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Consider a communication source that transmits packets containing digitized speech. After each transmission, the receiver sends a message indicating whether the transmission was successful or unsuccessful. If a transmission is unsuccessful, the packet is re-sent. Suppose a voice packet can be transmitted a maximum of 10 times. Assuming that the results of successive transmissions are independent of one another and that the probability of any particular transmission being successful is \(p\), determine the probability mass function of the rv \(X=\) the number of times a packet is transmitted. Then obtain an expression for the expected number of times a packet is transmitted.

Short Answer

Expert verified
Pmf: \((1-p)^{x-1}p, x=1,...,9; (1-p)^9, x=10\); Expected value: \(E(X) = \sum_{x=1}^{9}x(1-p)^{x-1}p + 10(1-p)^9\).

Step by step solution

01

Understanding the Problem

We are given a communication system where a packet can be transmitted up to 10 times. The receiver lets us know if the transmission is successful or not. We are tasked with finding the probability mass function (pmf) of the random variable (rv) \(X\), which represents the number of times a packet is transmitted until successful. Additionally, we need to find the expected value of \(X\).
02

Identifying the Random Variable

The random variable \(X\) is defined as the number of times a packet is transmitted, which could be a minimum of 1 (when it's successful on the first try) to a maximum of 10 (if the packet fails for the first 9 attempts and finally succeeds or remains unsuccessful on the 10th attempt).
03

Constructing the Probability Mass Function

The process can be formalized as a Geometric distribution with parameter \(p\) for the first 9 attempts. Thus, for the first 9 attempts (where the packet is successfully transmitted at the \(x^{th}\) attempt), the probability mass function is: \[ P(X = x) = (1-p)^{x-1}p, \quad x = 1, 2, ..., 9 \]On the 10th attempt, the packet either is successfully transmitted (with probability \(p\)), or it has failed 10 times (with probability \((1-p)^{9}\)).Thus, \[ P(X = 10) = (1-p)^9 \].
04

Setting Up the Expected Value Calculation

The expected value \(E(X)\) for \(X\) follows standard geometric distribution for the first 9 transmissions and an adjusted case at \(X = 10\). Therefore, \[ E(X) = \sum_{x=1}^{9} x (1-p)^{x-1}p + 10 (1-p)^9 \]. This requires calculating terms separately for each part and summing them up.
05

Calculating the Expected Value

Calculate \(E(X)\) using derived expressions: 1. Calculate the sum for \(x\) from 1 to 9 using the geometric series formula: \[ E(X)_{1-9} = \sum_{x=1}^{9} x (1-p)^{x-1}p \].2. Then, add the value corresponding to \(x = 10\), which results in an additional component: \[ 10(1-p)^9 \]. Combining gives the complete expected value.
06

Final Expression for Expected Value

After calculations, the final expression for the expected number of transmissions, taking into account all successes and failures up to 10 attempts, will be: \[ E(X) = \sum_{x=1}^{9} x (1-p)^{x-1}p + 10(1-p)^9 \]. If \(p > 0\), \(E(X)\) will simplify down to find precise computational value.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Geometric Distribution
The geometric distribution is a probability distribution that models the number of Bernoulli trials needed for a success. In simpler terms, it's about how many times you have to keep trying something before you succeed at it. In the context of our communication system, each attempt to successfully transmit a packet represents a Bernoulli trial, while a complete successful transmission of a packet corresponds to a success.- The key feature of the geometric distribution is that all trials are independent, and the probability of success, denoted as \(p\), remains constant on each trial.- The probability mass function (pmf) for the geometric distribution is given by \(P(X = x) = (1-p)^{x-1}p\), where \(X\) represents the number of trials until the first success.In this exercise, since a maximum of 10 attempts is possible, we see the geometric distribution in play up to 9 trials, with an adjustment needed for the 10th trial where it is either the final success or a total failure.
Expected Value
The expected value, often denoted as \(E(X)\), is a measure of the center of a probability distribution and represents the average outcome one can expect over many trials. In our packet transmission system, \(E(X)\) represents the average number of times we would expect to attempt packet transmission before achieving success.To calculate the expected value in this scenario:- For the first 9 transmissions, this follows the expected number of trials in a geometric distribution, typically calculated as \(E(X) = \frac{1}{p}\) for geometric cases.- We also adjust to account for the constraint of up to 10 attempts, adding a component for the 10th trial if all previous attempts have failed.Thus, the expected value formula becomes a combination of expected values for these conditions, calculated as:\[E(X) = \sum_{x=1}^{9} x (1-p)^{x-1}p + 10(1-p)^9\] This covers both the average scenario up to the 10th attempt under varying probabilities.
Independent Events
In probability, independence refers to events that do not affect each other's outcomes. In the context of this exercise, each packet transmission is an independent event. This means that whether a packet was successfully sent in previous attempts does not change the likelihood of success for the next attempt.- Independent events have a distinct characteristic where the probability of succeeding in each trial remains the same, represented by \(p\).- Calculating probabilities using the geometric distribution relies heavily on this independence, ensuring the calculations accurately reflect the probability across multiple trials.In practice:- This property allows us to model each packet transmission discretely without worrying about past outcomes, thus simplifying the calculation and expression of the probability mass function.
Random Variable
A random variable (rv) is a variable whose possible values are numerical outcomes of a random phenomenon. In this scenario, the random variable \(X\) represents the number of transmissions needed to successfully send a packet, ranging from 1 to a maximum of 10.- \(X\) is dictated by the principles of the geometric distribution, as it counts the trials up to and including the first success.- Random variables help in quantifying the uncertainty and variability associated with packet transmission, allowing us to use statistical tools to understand and predict outcomes.Understanding \(X\) as a random variable aids in defining characteristics like the probability mass function, which tells us the likelihood of different transmission counts needed, and the expected value, which informs the average required attempts in the long run.

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Most popular questions from this chapter

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