Interview Questions.

Question 1. What Are The Three Services Offered By Gsm? Explain Each Of Them Briefly?

Answer :

GSM offerings are classified in three teleservices, bearer, and supplementary services.

Teleservices (speak with other subscribers).
Bearer provider (presents the underlying community potential necessary for transmission to occur among two factors in the identical or distinctive networks).The bearer offerings describe what the community can provide (e.G. Speech, records and fax).
Supplementary provider is optional which subscriber can subscribe totally free. 
Ex: call forwarding, name waiting.

Question 2. What Is Ciphering? Why Do We Need It? Name The Algorithm(s) Used In It?

Answer :

The cause of ciphering is to encode the burst in order that it cannot be interpreted through any device aside from the meant receiver. The ciphering set of rules in GSM is known as the A5 algorithm. It does no longer add bits to the burst, meaning that the input and output to the ciphering system is the same as the enter.

Networking Interview Questions
Question 3. What Is Authentication? Why Do We Need It? Name The Algorithm(s) Used In It?

Answer :

Authentication is the system to confirm that this person belong to the community. The Authentication algorithm in GSM is known as the A8 set of rules.

Question 4. What Is Equalization? Why Do We Need It?

Answer :

Adaptive equalization is a solution specifically designed to counteract the trouble of time dispersion.

It works as follows:

A set of predefined known bit styles exist, known as training sequences. These are known to the BTS and the MS (programmed at manufacture). The BTS instructs the MS to encompass this type of in its transmissions to the BTS.
The MS consists of the schooling collection (proven in the discern as “S”) in its transmissions to the BTS. However, due to the problems over the radio course, some bits can be distorted.
The BTS receives the transmission from the MS and examines the education series within it. The BTS compares the received education collection with the schooling sequence which it had instructed the MS to apply. If there are differences among the two, it can be assumed that the problems inside the radio path affected these bits have to have had a comparable have an effect on at the non-training sequence bits.
The BTS starts a system wherein it makes use of its understanding of what happened the schooling collection to accurate the alternative bits of the transmission.
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Question five. Explain Speech Coding?

Answer :

Speech Coding: Instead of the use of 13 bits according to sample as in A/D conversion, GSM speech coding makes use of 260 bits. This calculates as 50 x 260 = 13 kbits/s. This gives a speech pleasant which is appropriate for mobile telephony and similar with twine line PSTN phones.

The voice compression coding approach is broadly used in the present day digital communique structures. In this method, a voice coder is used to set up a version to simulate the voice and noise produced by human vocal organs. The parameters to form the model could be transmitted via the TCH channels.
The voice coder is based totally at the residual excited linear prediction (REIP) coder. 
Moreover, the long time predictor (LTP) is used to enhance the compression effect.
LTP can make the coding of residual statistics extra nice by using disposing of the vowels from the voice. With 20ms as the unit, the voice coder outputs 260bits after compressed coding. Therefore, the code rate is 13kbps. According to the exceptional classes of the importance of the facts, the output bits can be categorized into three categories: 50 very critical bits, 132 vital bits and seventy eight ordinary bits.
Comparing with the conventional PCM line on which the voice is coded without delay and transmitted (64kbps), the 13kbps voice price of the GSM machine is a good deal lower. The extra advanced voice coder within the destiny can further lessen the charge to 6.5kbps (half price voice coding).
The coding mode is known as Regular Pulse Excited-Long Term Prediction (RPE-LTP).
It works as comply with: 8KHZ of sampling is carried out first, after which divided into frames with 20ms; each body has four sub-frames; the length of every sub-body is 5ms; and the natural bit price is 13kbit/s.

Manual Testing Interview Questions
Question 6. What Do You Mean By Frequency Re-use?

Answer :

An operator purchases a few frequency band. This band is divided into channels (two hundred KHz). To cowl the whole united states or city, the operator generally tend to reuse the channels after a ways "D" which at this distance the interference can be below manage.

Question 7. Name The Interfaces Between A) Bts And Ms B) Bts And Bsc C) Bss And Msc?

Answer :

Um interface.
Abis interface.
An interface.
GSM Tutorial GSM Interview Questions
Question eight. What Is Ma?

