lecture data communication and internet · pdf filechapter 1: introduction page 1 data...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 1: Introduction

Data Communication and Internet Technology

Lehrstuhl für Informatik IVRWTH Aachen

Prof. Dr. Otto Spaniol

Dr. rer. nat. Dirk Thißen



Page 2Chapter 1: Introduction

• In principle, every 14 days

• Exercise is given on Tuesday• Frontal exercise

• Exact dates depend upon thelecture dates

Exercises

October 26th 2004

November 2nd 2003

November 16th 2003

Organization

Lecture• Lecture takes place on Thursday, 10:00 – 11:30

and 13:45 - 15:15

• The lecture is planned with 3 hours / week• Not each date is needed, some are skipped

• First lecture dates are planned, the further dates are announced in time

October 14th 2004

October 21th 2004

October 28th 2004

November 4th 2004

November 11th 2004




Organization

Dr. rer. nat. Dirk Thißen

Lehrstuhl für Informatik IV, RWTH AachenAhornstraße 55, 52074 Aachen

Room 4226Phone: 0241 / 80 - 21450

eMail: [email protected]

Contact Information for questions regarding lecture/exercises

http://www-i4.informatik.rwth-aachen.de/content/teaching/lectures/sub/datkom/WS04-05/index.html

Slide Copies

At the end of winter termWritten Exam

• Copies to the lecture slides as well as exercise sheets are placed on the web page to the lecture:




1. Introduction

• Networks and Network Topologies• Communication Protocols

2. Computer Networks

• Network principles

• Network Components (Cables, Repeaters, Hubs, Bridges, Switches, Routers)• Local Area Networks (Ethernet, Token Ring, FDDI, DQDB)

• Wide Area Networks (Frame Relay, ATM, SDH)• Wireless Networks (WLAN)

3. Internet Protocols

• Internet/Intranet: the TCP/IP Reference Model

• Network protocols (the Internet Protocol IP)• Transport protocols (TCP and UDP)

4. Application Protocols in the Internet

• Higher protocols (FTP, HTTP, E-Mail, ...)

Content




Literature

• A.S. Tanenbaum: Computer Networks. 4th Edition, Prentice Hall, 2002.

• J.F. Kurose, K.W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet. Addison-Wesley, 2002.

• Cisco Systems: Internetworking Technologies Handbook. 3rd Edition, Cisco Press, 2001.

• J. Schiller: Mobile Communications. 2nd Edition, Addison Wesley, 2003.




Data communication is the processing and the transport of digital data over connections between computers and/or other devices

(generally over large distances)

Computer Networks

→ How to connect several computers?

→ Which media can be used for data transport?

→ How to represent digital data on the medium?

→ How to coordinate the access of several computers to the medium?

Communication Protocols (Internet Technology)

→ Design of uniform data units for transfer

→ How to achieve a reliable and efficient transfer?

Data communication comprises two topical areas:

Data Communication




The „driving power“ for the enormous growing importance of data communications:

• Continuously decreasing costs for hardware...

• ... while computing power is increasing.

Example for comparison:

• A PC today costs less than € 1.000,-• It has more computing power than a 10 years old mainframe

• It contains more than 100 Million transistors• A comparable number of other components would be

prohibitively expensive – e.g. 100 Million sheets of paper would cost more than € 50,000,-.

Computing power is very

cheap

Evolution of Data Communication




Increasing computing power leads to new possibilities in data processing:• Speech processing

• Image processing• Multimedia authoring

• Video conferencing• ......

Increasing system diversity

Increasing number of applications and users

Wide range of usage: offices, factories, at home, …

Applications




Sharing resources lowers costs

• Access to foreign resources by communication networks to achievereasonable usage

• Essential:

Efficient methods to share data between the components of a distributed system

• Procedures for efficient interworking(CSCW = Computer Supported Cooperative Work)

• Agreements for shared usage of devices which are too expensive to buy for one single organization and/or have no use for the total capacity

Example for interworking of two parties: Client/Server principle

Reducing Costs




Client Server

ClientProcess

ServerProcess

Request

Reply NetworkNetwork

→ Cost reduction→ Better usage of resources

→ Modular extensions→ Reliability by redundancy

Advantages

The Client/Server Principle




Client/Server Systems

Client Server

WWW Browser WWW Server

eMail Program Domain Name System(DNS)

FTP Client FTP Server

Examples for Client/Server systems

ServerProgram (process) which offers a service over a network. Servers receive requests and return a result to the inquiring party. The services offered include simple operations (e.g. name server) or a complex set of operations (e.g. web server).

ClientProgram (process) which uses a service offered by a server.




