What is GSM Tech.
Principle behind this?
Ans: - GSM (Global
System for Mobile Communication.) It is a
digital cellular mobile communication system. This system meets the criteria as
follows.
- Spectrum efficiency.
- International Roaming.
- Low mobile and base station cost.
- Good Subjective Voice Quality.
- Compatibility with other systems such as ISDN.
- Ability to support new services.
|
MS
|
Mobile Station
|
|
BTS
|
Base Transceiver Station
|
|
BSC
|
Base Station Controller
|
|
BSS
|
Base Station Sub-system
|
|
MSC
|
Mobile services Switching Center
|
|
HLR
|
Home Location Register
|
|
VLR
|
Visitor Location Register
|
|
AuC
|
Authentication Center
|
|
EIR
|
Equipment Identity Register
|
|
OMC
|
Operations and Maintenance Center
|
|
NMC
|
Network Management
Center
|
|
ADM
|
Administration Center
|
·
Mobile Station
The mobile station comes in a
number of different forms, ranging from the traditional car-mounted phone
operating at 20W, through transportables operating at 8W and 5W, to the
increasingly popular hand portable units which typically radiate less than 2W.
A fifth class for hand portables operating at 0.8W has been specified for Micro
Cellular versions of the network.
One of the main factors governing
the hand portable size and weight is the battery pack. Several features of the
system are designed to allow this either to be smaller or to give a
substantially longer life between charges. Chief among these is Discontinuous
Receive (DRX). This allows the mobile to synchronize its listening period to a
known paging cycle of the network. This can typically reduce the standby power
requirements by 90%.
6.3.2.2. The Radio Sub-system
When the mobile user initiates a
call, his equipment will search for a local base station. A base station
sub-system (BSS) comprises a base station controller (BSC) and several base
transceiver stations (BTS), each of which provides a radio cell of one or more
channels. The BTS is responsible for providing layers 1 and 2 of the radio
interface, that is, an error-corrected data path. Each BTS has at least one of
its radio channels assigned to carry control signals in addition to traffic.
The BSC is responsible for the management of the radio resource within a
region. Its main functions are to allocate and control traffic channels,
control frequency hopping, undertake handovers (except to cells outside its
region) and provide radio performance measurements. Once the mobile has
accessed, and synchronized with, a BTS the BSC will allocate it a dedicated
bi-directional signaling channel and will set up a route to the Mobile services Switching
Center (MSC).
6.3.2.3. The Switching
Sub-system
The MSC routes traffic and
signaling within the network and interworks with other networks. It comprises a
trunk ISDN exchange with additional functionality and interfaces to support the mobile application. When a
mobile requests access to the system
it has to supply its IMSI (International Mobile Subscriber Identity). This is a
unique number which will allow the system to initiate a process to confirm that
the subscriber is allowed to access it. This process is called authentication.
Before it can do this, however, it has to find where the subscriber is based.
Every subscriber is allocated to a home network, associated with an MSC within that
network. This is achieved by making an entry in the Home Location Register
(HLR), which contains information about the services the subscriber is allowed.
The HLR also contains a unique authentication key and associated
challenge/response generators.
6.3.2.4. Mobility Management and
Security
Whenever a mobile is switched on,
and at intervals thereafter, it will register with the system; this allows its
location in the network to be established and its location area to be updated
in the HLR. A location area is a geographically defined group of cells. On
first registering, the local MSC will use the IMSI to interrogate the
subscriber's HLR and will add the subscriber data to its associated Visitor
Location Register (VLR). The VLR now contains the address of the subscriber's
HLR and the authentication request is routed back through the HLR to the
subscriber's Authentication
Center (AuC). This
generates a challenge/response pair which is used by the local network to
challenge the mobile. In addition, some operators also plan to check the mobile
equipment against an Equipment Identity Register (EIR), in order to control
stolen, fraudulent or faulty equipment.
