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Chapter 1
Computer Networks and the Internet
Internet traffic
Introduction 1-3
What’s the Internet? (hardware)
millions of connected
computing devices:
hosts = end systems
running network
apps Home network
Institutional network
Mobile network
Global ISP
Regional ISP
router
PC
server
wireless laptop
cellular handheld
wired links
access points
communication links
fiber, copper, radio, satellite
transmission rate = bandwidth
routers: forward packets (chunks of data)
Internet appliances
Your work
Find me very interesting an Internet
appliance
Present me with picture and description
If your friends buy it, you get a point
You can invent a new one also whether it is not for real
What’s the Internet? (software)
communication infrastructure enables distributed
applications:
Web, VoIP, email, games, e-commerce, file
sharing
communication services provided to apps:
reliable data delivery from source to destination
“best effort” (unreliable) data delivery
Too many types of computing devices!
How to make networks which allow different
devices to communicate?
Analogous to human languages
Need speak the same language
Network protocol
Q: Other human protocols?
Hi
Hi
Got the time?
2:00
TCP connection request
TCP connection response
<file>
time
Who make network protocols?
Internet Engineering Task Force (IETF)
Internet standards
Request for Comments (RFCs)
Introduction 1-10
A closer look at network structure:
Network edge: applications and hosts
Access networks, physical media: wired, wireless communication links
Network core: interconnected routers
network of networks
Introduction 1-11
The network edge:
End systems (hosts): run application programs
e.g. Web, email
at “edge of network”
client/server
peer-peer
Client/server model client host requests, receives
service from always-on server
e.g. Web browser/server; email client/server
Peer-to-Peer model: minimal (or no) use of
dedicated servers
e.g. Skype, BitTorrent
Introduction 1-12
Access networks
Home network
Institutional network
Mobile network
Global ISP
Regional ISP
Three categories:
Residential access
Company access
Wireless access Help me identify them
Physical media
Guided media:
signals propagate in solid media: copper, fiber, coax
Unguided media:
signals propagate freely, e.g., radio
Twisted-pair copper wire (crosstalk reduced) Unshielded twisted pair (UTP)
Shield twisted pair (STP)
Guided media
Coaxial cable Guided media
Fiber optics
Guided media
Radio links
Microwave
WiFi
3G
Satellite
Unguided media
Network core (review slide 10)
The fundamental question: how is data transferred
through net?
circuit switching: dedicated circuit per call:
telephone net
packet-switching: data sent thru net in discrete
“chunks”
19
Circuit switching (telephone system)
Packet switching (store-and-forward)
Introduction 1-21
Packet-switching: store-and-forward
takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
store and forward: entire packet must arrive at router before it can be transmitted on next link
delay = 3L/R (assuming zero propagation delay)
Example:
L = 7.5 Mbits
R = 1.5 Mbps
transmission delay = 15 sec
R R R
L
Introduction 1-22
Packet switching versus Circuit switching
1 Mb/s link
each user:
100 kb/s when “active”
circuit-switching:
10 users
packet switching:
>> 10 users
Packet switching allows more users to use network!
N users
1 Mbps link
Packet switching versus Circuit switching Packet
Shared resources
No call setup
Long delay when traffic
is high
N users supported
Low cost
Circuit
Reserved resources
Call setup needed
Delay is known
s
< N users supported
High cost
1-24
How do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction 1-25
Four sources of packet delay
1. nodal processing:
check bit errors
determine output link
A
B
propagation
transmission
nodal processing queueing
2. Queuing:
time waiting at output link for transmission
depends on congestion level of router
Introduction 1-26
Delay in packet-switched networks
3. Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into
link = L/R
4. Propagation delay:
d = length of physical link
s = propagation speed in
medium (~2x108 m/sec)
propagation delay = d/s
A
B
propagation
transmission
nodal processing queueing
Note: s and R are very different quantities!
