What is 3G? Definition: 3G, known as 3rd Generation, is a standard for mobile / cellular phones.
It provides the services that fulfill the International Telecommunication Union specifications. The application services of 3G include wireless voice telephone, mobile internet access, video calls and mobile television all in a mobile environment. 3G allows simultaneous use of speech and data services 3G provides peak data rates of at least 200 KBits according to the IMT-2000 specification. Mobile broadband access of several MBits to laptop computer systems, smart phones is provided by latest 3.5G, 3.75G versions. What are the advantages and disadvantages of 3G? Advantages of 3G:
Overcrowding is relieved in existing systems with radio spectrum Bandwidth, security and reliability are more Provides interoperability among service providers Availability of fixed and variable rates Support to devices with backward compatibility with existing networks Always online devices 3G uses IP connectivity which is packet based Rich multi media services are available
Disadvantages of 3G:
The cost of cellular infrastructure , upgrading base stations is very high Needs different handsets. Roaming and data/voice work together has not yet been implemented Power consumption is high Requires closer base stations and are expensive Spectrum-license costs, network deployment costs and handset subsidies subscribers are tremendous.
How is 3G different from 2G?
Packet data speed is higher in 3G, and it is up to 384 KBPS Voice and sms speed is also 384 KBPS in 3G 2G utilizes different switching techniques for voice and data, where as 3G uses single switching, irrespective of data 3G has at least 2MB of data link of a mobile, where in 2G the data rate is in KBPS 3G has WiMAX facility for faster VOIP and internet 2G uses GSM TDMA technology with narrowband 200Khz.
3G uses CDMA technology with broadband 5Mhz, with same frequency carrier and time.
Explain: a.) GPRS ( General Packet Radio Service): b.) GSM (Global System for Mobile Communications): c.) HLR (Home Location Register): e.) iDEN(Integrated Digital Enhanced Network): f.) i-appli GPRS ( General Packet Radio Service)
Packet oriented mobile data service available to the users of 2G cellular systems. It is global system for communicating through mobile phones using GSM as well as in 3G systems. GPRS data transfer is charged per MB of traffic transferred, where as in circuit switching, data transfer is charged per minute of connection time GPRS is better packet switching service, as opposed to circuit switching. 2G cellular systems are combined with GPRS and known as 2.5G. Provides moderate speed data transfer by using unused TDMA channels, such as GSM.
What is 4G?
A short name for Fourth Generation A stage of broadband mobile communications that supersede 3G 4G is expected to use radio spectrum in an efficient manner, and as a result, provides cable-modem transmission speeds to the end users. This feature enables high-quality streaming video Simultaneous connections to multiple high-speed networks, known as pervasive computing, are expected by 4G. Thus seamless handoffs are provided throughout a geographical area. Coverage enhancement technologies, like femtocell and picocell are expected to develop for addressing various needs of mobile users in homes, public buildings etc..
Explain UMTS.
UMTS stands for Universal Mobile Telecommunications Systems One of the emerging mobile technologies for 3G mobile phones. The main benefit of UMTS is, undoubtedly, its speed Current rates of broadband data transfer is 2MBits / second The speed of UMTS makes it possible to stream video that supports movie downloads and video conferencing. In a nut shell, UMTS provides almost all the facilities of a home computer system while on the move.
UMTS combines the technologies like wireless, satellite which provides seamless transitions between Wi-Fi and satellite.
What is International Telecommunications Union and what are its functions?
ITU is an intergovernmental organization Telecommunications are developed by using ITU by public and private organizations. ITU is responsible for adopting regulations, international treaties and standards that governs telecommunications
Functions of International Telecommunications Union:
Maintaining and extending the international cooperation for the improvement Rational usage of telecommunications, for promoting efficient use of technical facilities To harmonize the actions of nations.
1. What are the three services offered by GSM? Explain each of them briefly.
The services offered by GSM are 1) telephone service 2) data service 3) supplementary service lyk multi-party(conference),call waiting, call forwarding, call hold, call barring 2. Which uplink/downlink spectrum is allocated to GSM-900? a.) GSM 900: Uplink spectrum is 890.2914.8 and downlink spectrum is 935.2959.8 b.) DCS 1800: Uplink spectrum is 1710.21784.8, and downlink spectrum is 1805.2 1879.8 Name the algorithms used in following: a.) Ciphering b.) Authentication a.) Ciphering The algorithm 129-EEA3 and 128-EIA3, 3GPP Confidentiality and Integrity Algorithm is used for ciphering in GSM b.) Authentication The algorithm by name A3 is used for authentication in the GSM systems This algorithm is used in implementing Subscriber Identity Module . Q2: Which uplink/downlink spectrum is allocated to GSM-900? for GSM-900: 890 - 915 MHz is uplink and 935 - 960 MHz is downlink
Q3: Which uplink/downlink spectrum is allocated to DCS(digital cross-connect system)1800? for GSM-1800: 1710 - 1785 MHz is uplink and 1805 - 1880 MHz is downlink Q4: How many carrier frequencies are there in GSM-900 & DCS-1800? 124 frequency channels with a bandwidth of 200 KHz are available for both the uplink and downlink direction for GSM 900 374 frequency channels with a bandwidth of 200 KHz are available for both the uplink and downlink direction for the DCS 1800 Q5: How much is the separation b/w the carrier frequencies? 200 KHz frequency separation is available for uplink and downlink direction for both GSM 900 and DCS 1800 Q6: What is equalization? Why do we need it? Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal Q7: What is interleaving? Why do we need it? Interleaving is used to obtain time diversity in a digital communications system without adding any overhead. The interleaving decreases the possibility of losing whole bursts during the transmission. 10.) 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.
