Telecom Interview Preparation

Que. :- Explain the structure of entire GSM.Ans :-The gsm techlogy is divided in two parts.1.BSSwhich consistBTSBSCTRAU2.NSSMSCHLRVLREIRAUCone or more numer of MS( Mobile stations are connected fromone BSC and the interface between MS and BTS is air or Uminterface,one or more bts is connected from one bsc and theinterface is A-bis,one or more BSC and TRAU is connected byone.The interface between MSC and BSC is called Airinterface and the interface between MSC and Trau is calledAter interface.MSc is connected from the HLR,VLR,EIR,Auc,GMSC,PSTN.Que. :- What is the function of Duplexer?Ans :-Duplexer unit perform duplex operation of TX and RX signalsthrough a common antenna and filters and amplifies main anddiversity received signals before they pass through theMulti coupler unit to the TRx unit.It contains a variable-gain LNA for optimal amplification ofthe received signal.Que. :- What is the function of TRAU?Ans :-TRAU, compress/decompress voice data 64kb data into 16kband vice-versa. so that we can trasport traffic of 4 callsin 1 single 64kbs Time Slot(E1/T1 carrier). Trau converts a 16kbps speech data into 64 kbps data onater interface. TRAU Means the Transmission Rate AdaptationUnit.Actually,it is a Transcoder unit(XCDR).it uses as aninterface B/W BSC & MSC.

Que. :- What is MTU?

Ans :-In computer networking, the term Maximum Transmission Unit

(MTU) refers to the size (in bytes) of the largest packetthat a given layer of a communications protocol can passonwards. MTU parameters usually appear in association with acommunications interface (NIC, serial port, etc.). The MTUmay be fixed by standards (as is the case with Ethernet) ordecided at connect time (as is usually the case withpoint-to-point serial links). A higher MTU brings higherbandwidth efficiency. However large packets can block up aslow interface for some time, increasing the lag on otherpackets. For example a 1500 byte packet, the largest allowedby Ethernet (and hence most of the Internet), would block upa 14.4k modem for about one second.

Que. :- What is the function of BOI card?

Ans :-BOIA (Base Operation and Interface unit)It handles the control functions common among other units inthe BTS.BOIA Card is like Brain of BTS.These functions include:1-BTS initialisation and self-testing2- Configuration .3- O&M signalling.4- software download .5- Main clock functions.6- Timing functions .7- Collection and management of external and internal alarms8- Message delivery to the BSC(Through FXC unit), and9- Cabinet control

Que. :- which slot is reserve for GPRS ?Ans :-Total time slots= 0 to 31= 32 slots. In which 27,28,29 & 30are reserved for GPRS. Each time slot is of 64kbps and totalis 64*4=256kbps. Thats why we are getting 256kbps whenconnecting to internet through mobile. This is standard inGSM/Edge BTS. We can also reduce the speed to 64, 128 or 192by changing the time slot.

Que. :- what are the function of BB2F card?Ans :-The main purpose for BB2F/A is to perform the DSP(digitalsignal processing) and frequency hopping.base band dual frame basically used for reception purpose.Que. :-How many types of Transmission cards are in Flexi BTS?Ans:-There are seven transmission modules and they are:FEIA = 8 E1'sFIPA = 8 E1'sFIFA = 16 E1'sFIQA = 4 E1's + 1 EthernetFIYA = 4 E1's + 1 EthernetFIQB = 4 E1's + 1 EthernetFIYB = 4 E1's + 1 Ethernet

Que. :-How many maximum TRX capacity of Nokia Flexi BSC?

Ans:-4200 TRXs support a nokia flexi BSC &Voice: 25,200 erlangs , Data: 30,720 (E)GPRS 16 kbit/s channels.

Que. :-BCSU card in BSC can support how many TRXs?

Ans:-Depends on the version of the BSC.S12 BSC BCSU will Support 200 TRX,Flexi BSC BCSU will Support 500 TRX

Que. :- Why we are using -48v in equipments?

Ans:-when we have +ve voltage we cannot transmit for long distance cause that will come to 0 volt or voltage get attenuated but if we have -48 volt that voltage lower to ground potential losses will be very less cause always energy flows from higher potential to lower so so -48 lower potential simply never come to ground potential.

Que. :- What is the full form of EDGE?

Ans:-Enhanced Data rates for GSM Evolution.

Que. :- What is the function of BTS?

Ans:-BTS has a set of transceivers to communicate with mobiles in its area. One BTS covers one cell. BTS transmits a BCH signal on one of the channels, on time slot 0. The BCH helps mobiles to identify the network. The capacity of a cell depends upon the no of channels loaded on BTS. Each RF channel is shared by 8 users in TDMA mode. A BTS connects to BSC through Abis interface, which is a 2 MB/s link. A data rate at GSM air interface is 13KB/s. 4 channels of 13 KB/s map into a 64 KB/s channel before fitting into 2 MB/s data frame on Abis interface.it is the interface between mobile station and BSC. from MS to BTS is radio link and from BTS to BSC is abis interface through e1 lines.

Que. :-What is VSWR?Ans:-Standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinodes (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line. The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio.

VSWR=Vmax/Vmin

Que. :-What doesBase Station Controller (BSC)mean?Ans:-A base station controller (BSC) is a critical mobile network component that controls one or more base transceiver stations (BTS), also known as base stations or cell sites. Key BSC functions include radio network management (such as radio frequency control), BTS handover management and call setup.

A BSC works with a mobile switching center (MSC) component that is external to the BTS, enabling it to provide full mobile telephony and fulfill capacity requirements. Base stations must communicate with the MSC and data must be managed as information overflow, impacting MSC efficiency. A BSC eliminates MSC base station activity management requirements, allowing the MSC to handle critical tasks, such as traffic balancing and database management.

Que. :-What is the function ofBSC Signalling Unit (BCSU)?Ans:-The BSC Signalling Unit (BCSU) performs those BSC functions that are highly dependent on the volume of traffic. The BCSU is housed in a cartridge of its own. It consists of two parts, which correspond to the A and Abis interfaces. The second optional Packet Control Unit (PCU) is included in each BCSU.

The A interface part of the BCSU is responsible for the following tasks: performing the distributed functions of the Message Transfer Part (MTP) and the Signalling Connection Control Part (SCCP) of SS7 controlling the mobile and base station signalling (Base Station Subsystem Application Part, BSSAP) performing all message handling and processing functions of the signalling channels connected to it.

The Abis interface part of the BCSU controls the Radio interface channels associated with transceivers (TRXs) and Abis signalling channels. Every speech circuit on the Abis interface is mapped one-to-one to a GSM-specific speech/data channel on the Radio interface. The handover and power control algorithms reside in this functional unit.The hardware of the BCSU consists of the following modules: a microcomputer an SS7 (Signalling System No. 7) interface a LAPD (Link Access Protocol on the D-Channel) interface a LAPD interface for control of the ET the Message Bus interface the Packet Control Unit