Lecture 2 - Friday: Communication metrics and protocol layering

Question 1

What is a communication link characterised by?

Answer 1

Transmission rate R and Propagation speed S

Question 2

What is the Transmission rate R?

Answer 2

• Bits per second (bps) that can be put on the link
• Typically ranges from 10 Mbps to 100 Gbps
• Determines the Transmission delay d-trans = L/R for a packet of L bits

Question 3

What is the Propagation speed S?

Answer 3

• The speed with which a bit/signal moves along the link.
• Typically ranges between 2* 10^8 m/s (copper wire) to
  3 * 10^8 m/s (fiber optics)
• Determines the propagation deley d-prop = d / S for a link distance d.

Question 4

What is a nodal delay?

Answer 4

The nodal delay is the total delay experienced by a data packet at a node (packet switch or host)

Question 5

How do we calulate the nodal delay at a packet switch?

Answer 5

Question 6

What is the end-to-end delay?

Answer 6

The end-to-end delay is the total delay experienced by a packet from source to destination.

Question 7

What is traffic intensity I?

Answer 7

Traffic intensity I relates the capacity of outgoing links with the arrival rate of incoming packets.
- Transmission rate R bits/second on outging link.
- Packet size of L bits
- Average arrival rate of a packets per second for transmission.

Question 8

What happens when

I < 1?
and
I > 1?

Answer 8

I < 1 = Queue size will be bounded

I > 1 = Queue size grows without bound, can cause packet loss.

Network congestion arises as traffic intensity approaches 1.

Question 9

What is Throughput?

Answer 9

Throughput measures the rate at which data is being received at the destination.

Question 10

What is Instantaneous throughput?

Answer 10

It’s the current amount of data received measured in bits/seconds.

Question 11

What is Average throughput?

Answer 11

The average amount of data received per time unit for entire data transfer.

Question 12

Simultaneous transmissions

Answer 12

Senders and receivers are competing for the available bandwidth in the Internet.

Question 13

What are they layers in the TCP/IP stack?

Answer 13

Question 14

What part of the TCP/IP stack does routers and link-layer switches implement?

Answer 14

They typically imlement only the bottom layers.
Link-Layer switches implement layers 1 and 2 (Physical and Link)
Routers implement layers 1 to 3 (Physical, Link and Network)

This means, for example, that Internet routers are capable of implementing the IP protocol (a layer 3 protocol), while link-layer switches are not.