Chapter 2 : Data transmission

What are packets and their structure?

• small units of data

• contains header , payload and trailer

describe a packet structure

• header = contains the IP address of sender ,receiver and packet number

• payload= contains the actual data

• trailer = mark the end and contains error checking method

What is packet switching ?

• Data is broken/split/divided into packets

• Each packet can take a different route

• A router controls the route/path a packet takes

• Selecting the shortest/fastest available route/path

• Packets may arrive out of order

• Once the last packet has arrived, packets are reordered

• If a packet is missing/corrupted, it is requested again

How many methods of data transmission are there ?

• Serial  Data sent down a single wire, one bit at a time

  • Less chance of interference/crosstalk

  • No skewing

  • More reliable over long distances

  • Bits will be sent/arrive in sequence

  • Cheaper

• Parallel- Data sent down multiple wires, multiple bits at a time

  • Faster for short distances (limited to about 5 meters)

  • Good for PC components

• simplex-Data sent in only one direction

• half-duplex- Data sent in both directions but not simultaneously

• full-duplex - Data sent in both directions simultaneously

Explain about USB and it’s pros and cons?

• Standard connection using serial data transmission designed for high speeds

• Benefits:

  • Industry standard/universal

  • Does not need external power source (can charge/power devices)

  • Fast data transfer rate

  • Fits only one way (less connection errors)

  • Automatically detects devices

  • Backward compatible

  • Little chance of data being skewed

• Limitations:

  • Length of wire limited to 5m

  • Not as fast as Ethernet

How many error detection methods are there?

1. Parity Check

2. Checksum

3. echo check

4. ARQ( AUTOMATIC REPEAT REQUEST)

Describe the process of parity check

• Parity can be set to odd or even

• Sender and receiver agree on parity to use

• Data is split into bytes/blocks of 7 bytes

• Sender counts the number of 1s/0s in each group/byte

• Each group/byte is assigned a parity bit to match the parity (odd/even)

• Receiving device recounts the number of 1s/0s in each group/byte

• Compares to parity used; if it does not match, an error is reported

• (In block check) the location of errors can be identified at the intersection

• Limitation: If two bits are corrupted, parity check might not detect the error

Describe the process of checksum

• Value is calculated from the data using an algorithm

• Value is transmitted with the data

• Value is recalculated by receiver using the same algorithm

• If checksum values are different, there is an error

Describe the process of echo check

• Involves comparison of data

• Sending device sends the data

• Receiving device retransmits the data back to sender

• Sending device compares the returned data to what it sent

• If same data, no error; else, error

• Slow method and errors can occur during retransmission

Describe the process of Automatic Repeat Request (ARQ)

• Uses acknowledgment and timeout

• Sender starts a timer when data is transmitted

• Receiver uses an error checking method

• If no error detected, a positive acknowledgment is returned

• If error detected, negative acknowledgment is returned

• If sender gets no acknowledgment within the set time, it resends the data

What is check digit ?

• Used to check for errors in data entry

• A check digit value is calculated from data using an algorithm (e.g., ISBN-13)

• Number is stored with data

• When data is entered, value is recalculated

• If same, then it's correct; if not, error in entry

• A type of validation

What is Encryption?

• Done to make data secure

• Data before encryption is plain text

• Encryption key is the algorithm used

• Cipher text is the name after encryption

Describe symmetric and asymmetric encryption.

• Shared key used to encrypt and decrypt

• Both the sender and receiver know the key

• Used at small scale

• Less secure (easier for hackers to intercept both key and text)

• AE :

• Different keys: public key and private key

• Public key to encrypt the data and private key to decrypt

• Anyone can know the public key but only those intended know the private key

• Two different keys make it more secure

• Used at larger scale