Answer :

Mobile Allocation (all frequency available for frequency hopping inside the cellular).

Question 9. What Is Maio?

Answer :

Mobile Allocation Index offset. (The offset from the preliminary point in an array of frequency).

Wideband Code Division Multiple Access (WCDMA) Interview Questions
Question 10. What Is The Difference Between Synthesized Frequency Hopping And Base Band Frequency Hopping?

Answer :

In baseband hopping, the transmitter will trade its frequency on body basis.
In synthesizer hopping, the transmitter will exchange its frequency on time slot foundation. That is why they also said it is fast hopping.
CDMA Tutorial
Question 11. What Is Cycling Frequency Hopping?

Answer :

The hopping sequence occurs in a uniform way. (Not random).

CDMA Interview Questions
Question 12. What Is Hsn? How Do We Apply It?

Answer :

Hopping collection range, if its price (0) cycle hopping. Otherwise it's far random hopping.

Networking Interview Questions
Question thirteen. What Is Dtx? Why It Is Used?

Answer :

Discontinuous Transmission (DTX) is a mechanism which lets in the radio transmitter to be switched off most of the time during speech pauses.

DTX can be implemented independently to every course, in order that the manage of DTX should recollect two components:

The uplink mode
The downlink mode
DTX can be enabled or disabled for the uplink and/or downlink mode on a in step with-mobile foundation.

Reasons for DTX When DTX is applied, real transmission at the radio course is reduced. This will cause a lower of the interference level in co-channel cells (using the same frequency). Another advantage will appear whilst the use of DTX within the uplink mode: it saves battery strength for the cell station. However, a drawback of the DTX mode is that it slightly deteriorates the nice of transmission. Note that transmitting in DTX mode does no longer shop timeslots at the air-interface.

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Question 14. What Is Drx? Why Do We Need It?

Answer :

Discontinuous reception is approach used to preserve electricity on the MS. The paging channel, used by the BTS to sign an incoming name, is structured into sub channels. Each MS is assigned one of these sub-channels and wishes to pay attention most effective to its very own sub-channel. In the time among successive paging sub-channels, the cell can pass into “sleep mode”, when almost no energy is used.

Question 15. What Is The Gross Data Rate Of Gsm?

Answer :

270kbps.

5G Interview Questions
Question sixteen. What Is Erlangs? What Is Meant By Gos?

Answer :

Traffic refers to the use of channels and is commonly notion of as the maintaining time according to time unit (or the number of “call hours” in step with hour) for one or numerous circuits (trunks or channels). Traffic is measured in Erlangs (E), for example, if one subscriber is constantly at the cellphone, this would generate one call hour in line with hour or 1 E of site visitors. The traffic one mobile can deliver relies upon at the number of visitors channels to be had and the quantity of congestion that is perfect (to each the patron and the issuer), the so-known as Grade of Service (GoS).

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Question 17. We Use Two Different Bands For Gsm/dcs Communications; Gsm900 And Dcs-1800. Which One Is The Better Of The Two In Terms Of Coverage?

Answer :

G900 is better. Due to route loss components as frequency is accelerated, the losses which the sign will stumble upon can be extra.

LTE Interview Questions
Question 18. What Is Ta? Why Do We Need Ta?

Answer :

Time develop (alignment) procedure The RF conversation studies a propagation delay over the space among the BTS and the MS. In order to synchronize the MS to the BTS, a timing increase is used to align the time slots arriving at the BTS receiver:

The BTS measures the reception time of the incoming MS burst
The BTS requests the MS to strengthen its transmission to make amends for the delay over the gap. A 6-bit range shows what number of bits the MS have to boost its transmission.
The time develop value TA could have a cost between zero and sixty three bit lengths, which corresponds to a put off of between zero and 233 ms.
This results in a most mobile variety of 35 km, which is as an alternative decided via the TA than with the aid of the signal energy.
Manual Testing Interview Questions
Question 19. What Is Location Update? Why Do We Need Location Update?

Answer :

A Location Area (LA) is defined as a set of cells. Within the network, a subscriber’s area is thought by the LA which they are in. The identification of the LA in which an MS is presently placed is stored inside the VLR. When an MS crosses a boundary from a mobile belonging to one LA into a cell belonging to any other LA, it need to report its new region to the network1. When an MS crosses a cellular boundary within an LA, it does need to record its new region to the network. When there's call for an MS, a paging message is broadcast inside all cells belonging to an LA.

