| QUESTION | ANSWER |
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| Can you list me the interfaces of a GSM network? | Air, Abis, Ater, A, Gb, Gi, Gn, Gd, Gr, Gs |
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| Can you list me the network elements of a GSM network? | MS, BTS, BSC, MSC, SGSN, GGSN,BG,HLR,EIR,AUC,CG,LIG |
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| What is the IMSI? | Unique Identifier for a SIM.1 MS can be paged with a single PAGING message |
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| What is the TMSI? | TMSI in a Location Area. Shorter so you can page more MS |
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| When do you use IMSI and when TMSI? | Authentication, but basically we care about it for paging. Paging message always contains IMSI |
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| How many MS can you page with 1 paging message done w/ IMSI? | 2 |
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| How many MS can you page with 1 paging message done w/ TIMSI? | 4 |
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| Paging method? Can you change this method w/ BSS parameters? | Either IMSI and / or TMSI … you can change the method by NSS parameters |
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| Name some of the CCH channels | RACH, AGCH, PCH … there is also NCH but it is not used … |
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| Which are the feature we have in ----- in order to change the CCCH behaviour? | Default PCH has always priority over AGCH. Anyway you can reserve a certain number of blocks of AGCH. However, if AGCH =0, AGCH has priority over PCH |
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| Which are the CCCH in UL and which are in DL? | UL = RACH DL = AGCH, PCH, (NCH) |
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| Dedicated Channels in GSM | FACCH, SACCH, SDCCH, TCH |
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| Broadcast Channels in GSM | BCCH, FCH, CBCH, SCH |
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| Which is the difference between CONGESTION and BLOCKING? | Congestion = all resources in use. BLOCKING = Blocked requests |
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| How do you measure CONGESTION in -----? | seconds |
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| How do you measure BLOCKING in -----? | events |
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| Which is the unit of measure for the blocking measurement? | 1 event |
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| Which is the measurement period for the congestion measurement? | 10 msec |
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| Which is the measurement period for the traffic measurement? | 20 sec |
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| Which are the ----- features to help SDCCH congestion not to become SDCCH blocking? | Dynamic SDCCH, FACCH Call Set Up |
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| Which are the ----- features to help TCH blocking? | HR (EFR and AMR), DR, DADLB (actually this one avoids congestion to happen as well as traffic reason handover), traffic reason HO (MSC initiated), AMH (traffic reason HO, BSC initiated) |
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| Which is the difference between DADLB and DR? | DR is triggered when there is actual congestion in the TCHs. DADLB is triggered when the load of the BTS is above a certain threshold defined as a parameter. |
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| Which type of HO is triggered w/ DR? | SDCCH - TCH HO |
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| Is DR helpful for SDCCH or TCH congestion? | TCH Congestion |
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| There are 2 timers and 1 threshold you need to define when you activate DR … can you tell me which are these parameters and what are these for? | Min Time = Time in order to identify neighbours.
Max Time = max time to try HO.
Level Threshold = Minimum Threshold of the neighbours in order to try SDCCH-TCH HO |
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| Which are the differences between DR (directed Retry) and IDR (Intelligent Directed Retry)? | Basically IDR is an improvement of DR and it takes into account the Source Call Type, Target Cell Type and Subscriber type … anyway it is never used … |
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| Which are the DR parameters you need to set? | Threshold Level
Min time DR
Max Time DR |
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| Can you briefly explain to me the BSC INITIATED TRAFFIC HO? | Basically when the BTS load is above the load defined by a parameter (in AMH feature's parameter group). the AmhPBGT MARGIN for that specific neighbour relation is squeezed in order to help to lower load traffic … that's the reason why you call it TRAFFIC HO |
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| What happen in a network if there are legacy MS that don't support C2? | They just use C1 (it happens all the time in Europe) |
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| Why do you want to use C2? | Microcell and to bias a layer (it happens in CINGULAR for 850 in order to have dual band MS which are EDGE capable only on 850) |
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| Can you make practical examples of C2 usage? | traffic management in dualband, microcell |
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| What's the difference between Cell Selection and Cell Re-Selection? | Cell Selection is when you switch on the MS. Cell ReSelection when it is already on |
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| What are the 4 criteria of the suitable cells for MS to camp on | - MS is able to read BCCH
- MS is allowed to register to the PLMN and LA
- the cell is not barred
- signal level is higher than threshold (C1 > 0) |
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| What is the TIMING ADVANCE? | Measure distance MS -BTS w/ resolution of 550 meters |
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| How long is it in time and in space? | 1/2 bit where the bit rate is 270kbps ~ 550 meters |
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| Who calculates it? | BSC/BTS and sends back to the MS |
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| How is it calculated? | RACH delay |
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| How can you collect TA statistics in the ----- OSS? | There is the table … although we have only 10 bins … not 63 … |
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| Can you modify the setting of the TA stats in the ----- OSS? | Yes, BSC level, maximum is 10 bins |
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| Why do we need to use timing advance | BTS needs to know when MS signal is expected to arrive to avoid inter symbol interference |
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| Max distance you can keep a call in GSM?
