Computer Networks and
Communications
[Δίκτυα Υπολογιστών και Επικοινωνίες]
Lectures 2&3: What is the Internet?
Univ. of the Aegean
Financial and Management Engineering Dpt
Petros KAVASSALIS
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What you will learn in this course

A set of fundamental concepts for understanding Data
Networks and the Internet
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
What is the Internet?
Internet architecture and layers
Internet applications and services
New concepts in the evolution of the Internet
The Internet goes Wireless…
Familiarization with the structure and organization of Digital
Networks


Business and Social Networks
Electronic Markets and Online Feedback Mechanisms
<pkavassalis@atlantis-group.gr>
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Who am I?

PhD in Economics and Management (Univ. Paris Dauphine
& Ecole polytechnique)
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Research experience



Ecole polytechnique, Paris
MIT Center of Technology Policy and Industrial Development, MIT
CTPID (MIT Internet Telecommunications Convergence Consortium)
Current positions
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
Univ. of the Aegean (FME): Assoc. Professor
RACTI: Director of ATLANTIS Group
<pkavassalis@atlantis-group.gr>
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Communication tools
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e-mail: pkavassalis [at] atlantis-group.gr
Course web site: see fme website
<pkavassalis@atlantis-group.gr>
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Course Textbook
[http://books.google.gr/books?id=Pdz64SJRBAC&dq=tanenbaum+networks&printsec=frontcover&source=bn&hl=el&ei=mldSfH9L4S2jAeJ5L3ZBQ&sa=X&oi=book_result&resnum=4&ct=result]
<pkavassalis@atlantis-group.gr>
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Supplementary Texts & References


William Stallings, Computer Networking with Internet
Protocols, Prentice Hall, 2004
James F. Kurose and Keith W. Ross, Computer Networking:
A Top-Down Approach, Addison-Wesley, 2008
<pkavassalis@atlantis-group.gr>
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Students evaluation

Class Participation (20%)
+
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Assignments (20%)
+

Final Exam (60%)
<pkavassalis@atlantis-group.gr>
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What is a network?

A hardware and software communications system formed by
the interconnection of three or more devices
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Devices may include:
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Telephones
PCs
Routers
Other communications devices (please give examples)
<pkavassalis@atlantis-group.gr>
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The geography of the Internet
<pkavassalis@atlantis-group.gr>
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Internet in a nutshell
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Protocols control sending, receiving
of msgs
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loosely hierarchical
public Internet versus private
intranet
Standards
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Global ISP
e.g., TCP, IP, HTTP, IM, Ethernet
Composition: “network of networks”
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Mobile network
Home network
Regional ISP
Institutional network
RFC: Request for comments
IETF: Internet Engineering Task
Force
<pkavassalis@atlantis-group.gr>
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Overview of the Internet
The structure of the Internet is roughly hierarchical
<pkavassalis@atlantis-group.gr>
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A multilevel structure: Tier 1

At center: “Tier-1” ISPs (e.g., Verizon, France Telecom,
Deutche Telecom etc.), national/international coverage

Treat each other as equals / interconnect privately
Tier-1
providers
interconnect
(peer)
privately
<pkavassalis@atlantis-group.gr>
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
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A multilevel structure: Tier 2

Tier-2” ISPs: smaller (regional) ISPs (OTEnet, Forthnet)

Connect to one or more tier-1 ISPs, possibly other Tier 2 ISPs
<pkavassalis@atlantis-group.gr>
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A multilevel structure: Tier 3

“Tier-3” ISPs and local ISPs

Last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier ISPs
connecting
them to rest of
Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
<pkavassalis@atlantis-group.gr>
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
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As a result, packet passes through many
network infrastructures
Tier 1 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP

Which networks? Let’s discover the Internet…
<pkavassalis@atlantis-group.gr>
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The essential of Internet: infrastructures
but also applications…
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Communication infrastructure
enables various distributed
applications
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E-mail, Web browsing,
Skypying, file sharing, online
games
Communication applications
are supported by

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reliable data delivery from
source to destination
“best effort” (unreliable) data
delivery
<pkavassalis@atlantis-group.gr>
Mobile network
Global ISP
Home network
Regional ISP
Institutional network
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… “separated” in two blocks
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IP (spanning-layer)
separates information
bitways from applications
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Applications may work over
multiple substrates
(network techs) and these
substrates do not prespecify the development of
new applications
[I will come back!]

