1st Slide Set Computer Networks

Transcription

1 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 1/74 1st Slide Set Computer Networks Prof. Dr. Christian Baun Frankfurt University of Applied Sciences ( : Fachhochschule Frankfurt am Main) Faculty of Computer Science and Engineering

2 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 2/74 Agenda for Today Organizational Information Fundamentals of computer networks Network services, roles, transmission media and network protocols Classification of networks Parallel/ serial data, synchronous/asynchronous data transmission Directional dependence (anisotropy) of data transmission Devices and topologies Frequency, data signal and fourier series Bitrate, baud rate, bandwidth and latency Media access control Collision domain Protocols TCP/IP reference model Hybrid reference model OSI reference model

3 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 3/74 Prof. Dr. Christian Baun Academic Training and Career 2005: Diploma in Computer Science from the FH Mannheim 2006: Master of Science from the HS Mannheim : Scientist at Forschungszentrum Karlsruhe and KIT : D-Grid integration project Reference installation Ensuring sustainable operation : Open Cirrus Cloud Computing Testbed Operation and optimization of private clouds Development of cloud services and tools 2011: Doctoral degree from Universität Hamburg Evaluation and Development of Cloud Computing Services as a Basis for the Creation of a Marketplace : Substitute Professor at HS Darmstadt : Quality Specialist for HANA DB at SAP AG Since September 2013: Professor at FH Frankfurt

4 Organizational Information Tell me when problems exist at an early stage!!! Homepage: Check the course page regularly!!! The homepage contains among others the lecture notes Presentation slides in English and German language Exercise sheets in English and German language Sample solutions of the exercise sheers Old exams and their sample solutions Participating in the exercises is not a precondition for exam participation But it is recommended to participate the exercises The content of the English and German slides is identical, but please use the English slides for the exam preparation to become familiar with the technical terms Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 4/74

5 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 5/74 Computer Networks in Computer Science (1/2) Where would you place the computer networks?

6 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 6/74 Computer Networks in Computer Science (2/2) Computer networks belong to practical computer science and technical computer science

7 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 7/74 Situation today and Objective for this Semester

8 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 8/74 Topics of this Course Image source: unknown Organisational information, introduction Fundamentals of computer networks Reference models Line codes, protocols and services Wired and wireless networks The course includes > 500 slides. But you do not need to memorize them all in detail for the exam...

9 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS1617 9/74 The Way a good Course works... Image source: Google Mr. Miyagi says: Not only the student learns from his master, also the master learns from his student. Active participation please!

10 Things, which are bad in a Course... Attending late (regularly!) Noisy eating in the course Offensive-smelling food in the course = annoying and disrespectful = annoying = disgusting Noisy greeting of/by people attending late Teamwork at the laptop in the course Filming or photographing the course = embarrassing and disrespectful = annoying for the rows behind = embarrassing and disrespectful Image sources: Google image search. In detail: Antenne Niedersachsen, Ruhrnachrichten, Celantino, Tagesspiegel, adpic, Fudder Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74

11 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Literature (Computer Networks) Computernetze kompakt, Christian Baun, Springer Vieweg (2015) Computer Networks, Andrew S. Tanenbaum, David J. Wetherall, Pearson (2010) Distributed Systems: Principles and Paradigms, Andrew S. Tanenbaum, Maarten van Steen, Prentice Hall (2006) Ethernet, Jörg Rech, Heise (2014) Wireless LANs, Jörg Rech, Heise (2012) Computernetzwerke, Larry L. Peterson, Bruce S. Davie, dpunkt (2008) TCP/IP, Gerhard Lienemann, Dirk Larisch, Heise (2011) Grundkurs Verteilte Systeme, Günther Bengel, Vieweg (2004) Computernetzwerke, James F. Kurose, Keith W. Ross, Pearson (2008) Prüfungstrainer Rechnernetze, Aufgaben und Lösungen, Jörg Roth, Vieweg (2010) Computernetzwerke, Rüdiger Schreiner, Hanser (2009) Einführung in die Informatik, Heinz Peter Grumm, Manfred Sommer, Oldenburg (2011) The books from Andrew S. Tanenbaum are available in English and German language

12 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Generations of Computer Systems Generation Timeframe Technological progress 0 until 1940 (Electro-)mechanical calculating machines Elelectron tubes, relays, jack panels Transistors, batch processing Integrated circuits, time sharing Very large-scale integration, PCs/Workstations until? Distributed systems, the network is the computer In the 1960s and 1970s, first development efforts have been made in the area of computer networks Performance of computers before the 1960s was quite weak These computers were not used for universal purposes They were used for specific tasks, e.g. to calculate the flight path (trajectory) of projectiles