(what is the maximum value of timing advance in normal GSM cell? And what does it equivalent to? ) | 63 TA .. Equivalent to ~ 35 km |
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| Which network element uses TA to take certain decisions? MS or BTS? | Basically MS sends its bursts in advance in order to get to the BTS in the window expected … |
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| What is the extended cell feature? | Alows to have double maximum distance MS - BTS … theoretically 70 km … practically 65 km |
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| Is it available in -----? | Yes … |
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| Which is the cabinet / BSC you need to have in order to make Extended Cell work? | Ultrasite will support it in S11.5. Talk Family supports it since S7 |
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| Advantages and disadvantages of the MASTER-MASTER and MASTER-SLAVE configuration | Basically when you have a MASTER, if this goes down also the other cabinet goes down |
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| Do you need the Synch Cable between cabinets in order to share the T1? | no |
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| Which is the best configuration for cabinet synchronization (MASTER, SLAVE, INDEPENDENT) if you don't have CBCCH? | independent |
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| Why independent configuration is the best? | Because doesn't have a chain effec in case 1 BCF has problems |
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| Advantages and disadvantages of allocating 1900 radios on one cabinet and 850 on another cabinet | Basically a hybrid configuration is always an headache for implementation |
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| Briefly explain FULL and SUB measurements in the Air I/F? | FULL is when you don't use DTX, SUB when you use DTX. FULL Is measured on 104 frames, SUB on 12 frames |
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| Are these stats always sent? Both of them? | Yes., both all the the time |
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| How can the BSC understand which one to use? | There is a flag that says if DTX is used or not |
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| If you plot these stats what do you notice? | Sub stats are more spread, more standard deviation |
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| What happens when you enable UL DTX / disable UL DTX to your RxQuality? | RxQual has more samples in 0 and 7 |
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| How much of it is due to real interference? | Some is real interference but most is just statistical distribution |
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| Why you have a different distribution between SUB and FULL RxQual samples? (HINT - It is a statistical reason) | Less samples = Less reliable = More deviation |
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| What are BER and BEP? | Bit Error Rate and Probability |
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| Can you explain what is NBLO measurement and how it is used? | It is used in CBCCH and it measures the difference between BCCH and non-BCCH layer |
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| Which is the main advantage of CBCCH Vs. Multiple BCCH? | CBCCH advantage = Trunking Gain + you can assign the call to the slave (e.g. other layer) w/ call assignment w/o using HO
MBCCH advantage = Give BCCH channel to both bands … especially here in US where there are a lot of legacy phones in singleband @ 1900 |
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| Could you please explain the basic signalling of a Mobile Originated Call Setup? | RACH, AGCH, SDCCH, TCH … |
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| Could you please explain the basic signalling of a Mobile Terminated Call Setup? | PCH, RACH, AGCH, SDCCH, TCH … |
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| What are the important GSM specifications for radio performance? | 3.22, 4 series esp. 4.08 & 4.60, 5 series esp 5.08, 5.05 |
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| What is the channel spacing in GSM? | 200 kHz |
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| What is the duplex frequency of GSm850 and GSM1900? | 45 MHz for GSM850 and 80 MHz for GSM1900 |
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| in GSM, What is the multiple access and duplex used? | FDMA/TDMA with FDD |
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| What is ARFCN? What does it mean? | Absolure Radio Frequency Channel Number. Basically it is the frequency in use. |
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| Is ARFCN = 120 in GSM850 or GSM1900 band? | 850 |
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| How many ARFCN do you have in GSM850? | ch 128 -251 (124 Ch) |
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| How many ARFCN do you have in GSM1900? | Ch 512 - 811 (300 ch) |
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| Which is the number of frames in a SIGNALLING CHANNEL? | 51 |
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| Which is the number of frames in a TRAFFIC CHANNEL? | 26 |
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| What's the difference between MECHANICAL and ELECTRICAL DOWNTILT? | Electrical reduces also side lobes and back lobes. |
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| How does the ELECTRICAL DOWNTILT work? | Put a signal w/ different phases which deletes part of the initial one creating smaller lobes |
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| What is the difference in propagation between 850Mhz and 1900Mhz? | 7 dB. The higher the frequency, the more the loss. |
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| What is Slow fading? | Also known as shadowing, caused by large obstructions such as hils, building trees. Behaviour is a slow change in propagation |
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| What is Fast fading? | Also known as multipath or Rayleigh. Caused by constuctive or destructive interference as a result of reflections, multiple propagation paths. Fades as deep as 30 dB over distances of half a wavelength |
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| In the slow moving mobile environment,
what kind of fading has more impact to signal? | Fast Fading. |
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| What is Diffraction? | When the RF signal is distorted around an object. Happens to all EM waves, it seems as though the wave "bends" around an object, therefore can provide coverage behind an obstruction. Happens when there is no LOS or the 1st Fresnel zone is obstructed. Commonly modelled as knife edge, either single or double. Epstein-Petersein is a multiple edge model. |
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| What is the effect of transmit frequency on Diffraction? | None. Diffraction is independent of frequency |
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| What is the difference between "Service" and "Coverage"? | Coverage is provided by the signal propagating to an area. Service is useable coverage, affected by interference or GSM distance constraints |
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| What is the relationship between Coverage and Interference? | Interference is unwanted coverage. The same signal can be both coverage or interference depending on whether the signal is useful or not |
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| What is the required information from site survey activities? And what are their purpose? | Site location (Lat, long), Antenna configuration (diversity, HPBW, gain, type), Antenna line information (feeder length/loss, spiltter, diplexer, MHA), Rooftop layout.
This is used for link budge calc., coverage planning and prediction simulation, optimization analysis |
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| Customer wants to give to ----- a new service, coverage planning of a new market. You need to tell him which are the inputs you need in order to start your work. Please do so … which are the inputs you need to ask him for. | Coverage Polygons (Rural, SubUrban, Urban, Freeway) # Different RxLevel for each polygon # Coverage Probability |
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| What is the Coverage Probability? You have normally 2 Coverage Probabillity … which are their names? | Cell Edge. Cell Area. |
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| Which is a typical Coverge Probability you can use for coverage dimensioning purposes? | Cell Area Coverage Probability ~ 90% is a typical value you can use and that you have to put in the Propagation model as an input in order to have the Cell Range and from that the Cell Area. |
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| The customer has given you the input you have requested him (Polygons, RxLev, Coverage Probability) now you need to decide which are the tools you need in order to give him an initial understanding of how many sites he needs. Please tell me which are the tools you need | 1. You need to decide the type of equipment and configuration you want to have. Normally this is a standard one (e.g. ULTRASITE, w/ LNA, 2 TRXs) 2. Calculate the LINK BALANCE for that specific configuration. 3. Propagation Model (e.g. Okomura Hata) correction factors (e.g. mean and standarddeviation) |
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| Probably at this point you have to decide / ask which is the typical configuration you want in the sites (omni, 2 sectors, 3 sectors). Which are the advantages / disadvantages of OMNI? | PRO OMNI = Trunking Gain. Cover perfectly an Island for example, from the top of a hill. CONS OMNI = Painful when you need to make frequency plan and/or limit interference |
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| Probably at this point you have to decide / ask which is the typical configuration you want in the sites (omni, 2 sectors, 3 sectors). Which are the advantages / disadvantages of 2 sectors? | Probably you can use a 2 sector site when you have a freeway in a rural area. |
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| Probably at this point you have to decide / ask which is the typical configuration you want in the sites (omni, 2 sectors, 3 sectors). Which are the advantages / disadvantages of 3 sectors? | 3 sectors is probably the best combination for coverage and frequency plan and it gives good ability to control interference (w/ tilts) |
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| Related to the previous question … which is the best configuration is you have to cover a freeway? And if you need to cover an Island? And if you need to cover a city? | City = 3 sectors. Freeway = 2 sectors (but only if around there is really nothing else to cover). Island = Omni works just fine if you put it on the top of the hill |
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| Once you have decided which is the sector configuration to use, how can you use the Cell Range calculated w/ the Propagation Model correction factors in order to calculate how many sites to you need? | You use the typical hexagon model. With that you know how many sites you need in order to cover you polygon. |
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| What is the relationship of W, dB and dBm? | 10*log (W) = dBW,
10*log(W/1000[mW]) = dBm,
10 log (Power/ Reference) = dB or 20 log (Voltage / Reference) |
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| 10 dB + 14 dB | 24 dB |
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| 10 dBm + 14 dB | 24 dBm |
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| 10 dBm + 14 dBm | In order to do it you need to convert the dBm to Watt, sum the Watts and go back to dBm … in this case the results is 15.455 dBm |
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| Is the following statement correct?