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Question 20. What Is Meant By Imsi, Tmsi, Imei And Ms-isdn? Why They Are Needed?

Answer :

IMSI = International Mobile Subscriber Identity

TMSI = Temporary Mobile Subscriber Identity

IMEI = International Mobile Equipment Identity

MS-ISDN =Mobile Station ISDN Number

The MSISDN is the directory wide variety allocated to the mobile subscriber. It is dialed to make a phone call to the cellular subscriber.
A MS is diagnosed through its IMSI. The IMSI is embodied within the SIM of the cellular gadget. It is furnished by means of the MS anytime it accesses the network.
The TMSI is an identification alias which is used rather than the IMSI while feasible. The use of a TMSI guarantees that the authentic identity of the cell subscriber remains confidential by getting rid of the need to transfer an IMSI code unciphered over a radio link.
A VLR allocates a completely unique TMSI code to every cell subscriber this is operating in its vicinity. This code, that's most effective legitimate inside the area supervised by using the VLR, is used to pick out the subscriber in messages to and from the MS.
When a exchange of region vicinity also entails a trade of VLR vicinity, a new TMSI code is allocated and communicated to the MS. The MS shops the TMSI on its SIM.
IMEI codes that become aware of the mobile equipment deployed within the GSM machine.
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Question 21. What Is Arfcn?

Answer :

Absolute Radio-Frequency Channel Number (ARFCN) is a code that specifies a pair of bodily radio providers and channels used for transmission and reception on the Um interface, one for the uplink sign and one for the downlink sign.

Question 22. Explain Power Control?

Answer :

Power Control allows the cell station and/or the BTS to boom or decrease the transmission energy on a in step with-radio link basis.
Power Control is separately carried out for the uplink and downlink. In both instances the BSC is chargeable for starting up Power Control; the cell station and the BTS undertake transmit power in step with the BSC Power Control commands.
Measurements whilst a mobile station is active on a name, it has the obligation of providing dimension data approximately the performance of the air-interface to its serving BTS so that the serving BSC can decide if a strength manage should be done. Also the serving BTS measures the overall performance of the air-interface. Whereas the cellular station measures the performance of the downlink, the BTS measures the overall performance of the uplink.
Downlink measurements The cellular station measures and reviews the following measurements to the BSC regarding the performance of the downlink:

Strength of the signal being acquired from its serving BTS (in dBm).
Quality of the sign being acquired from its serving BTS (in bit error charge).
Uplink measurements The BTS measures and reports the subsequent measurements to the BSC concerning the overall performance of the uplink:

Strength of the signal being obtained from the cell station.
Quality of the signal being acquired from the cellular station.
Periodically measuring the mobile station measures periodically the performance of the downlink, and sends the measurements inside the SACCH (Slow Associated Control Channel) thru the serving BTS to the BSC every SACCH multi-frame. This corresponds to the transmission of records every 104 TDMA frames or 480 Ms. The base station measures the first-class of the uplink. Also, it transfers the measurements in the SACCH to the BSC every 480 ms.

Signal electricity whilst the BSC notices that the signal energy of a specific radio link measured at the uplink will become under the decrease pre-defined threshold because the mobile station moves away from the BTS, it sends a Power Control command to the cellular station to boom its transmit energy (MS_TXPWR) via a pre-defined step (generally 2 dB).

Question 23. What Is The Difference Between Fdd And Tdd?

Answer :

FDD = frequency department duplexing (transmitter and receiver operates on one-of-a-kind frequencies)
TDD = Time department Duplexing (transmitter and receiver operates on identical frequency).
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Question 24. What Is An Extended Cell? How Does It Impact The System? Channels And Tdma Structure?

Answer :

The contemporary problem on the variety of a GSM cell website online to 35km is mandated by way of the length of the standard timeslots described within the GSM specification. The most distance is given by the most time that the signal from the mobile/BTS needs to attain the receiver of the cell/BTS on time to be successfully heard. At the air interface the postpone between the transmission of the downlink (BTS) and the uplink (mobile) has an offset of three timeslots. Until now the cell station has used timing advance to catch up on the propagation put off as the space to the BTS changes.

