What
is GSM ?
Global System for
Mobile Communications |
Presented
by: |
Copyright 2000© |
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Radio
spectrum is very limited, that’s why we have only 10-25MHz dedicated
to wireless communication. Such narrow bandwidth allows 100-400 channels
of reasonable quality, which is not rational and commercially not
profitable to develop network for such small number of mobile
subscribers. Genius idea lead to division of the whole geographical area
to relatively small cells, and each cell may reuse the same frequencies
by reducing power of transmission. Each cell has its own antenna (base
station), and all base stations are interconnected using microwave or
cable communication.
Once
upon a time there was analog cellular communication that didn’t
support encryption, compression, and ISDN compatibility; in addition
each country (company) developed its own system, which was incompatible
with everyone else’s in equipment and operation.
So,
in early 80s Europeans realized that pan-European public mobile system
should be developed. The new system had to meet certain criteria:
Figure
1. Layout of generic GSM network
GSM
Network consists of three main parts:
Consists
of:
Consists
of:
Consists
of:
-
Manages
the location of mobiles
-
Switches
calls
-
Manages
Security features
-
Controls
handover between BSCs
-
Resource
management
-
Interworks
with and manages network databases
-
Collects
call billing data and sends to billing system
-
Collects
traffic statistics for performance monitoring
-
Home
Location Register (HLR)
contains all the subscriber information for the purposes of call
control, and location determination. There is logically one HLR
per GSM network, although it may be implemented as a distributed
database.
-
Visitors
Location Register (VLR)
is only a temporary storage while the particular subscriber is
located in the geographical area controlled by the MSC/VLR.
Contains only the necessary information provision of subscribed
services.
-
Authentication
Center (AuC) is a
protected database that stores the security information for each
subscriber (a copy of the secret key stored in each SIM).
-
Equipment
Identity Register (EIR)
is a list of all valid mobile equipment on the network.
As
we have mentioned above radio spectrum is very limited resource shared
by all users. The method to divide up the bandwidth among as many users
as possible, chosen by GSM, is a combination of Time-
and Frequency-Division
Multiple Access (TDMA/FDMA).
FDMA divides
frequency bandwidth of the (maximum) 25 MHz into 124 carrier
frequencies. Each Base Station (BS)
is assigned one or more carrier frequencies. Using a TDMA
scheme each carrier frequency is divided in time, which forms logical
channels.
Time
Division Multiple Access (TDMA) - the
users take turns (in a round robin), each one periodically getting the
entire bandwidth for a little burst of time.
Frequency
Division Multiple Access (FDMA)
- the frequency spectrum is divided among the logical channels, with
each user having exclusives possession of some frequency band.
Mobile
unit can be in two modes
There
are two kinds of channels:
Figure
2. Organization of bursts, TDMA frames, and multiframes for speech
and data
The fundamental unit of time in TDMA
scheme is called a burst period
and it lasts 15/26 msec. Eight bust periods are grouped in one TDMA frame
(120/26 msec), which forms a basic unit
of logical channels.
One physical channel is
one burst period per TDMA frame.
Traffic
channels are defined as 26-frame
multiframe. 26-frame multiframe lasts
120 msec (26 * 120/26). Out of 26 frames, 24 are for traffic, 1 is
used for Slow Associated Control Channel (SACCH), and 1 is currently
unused.
The
speech is analog, so in order to be transmitted over digital
communication it should be digitized. The method used by GSM is Regular
Pulse Excited – Linear Predictive Coder (RPE-LPC) with a Long Term
Predictor loop. The main
idea behind this smart-looking name is simple. Speech is divided into 20
millisecond samples; current sample may be predicted from previous
samples, that’s due to slow change of voice patterns. Predicted and
real information are compared and the difference is saved. Each
20-millisecond sample is encoded using 260 bits (that requires 13 kbps).
Testing let to distinguish three classes of bits out of 260, that are
classified by they sensitivity to errors. The most sensitive class has
CRC and together with moderate sensitivity class is encoded using ½
rate convolutional encoder of length 4 – each input bit is encoded as
two bits, based on 4 previous bits. Thus we have 456 bits per 20
milliseconds sample (that requires 22.8 kbps).
The
idea is based on the fact that a person speaks less than 40% of time in
normal conversation, so turning the transmitter off can save power. In
order to distinguish voice and background noise, very accurate Voice
Activity Detector should be used. While transmitter is off, the
receiving end will hear a total silence, that’s due to digital
transmission. To avoid this, comfort noise is generated trying to match
the characteristics of background noise.
