Questions & Answers


Cellular Networks - Mobile Phones


Rajkumar Periannan & Fadi Joseph Fahham

These questions gives the candidate a general view on the degree of bit error that occurred when receiving the data that we were transmitting through our articles



    Rating :

  1. The diagram below shows the hierarchical structure of the cellular network.

    What is (i) and (ii)?

    1. Mobile phone and Mobile Switching Centre(MSC)
    2. Public Switched Telephone Network(PSTN) and Mobile Switching Centre(MSC)
    3. Mobile Switching Centre(MSC) and Public Switched Telephone Network(PSTN)
    4. None of the above


    Rating :

  2. GSM is the digital standard for Europe; What do the letters GSM currently mean?
    1. Global Special Mobile
    2. Greater System's Mobile
    3. Global Systems for Mobile Communications
    4. none of the above!


    Rating :

  3. Why is a digital network preferred over an analogue one?
    1. It is newer
    2. It is smaller
    3. It has lower power consumption
    4. It has higher capacity for the same bandwidth


    Rating :

  4. In cellular networks, simultaneous users over the same channel is achieved by :
    1. Digital Technology.
    2. Frequency re-use.
    3. CDMA and TDMA.
    4. Using VLRs in each area and the HLR in the network switching centre.

    1. I and II
    2. I and III
    3. II and III
    4. IV only


    Rating :

  5. The (or one of the) difference(s) between GSM and PCN is :
    1. GSM operates in the frequency range of 900MHz while PCN operates in the 1800MHz range
    2. GSM is the standard used in Europe, while PCN is the one in North America
    3. GSM uses digital technology while PCN is analogue
    4. none! There is no difference


    Rating :

  6. Which of these statements are correct?

    1. Mobile Switching Centre(MSC) would process the call made by one mobile phone user to another mobile phone user.
    2. Public Switched Telephone Network(PSTN) would be involved in processing a call when the call is made by a mobile phone user to a land phone.
    3. Both MSC and PSTN processes a call made from a mobile phone to a land phone.
    4. Electronic Serial Number(ESN) is the telephone number of the cellular instrument assigned to the subscriber.

    1. III only
    2. I, II and IV
    3. III and IV
    4. I and II


    Question 7 and 8 are based on the dialogue below.

    Wallace and Grommit are having a conversation through their phones. Wallace is using a landphone while Grommit, who is slightly richer, is using a mobile phone.
    Wallace : How are you Grommit?
    Grommit : Fine thank you. How about you?
    Unknown person : Look Smith, you'd better get the money soon.
    Wallace : Grommit! Was that you?
    Grommit : No! It was someone else.

    The dialogue above describes a problem with cellular networks with respect to mobile phones.

    Rating :

  7. Why does the problem above occur?
    1. Multipath exists between the transmitter and receiver
    2. Cell size is too small.
    3. Grommit's mobile phone has passed the handover area but has not been assigned another base station. This resulted in a noticeable break in which another conversation is transmitted.
    4. The radio waves from two different conversation have added on to each other due to the humidity of air.


    Rating :

  8. How is this problem solved in analogue cellular network?
    1. Base Station continuously transmits the Mobile Identification Number(MIN) and received by the mobile phone.
    2. Mobile phone continuously transmits the Signal Audio Tone(SAT) and received by the base station.
    3. Overall size of the cluster is increased
    4. Increasing the bandwidth allocated to each user.

    1. II only
    2. III only
    3. I and IV
    4. II and III


    Rating :

  9. Why does GSM use TDMA, as opposed to CDMA?
    1. When GSM was planned, CDMA was not approved as a multiple access system.
    2. TDMA is better than CDMA.
    3. CDMA is not really needed in Europe.
    4. CDMA is too expensive to implement.

