1. Four sources of packet delayB
propagation
transmission
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dproc: nodal processing
check bit errors
determine output link
typically microseconds
dqueue: queueing delay
time waiting at output link for transmission
depends on congestion level of router
Slides by Kurose and Ross
Introduction 1-1

2. Propagation delay
Propagation delay = d/s, where d is the length of the link and s is the speed of light in the physical medium of the link
hence the dependence on
length of the link
bit travels at the speed
of light in the medium
(~2x108 to 3x108 m/sec)
Propagation delay is the time needed for a bit dropped in at one end of a link to travel to the far end of the link. The bit travels at the speed of light in the link medium. Therefore it depends upon the length of the link and the speed of light in the medium of the link. c, the speed of light in vacuum, is 300,000km/sec. Speed of light in air can be approximated to equal the speed of light in vacuum. In other media, such as optical fiber, the speed of light is lower (speed of light in vacuum/sqrt(dielectric constant of medium))
propagation delay
1 bit

3. Emission (transmission) delay
Emission (transmission) delay = L/R, where L is the size of the data unit being transmitted in bits, and R is the transmission rate in bits/sec
Let’s say the link transmitter can
emit out 10 million bits/sec; this is R,
the transmission rate of the link. Hence the size
of the packet, L, and the transmitter rate, R,
determine the emission (transmission) delay
1 packet of L bits
Emission delay, as the name implies, is the time taken to emit the data unit on to the link. A transmitter is present at the sending end of a link and a receiver receives data bits at the receiving end. The time to transmit a data unit, which could be a file or a packet, at the sending end is called emission delay or transmission delay.
Analogy: Use a flashlight to transmit a signal. Turn on for '1' and off for '0'. The
rate at which you can turn the flashlight on and off determines the transmission rate r.
emission (transmission) delay:
time to emit (transmit) the data unit on to the link

4. Pipelining bits in a packet
Why do we not need to multiply propagation delay by the number of bits in determining the total delay ?
Because of pipelining. As the transmitter is emitting bits on to the channel, the bits are immediately propagating across. Hence we only need to add the propagation delay for the last bit to the emission delay of the whole packet.

5. Caravan analogy: pipelining of packets
toll
booth
ten-car
caravan
100 km
cars “propagate” at 100 km/hr
toll booth takes 12 sec to service car (transmission time)
car~bit; caravan ~ packet
Q: How long until caravan is lined up before 2nd toll booth?
time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec
time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr
A: 62 minutes
Introduction 1-5

6. Caravan analogy (more)
toll
booth
ten-car
caravan
100 km
cars now “propagate” at 1000 km/hr
toll booth now takes 1 min to service a car
Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?
A: Yes! After 7 min, 1st car arrives at second booth; three cars still at 1st booth.
1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!
Introduction 1-6

7. “Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to
Three delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw ( ) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms
4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms
5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms
( ) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms
( ) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms
trans-oceanic
link
* means no response (probe lost, router not replying)
Introduction 1-7

8. End-to-end delay: multiple links, single-packet file
Introduction

9. End-to-end delay: multiple links, multi-packet file (pipelining)
File is divided into P packets
Each packet is of size L
There are N-1 routers between source and destination (which means there are N links)
Each link is of length d
Speed of light in medium is s
Link transmission rate is R (same for all links)
Neglect processing and queueing delays
Introduction

10. Queueing delay (E[T] in our analysis)
R: link bandwidth (bps)
L: packet length (bits)
a: average packet arrival rate
La/R: traffic intensity
average queueing delay
traffic intensity
= La/R
La/R ~ 0
La/R ~ 0: avg. queueing delay small
La/R -> 1: avg. queueing delay large
La/R > 1: more “work” arriving
than can be serviced, average delay infinite!
La/R -> 1
Introduction 1-10

11. Packet loss queue (aka buffer) has finite capacity
packet arriving to full queue dropped (aka lost)
lost packet may be retransmitted by previous node, by source end system, or not at all
buffer
(waiting area)
packet being transmitted A B
packet arriving to
full buffer is lost
Introduction 1-11