Chapter 1
Computer Networks and the Internet
Internet traffic
Whats the Internet? (hardware)
PC millions of connected Mobile network
server computing devices: Global ISP
wireless hosts = end systems
laptop
cellular
running network
handheld apps Home network
communication links Regional ISP
fiber, copper, radio,
access satellite
points
transmission rate = Institutional network
wired
links bandwidth
routers: forward packets
router (chunks of data)
Introduction 1-3
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
Whats 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
Hi
TCP connection
Hi request
TCP connection
Got the response
time?
2:00
<file>
time
Q: Other human protocols?
Who make network protocols?
Internet Engineering Task Force (IETF)
Internet standards
Request for Comments (RFCs)
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 1-10
The network edge:
End systems (hosts):
run application programs
e.g. Web, email
at edge of network peer-peer
Client/server model
client host requests, receives
service from always-on server
client/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 1-11
Access networks
Mobile network
Three categories: Global ISP
Residential access
Company access
Home network
Wireless access
Regional ISP
Help me identify them
Institutional network
Introduction 1-12
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)
Guided media
Unshielded twisted pair (UTP)
Shield twisted pair (STP)
Coaxial cable
Guided media
Fiber optics
Guided media
Radio links
Unguided media
Microwave
WiFi
3G
Satellite
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
Circuit switching (telephone system)
19
Packet switching (store-and-forward)
Packet-switching: store-and-forward
L
R R R
takes L/R seconds to Example:
transmit (push out) L = 7.5 Mbits
packet of L bits on to link R = 1.5 Mbps
at R bps
transmission delay = 15
store and forward: entire sec
packet must arrive at
router before it can be
transmitted on next link
delay = 3L/R (assuming
zero propagation delay)
Introduction 1-21
Packet switching versus Circuit switching
Packet switching allows more users to use network!
1 Mb/s link
each user:
100 kb/s when active
circuit-switching: N users
10 users 1 Mbps link
packet switching:
>> 10 users
Introduction 1-22
Packet switching versus Circuit
switching
Packet Circuit
Shared resources Reserved resources
No call setup Call setup needed
Long delay when traffic Delay is known
is high s
N users supported < N users supported
Low cost High cost
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
packet being transmitted (delay)
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
1-24
Four sources of packet delay
1. nodal processing: 2. Queuing:
check bit errors time waiting at output
link for transmission
determine output link
depends on congestion
level of router
transmission
A
propagation
B
nodal
processing queueing
Introduction 1-25
Delay in packet-switched networks
3. Transmission delay: 4. Propagation delay:
R=link bandwidth (bps) d = length of physical link
L=packet length (bits) s = propagation speed in
time to send bits into medium (~2x108 m/sec)
link = L/R propagation delay = d/s
Note: s and R are very different
quantities!
transmission
A
propagation
B
nodal
processing queueing
Introduction 1-26
Caravan analogy
100 km 100 km
ten-car toll toll
caravan booth booth
cars propagate at Time to push entire
100 km/hr caravan through toll booth
toll booth takes 12 sec to onto highway = 12*10 =
service car (transmission 120 sec
time) Time for last car to
car~bit; caravan ~ packet propagate from 1st to 2nd
toll both:
Q: How long until caravan
100km/(100km/hr)= 1 hr
is lined up before 2nd toll
booth? A: 62 minutes
Introduction 1-27
Caravan analogy (more)
100 km 100 km
ten-car toll toll
caravan booth booth
Yes! After 7 min, 1st car at
Cars now propagate at 2nd booth and 3 cars still at
1000 km/hr 1st booth.
Toll booth now takes 1 1st bit of packet can arrive
min to service a car at 2nd router before packet
Q: Will cars arrive to 2nd is fully transmitted at 1st
booth before all cars router!
serviced at 1st booth? See Ethernet applet at AWL
Web site
Introduction 1-28
Nodal delay
d nodal d proc dqueue d trans d prop
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
Introduction 1-29
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!
Introduction 1-30
Experiment delay
Traceroute (XP)
Tracert (Linux)
Provide delay measurement from source
to router along end-end Internet path
towards destination.
Packet loss See applet examples
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
buffer
(waiting area) packet being transmitted
A
B
packet arriving to
full buffer is lost
1-32
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,
server sendswith link
bits pipe capacity
that can carry link that
pipe capacity
can carry
file of
(fluid) F bits
into pipe Rs bits/sec
fluid at rate Rfluid
c bits/sec
at rate
to send to client Rs bits/sec) Rc bits/sec)
1-33
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
bottleneck link
link on end-end path that constrains end-end throughput
1-34
Throughput: Internet scenario
Rs
per-connection Rs Rs
end-end
throughput: R
min(Rc,Rs,R/10)
Rc Rc
in practice: Rc or
Rs is often Rc
bottleneck
10 connections (fairly) share
backbone bottleneck link R bits/sec
1-35
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
Internet protocol stack
application: supporting network
applications
FTP, SMTP, HTTP application
transport: process-process data
transfer transport
TCP, UDP
network
network: routing of datagrams from
source to destination
link
IP, routing protocols
link: data transfer between physical
neighboring network elements
PPP, Ethernet
physical: bits on the wire
Introduction 1-54
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
