DCCN – LECTURE – 01

INTRODUCTION

MUHAMMAD IRTAZA SALEEM

DEPARTMENT OF COMPUTER AND INFORMATION SCIENCES

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 1

 Rules and Policies
Mobile Phones:
• Switched off and in your bag/ pocket

Punctuality:
• Don’t be late
• Attendance will be done twice.
• Initially in first 5 Minutes of Lecture and at any random moment in between.
• I expect you to stay till the end of lecture. If for some reason you want to
leave early, you have to ask for it in advance.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 2

 Dishonesty, Plagiarism, Proxy
Plagiarism or a Cheating Case in:
Semester Project
Sessional Exam
Terminal Exam
may result in F grade in the course.
Plagiarism in an assignment may result in zero marks in the whole
assignments category.
Disciplinary action against attendance proxy cases.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 3

 Tentative Evaluation Breakdown
Assignments (4): 10 Abs
Quizzes (6-7): 5 Abs
Sessional – 1: 12.5 Abs
Sessional – 2: 12.5 Abs
Semester Project: 10 Abs
Terminal Exam: 50 Abs

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 4

 Course Outline
Introduction to Computer Networks
Application Layer
Transport Layer
Network Layer
Network Layer Control Plane
Link Layer
And much more.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 5

 Chapter 01: Introduction
our goal:
get “feel” and terminology
more depth, detail later in course
approach:
◦ use Internet as example

PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED

LECTURE – 01 - INTRODUCTION 1-6
SCIENCES
 Computer Network
Computer network - a group of computers connected
together to communicate, exchange data, and share
resources in real time

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 7

Network Enables…
Simultaneous access to data Personal communication
◦ Data files are shared ◦ Email
◦ Data are stored in a centralized place ◦ Instant messaging
◦ All users have access to identical, up-to-date ◦ Conferencing
information
◦ Videoconferencing
◦ Software can also be shared
◦ Voice over IP
◦ Site licenses
◦ Phone communication over network wires

Sharing of hardware resources

Easier data backup
◦ Printers and faxes are commonly shared devices
◦ Usually in business corporations
◦ Reduces the cost per user
◦ Employers keep the data on a shared storage
device
◦ The network manager makes regular backups of
Collaborative work by multiple people the data

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 8

 What’s the Internet: “nuts and bolts” view
mobile network
Internet: “network of networks”
◦ Interconnected ISPs
global ISP

home
network
regional ISP

institutional
network

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 What’s the Internet: “nuts and bolts” view
PC mobile network
millions of connected computing devices:
server
◦ hosts = end systems
global ISP
wireless
laptop
◦ running network apps
smartphone
home
 communication links network
regional ISP
wireless  fiber, copper, radio,
links satellite
wired
links  transmission rate:
bandwidth

 Packet switches: forward packets

router (chunks of data) institutional
 routers and switches network

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 What’s the Internet: a service view
mobile network
Infrastructure that provides
services to distributed global ISP

applications:
◦ Web,VoIP, email, games, e- home
commerce, social nets, … network
regional ISP

provides programming interface

to apps
◦ hooks that allow sending and
receiving app programs to
“connect” to Internet
institutional
network

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but how these systems and network apps communicate with each other?

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 Protocols
For proper communication, entities in different systems must speak the
same language
◦ there must be mutually acceptable conventions and rules about the content, timing
and underlying mechanisms

Those conventions and associated rules are referred as “PROTOCOLS”

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 What’s a protocol?
a human protocol and a computer network protocol:

Hi TCP connection
request
Hi TCP connection
response
Got the
time? Get http://www.awl.com/kurose-
ross
2:00
<file>
time

Q: other human protocols?

PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND
LECTURE – 01 - INTRODUCTION 1-14
APPLIED SCIENCES
 What’s a protocol?
human protocols: network protocols:
“what’s the time?” machines rather than humans
“I have a question” all communication activity in
Internet governed by
introductions protocols

… specific msgs sent

… specific actions taken
protocols define format, order
when msgs received, or of msgs sent and received
other events among network entities,
and actions taken on msg
transmission, receipt
PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND
LECTURE – 01 - INTRODUCTION 1-15
APPLIED SCIENCES
 “Fun” internet appliances
Web-enabled toaster +
weather forecaster

IP picture frame
http://www.ceiva.com/

Tweet-a-watt:
monitor energy use

Internet
refrigerator Internet phones
PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND
LECTURE – 01 - INTRODUCTION 1-16
APPLIED SCIENCES
 “Fun” internet appliances