13 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Development of Computer Networks (since the 1960s) Arpanet = Advanced Research Projects Agency Network First computer network without central control Developed in the 1960s Connected 4 universities in the US until December 1969 Goal: Robustness to survive the loss of some participants Solution: Decentralized structure and packet switching After 1969, several universities and research facilities in the US joined Arpanet Popular application protocols like Telnet and FTP have been developed in the Arpanet project The story that the development of Arpanet was driven by the fear of a nuclear war is an urban legend. Arpanet s objective was the installation of a robust computer network to make it simple to access the computer resources of different universities. Factors that influenced the development are the unreliable telephone lines that were used as transmission medium (solution: packet switching) and scalability (solution: no central components)

14 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Development of Computer Networks (since the 1970s) Networks for connecting terminals with mainframes via serial cables This technology made time sharing possible Example: IBM Systems Network Architecture (SNA) since 1974 In time sharing mode, multiple users work at the same time and in a competitive way via terminals on a single computer by sharing the available computing time of the processor. The distribution of the computing time is done via time slices Host-to-host communication, based of proprietary networks, became available Example: DECnet As of 1975: Direct link between two PDP-11 computers As of 1976: Connecting up to 32 computers Later network technologies based on packet switching, as well as open standards and protocols (TCP/IP)

15 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Development of Computer Networks (since the 1980s) Situation: High-performance Personal Computers (PCs) and Workstations Inexpensive network technologies, such as Ethernet and Token Ring Effect: Number of computer networks, built up at universities and companies, increased In the 1980s, the Arpanet switched over to the protocols TCP/IP and became a part of the internet In the late 1980s, the Arpanet was shut down After mid-1990s: Internet access became inexpensive Mobile systems e.g. laptops, tablet computers and mobile phones Computer networks became well-established also at home Paradigm change: Centralized systems = Distributed systems Computer networks are an important topic in computer science because of the growing computing power and storage capacity, as well as the availability of high-performance but inexpensive network technologies such as WLAN and LTE

16 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Largest Computer Network (global Internet) Global network for web applications based of TCP/IP

17 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Required Components to set up a Computer Network For setting up and running a computer network, these components are required: 1 2 terminal devices with network services running The devices are intended to communicate with each other or access shared resources A network service provides a service for communication or shared resources usage 2 Transmission medium to send and receive data (see slide set 2) Common used transmission media are based of copper wires (e.g. twisted pair cables or coaxial cables) and fiber-optic cables Wireless data transmission is also possible 3 Network protocols (see slides 54) Rules that specify, how computers can communicate The rules (network protocols) are mandatory. Without them, the communication partners cannot understand each other. Just imagine a phone call to a foreign country. The connection is established, but no participant understands the other s language. Only if all participants speak the same language, communication becomes possible

18 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Computer Networks distinguished by their Dimension (1/3) Depending on the dimension, different groups of computer networks are distinguished Personal Area Network (PAN) Network of small mobile devices, such as cell phones Technologies: USB, FireWire, WLAN, Bluetooth, IrDA Major dimension: Few meters Local Area Network (LAN) Local network Range covers an apartment, building, company site or university campus Major dimension: m Concrete values depend on the transmission medium used and when using wireless networks, also the environment and the transmission power Technologies: Ethernet, Wireless LAN (WLAN), Token Ring (outdated)

19 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Computer Networks distinguished by their Dimension (2/3) Metropolitan Area Network (MAN) Connects LANs Range covers a city or agglomeration area Major dimension: 100 km Technologies: Fiber-optic cables, WiMAX (IEEE ) Fiber-optic cables are used because of lesser attenuation (signal weakening) and higher data transmission rates Wide Area Network (WAN) Connects several networks Range covers a large geographic area inside a country or continent Major dimension: 1000 km Technologies: Ethernet (10 Gbit/s), Asynchronous Transfer Mode (ATM)

20 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Computer Networks distinguished by their Dimension (3/3) Global Area Network (GAN) Range can covers an unlimited geographic area Connects WANs The internet is a GAN but not every GAN is the internet Example for a GAN: Connects a company s globally distributed offices Technologies: Satellites or fiber-optic cables

21 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Parallel Data Transmission Image source: and Google Communication between computers is possible via parallel and serial data transmission With parallel data transmission, in addition to the control lines, multiple data lines exist Example: Parallel port which was the standard interface to connect printers until it was replaced by USB Via this interface, a complete byte of data can be transferred per time unit Benefit: Higher throughput Drawback: Lots of lines are necessary This is cost-intensive for long distances Usage: Local bus systems The image shows the parallel port (25 pins)