Okumura Hata model used in GSM850 is the same as GSM1900. | False. The A and B coefficient used in OH model is different if frequency below 1000 MHz and above1500MHz. Besides, the area correction factor is different between the 2 frequency |
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| What is the possible way to improve link budget? | BTS type (Rx sensitivity and Tx Power), Antenna gain, feeder length and types (site planning), using MHA and diversity (improve Rx), using Booster (improve Tx) |
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| Sensitivity and TRX output power for ----- METROSITE? | -112 to -115 dBm, 5W TRX |
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| Sensitivity and TRX output power for ----- ULTRASITE? | -115 typical (-112 to -118 dBm),
43dBm (guaranteed ~ minimum on 100% production)
47dBm TRX (nominal ~ on 90% of production) |
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| How does the link budget impact to number of the sites required in an area? | With the maximum allow pathloss from link budget with consideration of LP, STD, Slow fading margin and propagation model , it is used for defining the cell range. If the pathloss is bigger, the cell range could be higher and the number of sites require will be reduced. |
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| Please list 3 methods of capacity enhancement and their pros & cons | cell/site splitting: coverage enhancement but more cost to hardware & limitation of frequency
more TRX: easy but need frequency
micro cell layer: more easy to control interference but more site require
dualband: additional spectrum required, minimize impact to existing band coverage/capacity,
soft capacity features: hopping, IUO, DFCA, AMR, DTX
better cell planning &optimization for better interference control. |
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| How does LNA improve the sensitivity? | Basically the LNA is helping to improve the effective noise figure of the receiving path which then reduces the Rx Sensitivity of receiving path. The simple way to do is the compensation of feeder loss (but not more than LNA gain). However, the calculation have to be calculated on the effective noise figure and effective gain of the receiving path. |
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| INTELLIGENT DOWNLINK DIVERSITY? | IDD is basically a DL booster. The way it works is basically putting together the signals of 2 TRXs with the same signal but w/ a different phase. The gain is ~ 3 dB. |
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| What is the problems with highrise buildings? | lack of coverage as the antennas are lower down. Increased interference from LOS to many sites. |
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| What is trunking gain? | TRUNKING GAIN IS THE GAIN THAT YOU HAVE WHEN YOU HAVE LESS ENTRY POINT TO THE RESOURCES (e.g. 1 line in front of the 10 airport check-in counters works much better than 10 lines in front of the same 10 airport check-in counters) … |
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| what does 1 Erlang mean? | One channel used for 3600 seconds in 1 h ~ 100% utilization of a resource (TCH or PDCH in GSM) |
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| Customer is complaining that w/ 1TRX he can serve only 3 Erlang and has 10% blocking while we have told him that according to the Erlang B table we should be able to serve 3 Erl w/ 2% blocking. What is happening? | Basically the traffic is not approaching the BTS w/ a Poisson distribution (which is the assumption that is the base of the Erlang B model) |
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| What is the Erlang C table? | It is a model which takes into account queuing |
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| If we have QUEUING active do we need to use the Erlang C table? Why? | No … because queue is not infinite … |
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| Is the Erlang B table the right model to use in case of AMR FR / HR? | NO |
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| 1 TRX w/ 2% blocking … according to the Erlang B table how many Erlang can I serve? | ~ 3 Erl |
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| What is the output from link budget? | Maximum allowed pathloss (UL and DL) |
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| Which is a typical PathLoss you can have in a GSM connection? | ~ 150 dB |
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| Do we use different LINK BUDGET for different type of environments? | No, maximum allowed path loss is not environment dependent. It is service dependent though |
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| What is the purpose of a radio planning tool? | To model the network. Allows the planner to see the effect of making changes to the network so the best solution can be chosen and implemented. Can also be used to store all the site data. |
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| How can you ensure the planning tool is as accurate as possible? | First start with accurate clutter, DTM and building data. Next calibrate the model correctly. All site data must be correct. This will give the propagation model of the network. |
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| What general power law do propagation models follow? | 3.5 power law (4th power law good enough) |
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| Name two methods of propagation modelling | Statistical and Deterministic (Ray tracing, virtual source) |
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| Name two statistical models commonly used | Okumra-Hata (General propagation) and Walfisch-Ikegami (canyon like environment) |
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| How do you calibrate a statistical propagation model? | Collect CW (not TEMS, noise floor isn't low enough) measurements from mulitple locations. Drive through as many clutter classes as possible. Input all data to a planning tool. Calculate SD. Use different values of first co-efficient to get different SD values. This is normally a square function, so get minimum. Continue with other coefficients. Repeat. Coefficient order: distance, diffraction, BTS height, BTS height*distance |
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| What are the main MML families in manipulating radio objects? | EA: Adjacency, EE: BSC, EQ: BTS, ER: TRX, EH: HOC, EU: POC |
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| How do you list current alarms for a BTS? | ZEOL |
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| What does the initial "Z" stand for in an MML command? | It makes MML interpret the command from the main level, even if you are inside another family |
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| What is the available tool for OSS data collection in -----? | TANO, NDW, EOSFLEX |
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| Name two drive test tools | TEMS, NEMO(TOM) |
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| What drive test equipment is needed for performance data collection? | Logging tool, 2 phones (one short call, one long), GPS, scanner, map, power supply for all |
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| What type of measurements should be used for model calibration? | Continuous Wave (CW), Greater dynamic range and lower noise floor |
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| Name some protocol analysers for monitoring the Abis, A or Gn interfaces | Nethawk, K1215, Agilent |
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| What is co-channel interference | When a cell is being interfered with by another cell with the same carrier frequency |
