GPRS Interview Questions and Answers

Here, we highlight some common questions you might be asked during a job interview for GPRS and telecom related positions. All the best for your interview Preparation.

Can you explain GPRS?

Short form of General Packet Radio System
GPRS is the next generation of GSM, and the basis of the 3G networks
A packet-oriented data service available to users of GSM and IS-136 type mobile phones
It complements the existing services like circuit-switched cellular phone connections and the SMS
Shortest set-up time is added for ISP connections
An optimized mobile service for sending and receiving information across a mobile telephone network, with less cost
GPRS provides data rates of mobile telephony and moderate speed data transfer, using unused TDMA channels
GPRS does not need dial-up modem connection
Sending and receiving e-mails, surfing internet are among the features of GPRS

What signals are transferred in GPRS?

GPRS uses 2.5 generation of GSM signals
The radio interface is the same that of GSM
GPRS uses 900 / 1800 Mhz, frequency band and GMSK modulation
The bit rates are EGPRS, similar to EDGE
Separate hardware and ports need to be added and availed.
How are GPRS terminals classified?
The GPRS services are classified into 12 service classes as per the duration of the time slots occupied / frame.
Usually 5 time slots per frame are occupied including transmission time slots and reception time slots
Increase in the number of occupied time slots certainly causes the increase in the entire duration of the occupied time slots, so that increased amount of data transmission and receipt are enabled
All the 12 service classes are further classified into higher and lower classes as per the relative duration of time slots
Higher speed data communication can be transmitted with high-class GPRS service by setting high transmission power and setting the duration of the time slots long.

What are the different coding schemes used in GPRS and their importance.?

Multiple Access Coding Scheme:

Multiple access scheme is used in GPRS based on the FDD and TDMA
One pair of up-link and down-link frequency channels are assigned to a user during a session
The Multiple Access Coding Scheme is combined with statistical multiplexing [packet mode communication], that allows several uses to share the same frequency channel
Downlink uses first-come first-served packet scheduling and uplink uses a scheme that is similar to reservation ALOHA.
Slotted ALOHA is utilized for reservation inquiries in a contention phase, followed by transferring data by utilizing dynamic TDMA with first-come, first-served scheduling

Channel Encoding Scheme:

Channel Encoding Scheme is based on a convolution code at various code rates and GMSK modulation
The table below summarizes the options:

Coding Scheme Speed

CS-1 8.0

CS-2 12.0

CS-3 14.4

CS-4 20.0

The least robust and fasted coding scheme is CS-4 and is available near a Base Transceiver Station
The most robust coding scheme is CS-1 and is used in case of mobile station is further away from a BTS
A speed of 20.0 Kbit/s/time slots can be achieved with CS-4 and has 25% normal cell coverage
A speed of 8.0 Kbit/s/time slots can be achieved with CS-1 and has 98% of normal coverage

Multislot Class:

Speed of data transfer available in the Uplink and Downlink directions is determined by Multislot Class
The speed varies between 1 and 45 for allocating by the network to allocate radio channels in the uplink and downlink channels
The representation of multi slot class is 5+2, for example, where the first number is the number of down link time slots and the second number is the number of uplink time slots that are allocated to use by a mobile station

What gross data rate does GPRS provide?

GPRS provide gross data rate of 22.80 KBPS / time slot
Voice uses only 13KBPS/ time slot and the rest is consumed by Error Correction Code
Error connection code is utilized for detecting and correcting the transmission errors
As voice is compressed, the usage of Error connection code is mandatory
How does GPRS architecture differ from GSM?
The following table depicts the architectural differences between GPRS and GSM

Mobile Station	New mobile station is needed for accessing GPRS services. They are backward compatible with GSM for voice calls
Base Transceiver Station	Software upgrade is needed in the existing BTS
Base Station Controller	Software upgrade and new hardware, Packet Control Unit are needed for GPRS.
GPRS Support Nodes	Installation of new core network elements, known as serving GPRS support node, and gateway GPRS support node are needed to deploy GPRS
Databases	Requires software upgrade to handle new models and functions to handle databases involved in the network

Differentiate between GSM and GPRS?

The differences between GSM and GPRS are depicted in the following table

GSM	GPRS
Uses one among 7 slots	Uses as many as 4+1 time slots
Connecting resources to each unit from remote location to the back office is done through a direct dial up	Does not claim any resources until some data is sent. The information is divided into packets
Circuit switched mode of operations	Packet switched mode of operations
Dedicated channel all the way to the destination is provided to the customer	One or more dedicated channels are assigned by the operator specifically for shared use

What is GSM?

Short form of Global System for Mobile Communications, is a wireless network system
A standard for digital cellular mobile communications
International roaming arrangements are enabled among mobile network operators, by providing the subscribers to use their personal mobile phones anywhere in the world.
GSM is considered as second generation mobile, as signaling and speech channels are digital
The carriers can be replaced without replacing mobile phones.
The ubiquity of GSM implementations is also enabling the switching between network operators, who can choose equipment from many GSM equipment vendors.
GSM pioneered low cost SMS implementation
The GSM standard includes a worldwide emergency telephone number feature.

Explain the services offered by GSM?