The authentication process is
very powerful and is based on advanced cryptographic principles. It especially
protects the network operators from fraudulent use of their services. It does
not however protect the user from eavesdropping. The TDMA nature of GSM coupled
with its frequency hopping facility will make it very difficult for an
eavesdropper to lock onto the correct signal however and thus there is a much
higher degree of inherent security in the system than is found in today's
analogue systems. Nevertheless for users who need assurance of a secure
transmission, GSM offers encryption over the air interface. This is based on a
public key encryption principle and provides very high security.
6.3.2.5. Call Set-up
Once the user and his equipment
are accepted by the network, the mobile must define the type of service it
requires (voice, data, supplementary services etc.) and the destination number.
At this point a traffic channel with the relevant capacity will be allocated
and the MSC will route the call to the destination. Note that the network may
delay assigning the traffic channel until the connection is made with the
called number. This is known as
off-air call set-up, and it can reduce the radio channel occupancy of any one
call thus increasing the system traffic capacity.
GSM Recievers.
1.
GSM 900 in Europe and Asia
Pacific.With 890 MHZ – 915 MHZ Uplink and 935 MHZ – 960 MHZ Downlink
Frequencies. There are 124 carriers per channel and carrier width is 200 KHZ,
Bandwidth 25 MHZ, Wavelength 33cm. and Channel separation 20 MHZ. (
Freq MHz = 890 + 0.2 * n ) where 1≤
n ≤124
2.
GSM 1800 in Europe, Asia Pacific and Australia. With
1710 – 1785 MHZ Uplink and 1805 – 1880 MHZ Downlink. The carrier width is 200
khz , Band width 75 Mhz, and channel Separation is 20 Mhz. There are 375
carriers per channel.
Freq. MHz = 1710 + 0.2 * (n – 512) , where 512 ≤ n ≤ 885
3.
GSM 1900 US
, Canada, Latin
America and Africa.With 1850 – 1910 MHZ Uplink and 1930 – 1990 MHZ
Downlink. There are 300 Careers per channel, 60 MHZ Band width, Channel
Separation is 20 MHZ.
Modulation Used in GSM 900 is GMSK (Gaussian Minimum Shift
Keying)
Principle Behind this is the Frequency Reuse. A Geographical
area is divided into several hexagonal cells. Each cell has some specific
radius and having a set of frequencies. The frequencies allotted to each cell
in such a way that after some distance these frequencies can again be reuse by
other cells without interfering each other.
2.Q How many channels used in
GSM .Explain each ?
Ans: - Data burst for
traffic
Data burst
for control
Two types of channels Physical and Logical.
Physical channel is combination of Time slot and Carrier
freq. One RF channel supports eight physical channels in time slots 0, 1, 2,
----7.
A logical channel carries information of specific type.
Traffic channel (TCH)
carries digitally encoded user speech or data and have same function in both
forward and reverse link.
Control channel
carries signaling and synchronizing commands between BS and MS. Certain type of
control channels defined for forward and reverse link.
TCH Traffic
Channel Full rate and Half rate.
When transmitted as full rate user data is contained within
1 time slot per frame. 22.8 Kbits/ps.
When transmitted as half rate user data is mapped onto same
time slot but in alternate frames. 11.4 Kbits/ps.
Four types of control
channels.
1.
Broadcast
Control Channels.
2.
Associated
control Channels.
3.
Dedicated
Control Channels.
4.
Common Control
Channels.
Broadcast Channels: -
operates on forward link and transmit data on first time slot. It Contains.
1.
SCH
(Synchronization Channel) it is used to identify the serving BS and
allowing each mobile to frame synchronize with the BS. The frame no. is sent
with the BSIC during SCH burst. And also 6 bit BSIC.
2.
FCCH (Frequency
Correction Channel) The FCCH allows each MS to synchronize its internal
freq. with exact freq. of the BS.
3.
BCCH (Broadcast
Control Channel) It carries information’s such as cell and network
identity. It also broadcast a list of channels that are currently in use within
a cell.
4.
CBCH (Cell
Broadcast Channel) Used to transmit short alphanumeric text msg. to all MS
within a cell.
Common Control
Channels (CCCH): - CCCH helps to establish the call from the MS. Three
different types of CCCH are defined.