11.) Channel coding
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.
12)Frequency reuse is the process of using the same radio frequencies on radio transmitter sites within a geographic area, which are separated by sufficient distance to cause minimal interference with each other. 13)Cell splitting is the process of dividing the radio coverage of a cell site in a wireless telephone system into two or more new cell sites. Cell splitting may be performed to provide additional capacity within the region of the original cell site. 15)LAPD - Link Access Protocol on D-channel, is a ISDN layer 2 protocol. It is also layer 2 protocol on BTS-BSC interface. LAPDm - Link Access Protocol on Dm-channel. It is slightly different from LAPD in the context of frame format, frame size etc. It is layer 2 protocol between on BTS-MS interface.
17. What is MA? MA List: set of frequencies (channels) assigned to a particular sector, ie are those channels that can be used to attend calls from users.
E1 it is a PCM carrier having a data rate of 2.048 Mbps. This carrier has 32 ,8-bit sample
The T1 carrier consists of 24 voice channels multiplexed at a rate of 1.544 Mbps
T1 carrier It is a PCM carrier having a data rate of 2.048 Mbps. This carrier has 32 8-bit samples packed into the basic 125 usec frame. It is a PCM carrier having a data rate of 2.048 Mbps. This carrier has 32 8-bit samples packed into the basic 125 usec frame. The T1 carrier consists of 24 voice channels multiplexed at a rate of 1.544 Mbps.
19. What is the difference between Synthesised Frequency Hopping and Base Band Frequency Hopping? a.) Synthesised Frequency Hopping
The GSMs 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.
b.) Base Band Frequency Hopping
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.
20.) Cycling Frequency Hopping
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.
22. What is DTX? Why is it used?
DTX is a Discontinuous Transmission. Generally used in mobile network. There is a speech time or silent time while conversation on mobile phone.This can be recongnised by using Voice Activity Detector(VAD).If there s a silent period during conversation DTX will off the transmitter and hence a mobile which has limited power supply capacity can be saved.And also Interference can also be minimised by DTX.
23. What is DRX? Why do we need it?
DRX is a discontinuous receiver.It also used inside the mobile.It is same as that of DTX for battery life saving purpose.It turns ON when the voice is present at the receiver side and OFF when no voice is present.
24 & 25 )What do you mean by following? a.) Erlangs b.) GoS 24) Erlangs
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
25) GoS
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.
26)We use two different bands for GSM/DCS communications; GSM900 and DCS-1800. Which one is the better of the two in terms of quality and coverage?
GSM900:-serves larger area when compared to dcs-1800. dcs-1800:- doesnt serve larger area when compared with gsm 900. when comes to coverage & quality: gsm900:- comes with good coverage and quality when compared to dcs-1800. dcs-1800: serves shorter distance.
27. What is TA? Why do we need TA?
* The Timing Advance is calculated by the delay of data bits in the access burst received by the BTS. A long guard period gives space for such a delay. * The TA signal is transmitted in the SACCH as a number between 0 and 63, in units of bit periods. * Each increment in the value of TA corresponds to a distance 550 between the MS and BTS. For example, TA = 0 means that MS is up to 550m from the BTS TA = 1 means it is between 550 and 1100m TA = 2, from 1100 to 1650m, and so on. * The maximum distance allowed by the TA between MS and BTS is 35 km (GSM 850 / 900). So, for example during a test drive, we can measure how far we are from the BTS through the value of TA. He does not give you exta the position, but gives an accuracy in a range of 550m. And using this parameter, and just check how far we? A good application is the control of handover.
Imagine you have a cell that uses two concentric bands. You can set as a condition to allow the handover from one band to another. More specifically: if you have a cell with 900/1800, you can configure the band 900 as BCCH, and in 1800 only to traffic. The TA threshold to control the terminal so it does not make it to the 1800 handover is far from the BTS. 28 & 29 )What do following mean? Why do we need them? a.) Location Area b.) Location Update a.) Location Area
Location Area is a group of cells. The subscriber is paged in this area One or more base station controllers are used to serve each Location Area but by a single MSC. Each Location Area has a unique Location Area Identity number
29) Location Update
IMSI attach or IMSI detach location update process is to be performed whenever a mobile is switched on or off Each mobile is needed to report regularly its location at a given time of interval by using a periodic location update process A random location update is required, whenever a mobile moves from one location to another while not on a call
Hence, the subscriber has reliability to access the network and freedom of mobility within the coverage area.
