### CS455 - Introduction to Computer Networks Homework 3

• Question 1 (20 pts)

• What is the bit pattern sent by an idle BISYNC sender ?

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• A BISYNC sender wish to transmit the following data:

 ``` Header: 01111110 00010000 Message: 00000011 00010000 01111110 CRC code: 11111111 00010000 ```

Give the BISYNC frame transmitted using character stuffing: (I already wrote 2 SYN characters for you in the answer)

 ``` 00010110 00010110 ```

• Question 2 (20 pts)

• An HDLC sender wants to transmit a HDLC frame containing the following information:

 ``` Destination address: 01111110 Send sequence number: 111 Receive sequence number: 000 Poll bit is not set Data: 11111111 11111111 FCS: 01111111 01111111 ```

Show the HDLC information frame of the sender before using bit stuffing (I already put a flag code in the answer for you):

 ``` 01111110 ```

Show the HDLC frame of the sender after using bit stuffing - provide your answer as follows:

1. Copy the frame in the above answer into the box below
2. Add the stuffed bits by writing the ^ symbol in the position where you will insert a 0 bit using the bit stuffing method

 ``` 01111110 ```

• Question 3 (20 pts)

• A sender is using Stop-and-Wait to communicate with a receiver.

Suppose a frame with send sequence number = 1 is received correctly.

Show the response of the received in the figure:

Suppose a frame with send sequence number = 1 is received incorrectly.

Show the response of the received in the figure:

• Question 4 (20 pts)

• The Stop-and-Wait protocol was once very popular; but when the bandwidth of the network increased to Mbps, the Stop-and-Wait protocol was abandoned.

This question will explore the reason why.

Around 1980's, we use modems to connect home PC to Emory's workstations

The transmission speed (= bandwidth) of a modem connection is 64 kbps (= 64,000 bps)

The one-way propagation delay between my house and Emory is 5 × 10-5 sec (I live about 15 km away)

Assume that there are no transmission errors (best possible case)

Suppose each data frame consists of 1000 bytes (= 8000 bits) and each ACK frame is 10 bytes (= 80 bits), compute the effective bandwidth utilization if the sender transmits continuously to the receiver

 The effective bandwidth utilization is the fraction of the available bandwidth that is used for data transmission (= transmitting data frames)

I.e.:

 What fraction/portion of the available 64 kbps bandwidth is used to transmit data frames

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• Around 1995, the telecom companies introduce high speed internet connections of speeds of about 100 Mbps (= 100,000,000 bps)

Assume that there are no transmission errors (best possible case)

Suppose each data frame consists of 1000 bytes (= 8000 bits) and each ACK frame is 10 bytes (= 80 bits), compute the effective bandwidth utilization if the sender transmits continuously to the receiver

In other words:

 What fraction/portion of the available 100 Mbps bandwidth is used to transmit data frames

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• Question 5 (20 pts)

A sender and a receiver is using the Stop-and-Wait protocol.

The one-way propagation delay between the sender and receiver is 1 msec.
The one-way propagation delay from the receiver to the sender is the same as the forward delay

The transmission data rate on the link is 1 Gbps.

Each data frame consists of 10000 bits and each ACK frame is 100 bits long. The sender needs to transfer a file of 1,000,000 bytes.

There are no transmission errors.

• How long will it take for the sender to completely transfer the file.

Show your derivation to get full credit.

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