Gprs Edge PPT
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Transcript of Gprs Edge PPT
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GPRS / EDGE
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Sometimes it is more cost effective to simply augment the existing system rather than completely creating a newer one. This is the case with GSM and
GPRS.
GPRS was only an extension to the older GSM technology.
As the demand for data services increased, GPRS was developed to support packet switching.
A feature that used GPRS technology was the Multimedia Messaging System or MMS. It allowed subscribers to send videos, pictures, or sound clips to
each other just like text messages.
GPRS also gave mobile phones the ability to surf the internet at dial-up speeds through WAP enabled sites.
WHY GPRS IS USED ?
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Difference between circuit switched
and packet switched
A
B C
D
Circuit-Switched Communication:
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Packet-Switched Communication:
A
B C
D
Info
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Gs
SMSC
BTSMTTE
BSC/TRC
GWMSC
ISDN
PLMN
PSTN MSC/VLR
HLR
AUC
SGSN GGSN
PLMN
X.25
Network
Corporate
LAN
InternetGd
Gp
Gf
Gr
Gb
Gi
Gn
IP Backbone network
BSC/TRC + PCU
EIR
Signaling
Traffic
GPRS ARCHITECTURE
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GSM Network
Element
Modification or Upgrade Required for
GPRS
Terminal Equipment
(TE)
A totally new subscriber terminal is required to access GPRS services.
These new terminals will be backward compatible with GSM for voice calls.
BTS A software upgrade is required in the existing base transceiver site (BTS).
BSC The base station controller (BSC) will also require a software upgrade, as
well as the installation of a new piece of hardware called a packet control unit
(PCU). The PCU directs the data traffic to the GPRS network and can be a
separate hardware element associated with the BSC.
Core Network The deployment of GPRS requires the installation of new core network
elements called the Serving GPRS Support Node (SGSN) and Gateway
GPRS Support Node (GGSN).
Databases
(VLR, HLR, etc.)
All the databases involved in the network will require software upgrades to
handle the new call models and functions introduced by GPRS.
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Serving GPRS Support Node (SGSN)
The SGSN forwards incoming and outgoing IP packets addressed to/from a mobile station that is attached
within the SGSN service area.
The SGSN provides:
Packet routing and transfer to and from the SGSN service area.
Ciphering and authentication.
Mobility management
Output of charging data, the SGSN collects charging information for each MS related to the radio network usage.
Gateway GPRS Support Node (GGSN)
The GGSN provides:
The interface towards the external IP packet networks.
From the external IP networks point of view, the GGSN acts as a router for the IP addresses of all subscribers served by the GPRS network.
Functionality for associating the subscribers to the right SGSN
Output of charging data, the GGSN collects charging information for each MS, related to the external data network usage.
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LIMITATIONS OF GSM DATA SERVICES
Enhancements of GSM data rates: HSCSD
Which allows the assignment of maximum 4 circuit switched time slots to the
same user over the air interface.
Thus the rate of 4 x 9.6(GSM data rate) = 38.6 kbps is achievable
Enhancements of GSM data rates: Coding Schemes
Coding Scheme 1 (CS1) was the first CS to be used and it adds a large number
of coding bits causing the user rate to be low.
Rate of data over GSM using this CS is 9.6 Kbps
To increase the rate more data will be sent Instead of strong error correction.
This makes the link less reliable but increases the rate.
Coding Scheme 2 (CS2) uses a less number of coding bits allowing the user rate
to reach 14.4 Kbps.
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LIMITATIONS OF GSM DATA SERVICES
Enhancements of GSM data rates: HSCSD + Coding
Schemes
Combining the effect of the HSCSD and CS2 will jump with the rate to reach
14.4 x 4 = 57.6.
Yet, this bit rate is still low for some applications and consumes large number of
resources, the service will be expensive to the user.
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Channel coding
in GPRS
Coding schemes
And
The Corresponding data rates
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CHANNEL CODING IN GPRS
Channel coding in a wireless cellular network is how the digital data
(either voice or data for non-voice applications) from the mobile or base
station is formatted to deal with the problem of transmitting information
across a radio channel.
Channel coding includes parity generation, convolution coding, puncturing
and interleaving. These processes are structured so that either the mobile
or the base station can receive a stream of data bits with corrupted values
and still have a high likelihood of decoding the bit stream correctly.
The GPRS data is protected by four different channel protection schemes:
CS1, CS2, CS3, and CS4.
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The main difference between the four coding schemes is the
level of protection from transmission errors that they offer
and the maximum throughput that can be obtained.
The GPRS system dynamically chooses the coding scheme
best suited for the transmission conditions at hand.