1.
The Paging Channel (PCH).
It is used to alert the MS of an incoming call.
2.
The Random Access Channel (RACH). Is used by MS to access the network.
3.
The Access Grant Channel (AGCH). Is used by the Base Station to inform the MS that which
channel it should use.
Dedicated Control
Channels (DCCH): - These channels are used for message exchange between
several mobiles or a mobile and network. Two types of DCCH are there.
1.
Stand Alone Dedicated Control Channels (SDCCH). Authentication, Registration,
Location area updation, SMS etc. needed for setting up a TCH.
2.
Slow Associated Control Channels (SACCH).
Associated Control
Channels: - Associated with the TCH.
1.
Slow Associated Control Channel (SACCH). Associated with TCH, Channel quality, Signal power level.
2.
Fast Associated Control Channel (FACCH). Uses time slots from TCH, Handover info.
4.Q What is Timing Advance ?
Ans.
Timing Advance
The timing of the bursts
transmissions is very important. Mobiles are at different distances from the
base stations. Their delay depends, consequently, on their distance. The aim of
the timing advance is that the signals coming from the different mobile
stations arrive to the base station at the right time. The base station
measures the timing delay of the mobile stations. If the bursts corresponding
to a mobile station arrive too late and overlap with other bursts, the base
station tells, this mobile, to advance the transmission of its bursts.
1 TA = 554m.
Calculation is given below.
Timing Advance:
T ´ T (bit) = (2d) ¤ c
Where T= Timing Advance
C = vel.of light 3´10^5
m /ms
T (bit) = 1 ¤ 270.833
(Raw bit rate per carrier is 270.833 Kbps. Each carrier is
shared by 8 users in TDMA Fashion.
There for bit rate for one user or one time slot is 1 /
270.833 Kbps ).
Now d = T ((T (bit) ´ c) ¤ 2)
= T
((1 ¤ 270.833) ´ 3 ´ 10^5) ¤ 2)
Now after calc. d= T ´ 554 m
TA is from 0 to 63.
5.Q What type of
modulation used in GSM ?
Ans.
Digital Modulation
The modulation chosen for the GSM system is the
Gaussian Minimum Shift Keying (GMSK). Figure 4 illustrates a GMSK modulator.
Q. What is handover? Explain it.
Ans.
Handover
The user
movements can produce the need to change the channel or cell, especially when
the quality of the communication is decreasing. This procedure of changing the
resources is called handover. Four different types of handovers can be
distinguished:
- Handover of channels in the same cell.
- Handover of cells controlled by the same BSC.
- Handover of cells belonging to the same MSC but controlled by different BSCs.
- Handover of cells controlled by different MSCs.
Handovers are mainly controlled
by the MSC. However in order to avoid unnecessary signaling information, the
first two types of handovers are managed by the concerned BSC (in this case,
the MSC is only notified of the handover).
The mobile
station is the active participant in this procedure. In order to perform the
handover, the mobile station controls continuously its own signal strength and
the signal strength of the neighboring cells. The list of cells that must be
monitored by the mobile station is given by the base station. The power
measurements allow deciding which the best cell is, in order to maintain the
quality of the communication link. Two basic algorithms are used for the
handover:
- The minimum acceptable performance algorithm. When the quality of the transmission decreases (i.e. the signal is deteriorated), the power level of the mobile is increased. This is done until the increase of the power level has no effect on the quality of the signal. When this happens, a handover is performed.
- The “power budget” algorithm. This algorithm performs a handover, instead of continuously increasing the power level, in order to obtain a good communication quality.
- Decibell Relation:
- db and dbm :-
1W = 30 dbm
2W = 33 dbm
dbm = 10 * log ( Pwr in Watts * 1000 )
OR 10 * log
(power in Watts) / 1 mW
- dbi and dbd : -
1 dbd = 2.14 dbi
dbi = dbd – 2.14
- Grade of Service GoS :
How much traffic can one cell carry? That depends on the
Number of traffic channels available and the acceptable
Probability that the system is congested, the so called
Grade of
Service (GoS).
