Wireless & Mobile Communications Multiple Choice Questions on “Packet Radio”.
1. In ___________ technique, many subscribers attempt to access a single channel in an uncoordinated manner.
a) Packet radio
b) Multiple access
c) Modulation
d) Spread spectrum
Answer: a
Clarification: In packet radio (PR) access techniques, many subscribers attempt to access a single channel in an uncoordinated (or minimally coordinated) manner. Transmission is done by using bursts of data.
2. Packet radio uses _________ and __________ signals for perfect feedback.
a) Forward, reverse
b) ACK, NACK
c) Active, passive
d) Linear, non linear
Answer: b
Clarification: Packet radio uses ACK and NACK signals for perfect feedback. The ACK signal indicates an acknowledgment of a received burst and NACK indicates that the previous burst was not received correctly by base station.
3. Packet radio has high spectral efficiency.
a) True
b) False
Answer: b
Clarification: Packet radio multiple access is very easy to implement, but has low spectral efficiency and may induce delays. The subscriber uses a contention technique to transmit on a common channel.
4. ___________ is defined as average number of messages successfully transmitted per unit time in packet radio multiple access technique.
a) Average delay
b) Figure of merit
c) Throughput
d) Efficiency
Answer: c
Clarification: The performance of contention technique can be evaluated by the throughput (T), which is defined as the average number of messages successfully transmitted per unit time, and the average delay experienced by a typical message burst.
5. What is the time period during which the packets are susceptible to collisions with transmissions from other users?
a) Delay time
b) Latency period
c) Average delay time
d) Vulnerable period
Answer: d
Clarification: Vulnerable period is the time period during which the packets are susceptible to collisions with transmissions from other users. It is used in order to determine the throughput.
6. For a PR multiple access technique, packet transmissions occur with __________ distribution.
a) Poisson
b) Gaussian
c) Pearson
d) Rayleigh
Answer: a
Clarification: For a PR multiple access technique, packet transmissions occur with Poisson distribution. To study packet radio protocols, it is assumed that all packets sent by all users have a constant packet length and fixed channel rate.
7. If λ is mean arrival rate and τ is the packet duration, then traffic occupancy of a packet radio network is equal to _________
a) λτ
b) λ/τ
c) λ-τ
d) λτ2
Answer: a
Clarification: If λ is mean arrival rate and τ is the packet duration, then traffic occupancy or throughput of a packet radio network is equal to λτ. The unit of R is in Erlangs.
8. Pure ALOHA is __________
a) Modulation technique
b) Multiple access technique
c) Random access technique
d) Spread spectrum technique
Answer: c
Clarification: The pure ALOHA protocol is a random access protocol used for data transfer. A user accesses a channel as soon as a message is ready to be transmitted.
9. For pure ALOHA protocol, the vulnerable period is ___________ the packet duration.
a) Double
b) Equal
c) Ten times
d) Not equal
Answer: a
Clarification: For the pure ALOHA protocol, the vulnerable period is double the packet duration. After a transmission, the user waits for an acknowledgement on either the same channel or a separate feedback channel.
10. The vulnerable period of slotted ALOHA is _________ packet duration.
a) Two
b) Ten
c) One
d) Three
Answer: c
Clarification: The vulnerable period of slotted ALOHA is one packet duration. Since, partial collisions are prevented through synchronization.
11. In slotted ALOHA, time is divided into different length of time slots.
a) True
b) False
Answer: b
Clarification: In slotted ALOHA, time is divided into equal time slots of length greater than the packet duration. The subscribers each have synchronized clocks and transmit a message only at the beginning of a new time slot.
12. CSMA stands for _________
a) Carrier sense multiple access
b) Code sense multiple access
c) Carrier sense modulation access
d) Carry sense multiple access
Answer: a
Clarification: CSMA stands for carrier sense multiple access. CSMA protocol is based on the fact that each terminal on the network is able to monitor the status of the channel before transmitting information.
13. __________ is the time required for a terminal to sense whether or not the channel is idle.
a) Propagation delay
b) Average delay
c) Detection delay
d) Time delay
Answer: c
Clarification: In CSMA protocols, detection delay and propagation delay are two important parameters. Detection delay is a function of the receiver hardware and is the time required for a terminal to sense whether or not the channel is idle.
14. Which of the following is based on time division multiplexing?
a) Slotted ALOHA
b) Pure ALOHA
c) CSMA
d) Reservation ALOHA
Answer: d
Clarification: Reservation ALOHA is a packet access scheme based on time division multiplexing. In this protocol, certain packet slots are assigned with priority, and it is possible for users to reserve slots for the transmission of packets.
15. Discrete packet time technique, PRMA stands for _________
a) Packet reservation multiple access
b) Photo reflector multiple access
c) Proton reflector modulating access
d) Packet reflection multiple access
Answer: a
Clarification: PRMA stands for Packet Reservation Multiple Access. PRMA uses a discrete packet time technique similar to reservation ALOHA and combines the cyclical frame structure of TDMA in a manner that allows each TDMA time slot to carry either voice or data, where voice is the priority.