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| What is the GSM rejection ratio for co-channel interference? | 9 dB |
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| What is adjacent channel interference? | When a cell is being interfered with by another cell with a carrier frequency which is seperated by 200kHz for the 1st adjacent interferer, 400kHz for the 2nd adjacent interferer, etc. |
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| What is the GSM rejection ratio for the first adjacent channel interference? | -9 dB (-18 dB for the second adjacent interferer) |
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| What is frequency reuse 4/12 ? | There are 12 frequency ch reuse on the 4x3-sector sites. |
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| When planning a frequency what is the channel separation required for the TRX in the same cell? | 3 ch. (600 kHz) for GSM850 and 4 ch (800 kHz) for GSM1900 |
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| When planning a frequency what is the channel separation required for the TRX in the same site? | 400 kHz |
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| what is the different between hopping scheme 1/1, 3/3, 1/3? | 1/1: there is 1 MA list and reuse on every cells
3/3: there are 3 MA list and each site (3 sectors) using same MA list and reuse every 3 sites
1/3: there are 3 MA list and each cells in same site using different MA list. And reuse same pattern for every site. |
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| What is the factor to determine which hopping scheme above to be used? | Number of TRX (too small number of TRX cannot gain much from BB hopping) and combiner type. (RTC is only can use BB hopping) |
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| Do we need to plan MAIO Step, MA List, MAIO Offset and HSN when use BB hopping? | There is no need to plan for MAIO and MAIO offset in BB hopping. However, HSN is required. |
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| Do we need to plan MAIO Step, MA List, MAIO Offset and HSN when use RF hopping? | yes, all of them. MA list is required to plan to ensure that there is no frequency collision within the site. HSN should be same for all sectors in the sites. |
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| Do you have MAL only w/ RF hopping or also w/ BB hopping? | only with RF hopping |
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| Which is the main difference between RF and BB hopping? | RF = TRX changes ARFCN. BB = Call moves from 1 RTSL to an other (in an other TRX) |
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| What is a MAL? | Mobile Alloction List. It is used for RF HOPPING |
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| How many MAL can you define in a ----- BSC? | 128 |
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| Which are the parameters you need t define in the BTS when you want to use a MAL? | HSN, Maio Step, MAIO Offset,hopping mode, MAlist |
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| Why do we use Frequency Hopping? (I mean which are the advantages of frequency hopping) | You have 2 advantages w/frequency hopping … FREQUENCY DIVERSITY (FADING) and INTERFERENCE DIVERSITY |
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| Which is the type of fading you are able to "beat" when you use HOPPING? | Fast Fading |
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| You have a BTS w/ 3 hopping TRXs and a MAL w/ 3 ARFCN … once you try to unlock the the site the BSC gaves you a DX error saying that you don't have enough frequencies … where is the problem? | Basically the sum of MAIO OFFSET and MAIO STEPS exceeds the number of ARFCN in the MAL … it is indeed a silly problem but you need to know it. Suggestion is to keep MAIO OFFSET = 0 and MAIO STEP =1 in such a case (where probably you are using ad HOC hopping) |
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| How many HSN do we have? | 64 (0 … 63) |
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| Is this a GSM standard or do you know if ----- has any plan to increase this number? | No. it can't … it is GSM specs |
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| How does HSN = 0 works? | sequential hopping (others are psuedo-random) |
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| Do you know which are the basics of the algorithm used from HSN? | Uses the parameters you set (MAIOs) and FN which is something you can't define … that is the reason why in a synch network you can better control the interference. |
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| How many HSN do I need to define for each BTS in RF HOPPING? | 1 |
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| How many HSN do I need to define for each BTS in BB HOPPING? | 2 |
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| What is EFL? | Effective Frequency Load. A way of measuring spectral efficiency with a hopping system. Formula: erls/(#freq*ave#(TCH/TRX)) |
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| What is spectral efficiency? | A way of measuring how effectively the spectrum is being used. The more traffic carried with less spectrum the more efficient the use. Formula: erls/MHz/Cell |
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| Is the following statement correct? Why?
It is good enough to use RF hopping reuse 1/1 with MA list length 15 in the area with 6+6+6 BTS. | No, because it is all frequencies are used all the time which could create the high interference level esp. if the site planning (dominant area) is not good enough. |
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| When customer want to build GSM1900 on GSM850 site or vise versa, what are the things that you would like to suggest him to check and what is the issue behind that? (not parameters) | Frequency use (intereference and intermodulation between 2 bands)
antenna solution (Isolation requirement and area spacing) |
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| When can you have paging thru the Gb I/f instead of the A I/f? | Gs interface is necessary |
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| What's the advantage of the paging thru the Gb I/f? | Can page a MS in data session |
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| Does the Abis I/f size change the paging capability? | Yes … if the LA is bigger you need to page more MS, so all these messages need to go thru the Abis … so you need a bigger Abis |
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| How many MS can you paging messages can you send in 1 hour w/ 16kbps TRXSig? | 100,000 paging messages … 200,000 MS if you use IMSI or 400,000 MS if you use TMSI |
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| How many MS can you paging messages can you send in 1 hour w/ 64kbps TRXSig? | 400,000 paging messages … |
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| Which is a parameter you can change in order to reduce the probability that a MS that went out of coverage will be paged? | Periodic Location Update … you need to reduce it … |
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| Why do we need BSIC in GSM? | Identify neighbour … couple BCCH, BSIC make you identify the neighbour |
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| How many bits are the BSIC? | 6 |
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| Which are the 2 components of the BSIC? | NCC, BCC |
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| Which is the relation between BCC and TSC? | They should be the same in the BCCH TRX, not in all the TRXs … NOKIA by default is usig the same but it is not mandatory in GSM. |
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| Can we have TSC <> BCC in -----? Since when? Do you know if CINGULAR has it activated?
