Data Communication and Internet Technology
Transcription
Data Communication and Internet Technology
Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Organization Exercises to the lecture • • • • Data Communication and Internet Technology Fortnightly Thursday 15:45 – 17:15 h Lecture hall AH 5 Presence exercise Note: first exercise date: November, 8th The dates for the following exercise hours are announced in the first exercise hour resp. on the lecture‘s web page Material (Slide copies, exercise sheets) http://www-i4.informatik.rwth-aachen.de/content/teaching/lectures/sub/datkom/WS07-08/index.html Lehrstuhl für Informatik 4 RWTH Aachen Written exam At the end of winter term Contact information Prof. Dr. Otto Spaniol Otto Spaniol / Dirk Thißen Lehrstuhl für Informatik 4 Phone: 0241 – 8021400 / 8021450 E-Mail: {spaniol, thissen}@informatik.rwth-aachen.de Dr. Dirk Thißen Chapter 1: Introduction Page 1 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Chapter 1: Introduction Page 2 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Content Literature and Related Courses 1. Introduction • A.S. Tanenbaum: Computer Networks. 4th Edition, Prentice Hall, 2002. • Communication Protocols • Computer Networks 2. Computer Networks • Network principles • Network topologies and components • Local Area Networks (Ethernet, Token Ring, Token Bus, FDDI, DQDB) • Wide Area Networks (Frame Relay, ATM, SDH, ISDN/DSL) 3. Internet Protocols • Internet/Intranet: the TCP/IP Reference Model • Network protocols (the Internet Protocol IP, Routing protocols) • Quality of Service in the Internet • Transport protocols (TCP and UDP) • J.F. Kurose, K.W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet. 3rd Edition, Addison-Wesley, 2005. • Cisco Systems: Internetworking Technologies Handbook. 3rd Edition, Cisco Press, 2001. Related courses: • Mobile Communications (starting Monday, 22th, 16:30 in AH 5) 4. Application Protocols in the Internet • Higher protocols (FTP, HTTP, E-Mail, ...) Chapter 1: Introduction Page 3 Chapter 1: Introduction Page 4 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Data Communication Computing Power is cheap... Data communication is the processing and the transport of digital data over connections between computers and/or other devices (generally over large distances) Data communication comprises two topical areas: • Today, “everybody” has a computer (at work as well as privately) • Possible applications: file sharing, efficient interworking (CSCW = Computer Supported Cooperative Work) • And: Sharing resources lowers costs Computer Networks Access to foreign resources by communication networks to achieve reasonable usage → How to connect several computers? 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 → Which media can be used for data transport? → How to represent digital data on the communication medium? • Essential: → How to coordinate the access of several computers to the medium? Efficient methods to share/transfer data between the components of a system of interconnected devices Communication Protocols (Internet Technology) → Design of uniform data units for transfer → How to achieve a reliable and efficient transfer? Example for interworking of two parties: Client/Server principle Page 5 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme The Client/Server Principle Client/Server Systems Client Server Client Process Server Process Server Program (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). Client Program (process) which uses a service offered by a server. Examples for Client/Server systems Request Network Network Reply Advantages Chapter 1: Introduction Page 6 Chapter 1: Introduction → Cost reduction → Better usage of resources → Modular extensions → Reliability by redundancy Page 7 Chapter 1: Introduction Client Server WWW Browser WWW Server eMail Program Domain Name System (DNS) FTP Client FTP Server Page 8 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Another principle: Peer-to-Peer Non-technical Aspects Communication networks enable a faster and cheaper exchange/distribution of information. There is however a large number of social, ethical, cultural, juridical, ... side effects. • 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) • ...... • 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. Gnutella, BitTorrent Page 9 Chapter 1: Introduction Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Page 10 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Why 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 Data Communication = Protocols A protocol is defined as the whole set of agreements between application processes with the purpose of a common communication Chapter 1: Introduction Page 11 Chapter 1: Introduction Page 12 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Example: Exchange of Ideas between Philosophers Implementation of Protocols Philosopher A 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 Language: Chinese additionally: English Language: Spanish additionally: English Technical Expert B Technical Expert A Recognizes single characters and sends them in Morse Uninterpreted characters Recognizes single characters and sends them in Morse in correct order Electrical signals Network The implementation takes place in layer models Page 13 Page 14 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Standards Organizations - ISO The ISO/OSI Reference Model Necessary for world-wide