Another principle: Peer-to-Peer

• Equal partners, no fixed client and server roles

• Connections between any pair of computers

• Establishment of a whole network of connections

• Best example: File Sharing, e.g. Napster, Gnutella




• Eventually dubious or forbidden contents

• Responsibility

• Juridical aspects (legislation)

• Potential censorship?

• Control over the productivity of employees,

of the whereabouts of people

• Annoyance through anonymous or unwanted messages (SPAM)

• ......

Communication networks enable a faster and cheaper exchange/distribution of information. There is however a large number of social, ethnical, cultural, juridical, ... side effects.

Non-technical aspects




Computer Networks




First Generation Computer Networks

MainframeOperator

PeripheralsTerminals

Rest of the world

Computing Center

Terminals

MultiplexerDemultiplexer

Telephone lines




Introduction of Local Area Networks

MainframeOperator

PeripheralsTerminals

Computing Center

Fixed lines

Router

Building C

Building B

Building A

Rest of the world




Global Networking

Mainframe

Peripherals

Computing Center

Fixed lines,

Router

Building A

Router

Switch

ClientsLocal

Server

Building B

Router

Switch

ClientsLocal

Server Switch

Router Server Network and system administrator

Backbone

Rest of the world

(Internet)

ISDN, Provider ...




Classification of Networks

Point-to-Point Network• A pair of computers is directly connected by one cable

Broadcast Network• One-to-all (e.g.: radio, television)• All connected stations are sharing one transmission channel• For ensuring that the data are sent the correct receiver, they have to

marked with the destination address of the receiving computer• Data are being packed into packets with the Unicast Address of the

receiver

• Every computer connected controls each received packet for its destination address. Only the addressed computer processes the data, all others are simply deleting them.

• To address all connected stations at once, so-called Broadcast Addresses are used




Classification of Networks

Classification by Distance

1 m

10 m Room

100 m Building

1 km Campus

10 km Town

100 km Country

1000 km Continent

10000 km Planet

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area Network (WAN)

Personal Area Network (PAN)

Internet




• Communication infrastructure for a restricted geographical area (10 m up to some km)

• Usually maintained by one local organization

• Linked are PCs/Workstations/...., for exchanging information and sharing peripherals and resources

• Transmission capacity up to 1,000 Mbit/s

• Transmission delay of a message in the range of milliseconds (~10 ms)

• Simple connection structures (“Simple is beautiful”)

LAN

Topologies

• Bus

• Star

• Ring

• Tree

• Meshed network

Local Area Networks




Bus

• Broadcast Network: if station A intends to send data to station B, the message reaches all connected stations. Only station B processes the data, all other stations are ignoring it.

- (+) Passive coupling of stations

- Restriction of the extension and number of stations to connected

+ Simple, cheap, easy to connect new stations

+ No choose of path to target (= routing) necessary

+ The breakdown of a station does not influence the rest of the network

Example: Ethernet

LANs: Bus

Terminating resistor

B

A

Ω Ω




Star

• Designated computer as central station: a message of station A is forwarded to station B via the central station

• Broadcast network (Hub) or point-to-point connections (Switch)

– Expensive central station

– Vulnerability through central station (Redundancy possible)

+ Definite path, no routing

+ N connections for N stations

+ Easy connection of new stations

Example: Fast Ethernet

LANs: Star

B

A




Tree

• Topology: Connection of several busses or stars

• Branching elements can be active (Router) or passive (Repeater)

+ Bridging of large distances

+ Adaptation to given geographical structure

+ Minimization of the cable length necessary

Backbone

Branch 1 Branch 2

Repeater Router

A

B

D

C

LANs: Tree




Ring

• Broadcast Network

• Chain of point-to-point connections

• Active stations: messages are regenerated by the stations (Repeater)

– Breakdown of the whole network in case of failure of one single station or connection

+ Large extent possible

+ Easy connection of new stations

+ Only N connections for N stations

• Variant: bidirectional ring

stations are connected by two opposed rings

LANs: Ring

Example: Token Ring, FDDI

B

A




Fully Meshed Network

• Point-to-Point connections between allstations

– For N stations, connections are needed

– Connecting a new station is a costly process

+ No routing

+ Redundant paths

+ Maximal connection availability through routing integration

Partly meshed network: cheaper, but routing, flow control

and congestion control become necessary (Wide Area Networks)

2

)1( −NN

LANs: Meshed Networks




Ethernet (IEEE 802.3, 10 MBit/s)- originally the standard network- available in an „immense number“ of variants

Token Ring (IEEE 802.5, 4/16/100 MBit/s)- for a long time the Ethernet competitor- extended to FDDI (Fiber Distributed Data Interface)

