CSE 4153 / 6153, Fall 2008, Assignment 7 Solutions
1. A satellite serving 100 stations can receive a maximum of 10,000 packets per second. If the channel is being utilized optimally
I. What is the average number of packets sent by every station? (for plain and slotted ALOHA) II. How many packets (per second) suffer collisions? (for plain and slotted ALOHA)
Plain ALOHAThe efficiency is 18% when total traffic rate in the channel is optimal
Or 1800 packets / sec will reach the satellite without collisionsTo operate at the optimal rate all station should (together) send 5000 packets per sec
Each station should send (on an average) 50 packets per secAbout 3200 packets suffer collisions
Peak efficiency is 32% when the traffic rate is optimalOr 3600 packets / per sec will reach the satellite without collisions
To operate at the optimal rate all station should (together) send 10,000 packets per secEach station should send (on an average) 100 packets per sec
About 6,400 packets / sec suffer collisions
2. Assume that you are designing a CSMA-CD protocol which permits
maximum distance of 100 m between any two stations
The velocity of propagation of electromagnetic waves in the medium is 250 million m/s. Specify other parameters of the system. What is the efficiency?
(There was an error in this question – the maximum distance was given as 1000 m which would mean the
minimum packet size is greater than the maximum packet size. I have provided the solution for a length
1 billion bits per second is the max bit rate2400 bits will take 2.4 micro seconds
RTTMaximum distance between stations is 100 m
Propagation delay is 0.4 micro secRTT is 0.8 micro seconds
3. Explain the hidden station and the exposed station problems in CSMA-CA. Is exposed station problem handled in 802.11? Why?
A is transmitting to B. C cannot hear A. If C transmits (say to D, or any other station), B will receive garbled transmissions.
802.11 uses MACAW. Both MACA and MACAW address this problem.
Before A transmits to B it will send a RTS packet. B will respond with a CTS packet. C which can hear the CTS will not transmit for some duration.
C is transmitting to D. B is in range of C desires to transmit to A. It can as A cannot hear C. But it will not
as B thinks the channel is not free.
In this case B heard the RTS from C but did not hear the CTS from D. So B knows that D is out of its range and can transmit.
802.11 does not address exposed station problem as D needs to send an ACK after C completes
transmission. If B transmits then the ACK from D may suffer collision at C. So when MACAW is used if a station hears a RTC or CTS it will not transmit for some time. On the other hand, if MACA is used stations
which hear an RTS will wait for a small duration to see if they can hear the CTS. If not they are free to transmit.
4. Briefly explain the concept of backward learning in bridges.
Bridges inspect the from address in MAC packets they forward to deduce which MAC addresses arrive on
which lines of the bridge and construct a table. Whenever a packet is received bridges inspect the table to see if the destination MAC address has an entry in the table. If not the packet is flooded.
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