Drahtlose Kommunikation
Wintersemester 2018/2019 Prof. Hannes Frey / Dr. Jovan Radak
Assignment 4
Name two disadvantages that result from using CSMA/CD (carrier sense multiple access with collision detection) in wireless networks.
Exercise 2
In the lecture (03 Medienzugriffskontrolle (VL11) p.64) the following formula has been shown for calculating the throughput S when using CSMA:
𝑆𝑆= 𝐺𝐺 ⋅ 𝑒𝑒−𝑎𝑎𝑎𝑎 𝐺𝐺 ⋅(1 + 2𝑎𝑎) +𝑒𝑒−𝑎𝑎𝑎𝑎 Answer the following questions:
a) Explain with few words the role of the variable G. What does it represent?
b) What unit does G have?
c) Assume that a node sends a packet. How large is the time frame, in which another node willing to send will cause a collision?
d) Name two reasons why the throughput S declines in CSMA when the load exceeds a certain value.
Exercise 3
a) How does the length of the CDMA chipping sequence affect the bandwidth?
b) Examine how CDMA is affected by arbitrary disturbances. Given is a disturbed signal that was received. Decode the signal like it was shown in the lecture (03 Medienzugriffskontrolle p. 75 ff).
Exercise 4
a) Given is a rechargeable battery with 1.2 V and 2000 mAh. Calculate how long a device using 1.2 watts can be powered with this battery.
a)
The given sensor network consists of sensor nodes, where each node gathers one measurement value. The highest measurement value shall be saved on the sink node.
Each transmission between two nodes consumes 1 Joule of energy. How much energy is needed in total if:
1) data is not aggregated? _____
2) data is being aggregated? _____
b) A message shall be transmitted from node A to node B. For a correct transmission the message has to be received with at least 1 mW of power. Nodes A and B are 100 m apart.
There are 9 relay nodes equally spaced in between. One transmission of the message takes 1 ms. (Note: Assume free space according to Friis)
1) How much energy (in Joule) is needed for direct transmission from A to B, without using the relay nodes?
2) How much energy (in Joule) is used in total for multi-hop transmission from A to B over the 9 equidistant relay nodes?
Sensor node Data sink
A B
Exercise 6
Which disadvantage of the S-MAC protocol is resolved in T-MAC and which new problem occurs?
Exercise 7
Given are three sensor nodes S1, S2 and S3 with following energy consumption values:
Sleep mode per ms: 0,1 J Receive mode per ms: 1 J Data transmission per ms: 2 J
Node S1 wants to send a data packet to node S3. The transmission takes 2ms.
For each of the following MAC protocols and the given time period of 10 ms, draw into the diagrams, how long each node is transmitting data (T), in receive mode (R), sending the preamble (P), or in sleep mode (S). Then calculate the overall energy consumption (in Joule).
a)
Nodes are not sleeping:
Energy consumed: ____
b)
For the following MAC protocols the sleep cycle of each node is 5 ms. The arrow indicates when the node wakes up from sleep mode for the first time.
1) All nodes are using the B-MAC protocol:
S2 S3
S1
0 1 2 3 4 5 6 7 8 9 10
time in ms
Assume a simplified strobe and acknowledgment length of 0 ms.
Energy consumed: ____
3) All nodes are using the Wise-MAC protocol:
Assume that node S1 already learned the schedule of node S3. Assume a simplified preamble length of 0 ms.
Energy consumed: ____
Exercise 8
Assume a scenario where all nodes have a fixed sleep/wake cycle of Δ. How long is the preamble sent on average when using the following protocols?
a) B-MAC:
b) X-MAC:
S2 S3
S1
0 1 2 3 4 5 6 7 8 9 10 time in ms
S2 S3
S1
0 1 2 3 4 5 6 7 8 9 10 time in ms