usage of common protocols: standardization Reduce the complexity of a communication process (all details to be considered) through layers. International Standards Organization - ISO 7 layers: • • • • Organisation, which is working on a volunteer basis (since 1946) Members: standards organizations in more than 150 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 International Telecommunication Union (ITU-T) regarding telecommunication standards • Pioneering work of ISO regarding data communication: the ISO/OSI reference model (OSI: Open Systems Interconnection) • Notice: only the concept is pioneering – not the products developed from those concepts! Chapter 1: Introduction Interpreter B Uninterpreted sentences, i.e. no knowledge about politics Accepted today: solution 2. www.iso.ch protocols Interpreter A 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 Philosopher B Language: Spanish service Language: Chinese Chapter 1: Introduction Thoughts about world politics Page 15 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical Standard interfaces for commonly used communication services Network-independent end-to-end data transfer Addressing and routing of “packets” Protection of “frames”; Flow Control Criticism of the model: Layer 5 and 6 are rarely being implemented Generally too much overhead – some details are unnecessary, some are overloaded Signal representation, character transmission Transmission medium (“Layer 0”) Chapter 1: Introduction Page 16 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Layer Tasks Layer Tasks 1. Physical layer This 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), data rate, … 2. Data Link Layer Ensures 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. Chapter 1: Introduction Page 17 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 4. Transport Layer 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 to detect missing or wrong ordered data units. In this flow control, the current network state is considered to not only adapt to the receiver, but to the network capacities as well. Again, addressing is a topic here as well. While on layer 3 the receiving host is addressed, here a single communication process on this host is addressed. Chapter 1: Introduction Page 18 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Layer Tasks Layer Tasks 5. Session Layer This layer offers the possibility for dialogue control, i.e. it can be defined which data streams on layer 4 together are forming a dialogue, in which pattern communication partners are allowed to send data, and (in case of a half-duplex communication medium) at which time data can be transferred in which direction. Part of this is the so-called token management. During the transmission tokens can be exchanged. With certain operations only the communication partner which owns the token is allowed to conduct the operation. That means, a set of tokens exists 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. Chapter 1: Introduction 3. Network Layer This 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 prerequisite 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 (so-called routers) manage tables with path information and use the uniform addresses to make a decision about the best path to the receiver. Page 19 6. Presentation Layer The task of this layer is to display the data to be transmitted in a 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 a specific abstract data format before the transmission and are being translated back by the receiver into its own personal data format. 7. Application Layer In this layer standardized interfaces are being provided for commonly used application services. One example is file transfer. On the application layer a universally valid protocol including an interface for 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 e-mail, remote operations etc. Chapter 1: Introduction Page 20 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Interplay of the Layers The whole Communication Process • 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). Application Layer • Two communication partners on layer n exchange PDUs by using the communication service of the nearest lower lying layer (n-1). Session Layer Layer n service received from Data Link Layer Layer (n-1) H Data H Application Layer Chapter 1: Introduction Transport Layer Network Layer T-PDU H N-PDU T Data Link Layer Layer (n-1) Physical Layer H: Header, e.g. control information of the layer Page 21 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Bit stream Transmission medium Page 22 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme The Communication Process The OSI Reference Model in the Network • Not necessarily a one-to-one mapping between layers • Depending on the protocol, a n-PDU can be segmented into several (n-1)-PDUs before transmission (or vice versa): Application process Application process Application Protocol Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Host A Chapter 1: Introduction Session Layer S-PDU Physical Layer (n-1)-PDU Presentation Layer P-PDU H Network Layer Data A-PDU H Transport Layer Application process Page 23 Chapter 1: Introduction Application Layer Presentation Protocol Session Protocol Transport Protocol Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Data Link Layer Router A Router B Host B Internal Protocols Application-oriented service request H Data Network Layer Physical