WHY GPRS IS USED ? 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.
Difference between circuit switched and packet switchedCircuit-Switched Communication: B C A D
GPRS ARCHITECTURE AUC HLR Gr Gd SMSC Internet EIR Gf PLMN Corporate Gs Gi LAN ISDN X.25 PSTN GWMSC MSC/VLR Network SGSN GGSN Gb Gn IP Backbone network BSC/TRC + PCU BSC/TRC Traffic GpSignaling PLMN TE MT BTS
GSM Network Modification or Upgrade Required for Element GPRS Terminal Equipment A totally new subscriber terminal is required to access GPRS services. (TE) 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 All the databases involved in the network will require software upgrades to handle the new call models and functions introduced by GPRS. (VLR, HLR, etc.)
Serving GPRS Support Node (SGSN)The SGSN forwards incoming and outgoing IP packets addressed to/from a mobile station that is attachedwithin 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 network’s 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.
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.
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.
Channel coding in GPRS Coding schemes AndThe Corresponding data rates
CHANNEL CODING IN GPRSChannel coding in a wireless cellular network is how the digital data(either voice or data for non-voice applications) from the mobile or basestation is formatted to deal with the problem of transmitting informationacross a radio channel.Channel coding includes parity generation, convolution coding, puncturingand interleaving. These processes are structured so that either the mobileor the base station can receive a stream of data bits with corrupted valuesand 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.
The main difference between the four coding schemes is thelevel of protection from transmission errors that they offerand the maximum throughput that can be obtained.The GPRS system dynamically chooses the coding schemebest suited for the transmission conditions at hand. Coding scheme -1 Coding scheme -4 Maximum protection to Maximum throughput error Lowest protection to Lowest throughput error
GPRS CODING SCHEMESFour coding schemes, CS-1 to CS-4, are used for the GPRSPDTCHs. They offer different levels of protection, and the CSto be used is chosen by the network according to the radioenvironmentScheme Code rate USF Pre- Radio Block BCS Tail Coded Punctured Data coded excl. USF bits bits rate USF and BCS kbps (Payload) 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
GPRS Data EncodingCS-2data in Block Check +16 bits268 bit Coding block 284 bits Add USF +3 bits 290 bits 287 bits USF add +4 bits pre-coding tail bits +3 bits 294 bits -132 bits Data out convolution x2 puncturing coding 588 bits 456 bits
GPRS ModulationModulation 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 withstandthe severe multipath fading but also the BER should at least be <10^-2 or better. (minimumacceptable BER for speech communication).
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 mainlobe width and the side lobe power). Then the output is passed on FM Modulator.
GMSK Performance:The performance of GMSK is measured by the 3 dB bandwidth-bit duration product ofthe Gaussian filter (BT) where as BT is lowered the amount of inter-symbol interferenceintroduced decrease.As BT decreases bandwidth increases.In GPRS we use BT= 0.3 which is the best compromise between increased bandwidthoccupancy and resistance to ISI.
EDGEEDGE system is quite similar to the GPRS system but with the capability for higher datarates.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.
Notes:• 3 Bits per symbol enables a data rate of 59.2 kbps per radio time slot.• Modulation amplitude variation changes the radio performancecharacteristics, so hardware changes in the base stations are mandatory. Diagram Showing EDGE modulation benefits
Another improvement that has been made to the EGPRS standard is the ability toretransmit 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 theproblem of choosing the wrong coding scheme for the next sequence of radio blocksbecause re-segmentation is possible.
EDGE Radio Network Planning:Coding Schemes:There are nine modulation and coding schemes (MCS-1 to MCS-9) that providedifferent 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 toMCS-9 it is 8-PSK.
Another advantage in EDGE networks is that the switching between differentcoding 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 notpossible to switch the coding scheme on reception failure, so the re-transmissiontakes 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 sentagain but with better protection than for its initial transmission. This is donethrough a process called link adaptation.Advantages of using link adaptation (LA):It leads to the highest throughput possible with the lowest amount of delay. Thisgives better link quality and makes EDGE a more efficient system.
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