Question

QUESTION 1: Error detection and Checksum Methods [15 Marks]

a. Critically analyse the role of flow and error control in a computer network. In your explanation, appropriate figures and examples are expected. [6 Marks]

b. Calculate the CRC for the X binary data shown as shown below, when the divisor is 1101. (A)2 = (200 + X)10 X = 1001100 [6 Marks]

c. Consider a scenario where a frame would be sent after appending the checksum to the original data. How would you prove that the frame received is error-free? [3Marks]

Answer 1

Hi, i have solved part a and c.

I would have solved b too but the way you wrote equation, i didn't get it properly please repost b part.

Flow Control

In computer networking, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from over running a slow receiver.
Flow control is important because it is possible for a sender to transmit information at a faster rate than the destination can receive and process it. This can happen if the receiver has a heavy traffic load in comparison to the sender, or if the receiver has less processing power than the sender. (Traffic Monitoring)

Flow control can be done either by control lines in a data communication interface or by reserving in-band control characters to signal flow start and stop (such as the ASCII codes for XON/XOFF). Common control lines are RTS (Request To Send)/CTS (Clear To Send) and DSR (Data Set Ready)/DTR (Data Terminal Ready), which is usually referred to as "hardware flow control". XON/XOFF is usually referred to as "software flow control". In the old mainframe days, data transmitted was called a "data set" and modems were called "data terminals", hence the survival of the term.

Software flow works by one side sending a XOFF when its buffer is full. The other side detects the XOFF and stops sending data. The first side then sends a XON when its buffer clears enough to allow further data transmission. The other side detects the XON and resumes sending data.

Hardware flow control works by the DTE, which signals the opposite end (the slave end such as a DCE) to begin monitoring its data input line. When ready for data, the slave end will raise its complementary line, CTS , which signals the master to start sending data, and for the master to begin monitoring the slave's data output line.  If either end needs to stop the data, it lowers its respective line. For PC-to-modem and similar links, DTR/DSR are raised for the entire modem session (say a dialup internet call), and RTS/CTS are raised for each block of data.

The most popular option today is to not use any flow control. Modern computers can easily cache the data sent over a serial communication before the buffer can fill up, so they do not need flow control.
In order to see which flow control type your device uses, read the device user manual's section on serial communications. If the settings do not match, buffer overruns can occur, resulting in lost data and garbled communication.

Error Control

Error control in the data link layer is based on ARQ (automatic repeat request),
which is the retransmission of data.

Error Control involves both error detection and error correction. It is necessary because errors are inevitable in data communication, in spite of the use of better equipment and reliable transmission media based on the current technology. In the preceding lesson we have already discussed how errors can be detected. In this lesson we shall discuss how error control is performed based on retransmission of the corrupted data. When an error is detected, the receiver can have the specified frame retransmitted by the sender. This process is commonly known as Automatic Repeat Request (ARQ). For example, Internet's Unreliable Delivery Model allows packets to be discarded if network resources are not available, and demands that ARQ protocols make provisions for retransmission.

There are basically two types of errors, namely, (a) Damaged Frame (b) Lost Frame. The key functions for error control techniques are as follows:

• Error detection
• Sending of positive acknowledgement (ACK) by the receiver for no error
• Sending of negative acknowledgement (NAK) by the receiver for error
• Setting of timer for lost frame
• Numbering of frames

Summary,

In case of data communication between a sender and a receiver, it may so happen that the rate at which data is transmitted by a fast sender is not acceptable by a slow receiver. IN such a situation, there is a need of flow control so that a fast transmitter does not overwhelm a slow receiver.

To ensure reliable communication, there needs to exist flow control (managing the amount of
data the sender sends), and error control (that data arrives at the destination error free).

• Flow and error control needs to be done at several layers.
• For node-to-node links, flow and error control is carried out in the data-link layer.
• For end-point to end-point, flow and error control is carried out in the transport layer.

c)

Steps to follow

• In checksum error detection scheme, the data is divided into k segments each of m bits.
• In the sender’s end the segments are added using 1’s complement arithmetic to get the sum. The sum is complemented to get the checksum.
• The checksum segment is sent along with the data segments.
• At the receiver’s end, all received segments are added using 1’s complement arithmetic to get the sum. The sum is complemented.
• If the result is zero, the received data is accepted; otherwise discarded.