- Key Performance Indicator ( KPI )
D1 ( Droop 1 ) or SD Droop < 1%
D2 ( Droop 2 ) or TCH Drop < 1.5%
SD Blocking < 0.5%
TCH Blocking < 0.5%
Congestion on SDCCH < 0.5%
Congestion on TCH < 1.5%
HOSR (Handover Success Rate) > 95%
TCH ASSR (TCH Assignment Success Rate) > 97%
CSSR (Call Setup Success Rate) > 98%
Setup Time = 3.5 Sec.
Availability = 99.9 %
CCR (Call Completion Rate) or CSR (Call Success Rate) >
96%
- Received Signal at MS and Path Loss:
= BTS (EIRP) – BTS to MS Path Loss + Donner Antenna Gain
(G1) – Feeder Loss +
Serving Antenna Gain (G2) – Avg. Fading Margin.
Where as Path Loss (db) = 20 log ( 4 Î d f / c )
Where ( d = Distance between antennas of BTS and MS.)
MS sensitivity = -102 dbm
BTS sensitivity = -104 dbm
Diversity Gain at BTS = 3.5 dbi
Antenna Gain at MS = 0.0 dbi
Slant Polarization Loss = 1.5 db
MS o/p Power = 2W or up to 0.8 W
EIRP = 53.7 db
Transmitted Power at BTS = 41 to 45 db
Duplex Loss at BTS = 0.8 db
Feeder loss and Jumper Loss at BTS = 3.00 db
Rayleigh fade margin without hopping = 3.4 db
Interference margin = 3.00 db
Car Loss = 6.00 db
Body Loss = 3.00 db
Dense urban loss = 6.00 db
- Erlang Traffic Theory :
Assuming that
one cell has two carriers, corresponding typically to 2x8-2=14 traffic
channels (two physical channels are needed for signaling)
and a GoS of 2% is acceptable, the traffic that can be offered is A=8.20 E. See
the table in Figure 3-1.
This number is interesting if an estimate on the average
traffic per subscriber can be obtained. Studies show that the average traffic
per subscriber during the busy hour is typically 15-20 or in special cases 40 -
50 mE. (this
can correspond to e.g. one call, lasting 54-72 seconds, per hour). Dividing the
traffic that one cell can offer, Acell=8.20
E, by the traffic per subscriber, here chosen as Asub=0.025
E, the number of subscribers one cell can support is derived as 8.20/0.025 =
328 subscribers.
When half rate is used it will theoretically
double the number of available traffic channels. In practice, however, live
networks will most likely consist of a mixture between half rate mobiles and
full rate mobiles.
Half rate will affect the SDCCH dimensioning
since more Signalling will be required when the number of TCHs is increased. An
important dimensioning factor is therefore the half rate penetration, i.e. the
percentage of half rate mobiles in the network.
When half rate TCH capacity calculations are
done it is assumed that the half rate mobiles are evenly spread among the
cells, i.e. all cells have the same half rate penetration. The TCH capacity
calculations made in this guideline are best illustrated with an example:
If for example a 2 TRX cell is used, it can
accommodate 14 full rate TCHs, i.e. 14 subscribers if one SDCCH/8 is used for
Signalling. A half rate penetration of 10 % would mean that 10 % of the 14
subscribers would be using a half rate connection, i.e. 1.4 subscribers (after
been rounded up = 2 subscribers). This would result in 13 timeslots used for
full rate and 1 timeslot used for half rate, resulting in 13 full rate TCHs and
2 half rate TCHs, i.e. 15 TCHs in total. The TCH capacity is then calculated for
15 TCHs with an Erlang B table with appropriate blocking figure.
Knowing the SDCCH holding times, with a given
number of performances during busy hour for every procedure, the generated
SDCCH traffic per subscriber can be calculated as follows:
For each type of procedure, multiply the
number of performances per busy hour and subscriber by the holding time of the
channel. By dividing the result by 3.6, the procedures contribution to the
SDCCH load in mErlang/subscriber is achieved.