Layer Network/transmission-oriented n-PDU H Presentation Layer • For layer (n-1), these PDUs are the data to be transmitted. Layer n Application process Page 24 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme First Generation Computer Networks Computing Center Operator Rest of the world Mainframe Telephone lines Computer Networks Demultiplexer (= Layer 1&2, also partly layer 3) Multiplexer Terminals Terminals Page 25 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Peripherals Page 26 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Introduction of Local Area Networks Global Networking Building A Building A Rest of the world Rest of the world (Internet) Clients Local Server Fixed lines, ISDN, Provider ... Fixed lines Switch Building B Computing Center Operator Computing Center Router Mainframe Router Network and system administrator Router Backbone Building B Router Server Clients Building C Local Server Switch Peripherals Terminals Peripherals Switch Mainframe Router Chapter 1: Introduction Page 27 Chapter 1: Introduction Page 28 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Important Terms Classification of Networks Switch A switch has several connectors, from each connector a cable can be drawn to a computer. These computers then are linked to a small network. The switch knows which computer is plugged in at which connector (address of the network interface card) and forwards data to a destination computer. Router A switch only knows which computers are connected to it directly; if someone wants to send data to a computer far away, some instance is needed which knows the way to the destination over several other computers or switches. Routers are used to manage global address information and forward data through complex networks. Backbone A backbone is a set of computers (usually routers) which are connected by point-to-point links over large distances. A backbone serves for covering a large region with a communication network which can interconnect small, local networks of single institutions. Page 29 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Point-to-Point Network • A pair of computers is directly connected by one cable Broadcast Network • One-to-all: when one station sends, all others receive • All connected stations are sharing only 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 Page 30 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Classification of Networks Networks Connection to a WAN Better classification scheme: Classification by Distance 1m Personal Area Network (PAN) 10 m Room 100 m Building 1 km Campus 10 km Town 100 km Country 1000 km Continent 10000 km Planet Chapter 1: Introduction Switch Router Local Area Network (LAN) Metropolitan Area Network (MAN) Local Networks (LANs) Wide Area Network (WAN) Metropolitan Network (MAN), backbone for a town or a region Internet Page 31 Chapter 1: Introduction Page 32 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Networks Networks Page 33 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Networks “Dark Fiber” rented wavelength GRE KIE HAM AWI ROS DES Central node Frankfurt – connection to the European research network Géant. EWE BRE POT HAN Surfnet BIE MUE BRA DUI KAS MAG TUB FFO HUB ZIB ADH GOE LEI FZJ DRE MAR JEN BIR AAC CHE GIE • Connection to other backbone networks (international as well as to other German backbone networks) Page 34 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Networks • German research network X-WIN – backbone based on optical fiber, for universities and research institutes with a data rate of 10 GBit/s • Additionally: DWDM (Dense Wavelength Division Multiplexing): up to 160 simultaneous transmissions on different wavelengths: 1.6 TBit/s capacity! Chapter 1: Introduction Geant2 FRA ILM BAY GSI WUE HEI SAA Renater Also in Frankfurt and Hamburg: intercontinental connections. ESF ERL REG FZK AUG KEH STU GAR Switch/GARR Chapter 1: Introduction Page 35 Chapter 1: Introduction Page 36 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Standards Organizations - IEEE IEEE 802.11 Variants Institute of Electrical and Electronic Engineers - IEEE • Standardization e.g. of the IEEE 802.XStandards for Local Area Networks • • • • • • • 802.1 802.2 802.3 802.4 802.5 802.6 802.7 Overview and Architecture of LANs Logical Link Control (LLC) CSMA/CD („Ethernet“) Token Bus Token Ring DQDB (Distributed Queue Dual Bus) Broadband Technical Advisory Group (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) Chapter 1: Introduction Each IEEE standard has lots of variants – e.g. here for Wireless LAN: www.ieee.org 802.11a • 802.12 Demand Priority (HP’s AnyLAN) • 802.14 Cable modems • 802.15 Personal Area Networks (Bluetooth) • 802.16 WirelessMAN (WiMAX) • 802.17 Resilient Packet Ring • 802.18 Radio Regulatory Technical Advisory Group (RRTAG) • 802.19 Coexistence Technical Advisory Group • 802.20 Mobile Broadband Wireless Access (MBWA) • 802.21 Media Independent Handover • 802.22 Wireless Regional Area Networks (WRAN) Page 37 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 54 MBit/s WLAN in the 5 GHz band 802.11b 11 MBit/s WLAN in the 2,4 GHz band 802.11c Wireless Bridging between Access Points 802.11d "World Mode", Adaptation to regional regulations (e.g. used frequency ranges) 802.11e QoS und streaming enhancement for 802.11a/g/h 802.11f Roaming for 802.11a/g/h (Inter Access Point Protocol IAPP) between Access Points of different vendors 