22 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Serial Data Transmission When serial data transmission is used, the bits are transmitted one after another via the bus Transferring a byte takes 8 times longer compared to parallel data transmission Benefit: Can be used for long range distances, because only few wires are required Drawback: Lesser throughput Usage: Local bus systems and computer networks Image source: The image shows the serial port (25 pins) The image shows the serial port (9 pins)

23 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Synchronous and Asynchronous Data Transmission (1/2) To read data from a bit stream, the incoming bit stream need to be sampled over a time window The time window is produced via a clock source Synchronous data transmission The communication partners synchronize the transmission via a clock signal The clock signal is... sent via a dedicated line or wire or it is recovered (= clock recovery) from the transmitted signals by the receiver Benefit: The data transmission does not need to be re-synchronized regularly Drawback: The realization of the clock recovery is expensive, because an additional line or wire is required

24 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Synchronous and Asynchronous Data Transmission (2/2) Asynchronous data transmission The communication partners use clock sources that are independent from each other When data is transferred, a start bit is inserted in front of the data The start bit indicates that the receiver is expected to start his clock source A stop bit at the end of the data indicates that the data transmission ends Benefit: No synchronization of the clock sources is necessary Drawbacks: Because the clock sources can differ, the maximum size of the data which can be transferred without interruption is limited The start and stop bits are overhead

25 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Data Transmission in Computer Networks In computer networks, the sender specifies the clock pulse and supplies it to the receivers within the bit stream For that reason, data transmission in computer networks is always synchronous Using an additional line or wire, just for the clock signal, is too expensive in computer networks in most cases

26 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Directional Dependence (Anisotropy) of Data Transmission Simplex The information transfer only works in one direction After the end of a transmission, the communication channel can be used by another sender Examples: Radio, TV, Pager Duplex (Full-duplex) The information transfer works in both directions simultaneously Examples: Phone, Networks with twisted pair cables because they provide separate wires to send and receive Half-duplex The information transfer works in both directions, but not simultaneously Only one direction at a time Examples: Networks with fiber-optic cables or coaxial cables, because there exists just a single line to sending and receiving Wireless networks with just a single channel

27 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Devices in Computer Networks (1/3) Repeater (layer 1) Most simple devices in computer networks Extends the range of LANs by cleaning and amplifying the received electrical or optical signals that are weakened in the transmission media Forwards signals (bits) but does not check their meaning and does not validates their correctness Provides only 2 ports For that reason, it is only suited to extend a LAN Hub (layer 1, Multiport Repeater) Can connect several terminal devices to a LAN Same functionality as a repeater, but provides > 2 ports Forwards incoming signals to all its ports Repeaters and Hubs have no network addresses, because they only forward signals For other network devices, they operate in a transparent way

28 Devices in Computer Networks (2/3) Modem (layer 1-2) Transmit signals over long distances by modulating them to a carrier frequency in the ultra low frequency band Recovers the signals by demodulating the transmitted information Modems are also Bridges (layer 2) Examples: (A)DSL or cable modems for broadband internet access and data modems Bridge (layer 1-2) Connects different physical networks Forwards frames between physical networks Forwards frames only when it is useful Validates the frames integrity via checksums Provides only 2 ports Forwards and filters frames but doesn t actively communicate with other devices = For that reason, Bridges don t need network addresses Examples: Modems, WLAN Access Points Layer-2-Switch (layer 1-2, Multiport Bridge) Same functionality as a Bridge, but provides > 2 ports Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74

29 Router (layer 1-3) Large network infrastructures usually consist of several smaller networks with separate logical address ranges Forwards packages between different logical networks Allows to connect the local area network (LAN) to a WAN Layer-3-Switch (layer 1-3) Similar functionality as a Router But is used only within local area networks in order to implement different logical address ranges Do not allow to connect the LAN to a WAN Gateway (layer 1-5) Enables communication between networks that use different protocols and/or different ways to address devices Host or Terminal Device Any device that communicates via computer networks Examples: PCs, mainframes, mobile phones, refrigerators,... All devices in computer networks are also called nodes and wireless network devices are also called stations Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Devices in Computer Networks (3/3)

30 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Topologies of Computer Networks The topology of a computer network... determines how the communication partners are connected with each other affects its reliability a lot The structure of large-scale networks is often a combination of different topologies Physical and logical topology can be different Physical topology: Describes the wiring Logical topology: Describes the flow of data between the terminal devices Topologies are graphically represented with nodes and edges