The services offered by GSM are ISDN compatible
GSM allows synchronous data and asynchronous data to be transported as bearer service, one of the telecommunication services offered by GSM, and forms ISDN terminal
The data could be either transparent service or nontrans parent service
Telephony is one of the services offered by GSM, which an emergency and the service provider is notified by dialling 3 digits
By using appropriate fax adaptor, G3 fax service is supported
One of the features of GSM is Short Messaging Service
Certain services, namely supplementary services are provided on top of teleservices or bearer services by GSM, such as, caller identification, call forwarding, multiparty conversations, barring outgoing calls and call waiting.

What is the maximum data rate supported by a GSM system?

Explain the Sectorization of a cell?

Explain the Frequency Reuse?

Frequency re-use is the key factor of cellular network ability for increasing both coverage and capacity.
Different frequencies are utilized by adjacent cells; however, problem does not arise to determine the cell sufficiently far apart using the same frequency.
Reuse distance and the reuse factor are the two elements that influence frequency reusability
The frequency is 1/K, where K is the number of cells that cannot use the same frequencies for transmission.

Explain the Transmitted Power Control?

Transmitted power control is one of the technical mechanism used within some networking devices
TPC is used for preventing too much unwanted interference among wireless networks
The central idea is to automatically reduce the transmission output power used, when other networks are within the same range.
TPC reduces interference problems and increases the battery capacity
The power levels of a single mobile device can be reduced by 6dB

Explain the Discontinuous Transmission?

Discontinuous Transmission, also known as DTX in short is a method of momentarily powering down a mobile set when there is no voice input to the set.
The overall efficiency of a wireless voice communication system is optimized by DTX.
Each speaker in a two-way conversation speaks slightly less than half of the time.
The workload of the components in the transmitter amplifiers are made easy and reduces interference.

Which uplink/ downlink spectrum is allocated to following? GSM – 900 and DCS – 1800?

What are the algorithms used in Ciphering and Authentication?

Explain Equalisation?

Explain Interleaving?

Time diversity in a signal communication system is obtained by using Interleaving.
The possibility of losing whole bursts will be decreased by interleaving
Total 456 bits from convolution encoder, including 20ms of speech, is subdivided into eight blocks. Each block consists of 57 bits.
All these blocks are transmitted in consecutive time slots.
Enough information is available in 7 blocks, in case one of the blocks is lost due to burst errors. This enables whole segment recovery by using error correction.

Explain Speech coding?

Speech coding is all about turning voice into digital form
Speech is inherently analog, as GSM is a digital system.
The digitization is employed by ISDN, and the current telephone systems that are used for multiplexing voice lines, with high speed trunks, optical fiber lines is done by Pulse Code Modulation(PCM)
The PCM output is 64 kbps, which is too high over a radio link in feasibility
The 64kbps signal is redundant
The algorithm used in conventional cellular is Vector Sum Excited Linear Predictive speech compression.

Explain Channel coding?

The data rate for the radio channel is 270 kbps
The data rate is split into 8 full rates or 16 half rate traffic channels, along with signalling channels
In order to have the maximum chance for detecting and correcting errors, the code is complex in a typical propagation path
Forward Error Correction is applied in order to get the speech coder encryption, coding and interleaving in a sophisticated way
The data is sent as bursts in 577 mus time slots. Each contains 116 encrypted bits
Every TDMA frame consists of 8 or 16 time slots
Transmit time slots are staggered, so that at some instant the mobile station will not receive the same instants while transmitting, which enables the simplifying the filtering requirements
At least one spare slot between transmit and receive is available with this scheme.

What is frequency re-use?

Frequency Reuse:

Frequency Reuse is one of the techniques for improving capacity and spectral efficiency
Commercial wireless systems are based on Frequency Reuse, that involves the partitioning of an RF radiation area into cell segments.
A frequency that is far enough away from the frequency in the bordering segment is used by one segment of the cell.
Similar frequency is used at least two cells apart from each other.
This practice enables various cellular providers to have several customers for a given site license.

What are the interfaces between BTS and MS?

Base station subsystem is a segment of cellular telephone network that is responsible for setting signals and traffic between mobile phone sets and network switching subsystems.
Transcoding of speech channels are carried out by BSS. BSS allocates radio channels to mobile phones, paging, transmission and reception over the air interface and many other tasks that are pertaining to the radio network.

What are the interfaces between BTS and BSC?

Base Station Controller provides the intelligence behind the BTSs. A BSC conrolls hundreds of BTSs.
Allocation of radio channels, receiving measurements from the mobile phones are some of the major task handlings by BSC.
BSC controls BTS to BTS.
BSC acts as a concentrator in which many different low capacity connections to BTSs will become reduced to few numbers of connections towards the mobile switching center.
BSC provides the required data to operation support subsystem(OSS) and also to the performance measuring centers
BTS supports the key features like, frequency hopping, sectorization, and GPRS.
The expansion and upgradation in the field are the features emphasized during its design.
The power output of BTS is up to 80W.

What are the interfaces between BSS and MSC?

What are the interfaces between TRAU and BSC?

TRAU (Transcoder and Rate Adaptation Unit) is an entity to perform a transcoding function for speech channels and RA (Rate Adaptation) for various data channels.
BSC and TRAU does not demand specific environmental conditions to perform operations.
With this the operator has an option for placing any one or both units at a central location.
BSC supports various BSC-BTS configurations like star, multidrop and loop, and star configurations towards TRAU
TRAU is a standalone unit which could be located close to a MSC (Mobile Switching Center), which enables the optimum utilization of 16 kbit/s channel sub multiplexing and saves line costs.