Fast Ethernet (IEEE 802.3u, 100 MBit/s)- at the moment the most widely spread network - extension of Ethernet for small distances

Gigabit Ethernet (IEEE 802.3z, 1,000 MBit/s)- very popular at the moment; 10 GBit/s are already in the

planning phase at the moment

LANs: Examples




• Designed for larger distances than a LAN, usage e.g. in a whole town

• Similar technologies as in a LAN• In general, only 1 or 2 cables without additional

components• Difference to LANs: Time slots

MAN

Metropolitan Area Network (MAN)

Example: Distributed Queue Dual Bus (DQDB, IEEE 802.6)




• Bridging of any distance• Connects LANs and MANs over large distances

• Irregular topology, based on current needs• Consists out of stations which are connected through point-to-point with

each other• Mostly quite complex interconnection of subnetworks which are owned by

independent organizations

WAN

Router

HostLAN

Wide Area Network (WAN)




Wireless Networks

• System Interconnections (PANs)• direct connection between the components

of a computer (Example: Bluetooth)

• Wireless LANs• Communication of computers connected by a base

station (Access Point) in a local area, or directconnection between computers

(Example: IEEE 802.11 Wireless LAN, WLAN)• Range of 10 – 100 meters

• Transmission capacity of up to 100 MBit/s

• Wireless MANs/WANs

• Common are telecommunication networks like GSM.• Range of several kilometers („worldwide")

• Transmission capacity below 1 MBit/s• IEEE WirelessMAN (IEEE 802.16) as MAN for data transmission




Institute of Electrical and Electronic Engineers - IEEE

• Standardization e.g. of the IEEE 802.X-Standards for Local Area Networks www.ieee.org

Standards Organizations - IEEE

• 802.1 Overview and Architecture of LANs

• 802.2 Logical Link Control (LLC)

• 802.3 CSMA/CD („Ethernet“)

• 802.4 Token Bus

• 802.5 Token Ring

• 802.6 DQDB (Distributed Queue Dual Bus)

• 802.7 Broadband Technical AdvisoryGroup (BBTAG)

• 802.8 Fiber Optic Technical Advisory Group (FOTAG)

• 802.9 Integrated Services LAN (ISLAN) Interface

• 802.10 Standard for Interoperable LAN Security (SILS)

• 802.11 Wireless LAN (WLAN)

• 802.12 Demand Priority (HP’s AnyLAN)

• 802.14 Cable modems

• 802.15 Personal Area Networks (Bluetooth)

• 802.16 WirelessMAN




Communication Protocols




To enable understanding in communication, all communication partners have to speak the same „language“.

→ Data formats and their semantics

→ Control over media access→ Priorities→ Handling of transmission errors

→ Sequence control→ Flow control mechanisms

→ Segmentation and composition of long messages

→ Multiplexing→ Routing

A protocol is defined as the whole set of agreements between

application processes with the purpose of a common communication

Why Protocols?




Implementation of Protocols

Solution 1:

Write one large „Communication Program“ which fulfills all requirements needed

to establish a communication process.

• Advantage: efficient data exchange for a given application.

• Disadvantage: No flexibility! Adoptions require large efforts.

Solution 2:

Write a set of small programs specialized to special tasks of the communication

process. For each application, the needed programs can be combined.

• Advantage: Very flexible, since single components can be exchanged.

• Disadvantage: Fixed structures of program interworking; adds more complexity

and overhead.

Accepted today: solution 2.

The implementation takes place in layer models.




Example: Exchange of ideas between philosophers

Philosopher A

Language: Chinese

Interpreter A

additionally: English

Recognizes single characters and sends

them in Morse

Network

Thoughts about world politics

Uninterpreted sentences,

i.e. no knowledge about politics

Uninterpreted characters

in correct order

Electrical signals

Technical Expert A

Language: Chinese

Philosopher B

Language: Spanish

Interpreter B

additionally: English

Recognizes single characters and sends in

them in Morse

Technical Expert B

Language: Spanish




Standardization

Indispensable for the area-wide practical use of communication systems:

• Consequence: Standardization processes are very slow (due to many, often non-technical reasons).

• On the national as well as the international level!

Complex technical problems have to be solved

The involved parties, e.g. companies are often working against each other

Confidentially restrictions hinder the information flow

• Successful standardization is quite difficult due to:

Standardization




International Standards Organization - ISO

• Organisation, which is working on a volunteer basis (since 1946).• Members: standards organizations in approx. 90 countries

• Deals with a very broad range of standards

• 200 Technical Committees (TC) for specific tasks (e.g. TC97 for computer and information processing)

• TCs consist of subcommittees comprising in turn several working groups

• Interworking with ITU-T regarding telecommunication standards, (ISO is a member of ITU-T).

