Introduction to Computer Networks Slides For Week 1.

Introduction to Computer Networks

Slides For Week 1

Course Goals

Understand the basic principles of computer networks

Understand the Internet and its protocols Understand the key design principles used to build

the Internet Experience building network systems Course is not about specific skills

E.g. configure a router from company X vs. learn principles of how all routers work

Success means you are confident to tackle a range of network programming, design and maintenance.

2 Introduction to Computer Networks

Networks Fundamentals

Why Study Networks?

Integral part of society Work, entertainment, community

Pervasive Home, car, office, school, mall …

Huge impact on people and society Person-to-person and group communication

email, blogs, chat Form and strengthen communities

chat rooms, newsgroups ... Commerce, services, entertainment, socializing


Concepts for this week

What is the Internet? What is a network? Core and Edge of the Internet Circuit, message and packet switching

Layering and encapsulation Network delay analysis


What is the Internet?

What is a network?

Lets start at the beginning…

7

What is a network for? To allow two or more endpoints to communicate Is Cable T.V. a network?

– yes.

What is a computer network? Nodes connected by links Nodes are computers:

Processing ability Receive and transmit information (unlike televisions).

Is this a Network?

Yes, not an interesting one..


Other Networks (network topologies)

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Ring Star (Hub and Spoke)

Hierarchical Broadcast


Data Networks

9

Data networks are Packet Switching Networks Data is carried in packets and is switched between

routers Every packet carries its destination address and is routed

according to current network conditions. There is no fixed route between every pair of end points.

Telephone networks are Circuit Switched Networks Voice is carried over a predetermined circuit between

two phones. The route is established before any of the data is sent, and

then the data is sent (like a phone call – you first call, have a connection, and then talk).

Data is also carried (e.g., fax). But it is not a data network!


Data Types

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Non-Isochronous Isochronous

Examples: mail, web files, P2P traffic, etc.

Examples: voice, live broadcast

Arrival rate is undetermined, bursty. Jitter (variation of delay between subsequent packets) is unknown, and not important.

Arrival rate is predetermined and jitter should be fixed. E.g., packet every 0.25 msec.

Delay is possible. For example, your email cab be delayed by a few seconds.

Delay is critical and bounded. Maximal possible delay is ~ 30-70 msecs.

Packet loss can be taken care of by the network, and missing packets can be retransmitted, so that the receiving side receives the complete data sent.

Packets lost cannot be resent due to the maximal delay limitation.

Ideal for packet switching Ideal for circuit switching


The Internet

11

Built initially for non-isochronous data Hence, a packet switching architecture.

Today we see that the world moves towards integration of networks: carry data, voice, TV over the Internet. Termed: everything over IP

(We will see later what that means)

Why not use circuit switching (telephone network)? There was a big debate, of course, but telephone

networks are less efficient for routing bursty data (Internet data).

In this course we study a packet switching network:The Internet.


What is Internet Technology? What is an internet?

Network of networks What is the Internet?

A global internet based on the IP protocol

To what does “Internet technology” refer? Architecture

Services We will learn the protocols which enable

the services, and understand the architecture it is built over.

Protocols


Architecture-wise

Host: Machine running user application

Media: Physical process used (copper wire, fiber optics, satellite link)

Company A

Company B

Internet Service Provider 1

ISP 2


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Architecture-wise

Network : Collection of interconnected machines or interconnected LANs (subnetworks)

Router: decide where to send data next

Company A

Company B

Internet Service Provider 1 (ISP 1)

ISP 2


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Architecture-wise

Company A

Company B

ISP 2

Core Networks (ISP tiers)•Tier 1: Biggest ISPs •Tier 2 and 3: Regional and very small.

