GSM Interview Preparation Guide

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★ The cell radius could be double or even more, based on the antenna, type of terrain and timing advance.
★ Indoor coverage is supported by GSM and could be achieved with the usage of picocell base station or an indoor repeater along with distributed indoor antennas.

★ Extended cell feature enables the operators for extending the coverage without installing new cell cites.
★ Extended cell feature is designed specifically for providing covering in rural areas, where density of population is low and installation of additional cell sites.
★ New features can be deployed in coastal areas for providing GSM coverage in offshore waters.

★ Time Division Duplexing, a 3GPP standardized version of Universal Mobile Telecommunications System networks.
★ The designated mobile sends the data in a stipulated time and receives data from the BTS in another time.
★ The time is divided into various time slots and user sends data in few time slots and receives the data in the rest of the time slots.
★ Alternative downlink and uplink directions are used.

★ Frequency Division Duplex is one of the techniques, in which one frequency band is utilized for transmitting and another used for receiving.
★ To up-link the carrying data from mobile phones to the base station, one electromagnetic block spectrum is allocated.
★ Various blocks of spectrum is allocated to down-link carrying data from the base station to the mobile telephone hand sets.

★ Power control is the intelligent selection of transmit power in a communication system for achieving best performance within the system.
★ The performance is depend on context and there are chances to include optimizing metrics like link data rate, network capacity, geographic coverage and range.
★ A higher transmit power translates into a higher signal power at the receiver.
★ The bit error rate will be reduces at the receiver of a digital communication link, by having higher signal-to-noise ratio.
★ Higher transmit power provides more protection against a signal fade in a wireless fading channel.

★ Absolute Radio Frequency Channel Number, a code to specify a pair of physical radio carriers and channels within the spectrum that is allocated for cellular mobile communications.
★ ARFCN is used for transmitting and receiving on the UM Interface, one to uplink signal and one to downlink signal.
★ ARFCN's bandwidth is 270.833 kHz; channel spacing is 200 kHz in a given GSM band.
★ The uplink-downlink spacing is generally 45 / 50 MHz.
★ If Frequency Division Duplex is operated in the system, then the channel number is associated with both uplink and downlink radio channels.
★ ARFCN is 1-124 within GSM900 spectrum, 512-885 within GSM1800, 512-810 within GSM1900, are used.

★ Mobile Subscriber Integrated services Digital Network Number, a unique number identifying a subscription in a GSM / UMTS mobile network.
★ It is the telephone number of the SIM card in a mobile phone.
★ MSIDN and IMSI are important numbers used to identify a mobile subscriber
★ MSIDN is used for routing the calls to the subscriber, where as IMSI identifies the SIM.
★ In the HLR, the IMSI is the key, and MSIDN is the number that is dialed for connecting a call to the mobile phone.

★ International Mobile Equipment Identity is a number usually unique for identifying GSM,WCDMA and iDEN mobile phones.
★ It is generally printed inside the battery compartment of the phone.
★ The number can be displayed on the screen by entering *#06# into the keypad on most of the phones.
★ A stolen phone can be stopped from accessing the network within the country, by using IMEI number for valid devices.
★ Unlike CDMA/MEID's electronic serial number, the IMEI is only used for identifying the device and has no permanent relation to the subscriber
★ The subscriber is identified by IMSI number, stored on the SIM card, which then can be transferred to any other handset.

★ Temporary Mobile Subscriber Identity for most commonly sent between the mobile and the network.
★ TMSI is assigned randomly by VLR to each mobile in the area.
★ The TMSI number is local for a specific local area, and need to be updated every time when the mobile moves to a new geographical area.
★ Paging a mobile is the key use of the TMSI.
★ The communication between the mobile and the base station is known as paging.
★ The emphasis of broadcast information is top set up channels for paging.
★ Broadcast mechanism is available in every cellular system for distributing broadcast information to a plurality of mobiles.

★ International Mobile Subscriber Identity - a unique identification that is associated with all GSM and UMTS network mobile phones.
★ The SIM inside the phone contains it as a 64 bit field.
★ Used for acquiring certain details in Home Location Register or Visitor Location Register.
★ IMSI is rarely sent for preventing eavesdroppers to identify and track the subscriber on the radio interface.
★ It is presented as 15 digit long number, where first 3 digits represent the Mobile Country Code and followed by the Mobile Network Code.
★ Remaining digits are mobile station identification number within the customer base network.