Coding scheme -1Maximum protection to
error
Lowest throughput
Coding scheme -4Maximum throughput
Lowest protection to
error
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GPRS CODING SCHEMES
Four coding schemes, CS-1 to CS-4, are used for the GPRS
PDTCHs. They offer different levels of protection, and the CS
to be used is chosen by the network according to the radio
environment
Scheme Code rate USF Pre-coded USF
Radio Block excl. USF and BCS (Payload)
BCS Tail Coded bits
Punctured bits
Data rate kbps
CS-1 1/2 3 3 181 40 4 456 0 9.05
CS-2 2/3 3 6 268 16 4 588 132 13.4
CS-3 3/4 3 6 312 16 4 676 220 15.6
CS-4 1 3 12 428 16 - 456 - 21.4
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Block Check Coding
USF
pre-coding
add
tail bits
convolution
codingpuncturing
data in
Data out
290 bits
294 bits
588 bits 456 bits
287 bits
CS-2
+3 bits
+3 bits
+4 bits
x2
-132 bits
Add USF
268 bit
block
+16 bits
284 bits
GPRS Data Encoding
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Coding schemes performance
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GPRS Modulation
Modulation scheme used in GPRS is GMSKNow the question here is why GMSK?
Power efficiency.
Bandwidth efficiency.
Cost and the complexity of the receiver is low.
Good BER performance.
Lower values of the C/I ratio (Carrier-to-Interference ratio).
A higher traffic-carrying capacity.
Efficient utilization of available dc power using a class C power amplifier.
In view of this, the MSK and GMSK are good choices. It must not only be able to withstand
the severe multipath fading but also the BER should at least be
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MSK (Minimum shift keying):Is a special case of coherent FSK modulation where modulation index is equal to 0.5.
But Unfortunately, the main lobe of MSK is wide and it has a lot of side lobes
(unwanted) so it is not suitable for narrowband application.
GMSK (Gaussian Minimum Shift keying):GMSK is the MSK derivative.
In GMSK the base band binary data is passed on a Gaussian filter first (reduces the main
lobe width and the side lobe power). Then the output is passed on FM Modulator.
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The performance of GMSK is measured by the 3 dB bandwidth-bit duration product of the Gaussian filter (BT) where as BT is lowered the amount of inter-symbol interference
introduced decrease.
As BT decreases bandwidth increases.
In GPRS we use BT= 0.3 which is the best compromise between increased bandwidth occupancy and resistance to ISI.
GMSK Performance:
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EDGE
EDGE system is quite similar to the GPRS system but with the capability for higher data
rates.
The most important change is the new modulation scheme.
In GSM and GPRS-> the GMSK modulation scheme -> one bit per symbol is used + constant amplitude modulation.
In an EDGE network-> 8-PSK modulation-> three bits per symbol + variation in modulation amplitude.
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Notes:
3 Bits per symbol enables a data rate of 59.2 kbps per radio time slot.
Modulation amplitude variation changes the radio performance
characteristics, so hardware changes in the base stations are mandatory.
Diagram Showing EDGE modulation benefits
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Another improvement that has been made to the EGPRS standard is the ability to
retransmit a packet that has not been decoded properly with a more robust coding scheme,
IN GPRS-> no re-segmentation i.e. once packets have been sent, they must be
retransmitted using the original coding scheme even if the radio environment has
changed.
IN EGPRS-> re-segmentation i.e. Packets sent with little error protection can be
retransmitted with more error protection, if required by the new radio environment.
Conclusion:In EDGE, the rapidly changing radio environment has a much smaller effect on the
problem of choosing the wrong coding scheme for the next sequence of radio blocks
because re-segmentation is possible.
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EDGE Radio Network Planning:
Coding Schemes:There are nine modulation and coding schemes (MCS-1 to MCS-9) that provide
different Throughputs. The MCS scheme carries data from 8.8 kbps to 59.2 kbps.
For coding schemes MCS-1 to MCS-4, modulation is still GMSK; for MCS-5 to
MCS-9 it is 8-PSK.
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Another advantage in EDGE networks is that the switching between different coding schemes can take place easily, which was not possible in a GPRS network.
When data transmission takes place in a GPRS coding scheme, it is not possible to switch the coding scheme on reception failure, so the re-transmission
takes place with exactly the same protection as for its initial transmission.
In EGPRS, it is possible to change the MCS, i.e. the data block can be sent again but with better protection than for its initial transmission. This is done
through a process called link adaptation.
Advantages of using link adaptation (LA):
It leads to the highest throughput possible with the lowest amount of delay. This
gives better link quality and makes EDGE a more efficient system.