This timing strengthen is defined within the GSM specification as 64 bits, which gives the theoretical maximum BTS/mobile separation as 35km.

With Extended Range Cell Feature, the BTS is able to receive the uplink signal in  adjacent timeslots as opposed to one. When the mobile station reaches its maximum timing increase, i.E. Maximum range, the BTS expands its hearing window with an internal timing advance that offers the vital time for the mobile to be heard with the aid of the BTS even from the extended distance. This greater increase is the duration of a single timeslot, a 156 bit duration.

GSM Interview Questions
Question 25. Why Do We Use Multiple Access Schemes? What Is The Difference Between Fdma, Tdma And Cdma?

Answer :

Multiple Access schemes permit for plenty customers to get right of entry to the network.

FDMA= Users get right of entry to the community thru frequency division (separation among users is made through frequency). (1G cellular structures).
TDMA= Users get entry to the community thru time division (separation between customers is made thru time). (2G cellular systems).
CDMA= Users get right of entry to the community via code department (separation between users is made via code). (3G cellular systems).
Question 26. Which Channel(s) Is Used For Sms?

Answer :

Either SDCCH or SACCH.

Question 27. Which Channel Is Used By Ms To Request Access To The Network?

Answer :

RACH = random get right of entry to channel.

Wideband Code Division Multiple Access (WCDMA) Interview Questions
Question 28. What Is Agch?

Answer :

AGCH (Access Grant Channel) assigns a signaling channel (SDCCH) to the MS.

Question 29. Why Do We Need Sdcch?

Answer :

The BTS switches to the assigned SDCCH. The call set-up process is achieved in idle mode. The BSC assigns a TCH. (SDCCH is also used to transmit textual content messages).

Question 30. What Is A Physical Channel? How Do We Differentiate Between Physical And Logical Channels?

Answer :

Each time slot on a TDMA body is called a physical channel. Therefore, there are 8 physical channels in step with service frequency in GSM.

Physical channels can be used to transmit speech, statistics or signaling records. A bodily channel may additionally carry one-of-a-kind messages, relying on the statistics which is to be sent. These messages are called logical channels. For example, on one of the physical channels used for visitors, the site visitors itself is transmitted the use of a Traffic Channel (TCH) message, even as a handover coaching is transmitted the use of a Fast Associated Control Channel (SACCH) message.

Question 31. Why Do We Need Fcch, Sch And Bcch?

Answer :

FCCH: Identifies BCCH provider via the provider frequency and synchronizes with the frequency.

Synchronization Channel (SCH): Transmits records approximately the TDMA frame shape in a cell (e.G. Frame wide variety) and the BTS identification (Base Station Identity Code (BSIC)).

BCCH: Broadcasts some general cellular information which includes Location Area Identity (LAI), maximum output strength allowed inside the mobile and the identity of BCCH companies for neighboring cells.

Question 32. Why Do We Need Sacch?

Answer :

Instructs the MS the transmitting energy to use and offers instructions on timing strengthen.

Question 33. What Is The Purpose Of Pch And Cbch?

Answer :

Paging Channel (PCH): Transmits a paging message to signify an incoming name or quick message. The paging message carries the identification variety of the cellular subscriber that the network desires to contact.

Cell Broadcast Channel (CBCH): BS uses this logical channel to transmit brief message service cellular broadcast.

CDMA Interview Questions
Question 34. Explain The Structure Of A Traffic Multi Frame. Why Do We Need Sacch And Idle Bursts In A Traffic Multi Frame?

Answer :

Traffic Multiframe Structures - The 26 traffic Multiframe shape is used to send facts at the traffic channel. The 26 site visitors Multiframe shape is used to combine person data (traffic), sluggish manipulate signaling (SACCH), and idle term.

The 12th body (no. Thirteen) inside the 26-body traffic channel Multiframe is used by the Slow Associated Control Channel (SACCH) which consists of link manage records to and from the MS–BTS. Each timeslot in a mobile allotted to visitors channel usage will follow this format, that is, 12 bursts of site visitors, 1 burst of SACCH, 12 bursts of traffic and 1 idle.