    1. I and III
    2. I only
    3. II and III
    4. I and IV


    Rating :

  10. Which of the following statement is true about spread spectrum:
    1. It uses a narrow band frequency
    2. Spread Spectrum allocates disjoint resources (frequency or time slots depending on the access system) to each user.
    3. Spread Spectrum signals can be picked up by simple receivers
    4. Spread spectrum signals are hard to jam and identify


    Rating :

  11. Which of the four cellular networks in the UK is the best and why?
    1. Mercury, because it offers good service within its covered area.
    2. Orange, because of its good "inside building" coverage.
    3. Vodafone, because it uses GSM
    4. Cellnet , because it offers a wider range of phones-sets


    Rating :

  12. The diagram below shows a fixed channel assignment method with borrowing strategy. Cells marked X are cells that are prohibited from using the a4 channel before borrowing occurs.

    Which one of these statements are correct?
    1. The diagram above shows two adjacent cells using the a4 channel after borrowing.
    2. Channel a4 is possibly locked to cells marked N.
    3. The handover rate will increase after borrowing
    4. The borrowing strategy illustrated above leads to poorer voice reception.

    1. I, II and III
    2. II only
    3. II and IV
    4. I and II


    Question 13 and 14 are based on queuing of handover requests.

    Rating :

  13. On the surface, queuing of handover requests seems to be a bad scheme and not possible since handover must not be noticed(delay sensitive) by the user. But it turns out that queuing handover request is possible.

    Why is handover request queuing possible?
    1. Handover request taking up to 800ms would not be noticed by the user. Queuing is possible within this time.
    2. There exist a time interval between the mobile phone reaching the handover threshold and the receiver threshold.
    3. Handover request can be made small packets. Therefore the queue can be processed very quickly.
    4. The alternative to queuing handover request is queuing new originating calls which is far more delay sensitive. Therefore queuing handover request(although delay sensitive) is a better idea.


    Rating :

  14. There are five cars(C1, C2, C3, C4 and C5) moving away from cell A into cell B. The table below shows the time at which each car arrives at the handover threshold and the speed at which they are travelling.

    Car Arrival time(ms) Speed(km/h)
    C1 0 10
    C2 10 40
    C3 15 50
    C4 25 20
    C5 100 360

    There are several assumption that the candidate is allowed to make about this question. They are :-
    • All the cars are travelling away from the handover threshold towards the receiver threshold.
    • none of the cars have reached the receiver threshold at 100ms.
    • the handover area is divided into infinite power level ratios.

    The choices below shows the possible arrangement of the FIFO queue and the MBPS queue of the handover requests at 100ms. C1, C2, C3, C4 and C5 represents the handover requests made by the mobile phones from the cars of the same name. Choose the correctly arranged queue.



    1. Answer : B

    Topic : What is a Cellular Network?

    Each cell has a base station. Cells when grouped together forms a cluster. MSC is connected to all the base stations in a cluster. MSC itself is connected to MSCs of other clusters and to the PSTN switching centre. The full answer is shown on the left.

    For further reading :


    2. ANSWER : C

    Topic : GSM

    GSM is currently the Global system for Mobile Communications. When it was first developed it was called a French name meaning Group Special Mobile. GSM was later renamed to its current name by the European Telecommunication Standards Institute (ETSI) when it gained world wide acceptance and became the European standard.

    For further reading :

    ARTICLE 1.Cellular Communication Networks - GSM -> GSM

    3. Answer : D

    Topic : Multiple Access Systems

    The digital network uses various methods to maximise the efficiency of bandwidth utilisation, such as TDMA and CDMA.

    4. Answer : C

    Frequency re-use in non-adjacent cells allows a small bandwidth to be used by many subscribers. Now TDMA/CDMA allows further sharing, using spread spectrum technologies or (for TDMA) by dividing the available bandwidth into time slots for different users. Digital technology has led to these multiple access systems but on its own, digital technology didn't achieve simultaneous users.

