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Assignment #6  ECE 487 (Data Communications Networks)

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Assignment #6
ECE 487 (Data Communications Networks) Section B1
Your Last Name: ____________________ Your Fist Name: _____________________
Your Student ID: ________________________
in the assignment box at 2nd Floor – Pedway
between ICE and ETLC
1. Give Manchester code for the following data. (4 point)
2. (16 points) Consider a wireless LAN consisting of one access point and three stations:
Stations A, B, and C. All the stations can hear each other. Each station has one and only one
DATA frame to be sent to the access point. The access point does not have DATA frame to be
sent. Distributed coordination function, which is based on CSMA-CA, is implemented. At time
instant 0 microsecond, the channel becomes idle, and the three stations start to contend for the
channel, with contention window being the initial contention window, i.e., [0, 31]. At time
instant 0 microsecond, the three stations pick up random backoff timers as 10, 30, and 30,
respectively.
Assume DIFS=50 microseconds; SIFS=10 microseconds; a backoff time slot=20 microseconds;
RTS transmission time=CTS transmission time=ACK transmission time=100 microseconds; A
DATA frame transmission time = 2000 microseconds. If a station sends an RTS frame, it will
wait for a TIMEOUT duration (which is equal to the duration of CTS plus SIFS) to get the
corresponding CTS. If a station hears a collision (i.e., senses a busy channel but cannot decode
information), after the channel becomes idle again, the station will also wait for a TIMEOUT
duration. Assume the propagation delay is negligible. We also have the following assumption: if
Station A needs to pick up a new backoff timer from its contention window denoted [X1, Y1], it
will pick up Y1-5; if Station B needs to pick up a new backoff timer from its contention window
denoted [X2, Y2], it will pick up Y2-10; if Station C needs to pick up a new backoff timer from its
contention window denoted [X3, Y3], it will pick up Y3-15.
In the system, we have transmission events. A transmission event is a successful transmission if
only one station transmits, or a collision if two or more stations transmit. How many
transmission events are there in the system? What is the duration for each event? Here the
duration of a successful transmission is the duration from the beginning of DIFS until the end of
ACK, and the duration of a collision is the duration from the beginning of DIFS until the end of
TIMEOUT.

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