Error Checking

What risk is present after data transmission?

That data has been corrupted or changed in some way.

How can the risk of data corruption be avoided?

With error checking systems

When is parity checking used?

In data transmission and when storing data (on the parity track on an HDD)

What must be agreed on by communicating devices?

Odd or even parity.

What is odd parity?

In total, there will be an odd number of 1s.

What is even parity?

In total, there will be an even number of 1s.

How is one byte split?

7 bits are the data, 1 is the parity bit.

What does the parity bit allow?

Adjustment so the overall number of 1s adds to the agreed parity. (The last bit will change to a 1 or a 0)

What are two problems with parity bits?

1) A parity bit cannot detect when there are an even number of errors (multiple incorrect bits)

2) It cannot detect whether transposition of bits has occurred (order of the bits)

What happens if parity is incorrect?

An error is detected. A request can be sent for the data to be resent, or the received bits could be re-read.

What is the Time to Live variable?

TTL is an expiration called a time-to-live or hop limit, which prevents information packets from passing from router to router indefinitely

What do packets of information which are created and sent out across the Internet rely on?

Routers to direct it to the correct destination.

What does each information packet have?

A place to store the TTL value

What happens every time a router receives a packet?

It subtracts one from the TTL count and then passes it onto the next location in the network.

What happens when the TTL count reaches zero after subtraction?

The router will discard the packet and send a message back to the originating host

What is a common initial value of a TTL variable?

64