GSM Interview Preparation Guide

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GSM, which stands for Global System for Mobile communications,
reigns as the world’s most widely used cell phone technology.
Cell phones use a cell phone service carrier’s GSM network by
searching for cell phone towers in the nearby area.

The origins of GSM can be traced back to 1982 when the Groupe
Spécial Mobile (GSM) was created by the European Conference of
Postal and Telecommunications Administrations (CEPT) for the
purpose of designing a pan-European mobile technology.

mobile station is normally mobile in nature which is an
advantage over pstn network . and in order to provide
connection to mobile station we use radio resource (air)...
in order to provide interface between mobile and bss . bts
is used.. a bts is responsible to cover a small geographical
area called cell...

Rx_Level_Full is measured when DTX is off & Sub is when DTX
is on.
RX Lev Full: Its is nothing but the Mobile transmit the
measurment report(SACCH multiframe) for every 480ms. this
multiframe containes 104 TDMA frames, in 104 TDMA frames 4
TDMA frames for Decode the BSIC and remaining 100 TDMA
frames for Average measurment of serving cell and
neighbouring cell.This average measurment of 100 TDMA frames
are RX Lev Full

RX Lev Sub: DTX is a discontinouse trasmission, When the
mobile conversation 40% of the time either Trasmitter or
Receive is idle. When DTX is ON, DTX will switch off the
Trasmitter or Receiver when they is no speech Pulses. only
few TDMA frames will trasmit, the average of this TDMA
frames is called RX Lev Sub, give you proper measurment of
RX level

Error ratios used in conjunction with GSM speech channels:
· Frame Erasure Rate, FER, is defined as the amount of swept speech frames (260 bits each)
divided by the amount of transmitted speech frames. The speech frame is swept if even one of
its most important 50 bits is observed not to be correct. The three parity bits following the 50
class Ia bits are used for error detection.
· Bit Error Rate, BER, is the ratio of erroneously received bits to all received bits. It is
important to notice that BER is evaluated before channel decoding, i.e. after equaliser. BER is
used for defining the RXQUAL value

Rx level 0 to 63 is GSM unit. While -110 to -47 is dbm
Rxlevel unit of GSM. When we will add 110 to Rxlevel
[dbm],it will get convert into GSM unit 0 to 63.

Long Term Evolution (LTE) is the next step from 3G/WCDMA &
HSPA for many already on the GSM technology curve but also
for others too, such as CDMA operators. This new radio
access technology will be optimized to deliver very fast
data speeds of up to 100Mb/s downlink and 50Mb/s uplink
(peak rates).

Designed to be backwards-compatible with GSM and HSPA, LTE
incorporates Multiple In Multiple Out (MIMO) in combination
with Orthogonal Frequency Division Multiple Access (OFDMA)
in the downlink and Single Carrier FDMA in the uplink to
provide high levels of spectral efficiency and end user data
rates exceeding 100 Mbps, coupled with major improvements in
capacity and reductions in latency. LTE will support channel
bandwidths from 1.25 MHz to 20 MHz and both FDD and TDD
operation.

Although both LTE and WiMAX use the OFDMA air interface, LTE
has the advantage of being backwards compatible with
existing GSM and HSPA networks, enabling mobile operators
deploying LTE to continue to provide a seamless service
across LTE and existing deployed networks.

Several major mobile operators, including some running CDMA
networks today, have indicated they will adopt LTE in the
next few years. Japanese mobile operator NTT DOCOMO has said
that it is aiming to launch a commercial LTE network by the
end of 2009, while in the U.S., the largest CDMA operator,
Verizon Wireless, is currently trialing LTE with a view to
launching a commercial LTE service in 2010.

LTE- Advanced
LTE-Advanced extends the technological principles behind LTE
into a further step change in data rates. Incorporating
higher order MIMO (4x4 and beyond) and allowing multiple
carriers to be bonded together into a single stream, target
peak data rates of 1Gbps have been set.

LTE-Advanced also intends to use a number of further
innovations including the ability to use non-contiguous
frequency ranges, with the intent that this will alleviate
frequency range issues in an increasingly crowded spectrum,
self back-hauling base station and full incorporation of
Femto cells using Self-Organising Network techniques.

LTE-Advanced will be 3GPP’s technology as a candidate for
the ITU-R IMT-Advanced process, which is intended to
identify ‘4G’ technologies.

The main difference between GSM and CDMA is: In GSM, the
entire frequency band is not available to the end-user,
while in CDMA the entire frequency band is available to the
end-user. So, the Frequency Re-use factor is 1 in CDMA.

GSM Works as follows:
---------------------
The entire frequency band is divided into chunks and
each such chunk is divided into timeslots and each such
portion is made available to a user.

CDMA Works as follows:
-----------------------
The entire frequency band is available to the user. So,
in order to differentiate, the transmission from each user
is "spread" or coded using an unique code given to
individual user. At the receiving end, the spread
information is decoded.

Currently the digital modulation used in telecom is GSMK
GMSK is a form of modulation used in variety of digital
radio communication systems.It has an advantage4s of being
able to carry digital modulation while still using the
spectrum efficiently.
One of the problems with other forms of phase shift keying
is that the sidebands extend outwards from the main
carrierand this can cause interference to the other radio
communication systems using nearby channels.

these are the various block in gsm architecture -
1>BSS-Base Station Subsystem
a.MS
b.BTS
c.BSC
2>NSS-Network Swithing Subsystem
a.HLR
b.VLR
c.AUC
d.MSC
e.OMC
3>Public Network
a.PSTN
b.ISDN
c.Data Networks

TRAI-Telecom Regulatory authority of India
it gives spectrum,it allocate new operator,and for which
circle u hve to provide your Network.
If operator cross thatCircle then TRAI can also ban that
operator or take Fine also.