• Pioneering work of ISO regarding data communication: the ISO/OSI reference model

• Notice: only the concept is pioneering – not the products developed from those concepts!

www.iso.ch

Standards Organizations - ISO

(OSI: Open Systems Interconnection)




Layer 5 and 6 are rarely being implemented

Application7

Presentation6

Session5

Transport4

Network3

Data Link2

1

Common services for the end user

Network-independentend-to-end data transfer

Addressing androuting of “packets”

Securing of “frames”;Flow Control

Physical Signal representation, character transmission

Criticism of the model:

7 layers:

Transmission medium („Layer 0”)

Generally to much overhead – some details are unnecessary, some are overloaded

Reduce the complexity of a communication process (all details to be considered) through layers.

The ISO/OSI reference model




Layer Tasks

1. Physical layerThis layer is responsible for transmitting single bits over the medium. Signal representation is defined here to ensure that a sent „1“ is understood by the receiver as „1“. For this, e.g. on a copper cable it is defined, which voltage is used to represent a „1“ resp. a „0“ and how long this voltage has to be for one bit.Moreover details are being defined like the type of cables, meaning of pins of network connectors, transmission direction on the cable (uni-/bidirectional), …

2. Data Link LayerEnsures an error-free data transmission between two neighbored hosts (e.g. in a sub-network). Therefore the incoming data are segmented into so-called frames which are being transmitted separately. The receiver, which identifies the start and the end of a frame e.g. with a bit pattern, checks if the transmission has been correct (e.g. with the help of a checksum). Additionally, flow control is used to control the re-transmission of corrupt frames and protect the receiver from overload.An additional task in broadcast networks is the control of medium access, i.e. the stations are coordinated in some way to prevent from access conflicts.




Layer Tasks

3. Network LayerThis layer is responsible for the data transmission over larger distances and between heterogeneous sub-networks. The main task is (worldwide) uniform addressing of hosts and choosing a path through the whole network (routing). A necessary pre-requisite for doing so is among other things a common address range and an agreement about a maximum size of the transferred data units. Intermediate stations (the routers) manage tables with routing information and use the uniform addresses to make a decision about the best path to the receiver.

4. Transport Layer (ISO/OSI)Layer 4 manages end-to-end communication between two processes. It is responsible for ensuring that the received data are complete and in correct order. For this, again flow control is used (sequence numbers, acknowledgements) to detect missing or wrong ordered data units. Beneath this, the current network state is considered to not only adapt to the receiver, but to the network capacities as well.

Addressing is a topic here as well. On the transport layer, a single communication process on receiver side is addressed.




Layer Tasks

5. Session LayerThis layer (like the transport layer) manages reliable data transport between the computers. However also additional services are being offered, like e.g. the possibility for dialogue control. I.e. it can be defined in which direction the transmission can take place.

Closely related with this topic is the token management which also belongs to level 5. During the transmission so called tokens can be exchanged. With certain operations only the communication partner which owns the token is allowed to conduct the operation.

Token management is also used here for other purposes, i.e. a set of tokens exist to coordinate several operations. One important operation is to set synchronization points in the communication process, to restart the transmission at the point it has ended in case of a connection loss.




Layer Tasks

6. Presentation Layer

The task of this layer is to display the data to transmitted that way, that they can be handled from a lot of different systems. So computers code a string with ASCII characters, others use Unicode, some for integers the 1-, other the 2-complement. Instead of defining a new transmission syntax and –semantics for every application, it is tried to provide a universally valid solution. Specific data are encoded in an abstract (and commonly recognized) data format before the transmission and are being translated back by the receiver into its own personal data format.

7. Application Layer (ISO/OSI)

In this layer (standard-) protocols are being provided which can be used from a whole set of applications/systems. One example is file transfer. On the application layer a universally valid protocol including an interface of file transfer is being provided. For systems from different manufacturers only the link-up into the local file system has to be realized. Other examples are file transfer, e-mail, remote operations etc.




Interplay between the Layers

Layer (n-1) Layer (n-1)

Layer n Layer nn-PDU

DataH

(n-1)-PDUH: Header, e.g. control information of the layer

• Layer (n-1) offers its functionality to the above lying layer n as a communication service.

• Layer n enhances the data to be sent with control information (Header) and sends the data together with the header as Protocol Data Units (PDU).

• Two communication partners on layer n exchange PDUs by using the communication service of the nearest lower lying layer (n-1).

• For layer (n-1), these PDUs are the data to be transmitted.