<pkavassalis@atlantis-group.gr>
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What is a protocol?
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Human protocols
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Machine protocols
“what’s the time?”
“I have a question”
Introductions (“this is…”)
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Specific msgs sent
Specific actions taken
when msgs received, or
other events
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<pkavassalis@atlantis-group.gr>
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Machines “talk each other”
(rather than humans)
All communication activity in
Internet governed by
protocols
Protocols define format, order
of msgs sent and received
among network entities, and
actions taken on msg
transmission, receipt
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Human and Computer protocols
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Make possible a series of interactions
http:www.atlantis-group.gr
<pkavassalis@atlantis-group.gr>
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The Internet path of a communication
(defined with the use of a protocol): end-core-end

Internet end and core
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mesh of interconnected
routers
how is data transferred
through net?
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circuit switching:
dedicated circuit per call:
telephone net
packet-switching: data
sent thru net in discrete
“chunks
<pkavassalis@atlantis-group.gr>
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Transmission speed
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Measured in bits per second (bps)
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Increasing factors of 1,000 …
Not factors of 1,024
Kilobits per second (kbps) - note the lowercase k
Megabits per second (Mbps)
Gigabits per second (Gbps)
Terabits per second (Tbps)
The rule for writing speeds (and metric numbers in general) in proper
form is that there should be 1 to 3 places before the decimal point
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23.72 Mbps is correct (2 places before the decimal point).\
2,300 Mbps has four places before the decimal point, so it should be rewritten
as 2.3 Gbps (1 place)
<pkavassalis@atlantis-group.gr>
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Circuit-switching (1)
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End-end resources
reserved for “call”
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Link bandwidth, switch
capacity
Dedicated resources: no
sharing
Circuit-like (guaranteed)
performance
Call setup required
<pkavassalis@atlantis-group.gr>
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Circuit-switching (2)
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Network resources
(e.g., bandwidth)
FDM
divided into “pieces”
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Pieces allocated to calls
frequency
Resource piece idle if
not used by owning call
TDM
(no sharing)
Dividing link bandwidth
frequency
into “service lines”
o
o
Frequency division (FDM)
Time division (TDM)
<pkavassalis@atlantis-group.gr>
Example:
4 users
time
time
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Packet-switching (1)
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Each end-end data
stream divided into
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Resource contention:
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packets
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User A, B packets share
network resources
Each packet uses full
link bandwidth
Resources used as
needed
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Aggregate resource
demand can exceed
amount available
Congestion: packets
queue, wait for link use
Mechanism: store and
forward: packets move
one hop at a time
o
<pkavassalis@atlantis-group.gr>
Node receives complete
packet before forwarding
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Packet-switching (2): statistical sharing
100 Mb/s
Ethernet
A
C
statistical multiplexing
1.5 Mb/s
B
queue of packets
waiting for output
link
D

E
Sequence of A & B packets does not have fixed pattern,
bandwidth shared on demand
<pkavassalis@atlantis-group.gr>
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Packet-switching (3): store-and-forward
L
R
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R
R
Takes L/R seconds to transmit (push out) packet of L bits on
to link at R bps
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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:
o L = 7.5 Mbits, R = 1.5 Mbps
o Transmission delay = 15 sec
<pkavassalis@atlantis-group.gr>
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Packet-switching v. Circuit-switching
N users
1 Mbps link
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1 Mb/s link
Each user:
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100 kb/s when “active”
active 10% of time
Circuit-switching: 10 users
Packet switching: more users can share the network
<pkavassalis@atlantis-group.gr>
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Packet-switching: pros and cons

Great for bursty data
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Excessive congestion: packet delay and loss
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Resource sharing
Simpler, no call setup
Protocols needed for reliable data transfer, congestion control
How to provide circuit-like behavior?
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Bandwidth guarantees needed for audio/video apps
(Still) a not fully unsolved problem
<pkavassalis@atlantis-group.gr>
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