132B_1_EE132B_MidtermSolution__Rubin_Spring2009_050209 -...

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UCLA ELECTRICAL ENGINEERING DEPARTMENT EE132B - DATA COMMUNICATIONS AND TELECOMMUNICATIONS NETWORKS Midterm Solution Professor Izhak Rubin Spring 2009 Problem 1   (50 points ) a. Define and state the key differences between fixed-assigned and demand-assigned TDMA schemes. Answer: Under fixed assigned TDMA, every station is allocated fixed slot(s)  (regardless of whether the resources are fully utilized); either at specified times, or  a fixed number in each time frame for a call duration.  Under DA/TDMA, a station  must first send to the controller a request (signaling) message and then if slots are  available, the channel is allocated slot or slots (either in each time frame for the  duration of a session for circuit switched systems, or a specified number of time  slots under a prescribed times). b. Define the slotted and unslotted ALOHA multiple access protocols. Assume that a communications channel that operates at a data rate of 8 Mbps is shared by a large number of stations through the use of a slotted-ALOHA protocol. Each station, when busy, has a packet of duration 6400 bits that it wishes to transmit across the shared channel. Calculate the maximum throughput rate achievable across this channel, as measured in units of packets/second. Answers:  With slotted Aloha, time is synchronized and divided into slots. Slot size  is equal to the packet transmission time. When packets are ready for transmission,  they   wait   until   the   start   of   next   slot.   With   unslotted   Aloha,   stations   start 
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This note was uploaded on 03/29/2011 for the course ELEC ENGR 132B taught by Professor A during the Winter '10 term at UCLA.

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132B_1_EE132B_MidtermSolution__Rubin_Spring2009_050209 -...

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