2.5 Detecting errors in data transmission

2.5 Detection errors in data transmission

Data transmission can have problems.

Data transmission errors can cause loss, alteration, or gain of information, leading to serious issues like login failures.

Multiple Choice

True or False: Data transmission is perfect and it never has problems.

• False
• True

Open Ended

The accuracy of data is often very important. Can you give one example where it's important that the data is sent correctly?

There are several error detection methods

We will learn some today, including:

• ​parity check

• checksum

• echo check

Parity check.

​The parity check simply counts the number of 1s in the data to see if the total is odd or even

Multiple Choice

Is the number 1 odd or even?

• even
• odd

Multiple Choice

Is the number 2 odd or even?

• even
• odd

Multiple Select

What numbers are even?

• 136

• 5

• 16

• 11

• 9

Multiple Select

What numbers are odd?

• 136

• 5

• 16

• 11

• 9

Parity check.

​The parity check simply counts the number of 1s in the data to see if the total is odd or even

​1010 0000

​two 1s so is even.

​1010 0000

hey it should be even. PERFECT!

Parity check.

​The parity check simply counts the number of 1s in the data to see if the total is odd or even

​1010 0000

​two 1s so is even.

​1000 0000

hey it should be even. HORRIBLE!

Parity check. How does it work?

​The parity check can use odd or even check method.

​Even

​Odd

Parity check. How does it work?

Example: We want to send 0001 0010 0010 00 01 0000

1. ​Select the method. We will use Odd method.

2. Add 0 or 1 to match the method. In this case the data has four 1s so we will add 1 at the beginning 1 0001 0010 0010 00 01 0000.

3. Send the data.

4. The destination will count the 1s and if its Odd it will be OK if its Even it will be Not OK

Multiple Select

Which of these bytes would have been transmitted incorrectly if an even parity check has been used?

• 1011 1011

• 0111 0111

• 1010 1000

• 0000 0001

Multiple Select

Which of these bytes would have been transmitted incorrectly if an odd parity check has been used?

• 0011 0001

• 1011 0101

• 1000 1000

• 0000 0001

Poll

Can you think of an instance in which an error could occur in the data, but this would not be detected by a parity check?

• Yes

• No

• I deserve 0% in participation.

Checksum

uses a calculated value to check for errors.

Checksum example:

• Original Data: 101 1001

• Checksum Calculation: The sum of the bits is 1 + 0 + 1 + 1 + 0 + 0 + 1 = 4 (binary 100).

• Checksum Appended: The original data with the checksum becomes 1011001 100.

• After Transmission: If the data received is 1011001 100, the checksum is recalculated, and if it matches, the data is accepted.

Multiple Choice

You have received the data 1101 0010 and a checksum of 011 (binary).

Is there an error in the data?

• yes

• no

Multiple Choice

You have received the data 1100 0010 and a checksum of 100 (binary).

Is there an error in the data?

• yes

• no

Multiple Choice

You have received the data 1100 0010 and a checksum of 010 (binary).

Is there an error in the data?

• yes

• no

Multiple Choice

You have received the data 1100 0010 and a checksum of 011 (binary).

Is there an error in the data?

• yes

• no

Echo check

An echo check involves a simple comparison of the data sent to the data received.

Echo check

​

​

​She sells seashells by seashore.

​Did you say "She sells seashells by seashore"?

YES!

Draw

Activity 2.8

Draw a diagram to represent how an echo check detects errors in data.

Multiple Choice

If the data does not match that which is sent back by the receiving device, will this always mean that the receiving device did not receive the data correctly?

• Yes, it always means that.
• The receiving device will always notify the sender of the error.
• The data is always lost during transmission.
• No, it does not always mean that.

Draw

ACTIVITY 2.9
Draw a diagram to represent how an ARQ system operates using positive acknowledgement.

​Check digit

Example:

Suppose we have a 4-digit number 4871, and we want to compute a check digit so that when we append it, the whole 5-digit number has a “digital sum mod 10 = 0” (i.e. sum of all digits ≡ 0 modulo 10).

Step 1. Sum the digits of 4871:

4 + 8 + 7 + 1 = 20

Step 2. Compute the remainder when this sum is divided by 10:

20 mod 10 = 0

Step 3. Pick the check digit d such that (sum + d) mod 10 = 0.

Since sum mod 10 is already 0, we need d = 0 so that (20 + 0) mod 10 = 0.

Thus the check digit is 0, so the full code is 48710.