(do we have any features that could allow TSC <> BCC?) | Yes, we have a S10 feature about it. CINGULAR doesn't use it. |
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| Which is the the reason to have TSC <>BCC? | Better HO SUCCESS RATE … but basically it doesn't change anything … |
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| Which is the relationship between NCC and PLMN? | There is no relationship. PLMN is MCC + MNC |
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| Why do we need TSC (Training Seuqence Code) in GSM? | This is for the MS and BTS to learn the interference. Basically the MS is expecting a certain TSC but receives an other if there is intereference. The difference between the expected and the measured one is used in order to reconstruct the rest of the bits of the burst. |
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| Which is a recommended size for the LA in -----? | Depends from Abis LAPD size … 1-2 BSC works fine w/ 16kbps |
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| Can you have smaller LA? | YES |
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| Can you have bigger LA? | YES |
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| Can you have a LA across 2 different MSC? | NO |
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| Why do you need the LA in GSM? | Paging reasons |
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| Can you have a LA across more than 1 BSC? | Yes |
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| Which is the trade off in the LA size? | Paging and SDCCH traffic |
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| Which are the KPI you need to monitor when you are modifying the LA size? | SDCCH, Paging Success Rate, congestion in Abis and Air for LAPD and CCCH |
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| Does roaming traffic affecting the LA size? | No |
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| Which are the parameters you can change in order to make harder or easier LA update? | Cell Reselection Hysteresis |
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| What is the PLMN permitted parameter? | It defines to which PLMNs the MS is permitted to report measurement results. |
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| Is "PLMN permitted" parameter the same as PLMN id? | No. |
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| What parameters need to be set as the equal to or subset of PLMN permitted? | NCC part of the BSICs. |
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| If it is not set, what will happen? | MS is not possible to measure the cells e.g. no HO. |
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| Where really do you need this parameter? | National Borders |
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| Do you know which is the PLMN setting used in CINGULAR? | All of them |
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| Can you use TRAFFICA tool in the CINGULAR network? Blue? Orange? Why? | Only if you have our core … means Orange |
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| If you increase the LA size do you expect an increase in the A I/f traffic? | NO |
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| If you increase the LA size do you expect an increase in the Abis I/f traffic? | YES |
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| If you increase the LA size do you expect an increase in the Air I/f traffic? | YES |
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| What do you use the FACCH for in UL and DL? | HO, CALL SETUP if you have the feature |
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| What do you use the SACCH for in UL and DL? | System Info (DL), Measurements (UL), SMS during a call |
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| How many SDCCH can you put in 1 RTSL? | 8 |
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| Which is the difference between COMBINED and SEPARATED signalling? | combined = CCCH + 4SDCCH; separated = CCCH and SDCCH are in different RTSL |
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| Which channels have no power control? | BCCH, SDCCH, GPRS TSL(DL) |
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| Measurement report is sent to BSC every period of which channel and how long does it equivalent to? | SACCH = 480 ms |
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| What is the different between sync and non-sync HO? And in which case it will happen? And what parameter is used to define this? | in non-sync HO, there is physical information is require for MS to perform HO. The information contains TA information and it requires when the cell HO to different BCF cell. The parameter is in each ADJ pair "SYNC" |
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| is following statement correct?
There is no problem if the neighbor list contains the same BCCH and same BSIC from different cells as long as the 2 neighbour are far away and not interfere to each other. | False. MS report only BCCH and BSIC to BSC, therefore BSC will not be able to differentiate which ADJ is the right one and that could cause HO failure and eventually call drop |
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| What is the Nx/Px parameters in handover parameters defined? And what is the impact on setting large number of Nx/Px? | Nx is the total number of sample in the evaluation.
Px is the number of samples within Nx that below or above threshold.
The large number could lead to the longer time for HO decision making |
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| What's the difference between HO ATTEMPT and HO command in -----? | HO Attempt is before the check if target has available RTSL. HO Command is after. There are more HO attempts than HO commands |
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| Which is the 1st thing you should check in the formula when the customer is telling yu he has a very high HO failure rate? | If counts HO attempts … |
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| Please name 5 types of HO reasons | 1) Interference (uplink or downlink)
2) Uplink quality
3) Downlink quality
4) Uplink level
5) Downlink level
6) Distance between MS and BTS
7) Turn around corner MS (special case of rapid field drop)
8) Any other rapid field drop
9) Fast / slow moving MS (special case of umbrella handover)
10) Any other umbrella or power budget handover
11) traffic reason ho |
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| What is the 4 criteria in ranking the ADJ for HO candidate? | 1) Adjacent Cell Load Threshold : Can be checked only for cells belonging to the same BSC as the serving one
2) Adjacent Cell Priority
3) Overloaded Cell -> Reduction of Priority
4) RX level (if same priority) |
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| What is typical value use for PBGT HO margin? | 6 dB |
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| What are the 3 parameters in BSS that need to change to make sure that dualband is working? | dualBandCell Y indicates cell as dual band cell
multiCellBandReporting 0..3 number of adjacent dual band cells taken into account for measurement report
earlySendingIndication Y enables MS to send classmark 3 message as early as possible |
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| What is the impact of setting higher number of mulitcellBandreporting? | it reduces the amount of same band reporting. Normally there are only 6 cells report from MS. Deducting by MBR, the rest will be used for same band. |
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| What feature could be used to help the situation? | EMR |
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| What is the RLT parameter? | RLT = Radio Link timeout. |
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| How does RLT works? | It is both in UL and in DL. Speaking about UL … if the BTS doesn't receive the SACCH in UL it decreases this counter by -1 and if receives the SACCH it increments by +2. If it gets to the maximum value it stops incrementing. When it gets to zero the BSC releases the call. |
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| How many RLT do we have in -----? UL / DL / AMR FR / AMR HR? | In NOKIA we have only one RLT … |
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| Why should we have different RLT for AMR FR and AMR HR and EFR? | Because the C/I protection in very different … so the RLT should take into account before dropping a call … |
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| What does EFR mean? | EFR = Enhanced Full Rate |
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| What is FAST AVERAGING? | It is a way to speed up the Power Control + HO decisions because you don't need to wait the complete window before start counting Nx/Px in your decision |
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| We have a WEIGHTING parameter in HOC, POC … what is this for? | It is used to overweight if you want) the FULL measurements rather than the SUB measurements |
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| How many parameters do you have for FAST AVERAGING?