★ Grade of Service is a probability of a cell in a circuit group which is blocked / delayed for more than a specified interval.
★ GoS is expressed as a vulgar fraction / decimal fraction.
★ It is with reference to the busy hour at the time of greatest intensity of the traffic.
★ GoS may be viewed independently from the perspective of incoming vs outgoing calls.
★ The GoS is not essentially equal between source to destination pairs.

★ Total traffic volume per hour / 3600 seconds is described by erlang
★ For example, 60 calls in one hour and each lasting 5 minutes results the following:
Minutes of traffic in an hour = Number of calls x Duration
Minutes of traffic in an hour = 60 x 5 = 300 minutes
Hours of traffic in the hour = 300 / 60 = 5 hrs
The traffic figure = 5 erlangs

★ The transceivers allows to serve several frequencies and different sectors of the cell.
★ Sends and receives signals from / to higher network entities.

★ DRX is referred as driver receiver.
★ DRX performs the transmission and reception of signals.
★ Sends and receives the signals to or from higher network entities, as in base station controller in mobile telephony.

★ Discontinuous Transmission is one of the methods which is based on the advantage of the fact that a person speak less
★ Nearly 40% of time in general conversation, the transmitter is turned off during silence periods
★ Mobile phone power conservation is an added benefit of DTX
★ Voice Activity Detection is the most important component of DTX
★ DT X distinguishes between voice and noise inputs.
★ The transmitter is turned off when a voice signal is misinterpreted as noise. This causes annoying effect known as clipping is heard at the receiver's end
★ Comfort noise is created at the receiving end, in order to ensure the non-connection.

★ Hopping sequence number is utilized, to spread the interference between all cells that uses the same hopping TCHs.
★ The correlation between closely located cells can be avoided by using HSN.
★ HSN of zero corresponds to the cyclic hopping sequence, and 1 through 63 corresponds to various pseudo random patterns.

★ In Cycling Frequency Hopping, the frequencies are changed, in every TDMA frame.
★ For ex: f1,f2,f3 …. Where f1,f2,f3 are frequency hopping sequence numbers
★ The specified setting parameter is HSN.
★ Hopping Sequence Number is set to 0 in a cyclic sequence hopping.
★ The sequence of frequencies starts from the lowest absolute frequency number in a set of frequencies specified for a specific channel group, to highest absolute frequency number.

★ Base Band Frequency Hopping is one of the frequency hopping techniques.
★ Time division multiplexing mapping is used in BBFH
★ BBFH is supported by a base station having a broadband transceiver
★ Physical channels are permitted by BBFH based upon mobile subscribers' communication with the base station.
★ In this, static transceiver frequencies are used while operating with broadband transceiver. This method is an exclusive of switching communication signals between transceivers.

★ The GSM's number of frequencies is 124
★ When the operators do not have enough frequencies, the range to be used is CDS 1800
★ Frequency hopping is used when the frequencies used are more with high interference.
★ Frequency hopping is utilized to equalize interference for the frequencies that are used in their range.
★ One of the frequency hoppings is Synthesised Frequency Hopping
★ Synthesized hopping allows the transmitter to change its frequency based on time slots. Because of this process SFH is known as fast hopping.

★ Mobile Allocation Index Offset is applied to same timeslot of different TRX.
★ Every time slot will have a separate / different MAIO, as they are using the same HSN
★ In case all time slot have the same HSN, a their sequence need to be started at different frequencies
★ The MAIO will set the index of the sequence for each timeslot.
★ The HSNs do not run the same pseudo random sequence at the same moment.

★ The Absolute Radio Frequency Carrier Number is used in hopping sequence pattern are determined by certain contents of the test set's Mobile Allocation Table.
★ The hopping sequence beginning point is known as Mobile Allocation Index Offset.
★ The first entry of Mobile Allocation is corresponds to zero.
★ An MA table is a list of ARFCNs. They present in Cell Allocation Table which shares the same frequency band.
★ A separate MA table is allocated for each frequency band.

★ The wireless equivalent of the Government Emergency Telecommunications Services is referred to WPS.
★ WPS provides access based on the priority to wireless telephone service during emergencies
★ Certain governing bodies such as authorized federal, officials of state and local, and certain key private sector officials can be put first in line for an open cellular channel by dialing 272 just before entering the number being called
★ It is T-Mobile, which began offering an initial operating capability in New York and Washington and had it deployed later.

★ The process of dividing the radio coverage of a cell site within a wireless telephone system into new cell sites is known as Cell Splitting
★ Additional capacity within the region can be achieved by using Cell Splitting

★ 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.

★ The data rate for the radio channel is 270 kbps
★ The data rate is split into 8 full rate or 16 half rate traffic channels, along with signaling 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.