    For further reading :

    ARTICLE 2. Mobile Phones in the UK and Multiple Access Systems -> Spread Spectrum

    5. ANSWER : A

    Topic : PCN phones

    Both GSM and PCN are digital mobile systems. The system used in North America is the PCS - personal communication system, not PCN - personal communication network. The difference is answer A - the frequency range.

    6. Answer :D

    Topic : What is a Cellular Network? AND Allowing Mobility of the Subscriber

    I and II are correct
    Each MSC is connected to MSCs of other clusters. Therefore any call made from a mobile phone to another mobile phone can be processed and directed to the appropriate mobile phone via the MSC. But a call made to a land phone would require the PSTN to process it as land phones are connected to the PSTN.

    III is incorrect
    ESN is the number assigned to the mobile phone by the manufacturer. The statement IV actually refers to the Mobile Identification Number(MIN).

    For further reading :


    Detailed explanation on call processing


    7. Answer : C

    This problem arises due to co-channel interference.
    In a cellular network, an area is divided into cells which when grouped together forms a cluster. In order to efficiently utilise the bandwidth allocated, the channel frequency used in one cluster is reused in another cluster. The pair of cells, located in these different clusters using this same channel frequency must be far away from each other so that no co-channel interference can occur.
    When the cell size is too small, the distance between two cells in different clusters using the same channel frequency would be too close to each other. This causes co-channel interference to occur.

    8. Answer : D

    The problem described above is co-channel interference.

    I is incorrect
    The mobile phone transmits MIN when it enters a new cell for registration purposes. This does not solve co-channel interference.

    II is correct
    The explanation below shows how SAT can be used to distinguish the correct user from other users who are using the same channel frequency.

    Diagram 1
    As shown in the diagram on the left, each cluster is assigned SAT at different frequencies. There are three different frequencies. They are 5970Hz(SAT 1),6000Hz(SAT 2) and 6030Hz(SAT 3).

    The base station of a cell would first send this SAT to the mobile phone when it enters this cell. After the mobile phone receives this SAT, it would re-transmit this SAT almost continuously to the base station. So any co-channel interference would be sensed as it would carry the wrong SAT frequency. In this case the unknown person's mobile phone would be transmitting the wrong SAT to Grommit's current base station. The base station can then mute the unknown user's conversation.

    III is correct
    Cells using the same channel frequency would be located in different clusters. So, increasing the size of the clusters would cause these cells using the same channel frequency to be further away from each other. Therefore co-channel interference is reduced.
    III is not an extremely accurate answer unlike II, but it is an acceptable answer owing to the fact that cluster size plays a major role in the degree of co-channel interference in both analogue and digital cellular networks.

    IV is incorrect
    Increasing the bandwidth can only make matters worst as it reduces the number of different channel frequencies. So, the same channel frequency would be re-used in cells that are closer together.

    9. Answer : A

    GSM uses time division multiple access since code division multiple access was not approved at the time GSM emerged.
    CDMA is supposed to give better long distance transmission quality. Europe consists of many small countries. The need to communicate with a user in another country from one country is not high. This is because phone users in different countries generally

    • do not speak each others language
    • not related to each other or even know each other
    Meanwhile in the USA which is a large country, it is essential that long distance calls have very good quality because
    • people from different states talk the same language
    • a very high chance that people from different state know each other or even related to each other.
    Therefore, CDMA is needed in the USA but not in Europe(not essential).

    For further reading :

    ARTICLE 2. Mobile Phones in the UK and Multiple Access Systems -> The CDMA cellular standard

    10. Answer : D

    Topic : Spread Spectrum

    Spread spectrum uses a wide frequency band and allocates all resources to all simultaneous users, controlling the power transmitted by each to a minimum required to maintain a signal-to-noise ratio for the required level of performance. In this way a simple receiver will pass the signal as background noise.

