CS 164 : Computer Networks
My name : Srikanth Krishnamurthy
TAs : Jianxia Ning
(jning@cs.ucr.edu)
Office hrs: W 10-11 a.m.
Web Site:TBA
Please make sure you subscribe to mailing
list of CS 164
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• Attendance to Labs are Compulsory
• Written homework assigned and collected one week
later.
• If you miss more than 2 homework assignments,
then there will be a penalty assessed.
• In Lab assignments need to be completed in the
lab.
• One project -- individual effort.
• No cooperation on anything in this course.
• Very Important : Please ask questions in class - if
you don’t understand something please let me know.
• Otherwise I have no way of knowing!
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• Three quizzes -- approximately 3 weeks apart.
• Each Quiz will be for 15 points
• We will only take your best two quizzes.
• Multiple choice
• Final Exam -- about 35 %
• Project -- 15 %
• Labs -- 10 %
• Homeworks 10 %
• More specific details will be on website in a day
or two.
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Roadmap for this lecture
o Introduction -- Chapter 1.
 Whatever I don’t cover is for self-study.
o The Internet and networks in general
o Basic types of connections between
computers
o How is information sent from one point to
the other ?
o What are protocols and what role do they
play ?
4
Internet and Stuff:The Basics
o What is the Internet?
o What does it consist of?
o Main terminology and lingo
o Fundamental Concepts
Re-using some slides from Kurose and Ross book -- a reference book
that you may want to refer to if you are interested.
Contents still reflect Chapter 1 from book.
5
Visualizing the Internet
o
millions of connected
computing devices: hosts, endsystems


pc’s workstations, servers
PDA’s phones, toasters
router
server
o
mobile
local ISP
running network apps
o communication links

workstation
regional
ISP
fiber, copper, radio, satellite
routers: forward packets
(chunks) of data thru network
company
network
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“Cool” Internet Appliances
IP picture frame
http://www.ceiva.com/
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
Web-enabled toaster+weather forecaster
http://dancing-man.com/robin/toasty/
7
Internet structure: network of networks
roughly hierarchical
o national/international backbone
providers (NBPs)
o


o
o
e.g. BBN/GTE, Sprint, AT&T,
IBM, UUNet
interconnect (peer) with each
other privately, or at public
Network Access Point (NAPs)
regional ISPs

local
ISP
connect into NBPs
regional ISP
NBP B
NAP
NAP
NBP A
local ISP, company

connect into regional ISPs
regional ISP
local
ISP
8
National Backbone Provider
e.g. Sprint US backbone network
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Links between computers
o Point to point link -- a direct cable or wire
that is dedicated for use between the
computers.
o Multiple access link -- shared between
many nodes -- example a bus, wireless
medium etc.
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Why structure ?
o Direct connectivity does not scale -- all
computers cannot be directly connected to
each other.
o Need organization


End hosts -- clients and servers -- usually house
information
Routers and switches -- nodes that are
primarily used for relaying information -sending information where it needs to go.
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Component networks of the
Internet
o Local Area Network -- LAN -- Extends up
to 1 Km or so -- example -- Ethernet,
Wireless LAN
o Metropolitan Area Network -- larger scale
-- tens of km.
o Wide Area Networks --- Largest -- could
consist of many component networks.
o System area networks -- single room.
12
Examples of networks
o Ad hoc and sensor networks -- specific
applications.
o Wireless hotspots Wi-Fi
o Of course, satellite networks have existed
for some time now.
13
End-hosts and applications
o Client -- requests information services.
o Server -- provides information services.
o Examples of applications -- Web (http),
File transfer (FTP), E-mail.
o Typically clients and server “processes” run
on different computers.
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The IETF
o Internet Engineering Task Force

Provides standards/ documents
• www.ietf.org
o Contains Internet Drafts and what are
called RFCs -- Request for Comments.

Quite detailed descriptions of what are called
protocols.
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What’s a protocol?
The definition of a behavior --set of rules
Here: the format of a communication exchange:
Sequence of actions, format of information, predefined
interpretation
Hi
connection
req.
Hi
connection
reply.
Got the
time?
2:00
<file>
time
Get http://gaia.cs.umass.edu/index.htm
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Centralized versus Distributed
Protocols
o Centralized: all information is collected in
one place and then processed
o Distributed: decisions are taken locally
with partial or summary of the information

Most of the protocols distributed for
scalability considerations.
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Packets
o Packets are similar to postal letters -basic units
of information



From, to, content
Postman handles all packets similarly
Addressing is hierarchical.
o The protocol that defines how packets are to be
“routed” is the Internet Protocol or IP.