(at what call phase Fast Averaging can be applied?) | 3 … after Call Set Up, After PC, After HO |
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| Why do we have the concept of PRIORITY in the HO? | Because based on the measurements we can take after the same SACCH different HO decisions and you want to give to certain HO more priority (e.g. Quality HO should have higher priority over PBGT HO because the problem is more urgent … and the way you choose the neighbours for the different HO types are different) |
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| What is a CLEAR CODE? | DX Cause w/ failure |
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| What is a DX CAUSE? Why this funny name? | Internal Cause in BSC. Because of DX architecture ofBSC and MSC |
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| After activation of Double BA list the sector doesn't make anymore HO … what's the problem? | Neighbour BCCHs are not in the list |
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| where to find information about ----- counter and KPI? | 1) NED 2) Jump page. 3) Database Description for BSC
Measurements (PDF document) |
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| What is the JUMP PAGE? | Intranet page w/ info about KPI, COUNTERS |
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| How can you use DX CAUSE and CLEAR CODE in order to optimize my network? | CLEAR CODES CAN BE MONITORED W/ OBSERVATIONS … YO CAN FOR EXAMPLE USE "DROP CALL OBSERVATION" OR "DROP CALL BREAKDOWN" … |
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| WHICH IS THE DIFFERENCE BETWEEN MEASUREMENT AND OBSERVATION | THEY ARE BOTH WAYS TO GET PERFORMANCE STATS … KPI AND COUNTERS ARE FROM MEASUREMENTS TABLES. OBSERVATION ARE GIVING US CLEAR CODES / DX CAUSE. |
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| Is the MS or the BTS able to measure the actual C/I of the Air I/f? | No … in GSM we can't measure the C/I … we can only measure the BER, FER … which are affected of the C/I |
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| In the ----- OSS I have seen there are interference measurements . Can you explain what they mean and how they is measured? | These are idle mode measurements and they are only in UL … basically in the RTSL not in use if the Level is above -110 dBm we assume it is interference and it is recorded in the OSS stats … |
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| In the ----- OSS measurements I can see HO due to interference? How do these HO work / are triggered? | INTERFERENCE HO are basically QUALITY HO in good RxLevel condition (which is set with a RxLevel parameter) … the reason is dual … 1) Have stats about bad Quality in good RxLevel conditions. 2) Trigger an intracell HO rather than a intercell HO |
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| What is AMR? | Adaptive Multirate voice coding |
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| How does it work? | AMR varies the proportion of voice coding and channel coding to provide more robustness in bad C/I at the expense of voice quality |
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| How many AMR codecs are there at FR and HR? | 8 at FR, 6 at HR (1 not used in HR). Only 4 can be used in a call |
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| What is MOS? | Mean Opinion Score: A subjective view of the quality of a voice call |
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| What is FER? | Frame Erasure Rate: Number of speech frames discarded due to errors. A better measure of quality than BER with AMR because BER is measured before decoding and FER after decoding |
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| What is the effect of AMR on a full rate channel? | It allows the call to maintain the same FER in worse C/I conditions |
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| What is the effect of AMR on a half rate Channel? | It allows the FER to improve in better C/I conditions while occupying half a timeslot |
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| How does AMR link adaptation work? | When certain C/I thresholds are met, the voice codec will be changed. There is no target FER to maintain and thresholds have to be carefully chosen |
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| What is a half rate channel? | A channel with 11.4 kbps. FR is 22.8 kbps. Each channel occupies alternate bursts of a multiframe. |
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| How does the BSC decide whether to use a HR or FR channel? | With cell load thresholds. When the lower limit is reached, calls are packed, when the upper limit is reached, new calls are sent to FR |
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| When is a HR unpacked to FR? | When the RxQual of the HR call reaches a pre-set limit |
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| Does ----- supports all the GSM AMR codecs? | No doesn't support HR 7.95kbps because it should use 16kbps TRAU and NOKIA doesn't have it. |
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| What is MAXCAP feature? | Name of a CINGULAR idea to maximize capacity. Basically while testing it they have found that our BSS had a problem and wasn't unpacking if a candidate for Quality HO wasn't available. When it was introduced we saw a sudden increase in drop call during unpacking. That is the reason why we want to have MAXCAP = OFF. |
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| Do you have more signalling channels in AMR HR? | In band singalling, but it is transparent |
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| Does the overall bit rate on the Air I/F changes w/ AMR? | No it doesn't … |
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| What is DFCA? | Dynamic Frequency and Channel Allocation, is a BSS radio resource management functionality that selects the radio channel individually for each connection from a dedicated channel pool based on C/I criteria. The different degrees of interference tolerence of different connection type such as EFR,AMR FR, HR are taken into account in the ch selection process. |
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| What are the DFCA HW requirement? | -BSC2i + CP2MX CPU cards or BSC 3i
-BSC-BSC connections for BSC using DFCA and have adjacent service area.
-Ultrasite or metrosite BTS
- Wideband combiner
- LMU in every BTS |
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| What are the DFCA SW requirement? | -S11.5
- CX4.1
-OSS4
-LMU4.1 |
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| Three frequency bands are defined in DFCA, what are they? | BCCH band, Regular band (optional), DFCA band |
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| What are the requirement/limitation in DFCA band? | -The range of Malist is 1-32
-The DFCA MA list must be defined the same way in al BSCs within each continous BSC area
-DFCA MA list do not contain consecutive frequency
-Any two DFCA MA lists containing adjacent frequency are required to be of the same length. |
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| What are the inputs for DFCA channel allocation? | -non real time information I.e. Background inteference matrix
-near real time information I.e. Mobile measurement report
- realtime information I.e. Radio channel usage information |
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| What are the 4 methods in DFCA C/I estimation? | incoming DL, Incoming UL, Outgoing DL, Outgoing UL |
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| What is the Soft Blocking in DFCA? | Soft Blocking is a user define parameter.