While
being in idle mode mobile station has to listen only
to Paging Channel, that uses almost
no power.
To
minimize co-channel interference and to conserve power, both the mobile
and BTS operate at the
lowest power level that will maintain an acceptable signal quality.
Mobile decides that power level is acceptable using bit errors ratio.
Figure
3. Signaling protocol structure in GSM
-
Layer
1 is the physical layer.
-
Layer
2 is the data link layer.
-
Layer
3 is the GSM signaling protocol.
We
have already seen structure used by physical
layer, so we won’t expand it any more. Data layer is modified
version of some protocol used in ISDN and in Signaling System Number 7.
So the only interesting thing that is left for us is Layer 3 - GSM
signaling protocol.
Layer 3 is itself divided into three sub-layers.
The
RR-Layer is concerned with the management of RR-session, which is the
time that a mobile is in dedicated mode, as
well as the configuration of radio channels. In addition RR-Layer
manages power control, discontinues
transmission and reception,
and handovers.
Handover
(handoff) is
switching of an on-going call to a different channel or cell.
There
are four types of handovers
-
Switching
channels in the same cell.
-
Switching
cells under control of the same Base Station Controller (BSC)
-
Switching
cells under the control of different BSCs,
but belonging to the same Mobil service Switching Center (MSC)
-
Switching
cells under control of different MSCs.
The
first two types of handover, called internal because they involve only BSC,
and MSC is notified only on
completion of the handover.
The
last two types of handover, called external because they involve MSC.
Handover
may be initiated by MSC
(traffic balancing) or by mobile unit.
The mobile unit always scans Broadcast
Control Channel of up to 16 neighboring cells, and forms a list of
the six best candidates for possible handover. This information is
transmitted to current Base Station at least once per second. BSC
and MSC use this
information for handover algorithm.
One
of the problems while making handover decision is whether the poor
signal quality is due to physical interference or mobile having moved to
another cell. There are two basic algorithms for making handover
decision:
-
Minimum
acceptable performance. If signal degrades beyond some point, then
transmission power is increased. If power increase does not lead to
improve then handover is performed. Disadvantages: increasing
transmission power may cause interference with neighbor cell.
-
Power
budget. Uses handover to improve transmission quality in the same or
lower power level. This method avoids neighbor cell interference,
but is quite complicated.
Manages
problem that arise from mobility of the subscriber. The ideal situation
is when system always knows where the subscriber is located (what cell)
in each moment. But this will cause the subscriber to update the system
on every move, and this means a lot of obsolete update messages, wasting
bandwidth. Another extreme situation is when system never knows
subscriber’s position, but this will cause the system to look for the
user over the whole geographical area, that means a lot of paging
messages on every terminating call. Strategy used by GSM is as
following, group of neighbor cells is grouped in one location area and
subscriber updates its position when moving from one location area to
another. Paging is done only in the current location area.
The only question is "what division of cells to location areas is
optimal?". There are various algorithm for solving this problem,
they are mostly based statistical data.
Figure
4. Registering to Mobile Switching Center (MSC).
As
seen form Figure 4, when a subscriber registers to MSC
it sends registration message that contains subscriber’s information. MSC
updates its VLR and
sends a message to subscriber’s HLR.
Since
the radio medium may be accessed by anyone, authentication is used to
prove that the users are who they claim to be. Each subscriber is given
a secret key that is recorded in subscriber’s SIM
and Authentication Center (AuC),
during authentication AuC
generates a random number that is sent to mobile. Using the secret key
and this random number mobile produces a response using ciphering
algorithm A3. The response number should be equal to the one calculated
by AuC.
The
same initial random number in conjunction with secret key is used to
generate the ciphering key using A8 algorithm. This ciphering key
together with TDMA frame number is used compute a sequence that is XORed
with the sent data.
Figure
5.
Paging process.
An
incoming mobile termination call is directed to Gateway
MSC (GMSC). GMSC is basically a switch,
which is able to interrogate the subscribers HLR
to obtain routing information. The routing information that is returned
to GMS is the Mobile Station Roaming Number
(MSRN). MSRN are related to the
geographical numbering plan, and not assigned to subscribers. To obtain
subscriber’s MSRN, subscriber’s HLR
have to query subscriber’s current VLR.
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