Wifi Bathroom Scales

The WifiRobin Autohacking Router

Nobaztag

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 What’s a protocol?

protocols define format, order of msgs sent and

received among network entities, and actions
taken on msg transmission, receipt

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 18

Roadmap
We have now a basic idea about what the internet is.
In this lecture, we would focus on
◦ How the hosts at network edge connect
◦ How the network core operates

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 19

Roadmap
We have now a basic idea about what the internet is.
In this lecture, we would focus on
◦ How the hosts at network edge connect
◦ How the network core operates

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 20

 Access networks and physical media
Q: How to connect end
systems to edge router?
residential access nets
institutional access networks (school,
company)
mobile access networks

PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND

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APPLIED SCIENCES
 Dial up connection
A dial-up connection is a fixed Internet connection that uses a
voice-band modem and telephone lines to transport data
between your computer and your ISP
◦ A voice-band modem converts the digital signals from your computer
into analog signals that can travel over telephone lines

Dial-up modems typically have a maximum theoretical transfer speed of 56 kbit/s

PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND

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APPLIED SCIENCES
 Access net: digital subscriber line (DSL)
telephone
central office network

DSL splitter
modem DSLAM

Internet
voice, data transmitted
at different frequencies over DSL access
dedicated line to central office multiplexer

use existing telephone line to central office DSLAM

(digital subscriber line access multiplexer )
 The home’s DSL modem takes digital data and translates it to high
frequency tones for transmission over telephone wires to the CO
 data over DSL phone line goes to Internet
 voice over DSL phone line goes to telephone net
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 Access net: digital subscriber line (DSL)

The residential telephone line carries both data and traditional telephone signals
simultaneously, which are encoded at different frequencies:

 A high-speed downstream channel, in the 50 kHz to 1 MHz band

 A medium-speed upstream channel, in the 4 kHz to 50 kHz band

 An ordinary two-way telephone channel, in the 0 to 4 kHz band

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 Access net: digital subscriber line (DSL)

• On the customer side, a splitter separates the data and telephone

signals arriving to the home and forwards the data signal to the DSL
modem.
• On the telco side, in the CO, the DSLAM separates the data and
phone signals and sends the data into the Internet.
• Hundreds or even thousands of households connect to a single
LECTURE – DSLAM
01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 1-25
 Access net: cable network
Cable Internet access makes use of the cable
television company’s existing cable television
infrastructure.

Cable internet access requires special modems,

called cable modems.

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 Access net: cable network
cable headend

cable splitter
modem cable modem termination system (CMTS)

C
O
V V V V V V N
I I I I I I D D T
D D D D D D A A R
E E E E E E T T O
O O O O O O A A L coaxial cable
1 2 3 4 5 6 7 8 9

Channels

frequency division multiplexing: different channels transmitted

in different frequency bands
PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND
LECTURE – 01 - INTRODUCTION 1-27
APPLIED SCIENCES
 DSL vs cable network

• In DSL phone and voice data is transmitted at low frequencies

while the Internet data uses higher frequencies. The same line
can handle both tasks without one interfering with the other.
However, DSL users will have to install a filter, or a device that
separates the signals.
• Cable modems use the coaxial cables that carry television
signals rather than phone lines. The cable uses a separate
signal for each channel and treats Internet signals in the same
way it does other channel information.

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 DSL vs cable network

• DSL Internet transfers data directly between the ISP and the
home over the phone line, so bandwidth is never shared with
anyone else. This means that users will see overall consistent
performance.
• Cable Internet, on the other hand, delivers a block of
bandwidth to an entire neighborhood and then shares it
among the homes. This means that during peak hours, when
many homes in the neighborhood are online, Internet speeds
may be drastically reduced, especially if others in the
neighborhood use their connections heavily.

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 Enterprise access networks (Ethernet)

institutional link to
ISP (Internet)
institutional router

Ethernet institutional mail,

switch web servers

typically used in companies, universities, etc

 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates
 today, end systems typically connect into Ethernet switch

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 Enterprise access networks (Ethernet)

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 Wireless access networks
shared wireless access network connects end system to router
◦ via base station aka “access point”

wireless LANs: wide-area wireless access

 within building (100 ft)  provided by telco (cellular)
 802.11b/g (WiFi): 11, 54 Mbps operator, 10’s km
transmission rate  between 1 and 10 Mbps
 3G, 4G: LTE

to Internet

to Internet

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 32

 Physical media
bit: propagates between
transmitter/receiver pairs
physical link: what lies between transmitter & receiver
guided media:
◦ signals propagate in solid media: copper, fiber, coax
unguided media:
◦ signals propagate freely, e.g., radio