The whole Communication Process

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application process

Data

DataH

A-PDUH

P-PDUH

S-PDUH

T-PDUH

N-PDUH

Transmission mediumBit stream

T

Application process

Application Layer




The Communication Process

• Not necessarily a one-to-one mapping between layers

• Depending on the protocol, n-PDUs can be segmented into several (n-1)-PDUs before transmission:




Physical Layer

Data Link Layer

Network Layer

Physical Layer

Data Link Layer

Network Layer

Host A Host BRouter A Router B

Transport Protocol

Session Protocol

Presentation Protocol

Application Protocol

The OSI Reference Model in the Network

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application process

Application Layer

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application process

Application Layer

Internal Protocols




Standards Organizations - IETF

Internet Engineering Task Force - IETF

• Forum for the technical coordination of the work regarding Arpanet, the precursor of the Internet (since 1986).

• Evolution to a large, open, and international community of administrators, vendors and researchers.

• Works on evolution of the Internet architecture and the smooth operation of the Internet.

• Several working groups on Internet protocols, applications, routing, security, …

• Standard draft proposals can become a full standard only if an implementation of the proposal is successfully tested at two independent locations for at least four month.

• Result of such a standardization process: the resounding success of the Internet protocols TCP/IP

www.ietf.org




The TCP/IP Reference Model

Don´t exist

Host-to-Network Layer

Application Layer Application Layer

Presentation Layer

Session Layer

Transport Layer Transport Layer

Network Layer Internet Layer

Data Link Layer

Physical Layer

ISO/OSI TCP/IP




The Tasks of the TCP/IP Layers

Host-to-Network Layer (corresponds to ISO/OSI 1-2)Not defined exactly. The design does not matter, it is only defined that a host must be connected to the network via a protocol in a way that it is able to send and receive IP datagrams. The protocol design is left over to other standards to cover heterogeneous networks of all kinds.

Internet Layer (corresponds to ISO/OSI 3)The term Internet refers here to the interworking of different networks, therefore not on the Internet itself. The protocol enables communication between hosts over the own network borders. In the Internet, the transmission is connectionless, meaning that the data are segmented into packets which are addressed and sent independently into the network. On each network border, a router takes over the forwarding of the packets. The choice of path can be dynamic, depending on the current network load. As a result, single packets can get lost by overload situations or received in wrong order. Such faults are not handled (this task is left over to the transport layer).In contrast to ISO, only one packet format is defined, together with a connectionless protocol, the Internet Protocol (IP).




The Layers of TCP/IP

Transport Layer (corresponds to ISO/OSI 4)This layer covers the communication between the end systems. To adapt to different applications, two protocols are defined.

TCP (Transmission Control Protocol) is a reliable, connection-oriented protocol to protect the transmission of a byte stream between two hosts. The byte stream is segmented to fit into IP packets. On the receiving side the packets are re-assembled in the original order with the purpose of restoring the original data stream. It also includes flow control to adapt to the receiver‘s capabilities and to overcome the faults caused by the connectionless IP. UDP (User Datagram Protocol) is an unreliable and connectionless protocol („best effort“). No error correction is integrated, thus the transmission is used when the speed of the data transmission is more important than the reliability (speech, video).

Application Layer (corresponds to ISO/OSI 7)This layer defines common communication services. This comprises TELNET (remote work on another computer), FTP (file transfer), SMTP (electronic mail), DNS („phonebook“ for the Internet), HTTP (used for World Wide Web), etc.




1. Time

The TCP/IP protocols were already widely used before OSI had finished the standardization activities.

3. Complicatedness

Very high and partly unneeded expense in the OSI specification (thousands of pages of specification descriptions).

By the wish to consider all special cases, lots of options were included, making the products lavish, unhandy, and for too expensive - “The option is the enemy of the standard”!

2. Freedom from obligation

A „reference model“ like OSI is free from obligation. It only defines whatis to be done, but not how to do it. Result: incompatibility of products.

OSI vs. TCP/IP




5. Hurriedly product implementation

The first OSI products were implemented too fast (driven by the success of TCP/IP protocols), were covered with faults, and had an overall low performance.

In contrast, the “theoretically far more unmodern“ TCP/IP protocols were continuously modified and improved. They were of a high quality level and successfully tested before deployment and cheap to buy due to high production numbers.

4. Political reasons

OSI was dominated too much by Europe – especially from the national telecommunication companies which had lucrative monopolies. The real market power was in the USA – nobody was interested in OSI over there.

OSI vs. TCP/IP

lecture data communication and internet · pdf filechapter 1: introduction page 1 data...

Documents