Internet Service Provider 1 (ISP 1)

Edge Networks: Companies, organizations with a “default route” Introduction to Computer Networks

Networks Today

16

Default Route Networks: Campus, Company

Local Area Networks (LANs) for labs, or certain floors

Virtual LANs

Edge Networks:

Small Internet Service Providers

Metropolitan

Core Networks:

Wide Area Network

The Internet: Core + Edge Networks


Networks Legendary Scaling

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Inter-processorDistance

Processors LocatedIn Same

1 m Square meter Personal Area Network

10m

100m

1km

Room

Building

Campus

Local Area Network

10 km City Metropolitan Area

Network

100 km

1000km

Country

ContinentWide Area Network

10000 km Planet The Internet


Local Area Network (LAN)

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Limited Size Run at speeds of 10Mbs, 100 Mbs, 1Gbps, 10

Gbps Legend topology: Ethernet (broadcast) Example: PC farm with servers and printers. Today:

Switched computers and nodes.


Service-wise (applications) Electronic mail Remote terminal File transfer Newsgroups File sharing Resource distribution World Wide Web Video conferencing Games


Enabling A Service

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The user, at home, is requesting page www.mta.ac.il The network (Internet) should figure:

1. Which server is www.mta.ac.il? (all computers use numbers. Most people seem to prefer names).

2. Which Route to use for user’s request?3. What format to use for sender request and receiver’s

reply? (maybe data should be encrypted) 4. What to do if the message gets corrupted along the

way?5. What to do if a routing computer fails along the way?6. Maybe the server restricts access to this page, is this

user ok?7. What if the page content was moved to another server,

or was deleted?8. What if user logged off or pressed “refresh” before

transmission was completed? ….Introduction to Computer Networks

Protocols The Internet uses the TCP/IP protocol suite

Architecture Service Rules of communication Divided to layers

…

FTP HTTP RTP TFTP

TCP UDP

IP

Ethernet 802.11 PPP

CAT-5 Single-ModeFiber

RS-232


Layering

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An important and basic principle in computer networks

Divide the work hierarchically: each layer is a separate entity that maintains its part.

Application

Transport

Network

Host-to-Net *(Link layer)

Host

The application layer receives or transmits the data to/from the application

The Transport layer is responsible to packetize the data and transmit it without errors to the end host

The Network layer is responsible to choose the next hop(a neighbor network node) and route the packets to it

The Host-to-net layer is responsible to send the data over the attached physical link in its format and * host-to-net layer is also called the link layer


TCP/IP Layering Architecture

Application Protocolend-to-end communication

Transport Protocol (TCP)end-to-end communication

ApplicationLayer

TransportLayer

NetworkLayer

Link Layer

Host A Host B

ApplicationLayer

TransportLayer

NetworkLayer

Link Layer

NetworkLayer

Link Layer

NetworkLayer

Link Layer

IPIP IP


Layering and Encapsulation

24

Use layers to hide complexity, each layer implements a service:

Layer N uses service provided by layer N-1 layer N-1 provides a service to layer N

This is done using encapsulation: Treat the neighbouring layer’s information as a

“black box”, can’t look inside or break message Sending: add information needed by the current

layer “around” the higher layers’ data headers in front trailers in back

Receiving: Strip off headers and trailers before handing up the stack


Layering (cont.)

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data

dataTransport layer header

Network layer headerdata

Transport layer header



Host-to-net layer header

At the sending host

A Packet

A Frame

Host sends the frame on the attached linkIntroduction to Computer Networks

Layers (cont.)

27

At the receiving host The frame arrives on the attached link

data

dataTransport layer header





Host-to-net layer header

Data is delivered to the application at the receiving hostIntroduction to Computer Networks

ISO OSI Layering Architecture –The reference model

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Application Protocol

Transport Protocol

Presentation Protocol

Session Protocol

Host A Host BApplication

Layer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Router Router28 Introduction to Computer Networks

ISO’s Design Principles

A layer should be created where a different level of abstraction is needed

Each layer should perform a well-defined function

The layer boundaries should be chosen to minimize information flow across the interfaces

The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy


Who is who?

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ISO - International Organization for Standardization ISO 9000, etc.

