Doubt in this computer network question

Question

Assume a system with fixed-length 250-byte packets where each packet contains 25 bytes of headers and trailers (thus, the data payload is 225 bytes). For a 600 byte message, what is the utilization (in %) in this system is :

a. 60% 

b. 70%

c. 80%

d. 88%

My approach: in 600 byte message 75 bits for header & trailer, so, $75/600 * 100 = 87.5%$

Is my approach correct??

Answer 1

It is mentioned in question that system implements fixed-length packet of 250B, which means that

data should always be 225B and total packet size should always be 250B(including header and trailers)

and if data is less than 225 then we have to make data to be 225B by padding extra bits.

 

So, 

$1^{st}$ Packet : 225B data + 25B(header and trailer)

$2^{nd}$ Packet : 225B data + 25B(header and trailer)

$3^{rd}$ Packet : 150B data + 75B padded data +25B(header and trailer)

 

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Total Packet data : 750B

Useful data(message) : 600B

$\eta$ = (Useful Data / Total Data)*100

$\eta$ = (600/750)*100 = 80%

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Option C is correct Answer.

Comments

@Bhaskar_Saini thanks I didn't see fixed-length packet & not added extra padding bits