The physical medium can take many shapes and forms and does not have
to be of the same type for each transmitter-receiver pair along the path.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 33

Roadmap
We have now a basic idea about what the internet is.
In this lecture, we would focus on
◦ How the hosts at network edge connect
◦ How the network core operates

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 34

What’s the network core
mesh of interconnected
routers

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 35

 Packet and Circuit Switching
Two basic ways to move data

1. Packet Switching
2. Circut Switching

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 36

The network core: Packet Switching
packet-switching: hosts
break application-layer
messages into packets
◦ forward packets from one
router to the next, across links
on path from source to
destination
◦ each packet transmitted at full
link capacity

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 37

 Packet switching versus circuit switching
packet switching
More efficient use of overall network bandwidth due to flexibility in routing
the smaller packets over shared links.
◦ resource sharing - simpler, no call setup

excessive congestion possible: packet delay and loss

◦ protocols needed for reliable data transfer, congestion control
Circuit Switching
◦ Constant rate data stream(packed) over some dedicated connection
◦ Wasted bandwidth - characterized by a fee per time unit of connection
time, even when no data is transferred.
◦ Highly reliable
◦ Blocked connection - Connection refused when resources are not sufficient
◦ Initial setup - it requires an initial channel set up that is usually time-
consuming. It also has a single point of failure that can disrupt all
communications.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 38

 Two key network-core functions
routing: determines source- forwarding: move packets from
destination route taken by packets router’s input to appropriate router
 routing algorithms output

routing algorithm

local forwarding table 1

header value output link
3 2
0100 3
0101 2
0111 2
1001 1

dest address in arriving

packet’s header

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 Two key network-core functions
routing: determines source- forwarding: move packets from
destination route taken by packets
router’s input to appropriate router
 routing algorithms
output

Routing means finding a suitable path for a packet from

sender to destination

Forwarding is the process of sending the packet toward

the destination based on routing information.

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 Circuit switching: FDM versus TDM
Example:
FDM (Frequency Division Multiplexing) 4 users

frequency

time
TDM

frequency

time
PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND
LECTURE – 01 - INTRODUCTION 1-41
APPLIED SCIENCES
 Protocol “layers”

Networks are complex,

with many “pieces”:
◦ hosts Question:
◦ routers is there any way of organizing
◦ links of various media structure of network?
◦ applications
◦ protocols
…. or at least our discussion
◦ hardware, software of networks?

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 42

 TCP

MAC
FTP
applications
IPv4
HTTP
DHCP DNS UDP
SSH
ICMP
ARP
Routing protocols hosts
RIP
hardware, software
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 43
 Organization of air travel

ticket (purchase) ticket (complain)

baggage (check) baggage (claim)

gates (load) gates (unload)

runway takeoff runway landing

airplane routing airplane routing

airplane routing

a series of steps

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 44

 Layering of airline functionality

ticket (purchase) ticket (complain) ticket

baggage (check) baggage (claim baggage

gates (load) gates (unload) gate

runway (takeoff) runway (land) takeoff/landing

airplane routing airplane routing airplane routing airplane routing airplane routing

departure intermediate air-traffic arrival

airport control centers airport

layers: each layer implements a service

◦ via its own internal-layer actions
◦ relying on services provided by layer below

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 45

Why layering?
dealing with complex systems:

modularization eases maintenance, updating of system

◦ change of implementation of layer’s service transparent to rest of
system
◦ e.g., change in gate procedure doesn’t affect rest of system

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 46

 We use the concept of layers in our daily life. As an example, let us consider two friends
who communicate through postal mail. The process of sending a letter to a friend would be
complex if there were no services available from the post office.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 47

 Tasks involved in sending a letter

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 48

 Internet protocol stack
application: supporting network applications
◦ FTP, SMTP, HTTP
transport: process-process data transfer application
◦ TCP, UDP
transport
network: routing of datagrams from source
to destination
◦ IP, routing protocols network
link: data transfer between neighboring
network elements link
◦ Ethernet, 802.111 (WiFi), PPP
physical
physical: bits “on the wire”

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 49

 Note

the application layer provides services for an application

program to ensure that effective communication with
another application program in a network is possible

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 50

 application: enable users/applications to access
network resources
HTTP protocol (which provides for Web document application
request and transfer)

transport
FTP protocol ( transfer of files between two end
systems) network

link
SMTP (transfer of e-mail messages)

physical

DNS (Domain name system)