Idle Bursts: The idle term permits a mobile tool to carry out different vital operations which includes monitoring the radio signal electricity degree of a beacon channel from different cells. The time interval of a 26 frame traffic Multiframe is 6 blocks of speech coder information (120 msec).

Question 35. How Is A Facch Formed? When Is A Facch Used?

Answer :

Fast Associated Control Channel (FACCH): Transmits handover records.

Question 36. What Do You Mean By Eirp?

Answer :

Effective isotropic radiated energy (EIRP) is the amount of electricity that a theoretical isotropic antenna (that frivolously distributes strength in all guidelines) might emit to produce the height energy density observed inside the path of most antenna benefit.

EIRP can recall the losses in transmission line and connectors and consists of the gain of the antenna. The EIRP is regularly said in phrases of decibels over a reference energy emitted with the aid of an isotropic radiator with equivalent signal power. The EIRP permits comparisons among special emitters no matter type, length or shape.

5G Interview Questions
Question 37. What Is Polarization? What Are The Types Of Polarization?

Answer :

The polarization indicates the plane wherein the wave is vibrating. The polarization plane is taken to be that of the electric thing.

Vertical and horizontal are the handiest varieties of polarization, and they each fall into a class called linear polarization. However, it is also viable to apply round polarization. This has a number of benefits in regions together with satellite tv for pc programs, wherein it helps to triumph over the results of propagation anomalies, floor reflections and the spin that arise on many satellites. Circular polarization is a little greater difficult to visualize than linear polarization; but, it may be imagined through visualizing a signal propagation from an antenna that is rotating. Another shape of polarization is referred to as elliptical polarization.

Question 38. What Is Fading?

Answer :

Fading is the variation of the obtained signal with time, it happens due to propagation distance.
Short time period (fast) fading: resulting from multipath propagation.
Long time period (slow) fading: due to shadowing.
Question 39. What Is Rayleigh Fading?

Answer :

Rayleigh fading is an inexpensive version whilst there are many objects within the environment that scatter the radio sign earlier than it arrives at the receiver. 

Rayleigh fading fashions assume that the significance of a sign that has passed thru any such communications channel will range randomly, or fade, in keeping with a Rayleigh distribution the radial issue of the sum of two uncorrelated Gaussian random variables.

Rayleigh fading is maximum relevant while there's no dominant propagation alongside a line of sight between the transmitter and receiver. If there may be a dominant line of sight, Rician fading can be more relevant.

Question forty. What Is Multi Path Fading?

Answer :

Multipath fading is receiving multiple copies of the signal at receiver because of reflections. The copies attain the receiver with specific stages, so summation either positive or adverse. This influences the quality of obtained sign in phrases of BER.

LTE Interview Questions
Question 41. What Are The Different Types Of Diversity?

Answer :

Space Diversity.
Polarization Diversity.
Time Diversity.
Frequency Diversity.
Question 42. Explain Various Types Of Antenna Diversity?

Answer :

Antenna variety will increase the acquired signal energy by means of taking benefit of the herbal homes of radio waves.

There are  number one range techniques: area range and polarization range.

Space Diversity: Increased obtained sign electricity on the BTS can be completed by using mounting  receiver antennae as opposed to one. If the 2 Rx antennae are physically separated, the possibility that each of them are tormented by a deep fading dip on the identical time is low. At 900 MHz, it is feasible to advantage about three dB with a distance of 5 to six meters between the antennae. At 1800 MHz the space may be shortened due to its reduced wavelength.

By choosing the satisfactory of each sign, the impact of fading can be decreased. Space variety offers slightly higher antenna advantage than polarization range, but calls for extra space.

Polarization Diversity: With polarization diversity the 2 space range antennae are changed via one twin polarized antenna. This antenna has regular size but carries two in another way polarized antenna arrays. The most common kinds are vertical/horizontal arrays and arrays in forty five diploma slant orientation. The  arrays are related to the respective Rx branches inside the BTS. The two arrays can also be used as mixed Tx/Rx antennas. For most programs, the distinction among the range gain for area variety and polarization diversity is negligible, however polarization variety reduces the distance required for antenna.

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