Introduction 1-27
Caravan analogy
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
toll booth
toll booth
ten-car caravan
100 km 100 km
Introduction 1-28
Caravan analogy (more)
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?
Yes! After 7 min, 1st car at
2nd booth and 3 cars still at
1st booth.
1st bit of packet can arrive
at 2nd router before packet
is fully transmitted at 1st
router!
See Ethernet applet at AWL
Web site
toll booth
toll booth
ten-car caravan
100 km 100 km
Introduction 1-29
Nodal delay
dproc = processing delay
typically a few microsecs or less
dqueue = queuing delay
depends on congestion
dtrans = transmission delay
= L/R, significant for low-speed links
dprop = propagation delay
a few microsecs to hundreds of msecs
proptransqueueprocnodal ddddd
Introduction 1-30
Queueing delay (revisited)
R=link bandwidth (bps)
L=packet length (bits)
a=average packet
arrival rate
traffic intensity = La/R
La/R ~ 0: average queueing delay small
La/R -> 1: delays become large
La/R > 1: more “work” arriving than can be serviced, average delay infinite!
Experiment delay
Traceroute (XP)
Tracert (Linux)
Provide delay measurement from source
to router along end-end Internet path towards destination.
1-32
Packet loss
queue (aka buffer) preceding link in 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
A
B
packet being transmitted
packet arriving to full buffer is lost
buffer (waiting area)
See applet examples
1-33
Throughput
throughput: rate (bits/time unit) at which
bits transferred between sender/receiver
instantaneous: rate at given point in time
average: rate over longer period of time
server, with file of F bits
to send to client
link capacity Rs bits/sec
link capacity Rc bits/sec
pipe that can carry fluid at rate Rs bits/sec)
pipe that can carry fluid at rate Rc bits/sec)
server sends bits (fluid) into pipe
1-34
Throughput (more)
Rs < Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
link on end-end path that constrains end-end throughput
bottleneck link
1-35
Throughput: Internet scenario
10 connections (fairly) share backbone bottleneck link R bits/sec
Rs Rs
Rs
Rc
Rc
Rc
R
per-connection
end-end
throughput:
min(Rc,Rs,R/10)
in practice: Rc or
Rs is often
bottleneck
Layered architecture
Layered architecture is our day life
Why layer?
Things are too complex
Computer networking is too complex
Sending a mail
Encapsulation
Decapsulation
In-class assignment
Please spend a few minutes to find other
example of a layered system
Show me to get a point
ISO issues OSI (7-layer architecture)
The International Standards Organization
(ISO)
Open Systems Interconnection (OSI) model
Seven layers of the OSI model
The interaction between layers in the OSI model
An exchange using the OSI model
Physical layer
The physical layer is responsible for
movements of individual bits from one hop
(node) to the next
Data link layer
The data link layer is responsible for
moving frames from one hop (node) to the
next
Hop by hop delivery
Network layer
The network layer is responsible for the
delivery of individual packets from the
source host to the destination host
Source to destination delivery
Transport layer
The transport layer is responsible for the
delivery of a message from one process to
another
Reliable process-to-process delivery of a message
Session layer
The session layer is responsible for dialog
control and synchronization
Presentation layer
The presentation layer is responsible for translation, compression, and encryption
Presentation layer
The application layer is responsible for providing services to the user
Summery
Introduction 1-54
Internet protocol stack
application: supporting network
applications
FTP, SMTP, HTTP
transport: process-process data
transfer
TCP, UDP
network: routing of datagrams from
source to destination
IP, routing protocols
link: data transfer between
neighboring network elements
PPP, Ethernet
physical: bits “on the wire”
application
transport
network
link
physical
Internet protocol stack and OSI
Homework Assignment
Chapter 1 problems:
P2
P4
P5
P6
P9
P12
P13
Discussion Questions
Make a group of two persons and choose
one question from the list below
D1
D7
D11
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