Chapter 1

Computer Networks and the Internet

Internet traffic

Introduction 1-3

What’s the Internet? (hardware)

millions of connected

computing devices:

hosts = end systems

running network

apps Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wireless laptop

cellular handheld

wired links

access points

communication links

fiber, copper, radio, satellite

transmission rate = bandwidth

routers: forward packets (chunks of data)

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

What’s 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

Q: Other human protocols?

Hi

Hi

Got the time?

2:00

TCP connection request

TCP connection response

<file>

time

Who make network protocols?

Internet Engineering Task Force (IETF)

Internet standards

Request for Comments (RFCs)

Introduction 1-10

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 1-11

The network edge:

End systems (hosts): run application programs

e.g. Web, email

at “edge of network”

client/server

peer-peer

Client/server model client host requests, receives

service from always-on 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 1-12

Access networks

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

Three categories:

Residential access

Company access

Wireless access Help me identify them

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) Unshielded twisted pair (UTP)

Shield twisted pair (STP)

Guided media

Coaxial cable Guided media

Fiber optics

Guided media

Radio links

Microwave

WiFi

3G

Satellite

Unguided media

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”

19

Circuit switching (telephone system)

Packet switching (store-and-forward)

Introduction 1-21

Packet-switching: store-and-forward

takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

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:

L = 7.5 Mbits

R = 1.5 Mbps

transmission delay = 15 sec

R R R

L

Introduction 1-22

Packet switching versus Circuit switching

1 Mb/s link

each user:

100 kb/s when “active”

circuit-switching:

10 users

packet switching:

>> 10 users

Packet switching allows more users to use network!

N users

1 Mbps link

Packet switching versus Circuit switching Packet

Shared resources

No call setup

Long delay when traffic

is high

N users supported

Low cost

Circuit

Reserved resources

Call setup needed

Delay is known

s

< N users supported

High cost

1-24

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

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

Introduction 1-25

Four sources of packet delay

1. nodal processing:

check bit errors

determine output link

A

B

propagation

transmission

nodal processing queueing

2. Queuing:

time waiting at output link for transmission

depends on congestion level of router

Introduction 1-26

Delay in packet-switched networks

3. Transmission delay:

R=link bandwidth (bps)

L=packet length (bits)

time to send bits into

link = L/R

4. Propagation delay:

d = length of physical link

s = propagation speed in

medium (~2x108 m/sec)

propagation delay = d/s

A

B

propagation

transmission

nodal processing queueing

Note: s and R are very different quantities!

Introduction 1-27

Caravan analogy

cars “propagate” at

100 km/hr

toll booth takes 12 sec to

service car (transmission

time)

car~bit; caravan ~ packet

Q: How long until caravan

is lined up before 2nd toll

booth?

Time to “push” entire

caravan through toll booth

onto highway = 12*10 =

120 sec

Time for last car to

propagate from 1st to 2nd

toll both:

100km/(100km/hr)= 1 hr

A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km 100 km

Introduction 1-28

Caravan analogy (more)

Cars now “propagate” at

1000 km/hr

Toll booth now takes 1

min to service a car

Q: Will cars arrive to 2nd

booth before all cars

serviced at 1st booth?

Yes! After 7 min, 1st car at

2nd booth and 3 cars still at

1st booth.

1st bit of packet can arrive

at 2nd router before packet

is fully transmitted at 1st

router!

See Ethernet applet at AWL

Web site

toll booth

toll booth

ten-car caravan

100 km 100 km

Introduction 1-29

Nodal delay

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

proptransqueueprocnodal ddddd

Introduction 1-30

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!

Experiment delay

Traceroute (XP)

Tracert (Linux)

Provide delay measurement from source

to router along end-end Internet path towards destination.

1-32

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

A

B

packet being transmitted

packet arriving to full buffer is lost

buffer (waiting area)

See applet examples

1-33

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, with file of F bits

to send to client

link capacity Rs bits/sec

link capacity Rc bits/sec

pipe that can carry fluid at rate Rs bits/sec)

pipe that can carry fluid at rate Rc bits/sec)

server sends bits (fluid) into pipe

1-34

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

link on end-end path that constrains end-end throughput

bottleneck link

1-35

Throughput: Internet scenario

10 connections (fairly) share backbone bottleneck link R bits/sec

Rs Rs

Rs

Rc

Rc

Rc

R

per-connection

end-end

throughput:

min(Rc,Rs,R/10)

in practice: Rc or

Rs is often

bottleneck

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

Introduction 1-54

Internet protocol stack

application: supporting network

applications

FTP, SMTP, HTTP

transport: process-process data

transfer

TCP, UDP

network: routing of datagrams from

source to destination

IP, routing protocols

link: data transfer between

neighboring network elements

PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

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