If in the C/I estimation phase any of the four C/I estimates produced for each radio channel candidate does not exceed the soft blocking limit of the interfered connection, then the radio channel candidate is deemed soft blocked.
If there are no acceptable assignment candidates (i.e. all the candidates breach the connection type specific soft blocking C/I limit), the assignment will be directed to a regular TRX if available.
If there are no free TSLs on any regular TRXs of the cell, the call will be finally rejected (DFCA soft blocking situation). Directed retry could still be used and if successfull the directed retry will move the call to another cell in which case the call is not rejected.
In case of handovers soft blocking is not applied except if the handover is for DR or DADLB then the soft blocking is applied as in normal call setup. |
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| What is DFCA forced HR mode? | If the current incoming DL C/I average is below the Force HR mode C/I threshold for the corresponding channel type (non-AMR/AMR), then the force HO mode is switched on. After HR mode switched on, HR channel/AMR HR channel will be assigned to a speech connection that is placed on a DFCA TRX until the HR mode switched off.
DFCA Force HR mode C/I threshold and Force HR C/I threshold AMR are user defined parameters. |
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| What is the restrictions to use DFCA? (features interworking limitation) | · Packet switch territory is not allow in DFCA TRX
· Dynamic SDCCH is not allowed in DFCA TRX
· FACCH call setup to DFCA TRX is not possible.
· Antenna hopping cannot be used in DFCA TRX |
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| Apart from DFCA parameter, what are the parameters shall be checked for DFCA activation? | BSS Synchronization
-Sync MUST be on prior to switching to DFCA Mode.
-FN and TS offsets should be set (not left at 0)
Interference Handovers
-nterference based handovers should be enabled on all DFCA BTS
Dynamic Power Control
-Should be enabled on all DFCA BTS for UL and DL
Timeslot Type
-TCH timeslots only supported on DFCA TRX’s
-All signaling (e.g. SDCCH) and GP timeslots must be removed.
Idle & Active BA List Usage
-BA list must be created and attached for Idle & Active use in each DFCA BTS |
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| What additional network elements are needed for Data? | SGSN, GGSN, CG, DNS, (BG, LIG) |
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| What elements need to be upgraded to activate EGPRS? | TRXs. A modulation scheme is used and modulation is performed by the TRX with hardware |
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| How does an MS handover from one cell to another during a data session? | It doesn't. MS performs cell re-selection exactly like idle mode |
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| What is a Flush? | When a cell re-selects, the data stored in the BVC buffer is Flushed (Deleted). If the new cell is in the same PCU than the data is moved to the new BVC. If not it is deleted and there is a bigger break in downloading. |
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| What modulation scheme does GRPS use? | GMSK |
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| How many GPRS coding schemes are there currently in ----- and what are they called? | CS1, CS2 |
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| What modulation scheme does EGPRS use? | GMSK(mcs1-4), 8PSK |
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| How many EGPRS coding schemes are there? | 9: MCS1 - MCS9 |
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| What is the maximum bitrate per timeslot with GPRS and EGPRS? | GPRS: 13kbps with CS2, EGPRS: 59.62 kbps with MCS9 |
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| What is timeslot capacity? | Maximum TSL capacity after C/I has reduced the MCS but before TBF multiplexing |
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| What are the MCS families used for? | Re-transmissions are sent in MCSs of the same family as each family has a different packet length. |
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| What functionality counters fast fading in EGPRS? | Incremental Redundency |
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| How does it work? | Data is punctured after coding. This involves reducing the number of bits needed to send the data. 3 puncturing codes are used. If the first packet can't be fully decoded, it is sent again with different puncturing scheme. The packets are combined until it is decoded. |
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| What functionality counters slow fading in EGPRS? | Link Adaptation |
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| How does it work? | PCU tries to keep best throughput by using BEP to calculate the probabilty of the packet being decoded. PCU uses initial MCS, then once it has BEP values, chooses modulation, then coding scheme according to whether it is initial transmission or re-transmission. User only has to set initial MCS |
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| What is timeslot multiplexing? | When there is more than one TBF allocated to the same timeslot |
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| What is TBF multiplexing? | When there is a mix of GPRS and EGRPS TBFs on a timeslot. Especially bad with UL GPRS and DL EGRPS |
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| How many different types of Network Mode of Operation are they and how are they different? | 3, NMO1: Needs a Gs link, all CS and PS paging through SGSN and PCCCH (or CCCH). NMO2: No Gs link, CS paging through MSC and CCCH, PS paging through SGSN and CCCH, NMO3: No Gs link, CS paging through MSC and SGSN, PS paging through SGSN and PCCCH (not supported in Nokia) |
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| What is the difference between the RLC and LLC protocols? | Radio Link Control is between the PCU and the MS. Logical Link Control is between the SGSN and the MS. RLC gives network throughput, LLC gives user throughput. |
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| What is a territory? | A territory is an area of the cell that is used for a certain purpose |
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| How many are there and what are they called? | 4: CS, Dedicated data, Default data, Additional data |
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| How does each behave? | Dedicated data is only used for data, default can be used for both but CS is kept out of it unless there is no more CS territory available, additional is data territory beyond default and is released as soon as possible, CS can be used to carry CS and includes default and additional |
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| Is there power control in EGPRS? | Only in Uplink |
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| What is an EDAP and why is it needed? | EGPRS Dynamic Abis Pool: A part of the Abis set aside for EGPRS use. It is shared amongst all TRXs attached to it and provides a 30% saving on Abis capacity compared to fixed allocation. The EDAP is needed because the higher bitrates with EGPRS need more than 1 16kbps Abis subslot |
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| What is the difference between one-phase and two-phase access? | In one phase, the RACH request asks for a TBF. In two phase, the RACH request ask for a single block and uses this to ask for the TBF. |
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| What is a RA? | Routing Area: serve the same function for data as the LA for voice: Allows the SGSN to page the MS |
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| What is its relationship to an LA? | A RA is a subset of an LA. Maxiumum size is the LA size. An LA can have more than one RA. |
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| Which is the trade off in the DEFAULT TERRITORY size? | Small default territory = Few GPRS reason Ho. Big default territory = Better Throughput |
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| Can you have separated (from the VOICE one) signalling for GPRS / EGPRS? | Yes, you can although it is not really used |
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| Which is the channel you are going to use? | PBCCH |
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| Which is the trade off in term of signalling using PBCCH? | Need to use a dedicated RTSL for it |
