobile Services Switching Center (MSC) - The MSC is the Network Subsystem's most important component. The MSC manages mobile services such as registration, authentication, location updates, handovers, and call routing to a roaming subscriber, as well as switching calls between mobile and other fixed or mobile network users. It also handles toll ticketing, network interfacing, and common channel signaling, among other things. A unique ID is assigned to each MSC.
• Home Location Register (HLR) – The HLR is a database used for storage and management of subscriptions. The HLR is the most significant database since it contains permanent information about users, such as their service profile, location, and activity status. When a person purchases a SIM subscription, all of the information about that subscription is stored in the HLR of that operator.
• Visitor Location Register (VLR) – The VLR is a database that the MSC uses to store temporary information on subscribers in order to provide service to visiting subscribers. The MSC and the VLR are always linked. When a mobile station roams into a new MSC area, the VLR associated with that MSC will ask the HLR for information about the mobile station. If the mobile station makes a call later, the VLR will have the information it needs to set up the call without having to query the HLR every time.
Types of handover within the GSM network
•
Intra-BTS handover: This form of GSM handover occurs if it is required to
change the frequency or slot being used by mobile because of interference, or
other reasons. The mobile phone remains connected to
the same base station transceiver, but the channel or slot is switched.
•
Inter-BTS Intra BSC handover: This form of GSM handover or GSM handoff occurs
when the mobile moves out of the coverage area of one BTS but into another
controlled by the same BSC.
The BSC is able to complete the
handover in this case, and it assigns a new channel and slot to the mobile
before freeing the old BTS from talking with the mobile.
•
Inter-BSC handover: When the mobile moves out of the range of cells controlled
by one BSC, a more involved form of handover has to be performed, Not
only from one BTS to another, but also from one BSC to another. The MSC is in
charge of the handover in this case.
•
Inter-MSC handover: This form of handover occurs when changing between
networks. The two MSCs involved negotiate to control the handover.
Limitations
• If there is no network coverage in a
certain place, the digital signals may be faint. In less populated places, the
weaker digital signal transmitted by the cellular phone may not be adequate to
reach the cell tower.
• Provides lower data rates ranging from
9.6 Kbps to 28.8 Kbps
• The weaker digital signal transmitted
by the cellular phone can not be sufficient to reach the cell tower in less
populous areas
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