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 51

 Transport layer
• This layer divide the data into segments.
• Transport Layer is where the decision to use TCP/UDP is made.
Among commonly used protocols in this layer, TCP is reliable,
UDP isn't.
• Depending upon the choice made, the respective headers are
attached to your packet.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES

 Transport layer (TCP/UDP)

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES

 Note

• Now, after TCP/UDP header being appended, it moves on to

the Network Layer. Till this step, the remote end-point's IP
address wasn't a part of the packet at all.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 54

 Network layer
• The Internet transport-layer protocol (TCP or UDP) in a source
host passes a transport-layer segment and a destination address
to the network layer.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 55

 Network layer

• The first physical equipment at the Network Layer would be

Routers

• It receives SEGMENTS from the upper layer and convert it into

PACKETS.

• Logical addressing: The network layer adds a header to the

packet coming from the upper layer, includes the logical
addresses (IP) of the sender and receiver.

• Makes “Best Path Determination” decision based on logical

addressing.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 56

 Link layer services
framing, link access:
specify the encapsulation of a packet into a frame and the techniques for
getting the encapsulated packet on and off each medium.
channel access if shared medium
◦ “MAC” addresses used in frame headers to identify source, dest (different from IP
address!

Error checking

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 57

 Physical layer

• Defines the physical and electrical medium for data

transfer.
• Physical layer components: cables, jacks, punch blocks,
hubs.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 58

 Physical layer

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LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 60
 The OSI Model
Established in 1947, the International Standards Organization (ISO) is a multinational body
dedicated to worldwide agreement on international standards. An ISO standard that covers all
aspects of network communications is the Open Systems Interconnection (OSI) model. It was first
introduced in the late 1970s.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 61

 Note

ISO is the organization.

OSI is the model.

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 62

 Seven layers of the OSI model

Data
Data
Data
Segment
Packet
Frame
Bits

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 63

 Seven layers of the OSI model

Application

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 64

 Seven layers of the OSI model

Data

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 65

 Seven layers of the OSI model

TH | Seg1 TH | Seg2

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 66

 Seven layers of the OSI model

TH | Seg2

NH|TH | Seg1

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 67

 Seven layers of the OSI model

NH|TH | Seg2

DH|NH|TH | Seg1

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 68

 Seven layers of the OSI model

DH|NH|TH | Seg2

01001010001010

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 69

 Seven layers of the OSI model

DH|NH|TH | Seg1

01001010001010

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 70

 Seven layers of the OSI model

NH|TH | Seg1

DH|NH|TH | Seg2

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 71

 Seven layers of the OSI model

TH | Seg1

NH|TH | Seg2

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 72

 Seven layers of the OSI model

TH | Seg2 TH | Seg1

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 73

 Seven layers of the OSI model

Data

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 74

Review
What is the Internet?
What is a protocol?
Network edge: hosts, access
network, physical media
Network core: packet/circuit
switching, internet structure
Protocol layers, service models

Introduction: 1-
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES
75
 A closer look at Internet structure mobile network

Network edge: national or global ISP

hosts: clients and servers

servers often in data centers
local or
Access networks, physical media: regional ISP

wired, wireless communication links home network content

provider
network datacenter
Network core: network

 interconnected routers
 network of networks enterprise
network

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-76
Access networks: data center networks
mobile network
 high-bandwidth links (10s to 100s national or global ISP
Gbps) connect hundreds to thousands
of servers together, and to Internet

local or
regional ISP

home network content

provider
network datacenter
network

Courtesy: Massachusetts Green High Performance Computing enterprise

Center (mghpcc.org) network

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-77
Packet-switching: store-and-forward
L bits
per packet
3 2 1
source destination
R bps R bps
72.14.211.72

packet transmission delay: takes L/R seconds to One-hop numerical example:

transmit (push out) L-bit packet into link at R bps  L = 10 Kbits
store and forward: entire packet must arrive at  R = 100 Mbps
router before it can be transmitted on next link  one-hop transmission delay
= 0.1 msec

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-78
Packet-switching: queueing
R = 100 Mb/s
A C

D
B R = 1.5 Mb/s
E
queue of packets
waiting for transmission
over output link

Queueing occurs when work arrives faster than it can be serviced:

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-79
Packet-switching: queueing
R = 100 Mb/s
A C

D
B R = 1.5 Mb/s
E
queue of packets
waiting for transmission
over output link

Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps)
of link for some period of time:
packets will queue, waiting to be transmitted on output link
packets can be dropped (lost) if memory (buffer) in router fills up