30)Explain: a.) IMSI b.) TMSI c.) IMEI d.) MS-IDN. a.) IMSI
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
b.) TMSI
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
c.) IMEI
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/MEIDs 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
d.) MS-IDN
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.
31. What is ARFCN? Which ARFCNs are allocated to Ufone?
ARFCN means absolute radio frquency channel number. The ARFCN is a number that describe a pair of frquencies, one uplink and one downlink.
32)What is power control?
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.
33)Explain: a.)FDD b.)TDD a.)FDD
Frequency Division Duplex is one of the techniques, in which one frequency band is utilized for transmitting and another used for receiving. To uplink the carrying data from mobile phones to the base station, one electromagnetic block spectrum is allocated Various blocks of spectrum is allocated to downlink carrying data from the base station to the mobile telephone hand sets
b.)TDD
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 .
34)What do you mean by Extended Cell? Does it have any impact on the system? If yes, what?
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 Its impact is : 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.
35)Why do we use Multiple Access Schemes? What is the difference between FDMA, TDMA and CDMA?
because it is necessary to have a scheme that enables several multiple users to gain access to it and use it simultaneously. difference between FDMA, TDMA and CDMA: TDMA: it is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots FDMA: it is used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels CDMA: allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies
36)Which channel(s) is used for SMS?
SDCCH= STABD ALONE DEDICATED CONTROL CHANNEL SACCH= SLOW ASSOCIATED CONTROL CHANNEL
82. Which modulation scheme is used in GSM? Explain.
GSM uses Gaussian-Fitered Minimum Shift Keying (GMSK) as it's modulation scheme. Before the GMSK can be explained, some fundamentals of Minimum Shift Keying (MSK) must be known. GSM Basics Speech Coding in GSM The GSM speech coder breaks up human voice into 20ms blocks that are transmitted over eightconsecutive TDMA frames. Speech data is divided into three different bit classes in the followingmanner:Class Ia: 50 bits Block + Convolution CodedClass Ib: 132 bits Convolution CodedClass II: 78 bits Unprotected (no additional coding) Class 1a 50 bits Class 1b 132 bits Class 2 78 bits Mobile Allocation (MA) : This is the set of frequencies the mobile/BTS are allowed to hopover. Two time-slots on a same transceiver of a cell may be configured to operate on differentMA. MA is the subset of the total allocated spectrum for the GSM operator and the maximumnumber of frequencies in a MA list is limited by GSM recommendation to 64. Mobile Allocation Index Offset (MAIO) : This is an integer offset that determines whichfrequency within the MA will be the operating frequency. If there are N frequencies in the MAlist, then MAIO = {0, 1, 2, N1}. 21)Hopping Sequence Number (HSN) : This is an integer parameter that determines how thefrequencies within the MA list are arranged. There are 64 HSN defined by GSM. HSN = 0 sets acyclical hopping sequence where the frequencies within the MA list are repeated in a cyclicalmanner.HSN = 1 to 63 will provide pseudo random hopping sequence. The pseudo random pattern willrepeat itself after every hyperframe, which is equal to 2,715,648 (26x51x2048) TDMA framesor about 3 hours 28 minutes and 54 seconds
Gateway GPRS Support Node (GGSN) The GGSN is responsible for the interworking between the GPRS network and external packet switched networks, like the Internet and X.25 networks. Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area
113. What is the basic difference between GSM and GPRS architecture?
GSM is primarily used to carry your voice on cell phone networks that uses that type of technology. GPRS is a way of carrying data on cell phone networks that are based on GSM technology. GPRS provided faster data transmission that what could be done over GSM. GPRS is for data only - it does not carry voice. There are now new and faster ways of carrying data on these networks - EDGE and HSDPA. These are nice acronyms, but to the end-user it simply means you can get faster data transmission which means when you surf the web it will give you a better experience. What are the interfaces between thee following?a.) BTS and MS b.) BTS and BSC c.) BSS and MSC d.) TRAU and BSC e.) BSC and PCU a.) 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.
b.) 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.
c.) BSS and MSC
BSS will send the called number to MSC(Mobile Switching Center) MSC checks the VLR and queries BSS for allocation of resources for the cell Then MSC routes the call to GMSC Switching nodes for base station controls is done by MSC
d.) 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 stand alone 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.
e.) BSC and PCU
The Packet Control Unit is an adaptor / handler unit which enables GPRS and EDGE functionality within Radio Access Network. The BSC from the GSM network is connected with packet core by PCU Motorola PCU is based on certain industry standards like Compact PCI and is highly scaleable.
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