Hierarchical addresses like in the postal world.
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Types of Communications
• Circuit Switching (reserve a slice of resources)
• Frequency division multiplexing
• Time division multiplexing
• Packet switching
• Virtual Circuits: routers keep per connection info
• Datagrams: no per connection information
• Connection oriented (state at end points, handshake TCP)
• TCP -- Transmission Control Protocol
• Connectionless (no state at end points - UDP)
• UDP -- User Datagram protocol.
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Types of Communications
o Advantage of packet switching:



Resource sharing
No need for reservations
Easier to implement in a distributed way.
o Advantage of circuit switching:
 Can guarantee performance (Quality of
Service) -- reserved resource.
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The Principles of the Internet
o Goal: Interconnect existing net technologies
 ARPA packet radio, and ARPANET
 Packet switching? Flexibility
 Trade-off: poorer non-guaranteed performance
 Higher layer protocols try to provide reliability.
o “The Design Philosophy of The DARPA Internet
Protocols”, David Clark, MIT.
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Secondary Internet Principles
1.
2.
3.
4.
5.
6.
7.
Fault-tolerance to component failures
Support multiple types of services
Interoperate with different technologies
Allow distributed management
Be cost effective (i.e. sharing)
Be easily extendible
Resources and entities must be
accountable (for security purpose)
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Internet Architecture
Characteristics
o Scalability to millions of users

Stateless routing: Routers cannot keep detailed
info per connection
o Best-effort service: no guarantees
o Decentralized control
o Self-configuration
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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
o a series of steps
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Organization of air travel: a different view
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
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Internet protocol stack
o
application: supporting network applications

o
transport: host-host data transfer

o
ip, routing protocols
link: data transfer between neighboring
network elements

o
tcp, udp
network: routing of datagrams from source
to destination

o
ftp, smtp, http
ppp, ethernet
physical: bits “on the wire” -representation, signal modulation.
application
transport
network
link
physical
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Other OSI layers
o OSI -- open systems interconnection had
two other layers.


Session layer -- bundles multiple transport
layer streams into a single session -- audio and
video.
Presentation layer -- representation of data to
the application
o Not much used.
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Why layering?
Dealing with complex systems:
o 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
o Isolating “functions” and interactions components
 layered reference model for discussion
o layering considered harmful?
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Layering: logical communication
Each layer:
o distributed
o “entities” implement
layer functions at
each node
o entities perform
actions, exchange
messages with peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
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Layering: logical communication
E.g.: transport
o
o
o
o
o
take data from app
add addressing,
reliability check
info to form
“datagram”
send datagram to
peer
wait for peer to
ack receipt
analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
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Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
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Message flow through stack
• At each layer,
headers added.
M
H
HH
M1
M1
H
M2
HH
M2
• A protocol defines
maximum packet size - might require higher
layer packet to be
fragmented.
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Protocol layering and data
Each layer takes data from above
o adds header information to create new data unit
o passes new data unit to layer below
M
Ht M
Hn Ht M
Hl Hn Ht M
source
destination
application
transport
network
link
physical
application
transport
Network
link
physical
M
message
Ht M
Hn Ht M
Hl Hn Ht M
segment
datagram
frame
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Basic Routing Concepts
o Packet = postal letter
o Router receives packet
o Needs to decide which link to send it to
o Scalability: decide on local information
o Routers keep “summary” of information
o Exploit the hierarchy in the IP address
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The network edge:
o end systems (hosts):



run application programs
e.g., WWW, email
at “edge of network”
o client/server model


client host requests, receives
service from server
e.g., WWW client (browser)/
server; email client/server
o peer-peer model:


host interaction symmetric
e.g.: Gnutella, KaZaA
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Network edge: connection-oriented service
Goal: data transfer between
end sys.
o handshaking: setup
(prepare for) data transfer
ahead of time


o
Hello, hello back human
protocol
set up “state” in two
communicating hosts
TCP - Transmission Control
Protocol

Internet’s connectionoriented service
TCP service [RFC 793]
o
reliable, in-order byte-stream
data transfer

o
flow control:

o
loss: acknowledgements and
retransmissions
sender won’t overwhelm
receiver
congestion control:

senders “slow down sending
rate” when network congested
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Network edge: connectionless service
Goal: data transfer between end systems
 same as before!
o UDP - User Datagram Protocol [RFC 768]: Internet’s
connectionless service
 unreliable data transfer
 no flow control
 no congestion control
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The Network Core
o mesh of interconnected
routers
o 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”
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Network Core: Circuit Switching
End-end resources
reserved for “call”
link bandwidth, switch
capacity
o dedicated resources: no
sharing
o circuit-like (guaranteed)
performance
o call setup required
o
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Network Core: Circuit Switching
network resources (e.g., bandwidth) divided
into “pieces”
o pieces allocated to calls
o resource piece idle if not used by owning call (no
sharing)
o dividing link bandwidth into “pieces”
 frequency division
 time division
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Circuit Switching: TDMA and TDMA
Example:
FDMA
4 users
frequency
time
TDMA
frequency
time
41
Network Core: Packet Switching
each end-end data stream divided
into packets
o user A, B packets share
network resources
o each packet uses full link
bandwidth
o resources used as needed,
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
o aggregate resource
demand can exceed
amount available
o congestion: packets
queue, wait for link use
o store and forward:
packets move one hop at
a time
 transmit over link
 wait turn at next link
42
Network Core: Packet Switching
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
45 Mbs
E
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Packet switching versus circuit switching
-- An example
Packet switching allows more users to use network!
1 Mbps link
o each user:
o


o
circuit-switching:

o
100 Kbps when “active”
active 10% of time
N users
10 users
packet switching:

1 Mbps link
with 35 users, probability
> 10 active less than .0004
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Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
o Great for bursty data
resource sharing
 no call setup
o Excessive congestion: packet delay and loss
 protocols needed for reliable data transfer,
congestion control
o Q: How to provide circuit-like behavior?
 bandwidth guarantees needed for audio/video
apps
 still an unsolved problem (see QoS, multimedia)

45
Classification of the Types of
Communication
Circuit Switching
Dedicated
Shared
hardware
(no per flow
Packet
State in routers)
Dedicated
hardware
Time
Frequency
Division
Division
Multiplexing Multiplexing
Datagrams
(IP)
ConnectionOriented
(TCP, end points
Keep state)
Switching
Virtual Circuits
(ATM)
Connectionless
(per flow state
in routers resource
reservation)
(UDP, endpoints
don’t -much- keep state)
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Packet-switched networks: routing
o
Goal: move packets among routers from source to destination

o
datagram network:



o
we’ll study several path selection algorithms (chapter 4)
destination address determines next hop
routes may change during session
analogy: driving, asking directions
virtual circuit network:



each packet carries tag (virtual circuit ID), tag determines next
hop
fixed path determined at call setup time, remains fixed thru call
routers maintain per-call state
47
Access networks and physical media
Q: How to connect end systems
to edge router?
o residential access nets
o institutional access networks
(school, company)
o mobile access networks
Keep in mind:
o bandwidth (bits per second)
of access network?
o shared or dedicated?
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Residential access: point to point access
o Dialup via modem
up to 56Kbps direct access to
router (conceptually)
o ISDN: integrated services
digital network: 128Kbps alldigital connect to router
o ADSL: asymmetric digital
subscriber line
 up to 1 Mbps home-to-router
 up to 8 Mbps router-to-home
 ADSL deployment: happening

49
Residential access: cable modems
o HFC: hybrid fiber coax
asymmetric: up to 10Mbps upstream, 1 Mbps
downstream
o network of cable and fiber attaches homes to
ISP router
 shared access to router among home
 issues: congestion, dimensioning
o deployment: available via cable companies, e.g.,
MediaOne

50
Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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Physical Media
o physical link:
transmitted data bit
propagates across link
o guided media:

signals propagate in
solid media: copper,
fiber
o unguided media:
 signals propagate
freely, e.g., radio
Twisted Pair (TP)
o two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
Ethernet
Category 5: 100Mbps
Ethernet
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Physical Media: coax, fiber
Coaxial cable:
o wire (signal carrier)
within a wire (shield)
o bidirectional
o common use in 10Mbs
Ethernet
Fiber optic cable:
o glass fiber carrying
light pulses
o high-speed operation:


100Mbps Ethernet
high-speed point-to-point
transmission (e.g., 5 Gps)
o low error rate
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Physical media: radio
o signal carried in
electromagnetic
spectrum
o no physical “wire”
o bidirectional
o propagation
environment effects:



reflection
obstruction by objects
interference
Radio link types:
o microwave
 e.g. up to 45 Mbps
channels
o LAN (e.g., WaveLAN)
 2Mbps, 11Mbps
o wide-area (e.g., cellular)
 e.g. CDPD, 10’s Kbps
o satellite
 up to 50Mbps channel
(or multiple smaller
channels)
 270 Msec end-end
delay
 geosynchronous versus
LEOS
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Delay in packet-switched networks
packets experience delay
on end-to-end path
o four sources of delay
at each hop
o
nodal processing:


o
check bit errors
determine output link
queueing


time waiting at output link
for transmission
depends on congestion
level of router
transmission
A
propagation
B
nodal
processing
queueing
55
Delay in packet-switched networks
Transmission delay:
o R=link bandwidth (bps)
o L=packet length (bits)
o time to send bits into
link = L/R
transmission
A
Propagation delay:
o d = length of physical link
o s = propagation speed in
medium (~3x108 m/sec)
o propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
nodal
processing
queueing
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Queueing delay
R=link bandwidth (bps)
o L=packet length (bits)
o a=average packet arrival
rate
o
traffic intensity = La/R
o La/R ~ 0: average queueing delay small
o La/R -> 1: delays become large
o La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
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Summary
Covered a “ton” of material!
o Internet introduction
o what’s a protocol?
o network edge, core, access
network
 packet-switching versus
circuit-switching
o performance: loss, delay
o layering and service
models
You now have:
o context, overview,
“feel” of networking
o more depth, detail
later in course
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What is to follow ?
o Complete Chapter 1:



Concepts of throughput and end to end latency
Bandwidth delay product -- network capacity
Brief intro to sockets
o Chapter 2: PHY Layer and Link Layer
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