10. Concepts and Terms (ii)

Question 1

Define simplex, (full) duplex and half-duplex communication. Give an example of each.

Answer 1

• simplex is a one way connnection, TV or radio, or a website with no user feedback
• half duplex is a two way connection, but it is not concurrent, WiFi
• full duplex is a two way connection that allows concurrent communication, similar to a phone call.

Question 2

What is bandwidth?

Answer 2

The number of bits delivered per second

Question 3

What is delay?

Answer 3

The time it takes the first byte to travel from source to destination.

Question 4

What is the usual rate bits travel?

Answer 4

2x10^8 meters per second

Question 5

What are the 4 sources of packet delay? What are their causes

Answer 5

• nodal processing delay, need to check for bit errors, determine output link
• queueing delay, time waiting to be transmitted, depends on the congestion of the router
• transmission delay. length of the packet in bits / link bandwidth in bits per sec
• propagation delay, length of physical link / propagation speed (2x10^8 m/sec)

Question 6

How are propagation delay and transmission delay calculated?

Answer 6

• Dprop = distance/speed of medium
• Dtrans = num of bits/bits per second
  add all delays together

Question 7

What is RTT? Why can’t we be sure about actual delay to anything other than an adjacent link?

Answer 7

Round trip time. That requires clock syncronisation. Used to calculate the time taken for a message to be send and received back. Can use ping command.

Question 8

What are causes of packet loss?

Answer 8

• physical transmission problems (corruption, interference)
• congestion causing timeouts
• too many mobile devices in one place can cause infrastructure overload
• network router buffers can suffer from transient overload

Question 9

What is throughput? What is goodput?

Answer 9

The amount of data passing through the system in a given amount of time. Goodput is the rate at which useful data is passing through a system. Can measure throughput using time wget -o get-file

Question 10

Difference between throughput and bandwidth?

Answer 10

Bandwidth is the maximum amount of data that can travel through a ‘channel’. Throughput is how much data actually does travel through the ‘channel’ successfully.