Operating Systems and Computer Networks L8 …...Operating Systems and Computer Networks L8 –Introduction to computer networks Prof. Dr.-Ing. Axel Hunger Alexander Maxeiner, M.Sc.

Operating Systems and Computer Networks

L8 – Introduction to computer networks

Prof. Dr.-Ing. Axel HungerAlexander Maxeiner, M.Sc.

Institute of Computer EngineeringFaculty of Engineering

University Duisburg-Essen

Dr.-Ing. Pascal A. Klein & Alexander Maxeiner M.Sc.

[email protected]

Alexander Maxeiner, M.Sc.University Duisburg-Essen

2Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering OSCN – Process Scheduling

ISO/OSI Model

Data Transmission

Network Transmission

Code efficiency

Transmission protocols

TCP/IP

Network topologies

Routing protocols

Routing algorithms

Overview of content



Computer networks consist of several autonomous devices interconnected through a series of wires, routing systems and/or wireless transmitters/receivers.

The origin of computer networks begin in the 1950’s. The American defense agency wanted a network capable of enduring a nuclear war.

Telephone services back then were organized like interconnected star networks.

Paul Baran of the RAND Corporation presented a mesh network as a solution to the problem.

AT&T was ordered to realize the network. But the didn’t want to.

History of networks I



In 1967 a network implemented at the National Physical Laboratory in GB presented a working design of connecting several computer systems at the campus.

The groundwork of this network then resulted in the ARPANET.

This network used IMP’s (Interface Message Processors) that stored the received data, divided them into packages and transmitted the data with 56 kbit/s.

The network was then realized and tested at several universities within the USA.

The experiment showed that ARPANET protocols were insufficient to connect multiple networks. TCP/IP was invented.

History of networks II



During the 80’s and 90’s several regional networks were connected to each others with the TCP/IP stack protokolls.

A computer that has an IP address and can send TCP/IP packages (and is physically connected to the network) is part of the internet.

At first this network was mainly for research, companies and governments.

CERN Physicist Tim Berners-Lee invented the WWW-application, that streamlined the transmission of data and in combination with the Mosaic-Viewer a GUI for websites were invented.

www.spacejam.com

History of networks III



Networks today:

LAN

Bluetooth

Modem

Network providers

Mobile Internet

WAN

Internet

Networks



Application on top level of computer starts a data request (browser)

Task is translated to lower level.

Data transmission protocols are activated, physical connection established, packages send/received.

Data received is processed, functions depending on data executed.

Tasks result in change of GUI -> Display is refreshed.

Connection closed.

Architecture



Example for the functions of networking

Visual example

Manager X wants to communicate with Manager Y

Secretary knows Address and

Department

English is taken as a common

understood language

Official

signature is

added

International Post-ZIP-Code added



Each layer does not know (need to know) how each other layer works.

Privacy between each layer. Work on a strict need-to-know basis.

Each layer can be changed due to technical inventions.

As long as there is an interface between each layer, they can be viewed as individual modules.

Necessary that each interface has established standards to re-translate the original data correct.

Analogy



Model -> Realisation

Department / Room = Port-Number

Page-Number = Sequence Number

House Number & Post-ZIP-Code

= IP-Address

ISO/OSI

ModelExample Protocols DoD Model



OSI Model = Open System Integration model.

Published 1984, first step to standardize several communication protocols.

Protocols are rarely used, was cast aside in favor of the TCP/IP routing protocols.

The model is still valid and based on:

– Knowledge of technology

– Human communication

– Link different systems in a transparent way

– Cover all fields from application to technical data

OSI Model



OSI layer model

data linkdata link

networknetwork

transporttransport

sessionsession

presenta-tion

presenta-tion

AppApp

physicalphysical

application protoccol

presentation protoccol

session protoccol

transport protoccol

network network

data link data link

1

3

2

4

5

6

7

bit

packet

frame

message

message

message

message

network (routing) protocol

Communication subnetA B

Unit

Data link protocol

physical physical



Layers & Interfaces

Layer 1Layer 1

Layer 2Layer 2

Layer 3Layer 3

Layer 4Layer 4

Layer 5Layer 5

physicalphysical

Layer 1 - protocol

Layer 5 - protocol

Layer 4 - protocol

Layer 3 - protocol

Layer 2/3-Interface

Layer 1/2-Interface

Layer 3/4-Interface

Layer 4/5-Interface

Layer 2 - protocol

Host 1 Host 2



Flow of information

M

MediaMedia

destinationSource

Layer 5 - protocol

Layer 4 - protocol

Layer 3 - protocol

L5

Layer 2 - protocol

Host 1 Host 2

Header 4

M1H4H3 M1H4H3

M1H4H3H2 T2

M1H4H3 M1H4H3

MHeader 4

M1H4H3H2 T2

L4

L3

L2



ARPANET connections between universities over telephone cables usually without problems, but satellite and wireless networks caused complications.

New reference was necessary to connect/disconnect devices more easy.

The new internet layer works on a package based network, where a constant connection is not necessary.

Data packages can be send to it’s destination without regards of correct order. The IP protocol is tasked with correct transmission.

The TCP (Transmission Control Protocol) disassembles the incoming bytes and relays them to the internet layer.

