EECS 148 Introduction to Computer Networks


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Problem Set #2

** NOTE: The "Walrand 2nd. ed." problems are in your textbook.

Walrand 2nd ed. #4.6:

"A new packet arrives and is transmitted in a time slot with probability epsilon" - This means that, in a particular time slot, the probability that exactly 1 of the computers on the LAN decides to attempt to transmit a new packet is epsilon. We are ignoring the probability that 2 or more of the computers on the LAN decides to attempt to transmit a new packet.

"Every old packet is transmitted with probability p." - This means that, in a particular time slot, the probability that a particular computer on the LAN with an old packet attempts to transmit that old packet is p.

Walrand 2nd ed. #4.8:

"Each file has an exponential length, with mean AxR" means that the file length is modeled as random, with an Exponential distribution with mean AR. (If you are currently taking ECE186, then ask someone who has already taken the course to translate this.)

"Each waiting time is also exponentially distributed, but with mean B" means that after receiving a file a user thinks for a random time with an Exponential distribution with mean B before requesting another file.

"How would you simulate this system ..." - Describe the major components (e.g. subroutines) of the simulation, and the functionality of each.

"Can you estimate ..." - Can you write a mathematical expression that gives the answer, or at least describe what terms this would include?

Walrand 2nd ed. #4.9:

"... as a function of the average number of back-to-back packets that the stations send" - It should turn out that the efficiency is not a single number, but depends on how many consecutive slots a station issues an inhibit signal for. Create some notation for this variable, and then efficiency should be a function of this variable.