Chapter 1
Introduction
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers).
They’re in PowerPoint form so you see the animations; and can add, modify,
and delete slides (including this one) and slide content to suit your needs.
They obviously represent a lot of work on our part. In return for use, we only
ask the following:
 If you use these slides (e.g., in a class) that you mention their source
(after all, we’d like people to use our book!)
 If you post any slides on a www site, that you note that they are adapted
from (or perhaps identical to) our slides, and note our copyright of this
material.
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
Thanks and enjoy! JFK/KWR
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
Introduction 1-1
University of Nevada – Reno
Computer Science & Engineering Department
Fall 2015
CPE 400 / 600
Computer Communication Networks
Lecture 2
Prof. Shamik Sengupta
Office SEM 204
ssengupta@unr.edu
http://www.cse.unr.edu/~shamik/
Introduction 1-2
Chapter 1: Computer Networks
and the Internet
What is computer network: “nuts and bolts” view
PC
1.
server
wireless
laptop
cellular
handheld
Numerous connected
Mobile network
computing devices: hosts
Global ISP
= end systems
 running network apps
2. communication links
fiber, copper,
radio, satellite
 transmission rate
= bandwidth
3. routers: forward
packets (chunks of
data)
access
points
wired
links
router
Home network
Regional ISP

Institutional network
1-4
Categorization of Computer Networks
•
•
•
1-5
Business Networks
Home Networks
Mobile Networks
Example Network Applications (1)
A network with two clients and one server
(typical client-server connection)
1-6
Example Network Applications (2)
The client-server model involves requests and
replies over the public/private network
Example Network Applications (3)
Peer-to-peer networking: no fixed clients and servers
Example wireless network (4)
network
infrastructure
6-9
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary (nonmobile) or mobile

wireless does not always
mean mobility
Categorization of networks by coverage scale
•
•
•
•
•
1-10
Personal area networks (PAN)
Local area networks (LAN)
Metropolitan area networks (MAN)
Wide are networks (WAN)
The Internet (Global network)
Personal Area Network (PAN)
Bluetooth PAN configuration
Local Area Networks (LAN)
Wireless and wired LANs. (a) 802.11. (b)
Switched Ethernet.
Metropolitan Area Networks (MAN)
A metropolitan area network
Wide Area Networks (WAN)
WAN that connects three branch offices in Australia
Coverage scale (contd.)
Classification of interconnected processors by scale
A different categorization of networks
In terms of communication technology
•
•
•
Unicasting
Broadcasting
Multicasting
What is computer networking: an operational view
Any communication is all about protocol
Hi
Connection req.
Hi
Connection
reply.
Got the
time?
Get http://www.cnn.com/slide.ppt
2:00
human protocol
<file>
time
networking protocol
1-17
What is computer networking: an operational view
human protocols:
… specific msgs sent
… specific actions taken
when msgs received, or
other events
network protocols:
 machines rather than
humans
 all communication activity
governed by protocols
protocols define format,
order of msgs sent and
received among network
entities, and actions
taken on msg
transmission, receipt
1-18
Protocol “Layers”
Networks are complex!
It is not just two machines communicating!


Millions of components:
 hosts
 routers
 Access networks
 Physical links
Numerous functionalities
Question:
How to manage such vast
amount of components?
Soln: Divide functionalities
among multiple layers.
1-19
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
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below and above

1-20
Another example: Postal Service!
What are the adv. of layering?
Network is a huge complex system.


Reduce the design complexity
Ease of updating the system
 change of implementation of layer’s service transparent to
rest of system
 e.g., Postal service (overnight flight or overnight ground)
Internet protocol stack

application
 support host/network applications
 Email, FTP, HTTP (HTML)


transport
application
 process-process data transfer
 TCP, UDP
transport
network
 routing of datagrams from src. to destn.
network
 IP address, routing protocols

link

physical
 data transfer between neighboring
linknetwork elements
 Ethernet
 bits “on the wire”
physical
(Compare with the Postal System!)
1-22
The TCP/IP Reference Model
1-23
ISO/OSI reference model
presentation: allow applications
to interpret meaning of data,
e.g., encryption, compression,
machine-specific conventions
 session: synchronization,
checkpointing, recovery of data
exchange

application
presentation
session
transport
network
link
physical
Introduction 1-24
Messages, Segments, Datagrams and Frames
source
message
segment
M
Ht
M
datagram Hn Ht M
frame Hl Hn Ht M
application
transport
network
link
physical
link
physical
switch
Encapsulation
destination
message
Ht
Hn Ht
Hl Hn Ht
M
M
M
M
application
transport
network
link
physical
Hn Ht
H l Hn Ht
M
M
network
link
physical
Hn Ht
M
router
1-25
Network core
 packet switching, circuit switching,
Network structure
Introduction 1-26
The network core


mesh of interconnected
routers
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
Introduction 1-27
Packet-switching: store-and-forward
L bits
per packet
source
3 2 1
R bps
takes L/R seconds to
transmit (push out) L-bit
packet into link at R bps
 store and forward: entire
packet must arrive at router
before it can be transmitted
on next link
 end-end delay = 2L/R
(assuming zero propagation delay)