TCP/IP Model



Comparison OSI – TCP/IP

Comparision OSI/TCP

Routing

Transport

Session

Visualisation

L7 Application

Security

OSI

Bit-transmission

L6

L5

L4

L3

L2

L1

Internet

Transport

Not part of model

Not part of model

L7 Application

Host-to-network

TCP/IP

L6

L5

L4

L3

L2

L1



TCP/IP has no visualization- and session layers. Most applications do not need them.

Earlier inventions of the application layer consists of TELNET, FTP, SMTP.

Later inventions include DNS and HTTP.

Below the internet layer the information are rare.

The TCP/IP reference model only states a protocol must maintain a connection to the network.

With network access IP packages can be transmitted to its destination.

Those protocols are undefined in the reference model and depend on the network.

TCP Layers



OSI and TCP/IP reference models have several similarities. Both act on a stack of independent protocols.

Layers up to the transport layers serve the availability for a network independent end-to-end transport service.

The OSI model was designed before the protocols were designed. Inventions for broad networks stood in contrast to the model, therefore the model needed to be changed.

The TCP/IP model was designed after the protocols were established. It was therefore more adapt to its structure but cannot be used to describe non-TCP/IP networks.

OSI- & TCP/IP-Reference



The lowest level of the OSI-reference model consists of the bit-layer.

This layer handles the physical transmission of signals from one source to one destination.

At the next physical destination, the transmitted bits are re-translated into data for further processing.

A transmission from one computer to another through a network will be translated into bits, send, received by a routing device, retranslated into bytes and its information content used for further transmission until the final destination.

This delay during the transmission is known as the network latency.

Bit layer



During the 19th century a French mathematician named Jean-Baptiste Fourier proved that a periodically function within reasonable parameters and period T can be portrayed as a (maybe indefinitely) function of sine and cosine functions.

This discovery allows for data transmission the conversion of discrete into continuous signals and also with the knowledge of the harmonics of the signal the correct interpretation of a continuous signal for its discrete values.

The frequency range of the transmission channel defines its bandwidth.

The bandwidth of the channel defines the highest possible harmonic that can be transmitted,

Fourier



H. Nyquist discovered that even perfect channels can only transmit a certain number of bits.

Any signal wandering through a LPF of bandwidth H can only have 2H exact sampling values.

With V discrete values in each signal the Nyquist theorem results in:

𝑀𝑎𝑥, . 𝐷𝑎𝑡𝑎𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 = 2𝐻 log2 𝑉 𝐵𝑖𝑡𝑠

Meaning a 2kHz Signal can only transmit binary signals with 4000 Bits/sec.

This works only for noise free channels, as soon as there is noise interfering with the transmission medium the maximum transmission rate of all channels is limited by the SNR.

Transmission rate



Damping and propagation speed are dependent on frequencies therefor a signal should not include too many frequencies

Rectangular signals have a broad frequency spectrum and are therefor only suitable for short distances and low transmission rates.

For long range transmission AC-signals are used, that can be modulated by:

– Frequency

– Amplitude

– Phase modulation

Transmission methods



Frequency modulation

Frequency modulation – Source: Wikipedia

Still used in radio transmission



Amplitude mod

Amplitude modulation – Source: Wikipedia

Still used in radio transmission



Phasenmodulation – Source: Elektronik-compendium.de



Phase modulation allows the coding of carrier wavesto specific binary values.

Every phase shift of X-degrees from its original phase depicts another binary value.

Every transmission interval the frequency shift is calculated and the original binary value decoded.

In modern transmission devices a combination of several modulation methods are used.

QAM-16 for example uses 4 phase shifts and 4 amplitude shifts. This means in every period a 4 Bit symbol can be transmitted.

This allows for the transmission of 9.600 Bits on a 2.400 Baud cable. (Baud = Symbols per second)

Phase modulation



As soon as there is a connection available on the physical level and all transmission parameters are set, the connected devices need to talk to each others.

Usually one device sends on one channel and the other responds to the received data.

The response of the receiver consists usually of a data package containing a NAK (not Acknowledged) or an ACK (Acknowledged)

Three basic transmission methods are introduced here:

– Stop-and-wait

– Go-back-n

– Selective repeat

Handshakes



Stop-and-wait

:Host A :Host B



Go-back-n

:Host AData A

Data B

Data C

:Host B



Go-back-n 𝒕𝒐𝒖𝒕 error

:Host A :Host B

Data B

Data A

Data B

Data C

Data D



Selective repeat

:Host AData A

Data B

Data C

:Host B



Selective repeat 𝒕𝒐𝒖𝒕 error

:Host A :Host B

Data B

Data A

Data B

Data C

Data D



The Go-back-n and Selective repeat protocols are also so called ‘sliding window’ protocols. The data is stored in a buffer of the size n (window).

When a data set from the window is acknowledged it gets marked ‘ACK’. Then if the first Data set within the buffer is marked ACK the window slides further, deleting the data and adding the next package from the pipeline.

In go-back-n only the first package within the window can be marked ACK.

In selective repeat the window slides through all ACK frames until it reaches an unacknowledged.

More @ the exercises.

Sliding window



Questions?



Tanenbaum, Andrew S., “Computer Networks”, 4th edition, Pearson Education Inc, Amsterdam, Netherlands, 2003.

Tanenbaum, Andrew S., “Computernetzwerke”, 4th edition, Pearson Education Inc, Amsterdam, Netherlands, 2003.

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Operating Systems and Computer Networks L8 …...Operating Systems and Computer Networks L8 –Introduction to computer networks Prof. Dr.-Ing. Axel Hunger Alexander Maxeiner, M.Sc.

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