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| What's the impact of GPRS traffic on the CCCH load? | There is an impact, but there is no evidence so far of any real bottleneck created from data signalling |
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| Why we don't we use PBCCH in CINGULAR? | Basically Blackberry 1st generation doesn't suport it |
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| Which are the other features you can use for cell reselection if you implement PBCCH? | C31, C32 |
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| Which is the meaning of C31 and C32? | C31 allows you to prefer a set of BTSs. C32 makes the cell reselection among the one prefered from C31 |
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| What is a PCU and what does it do? | Packet Control Unit. Receives the LLC PDUs from the SGSN and breaks them down into RLC Packets for the radio interface. Vice versa in the opposite direction. Responsible for all scheduling, LA and radio resource management for data |
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| How many are there in a BSC? | 2i: 2 per BSCU, 8+1 BCSU max, 3i: 4 per BSCU, 6+1 BCSU max |
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| What is a DSP core? | Digital Signal Processor core. Handles the individual Abis subslots and does the actual work. |
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| How many are there in a PCU? | 16 |
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| What is the maximum number of EDAPs a PCU can handle? | 16 |
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| What is the maximum number of 16 kbps Abis sub-slots a PCU can handle? | 256 |
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| What is the effect on the link budget of using 8PSK modulation? | As 8PSK has variable amplitude, the amplifier has to work in linear, not satuarated, mode. There has to be a power back-off to keep operation in linear mode. Back-off is 2 dB for BTS, 4-5 dB for MS |
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| Outline a dimensioning method for combined voice and data traffic? | Erlang B will provide enough spare TSL to guarentee GoS for voice. Data can use these. Formula is a choice between Erlang B and an approximation, which ever requires more timeslots. Number of TSL for data is data load divided by TSL capacity. Total TSL required given by Max(ErlangB(CS),CS traffic(E)+Guard TSL+TSL for data-Dedicated TSL)+dedicated TSL. From here work out TRXs and so on. |
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| What is rate reduction? | A way of including timeslot multiplexing in dimensioning by taking into account CS and PS traffic and PS territory |
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| How big can an EDAP be? | 12 * 64 kbps Abis TSL |
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| How many EDAPs can be attached to a site? | There is no limit. It could be one per site or one per TRX, although this is inefficient |
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| How many EDAPS can be mapped onto a single T1? | As long as there is space on the T1, there is no limit |
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| Which are the 3 states of a MS in GPRS? | Ready, Idle, Standby |
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| What does READY mean? | MS is transmitting data or it has just transmitted data … it is in active mode |
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| What does IDLE mean? | Basically the SGSN doesn't know where the MS is |
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| What does STANDBY mean? | SGSN knows where the MS is based on the RA resolution |
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| How do you move from IDLE to READY? And viceversa? | Attach and Detach |
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| How do you move from READY to STANDBY? And viceversa? | READY --> STANDBY / timer. STANDBY --> READY just if you send or receive data |
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| How do you move from STANDBY to IDLE? And viceversa? | GPRS Detach, either implicit or through a timer |
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| What is a TBF in EGPRS? | Temporary Block Flow |
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| Is the TBF in both directions? | no, unidirectional |
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| Can the same TBF be bi-directional (let's say like a voice call)? | no |
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| Which is the basic signalling in order to maintain an UL TBF? | UL TBF and in DL just ACK |
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| Which is the basic signalling in order to maintain an UL TBF? | DL TBF and in UL just ACK |
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| What is the effect of putting data on a hopping TRX? | With no power control there is increase interference to the hopping layer |
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| What is the main objective of EGPRS optimisation? | To maximise user throughput |
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| In EGPRS, how is this achieved? | Mostly through, capacity improvements. C/I improvements will affect the MCS but this is linked to GSM. For EGRPS only we can play with TRXs, Ded TSL, EDAP, PCU, Gb |
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| If a TRX is BLK-SYS when EGENA is turned on, what is the most probably cause? | GTRX=Y and the TRX is not attached to the EDAP |
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| Name some EGPRS KPIs | Throughput, TBF est fail, TBF fail, TBF lost due to flush or MS lost, GMSK share of EGPRS, TBF/TSL, Payload, EDAP congestion (mins/Gb), territory upgrade rejects, territory downgrades, PCU congestion |
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| What is PoC? | Push-to-Talk over Cellular. It is a real-time service |
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| What elements need to be added for PoC? | PoC Server, possible Presence server |
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| How is EGPRS used for PoC? | It is merely a transport layer, PoC is a service that uses EGPRS |
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| Are there re-transmission in PoC? | Not from the PoC server. EGPRS uses normal algorithm |
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| What KPIs are used in PoC? | Start to Talk (STT) time, Voice Through Delay (VTD),Glitches (delay in packet arrival), Lost data), Round Trip Delay (RTT) (depends on user reaction) |
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| What values are normally expected for STT and VTD? | STT: <2s, VTD: <4s |
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| What is the advantages/disadvantages of a low initial MCS | Adv: greater possibility of the data being decoded for MSs in bad quality. Dis: Less range if 8PSK due to power back-off |
|
| Define a Microcell | A small capacity cell with an antenna not more than 5m in height |
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| What is the best position for a microcell antenna? | In a grid system, on the side of a building. Not on a corner otherwise the canyon streets will propagate the signal too far. In a non gird system, side or corner of the building wherever best serves the target area. |
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| What are the disadvantages of a micro cell? | Fast moving mobiles may select it and then leave the coverage area very fast. Local congestion because of high traffic |
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| How can these be solved? | Use C2 penalty time to make the cell attractive only to slow mobiles. Run the cell as barred and use hierarchical handovers to move the calls from the overlaying macros |
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| Name some RF systems used in in-building cells | Active or passive Distributed Antenna Systems (DAS), leaky feeders, Yagis in the lift shaft. |
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| If you don't know an answer to a question where do you look? | NED, Jump, Quickplace, mailing lists, ETSI Specs, Nokia Feature Descriptions, Training course material. |
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| Why don't we suggest to have a bigger DEFAULT TERRITORY? | More intracell Ho. Eat up PCU capacity |
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