    For further reading :

    ARTICLE 2. Mobile Phones in the UK and Multiple Access Systems -> Spread Spectrum

    11. Answer : none??

    Topic : PCN Phones

    As discussed in the article, choosing a mobile phones depends on the subscriber; where will the phone be used, what sort of services are expected from, price and so on. For instance, a customer requiring a cheaper service within the London area should get a Mercury One2One as compared to someone who wants to use it in Europe - in which case the choice would be Vodafone or Cellnet (which both use GSM by the way).

    For further reading :

    For a comparison between the digital networks in the UK ->
    ARTICLE 2. Mobile Phones in the UK And Multiple Access Systems

    12. Answer : B

    Topic : Channel Assignment Strategies in Cellular Networks

    I is incorrect
    Let's say cell A and cell B are adjacent to each other.
    After a channel frequency in cell A is borrowed to cell B, cell A cannot use this channel frequency. This avoids co-channel interference.

    II is correct
    From the diagram it can be seen that the cell using the a4 channel is closer to the cells marked N after the borrowing had occurred. Now, more cells around the cells marked X would be unable to use the a4 channel in order to prevent co-channel interference. So, cells marked N are possibly the cells that are not allowed to use the a4 channel after borrowing had occurred.

    III is incorrect
    Borrowing is meant to increase the efficiency of spectrum utilisation and does not affect the handover rate.

    IV is incorrect
    There is no reason for poorer voice reception as there is no increase in co-channel interference or other related problems after borrowing occurs.

    For further reading :


    13. Answer : B

    Topic : Schemes to Minimise Forced Termination of a Call

    The two circle area which overlaps shows the handover area. The mobile phone reaches this circle when it arrives at the handover threshold point[t0] and leaves this area when it reaches the receiver threshold point[t1].

    The handover threshold is set at the point where the power received from a neighbouring cell site has started to exceed the power received from the current BS for certain amount and/or for a certain time.
    The receiver threshold is the point at which the received power from the BS is at the minimum acceptable level.
    A main requirement here in order to prevent forced termination of a call is that :-

    • The mobile phone must be assigned a new Base Station(thus the relevant channels needed to continue a call) within the time interval (t1 - t0).
    If queuing of handover request is done within the time interval (t1 - t0), the above mentioned condition will be satisfied. This is why queuing of handover request is possible.

    For further reading :


    14. Answer : C

    Topic : Schemes to Minimise Forced Termination of a Call

    In order to answer this question follow the steps below :-
    Step 1.
    Convert the units of speed from km/h to m/s. Convert the unit of time from ms to s. These steps above makes the units consistent.

    Step 2.
    Calculate the distance the car would have travelled away from the handover threshold at 100ms This is done for each car by using the formula :-

    speed x (0.1s - arrival time at the handover threshold)

    Step 3.
    Construct the table shown below.
    Car Distance from the handover threshold(m)
    C1 0.278
    C2 1.00
    C3 1.18
    C4 0.417
    C5 0

    FIFO queuing scheme
    In this queuing scheme, the handover request that arrives first would be at the top of the queue and the request arriving the last would be at the bottom of the queue. In other words, the position of a request in the queue is based on the time of arrival of the request.

    Measurement Based Prioritisation Scheme(MBPS)

    The handover request in the MBPS queue is arranged according to the power level ratio of the mobile phone to which the handover request belongs to. This power level ratio depends on where the mobile phone is in the handover area(since the handover area is divided into power level ratios). This power level ratio itself can be determined by the MSC by dividing the power level received from the mobile phone by the current base station to the power level received by the next base station. The further the mobile phone is from the current base station, the lower the power level ratio.

    Since it is assumed that the handover area is divided into infinite power level ratios, a small difference in distance between the cars(from the table it can be seen that this difference is as low as 0.18m) would be noticed by the system and this would result in sorting of the queue. So, a car(e.g. C3) that is slightly further away from another car(e.g. C2) would be assigned a higher priority.

    Another important point here is that when a handover request arrives it is given the lowest priority. Based on this and the criteria described before, the only correct MBPS queue is in C.