802.11g 54 MBit/s WLAN in the 2,4 GHz band 802.11h 54 MBit/s WLAN in the 5 GHz band with dynamic adaptation of channel and frequency choice as well as automatic adaptation of transmission power (enhancement of IEEE 802.11a for Europe) 802.11i Authentication/encryption for 802.11a/b/g/h 802.11j Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz 802.11k Improved measurement/evaluation/management of radio parameters (e.g. signal strength), e.g. for enabling location based services Page 38 Chapter 1: Introduction Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme IEEE 802.11 Variants 802.11m Summary of earlier enhancements, correction of errors in former specifications (maintenance) 802.11n Enhancement for a future, faster WLAN with data rate of 108 – 320 MBit/s 802.11p WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) 802.11r Fast roaming 802.11s ESS Mesh Networking 802.11t Wireless Performance Prediction (WPP) - test methods and metrics Recommendation 802.11u Interworking with non-802 networks (for example, cellular) 802.11v Wireless network management 802.11w Protected Management Frames 802.11y 3650-3700 Operation in the U.S. Internet Protocols (= Layer 3 – 7) Only few standards (the grey shaded) are “standalone” standards defining data exchange – the others are supplements for network variants, or enhancements like security, QoS, etc. Chapter 1: Introduction Page 39 Chapter 1: Introduction Page 40 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Standards Organizations - IETF The TCP/IP Reference Model Internet Engineering Task Force - IETF www.ietf.org • 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 Chapter 1: Introduction Page 41 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Application Layer Presentation Layer Don´t exist, integrated into layer 7, if needed Session Layer Transport Layer Transport Layer Network Layer Internet Layer Data Link Layer Host-to-Network Layer Physical Layer ISO/OSI Chapter 1: Introduction TCP/IP Page 42 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme The Tasks of the TCP/IP Layers The Layers of TCP/IP 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 organizations 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 beyond 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). But, several enhancements are done in the meantime, e.g. MPLS for a connection-oriented transfer of data (see QoS chapter). Chapter 1: Introduction Application Layer Page 43 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 reassembled 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), or when only very small messages have to be sent. 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. Chapter 1: Introduction Page 44 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme OSI vs. TCP/IP OSI vs. TCP/IP 4. Political reasons 1. Time The TCP/IP protocols were already widely used before OSI had finished the standardization activities. 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. 2. Freedom from obligation A „reference model“ like OSI is free from obligation. It only defines what is to be done, but not how to do it. Result: incompatibility of products. 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”! Chapter 1: Introduction Page 45 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme 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. The TCP/IP protocols are used, from ISO/OSI only the terminology remained Chapter 1: Introduction Page 46 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme Cross-Layer Protocol Design Cross-Layer Protocol Design ISO/OSI clearly defines independent layers, in the Internet protocols the layers theoretically are also independent (in reality, there are some mix-ups between TCP and IP) But sometimes it makes sense to violate the layer concept and to allow inter-layer exchange of information or even interaction: Examples for cross-layer interaction: • In wireless transmission, common control of data rate (layer 2) and sending power (layer 1) is useful • Routing protocols (layer 3) in wireless mesh networks can use information about signal-to-noise ratio and interference (layer 1) in path decisions Disadvantages: • Flexibility is lost – exchange of single protocols is not longer possible • Solutions are tailored to certain application scenarios, usage is restricted Goal: lightweight (i.e. simplified) protocols, specially tailored to a certain application Avoid overhead of layer concept: • Minimize energy consumption for small devices with low capabilities • Consider characteristics of communication medium at time of protocol design • Performance increase Chapter 1: Introduction 5. Hurriedly product implementation Page 47 Concept for low-capability devices / wireless communication Chapter 1: Introduction Page 48 Lehrstuhl für Informatik 4 Kommunikation und verteilte Systeme And now… 1. Introduction • Communication Protocols • Computer Networks 2. Computer Networks • Network principles • Network topologies and components • Local Area Networks (Ethernet, Token Ring, Token Bus, FDDI, DQDB) • Wide Area Networks (Frame Relay, ATM, SDH, ISDN/DSL) 3. Internet Protocols • Internet/Intranet: the TCP/IP Reference Model • Network protocols (the Internet Protocol IP, Routing protocols) • Quality of Service in the Internet • Transport protocols (TCP and UDP) 4. Application Protocols in the Internet • Higher protocols (FTP, HTTP, E-Mail, ...) Chapter 1: Introduction Page 49