R bps
destination
one-hop numerical example:
 L = 7.5 Mbits
 R = 1.5 Mbps
 one-hop transmission
delay = 5 sec
more on delay shortly …
Introduction 1-28
Packet Switching: queueing delay, loss
A
C
R = 100 Mb/s
R = 1.5 Mb/s
B
D
E
queue of packets
waiting for output link
queuing and loss:

If arrival rate (in bits) to link exceeds transmission rate of
link for a period of time:
 packets will queue, wait to be transmitted on link
 packets can be dropped (lost) if memory (buffer) fills up
Introduction 1-29
Alternative core: circuit switching
end-end resources allocated
to, reserved for “call”
between source & dest:




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
Introduction 1-30
Circuit switching: FDM versus TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
Introduction 1-31
Packet switching versus circuit switching
packet switching allows more users to use network!
example:
 1 Mb/s link
 each user:
• 100 kb/s when “active”
• active 10% of time
N
users
1 Mbps link
 circuit-switching:
 10 users
Introduction 1-32
Packet switching versus circuit switching
is packet switching a “clear winner?”



great for bursty data
 resource sharing
 simpler, no call setup
excessive congestion possible: packet delay and loss
 protocols needed for reliable data transfer, congestion
control
Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video apps
 still an unsolved problem (will discuss about this more
later…)
Introduction 1-33
Internet structure: network of networks

End systems connect to Internet via access ISPs (Internet
Service Providers)
 Residential, company and university ISPs

Access ISPs in turn must be interconnected.
 So that any two hosts can send packets to each other

Resulting network of networks is very complex
 Evolution was driven by economics and national policies

Let’s take a stepwise approach to describe current Internet
structure
Internet structure: network of networks
Question: given millions of access ISPs, how to connect them
together?
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
Option: connect each access ISP to every other access ISP?
access
net
access
net
access
net
access
net
access
net
access
net
access
net
connecting each access ISP
to each other directly doesn’t
scale: O(N2) connections.
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
Option: connect each access ISP to a global transit ISP? Customer
and provider ISPs have economic agreement.
access
net
access
net
access
net
access
net
access
net
access
net
access
net
global
ISP
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
But if one global ISP is viable business, there will be competitors
….
access
net
access
net
access
net
access
net
access
net
access
net
access
net
ISP A
access
net
access
net
access
net
ISP B
ISP C
access
net
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
But if one global ISP is viable business, there will be competitors
…. which must be interconnected
Internet exchange point
access
access
net
net
access
net
access
net
access
net
IXP
access
net
ISP A
IXP
access
net
access
net
access
net
access
net
ISP B
ISP C
access
net
peering link
access
net
access
net
access
net
access
net
access
net
Internet structure: network of networks
… and regional networks may arise to connect access nets to
ISPS
access
net
access
net
access
net
access
net
access
net
IXP
access
net
ISP A
IXP
access
net
access
net
access
net
access
net
ISP B
ISP C
access
net
access
net
regional net
access
net
access
net
access
net
access
net
Internet structure: network of networks


roughly hierarchical
at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and
Wireless), national/international coverage
 treat each other as equals
Tier 1 ISP
Tier 1 ISP
1-41
Tier 1 ISP
Tier-1 ISP: e.g., Sprint
1-42
Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPs
 Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
 tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
1-43
Tier 1 ISP
Tier-2 ISP
Tier-2 ISPs
also peer
privately
with each
other.
Tier-2 ISP
Internet structure: network of networks

“Tier-3” ISPs and local ISPs
 last hop (“access”) network (closest to end systems)
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
1-44
local
local local
Tier
3
local
ISP
ISP
ISP ISP ISP
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Internet structure: network of networks

a packet passes through many networks!
local
local local
Tier
3
local
ISP
ISP
ISP ISP ISP
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
1-45
Tier-2 ISP
local
local
ISP
ISP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Delay, loss, throughput in networks
Introduction 1-46
How do loss and delay occur?
packets queue in router buffers


packet arrival rate to link (temporarily) exceeds output link
capacity
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction 1-47
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dproc: nodal processing
 check bit errors
 determine output link
 typically < msec
dqueue: queueing delay
 time waiting at output link
for transmission
 depends on congestion
level of router
Introduction 1-48
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dtrans: transmission delay:
 L: packet length (bits)
 R: link bandwidth (bps)
 dtrans = L/R
dtrans and dprop
very different
dprop: propagation delay:
 d: length of physical link
 s: propagation speed in medium
(~2x108 m/sec)
 dprop = d/s
Introduction 1-49
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

buffer
(waiting area)
A
packet being transmitted
B
packet arriving to
full buffer is lost
Introduction 1-50
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,
withbits
server
sends
file of into
F bitspipe
(fluid)
to send to client
linkpipe
capacity
that can carry
Rs bits/sec
fluid at rate
Rs bits/sec)
linkpipe
capacity
that can carry
Rc bits/sec
fluid at rate
Rc bits/sec)
Introduction 1-51
Throughput: Internet scenario

per-connection endend throughput:


min(Rc,Rs,R/10)
Rs
Rs
Rs
in practice: Rc or Rs
is often bottleneck
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction 1-52
Metric Units (1)
The principal metric prefixes
1-53
Metric Units (2)
The principal metric prefixes
1-54