IETF – The Internet Engineering Task Force: Near term, standardized oriented group, divided into disciplines such as: applications, routing, addressing, etc.

IRTF – The Internet Research Task Force: Pursue long term research projects

RFCs: Requests For Comment Some become official standards http://www.rfc-editor.org Look for “aviation over IP” (aka birds)…


Host-to-net Layer

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Receives a data packet from the network layer

Creates a frame of bits Signal the start and end of a frame (usually by a

string of known bits)

Add error correcting codes to enable transmission even on noisy channels

Collision detection and retransmissions

What is the time it takes to transmit and receive a frame on a link?


Link Delay

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Two factors

Propagation delay = Distance / Speed of light Time for bits to “move across wires”

Transmission delay = Packet size / BandwidthTime to “get bits on wires”


Transmission vs. Prop. delay

A single transmission link as a water pipe

The thicker the pipe, the more water it can carry from one end to the other in each unit time

Water is carried from one end of the pipe to the other at constant speed, no matter how thick the pipe is

Water = Data bits

Thickness of the pipe = Channel capacity (bandwidth)

Speed of water through the pipe = Propagation speed (light speed)


Transmission vs. Prop. Delay (cont)

pipe

1. Propagation delay is how long takes to cross the pipe, irrespective of volume

2. Transmission (bandwidth delay) is related to how much water can be pushed in through the opening per unit time


1500 x 8 bits

Transmission TimeHow long does it take A to transmit an entire packet on the link?

Relevant information: packet length = 1500 bytes channel capacity = 100

Mbps

Another way to ask this question:If the link can transmit 100 million bits in a second, how many seconds does it take to transmit 1500 bytes (8x1500 bits)?

100 Mbits

1 sec=

t

Solving for t… t = 0.00012 sec (or 120 sec)


Propagation Delay

How long does it take a single bit to travel on the link from A to B?

Relevant information: link distance = 500 m prop. delay factor = 5 sec/km

Another way to ask this question:If it takes a signal 5 sec to travel 1 kilometer, then how long does it take a signal to travel 500 meters?

5 sec

1000 m=

500 m

t Solving for t…t = 2.5 sec


Units Bits are the units used to describe an amount of data in a

network 1 kilobit (Kbit) ~1 x 103 bits = 1,000 bits 1 megabit (Mbit)~1 x 106 bits = 1,000,000 bits 1 gigabit (Gbit) ~1 x 109 bits = 1,000,000,000 bits

Seconds are the units used to measure time 1 millisecond (msec) = 1 x 10-3 seconds = 0.001 seconds 1 microsecond (sec) = 1 x 10-6 seconds = 0.000001 seconds 1 nanosecond (nsec) = 1 x 10-9 seconds = 0.000000001 seconds

Bits per second are the units used to measure channel capacity/bandwidth and throughput bit per second (bps) kilobits per second (Kbps) megabits per second (Mbps)

Bytes are units that describe a series of eight bits 1 Byte = 8 bits Bytes per second (Bps)


Processing Delay Stylized format required to send data Analogy: adding and removing envelopes to

letters How long does it take to execute all these

layers?

ApplicationLayer

TransportLayer

NetworkLayer

Host-to-Net Layer

Host

Why is this time important?

NetworkLayer

Host-to-Net Layer

Router

ApplicationLayer

TransportLayer

NetworkLayer

Host-to-Net Layer

Host


Example

Protocol Processing Time = 40 sec packet length = 1500 bytes

channel capacity = 100 Mbpspropagation delay factor = 5 sec/km

A B

500 m

1. How long to format the data?

2. How long does it take a single bit to travel on the link from A to B?

3. How long does it take A to transmit an entire packet onto the link?


Timeline Method

Time

Protocol Delay

Transmission time

Propagation delay

Protocol Delay

Host A Host B

40

2.5

120

40

1st bit

last bit

Total time: 40+120+2.5+40 = 202.5 sec


Introduction to Computer Networks Slides For Week 1.

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