6 - Delay

Question 1

What is ASCII?

Answer 1

American Standard Code for Information Interchange

Question 2

What are the Internet Protocol Stack layers?

Answer 2

Applic. - message
Transp. - segment
Net. -datagram
Link - frame
Physical -bits

Question 3

Give Application layer examples

Answer 3

HTTP, FTP, SMTP

Question 4

Transport Layer Examples

Answer 4

TCP, UDP

Question 5

Netowkr Layer example

Answer 5

IP

Question 6

Link Layer example

Answer 6

Ethernet

Question 7

Define Bandwidth

Answer 7

Measure of rate or capcity of a link

Question 8

What is bandwidth units?

Answer 8

Size/time - bytes/second etc

Question 9

What are the four types of packet delay?

Answer 9

Transmission
Propagation
Queuing
Processing

Question 10

Transmission delay

Answer 10

n = packet length (size in bits)
B = bandwidth

n/B is time to send

Question 11

Propagation Delay

Answer 11

n = length of physical link (metres)
s = propagation speed in medium (~2x10^8 m/sec)

delay = n/s

Question 12

Processing delay

Answer 12

check bit errors

Examine packet headers and determine output link

Question 13

Queuing Delay

Answer 13

Time waiting at output link

Question 14

If a router puts a 1500Byte packet onto a 10Mb/s link what is the transmission delay?

Answer 14

(15008)/(10^7) = 1.210^-3 s

Question 15

Transmission delay for 500byte packet on 512kb/s cable?

Answer 15

(5008)/(51210^3) = 7812us

Question 16

Transmission delay for a 100b packet on a 2Mb/s link?

Answer 16

100/(2x10^6) = 0.00005seconds

Question 17

Define propagation delay as a concept

Answer 17

The time needed for the signal to travel from one end of the wire to the other

Normally use 2*10^8

Question 18

Propagation delay on a 100m cable in seconds?

Answer 18

100/(210^8) = 510^-7 seconds

Question 19

Propagation delay on a 40km fibre?

Answer 19

(4010^3)/(210^8) = 2*10^-4 seconds = 0.2ms

Question 20

A packet of 650 byte is sent

A-> R = 10Mb/s and 5metres
R -> B = 2Mb/s and 10km

What is the transmission delay at A?

Answer 20

(6508)/(10^7) = 5.210^-4

Question 21

A packet of 650 byte is sent

A-> R = 10Mb/s and 5metres
R -> B = 2Mb/s and 10km

What is the propagation delay A to R?

Answer 21

5/(210^8) = 2.510^-8 s

Question 22

A packet of 650 byte is sent

A-> R = 10Mb/s and 5metres
R -> B = 2Mb/s and 10km

What is transmission delay at R?

Answer 22

(6508)/(210^6) = 2.6*10-3s

Question 23

A packet of 650 byte is sent

A-> R = 10Mb/s and 5metres
R -> B = 2Mb/s and 10km

What is propagation delay at R?

Answer 23

(10^4)/(210^8) = 510^-5 s = 50 micro seconds

Question 24

What is the formula for Queuing delay?

Answer 24

I/R

I = average num of bits arriving per second

R is bandwidth

if more than 1, cannot function
else there is no queue

Question 25

traffic intensity

Answer 25

packet length (bits) * average arrival rate all divided by bandwidth in bps

Question 26

WHat happens to larger messages?

Answer 26

Split into smaller packets

Each packet has a transmission/propagation delay

Question 27

What is the total delay for larger messages?

Answer 27

one transmission delay for each packet plus the propagation delay for the last

Question 28

Stop and wait protocol

Answer 28

Next packet cannot be sent until an acknowledgement was received for the current one

Question 29

RTT

Answer 29

Round Trip Time

delay between sending packet and receiving the acknowledgement

Question 30

if RTT = 20ms and the 600 packet message uses stop and wait, how long would it need?

Answer 30

20ms x 600 = 12s

Question 31

If the delay from A to B is 30ms, what is the estimated RTT?

Answer 31

60ms

Question 32

Throughput

Answer 32

Amount of data per second that can be transferred

L/RTT

Question 33

Efficiency

Answer 33

Ratio between actual performance and max theoretical performance.

packets of L bits over link of bandwidth R

L/(R*RTT)