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-80
Alternative to packet switching: circuit switching
end-end resources allocated to, reserved for “call”
between source and destination
in diagram, each link has four circuits.
◦ call gets 2nd circuit in top link and 1st circuit in right
link.

dedicated resources: no sharing

◦ circuit-like (guaranteed) performance

circuit segment idle if not used by call (no

sharing)

 commonly used in traditional telephone networks

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-81
 Circuit switching: FDM and TDM
Frequency Division Multiplexing (FDM)
optical, electromagnetic frequencies 4 users

frequency
divided into (narrow) frequency bands

 each call allocated its own band, can

transmit at max rate of that narrow time
band

Time Division Multiplexing (TDM)

frequency
 time divided into slots
 each call allocated periodic slot(s), can
transmit at maximum rate of (wider) time
frequency band (only) during its time
slot(s)
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-82
Packet switching versus circuit switching
example:
 1 Gb/s link
N
 each user: users 1 Gbps link
• 100 Mb/s when “active”
• active 10% of time

Q: how many users can use this network under circuit-switching and packet switching?

 circuit-switching: 10 users
 packet switching: with 35 users, Q: how did we get value 0.0004?
probability > 10 active at same time
is less than .0004 *
A: HW problem (for those with
course in probability only)

* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-83
Packet switching versus circuit switching
Is packet switching a “slam dunk winner”?
 great for “bursty” data – sometimes has data to send, but at other times not
• resource sharing
• simpler, no call setup
 excessive congestion possible: packet delay and loss due to buffer overflow
• protocols needed for reliable data transfer, congestion control
 Q: How to provide circuit-like behavior with packet-switching?
• “It’s complicated.” We’ll study various techniques that try to make packet
switching as “circuit-like” as possible.

Q: human analogies of reserved resources (circuit switching) versus

on-demand allocation (packet switching)?
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-84
Internet structure: a “network of networks”
mobile network
hosts connect to Internet via access national or global ISP
Internet Service Providers (ISPs)
access ISPs in turn must be
interconnected
◦ so that any two hosts (anywhere!) local or
regional ISP
can send packets to each other
home network content
resulting network of networks is very provider
network datacenter
complex network

◦ evolution driven by economics, enterprise

network
national policies
Let’s take a stepwise approach to describe current Internet structure
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES 85
Internet structure: a “network of networks”
Question: given millions of access ISPs, how to connect them together?
access access
net net
access
net
access
access net
net
access
access net
net

access access
net net

access
net
access
net

access
net
access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-86
Internet structure: a “network of networks”
Question: given millions of access ISPs, how to connect them together?
access access
net net
access
net
access
access net
net
access
access net
net

connecting each access ISP to

each other directly doesn’t scale:
access
access
net O(N2) connections. net

access
net
access
net

access
net
access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-87
Internet structure: a “network of networks”
Option: connect each access ISP to one global transit ISP?
Customer and provider ISPs have economic agreement.
access access
net net
access
net
access
access net
net
access
access net
net

global
access
net
ISP access
net

access
net
access
net

access
net
access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-88
Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors ….

access access
net net
access
net
access
access net
net
access
access net
net ISP A

access
net ISP B access
net

access ISP C
net
access
net

access
net
access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-89
Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors …. who will
want to be connected
Internet exchange point
access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
peering link
access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-90
Internet structure: a “network of networks”
… and regional networks may arise to connect access nets to ISPs

access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
regional ISP access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-91
Internet structure: a “network of networks”
… and content provider networks (e.g., Google, Microsoft, Akamai) may
run their own network, to bring services, content close to end users
access access
net net
access
net
access
access net
net
IXP access
access net
net ISP A

Content provider network

access
net
IXP ISP B access
net

access ISP C
net
access
net

access
net
regional ISP access
net
access access
net access net
net

LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-92
 Internet structure: a “network of networks”
Tier 1 ISP Tier 1 ISP Google
IXP IXP IXP
Regional ISP Regional ISP

access access access access access access access access

ISP ISP ISP ISP ISP ISP ISP ISP

At “center”: small # of well-connected large networks

 “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage
 content provider networks (e.g., Google, Facebook): private network that connects its
data centers to Internet, often bypassing tier-1, regional ISPs
LECTURE – 01 - INTRODUCTION PIEAS, PAKISTAN INSTITUTE OF ENGINEERING AND APPLIED SCIENCES Introduction: 1-93