31 Bus Network All terminal devices are connected via a shared communication cable the bus No active components between the terminal devices and the shared communication cable If a participant fails, it does not affect the network itself Advantage: Cheap to implement In the past, Hubs and Switches have been expensive Shared communication cable fails = complete network fails Only one participant can send data at each point in time = otherwise, collisions will occur A media access control method like CSMA/CD is required Examples: 10BASE2 (Thin Ethernet) and 10BASE5 (Thick Ethernet): 10 Mbps Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74

32 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 10BASE2 (A Journey into the Past)

33 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Ring Network Connects participant to participant All data is transferred from participant to participant until the destination is reached Disruption of a single link = network failure Each participant is also a repeater, which amplifies the signal For that reason, large-sized rings (transmission medium dependent) are possible Maximum ring length for Token Ring: 800 m Examples: Token Ring (logical): 4-16 Mbps Fiber Distributed Data Interface (FDDI): Mbps FDDI implements 2 rings One is a secondary backup, in case the primary ring fails

34 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Star Network All participants are connected directly with a central component (Hub or Switch) Outage of the central component leads to an outage of the network itself The central component can be implemented in a redundant way Outage of a participant do not cause an outage of the network itself Advantages: Expandability and stability Examples: Ethernet: 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps Token Ring (physical): 4-16 Mbps Fibre Channel (storage networks): 2-16 Gbps InfiniBand (cluster): Gbps

35 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Media Access Unit Image source: Google Image Search Token Ring demonstrates that the physical and logical topology of a network can be different Token Ring implements a logical ring network Wiring is mostly done equal to a star network Using a Media Access Unit (MAU) is common Each device is connected with just a single cable with the MAU Realizes a star network from a technical point of few Still a ring network from a logical point of view A MAU is a ring in a box If a participant is not connected or does its connection fail, then the MAU bypasses this participant and the ring is still properly functioning

36 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Mesh Network Each participant is connected with one or more other participants In a fully connected mesh network, the participants are all connected to each other If participants or connections fail, the communication is typically still possible because the frames are redirected Benefit: Failure safe (depends on the cabling effort) Drawbacks: Cabling effort and energy consumption Furthermore, in not fully connected meshed networks, it is complex to identify the best way from sender to receiver during packet forwarding Examples: Logical topology between Routers Ad-hoc (wireless) networks

37 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Tree Network Image source: Google Image Search One or more edges are connected with the root Every edge leads to a leaf node or to the root of another tree Several star topology networks are hierarchically connected Benefits: Failure of a terminal device (leaf node) has no consequences Good expandability and long distances are possible Well suited for searching and sorting algorithms Drawbacks: When a node fails, the complete (sub-)tree behind is no longer accessible In a large tree, the root can be a bottleneck because the communication from one half of the tree to the other half always needs to pass the root Example: Connecting Hubs or Switches via uplink

38 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Cellular Network Implemented mainly by wireless networks Cell: Area where the participants can communicate with the base station Advantage: Outage of participants do not affect the network itself Drawback: Maximum dimension is limited by the number of base stations and their positions Examples: Wireless LAN (IEEE ) Global System for Mobile Communications (GSM) Bluetooth hotspots

39 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Current Situation Today, Ethernet (1-10 Gbit/s) with Switches (= star topology) is the standard for wired LAN Connecting Hubs and Switches implements a tree topology, if there are no loops in the cabling Cell topology is the standard for wireless networks Mesh topology is one possible use case of wireless networks and it is the logical topology between routers Bus and ring topologies are not longer used for new computer networks 10BASE2 (Thin Ethernet) and 10BASE5 (Thick Ethernet) are outdated May 2004: IBM sells his complete Token Ring product lineup

40 Frequency Image source: Electrical engineering distinguishes between 2 types of voltage: 1 Direct current voltage: Polarity of voltage and voltage level remain constant 2 Alternating current voltage: Polarity of voltage and voltage level change periodically Period: The time it takes for the periodic voltage curve Frequency: Number of oscillations per second The lower the period, the higher is the frequency Fig. A: Rectangular shaped alternating current voltage in theory Fig. B: Sinus shaped alternating current voltage in practice Frequency [Hz] = 1 Period [s] The unit for frequency is the hertz (Hz) 1 Hz means = 1 event (oscillation) per second Example: Alternating current voltage in Europe with 50 Hz Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74

41 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Data Signal Data exchange takes place through the exchange of binary data Despite the fact that a digital signal is put on the network cable, an analogous signal is sent The signals are subject to physical laws This includes the attenuation (signal weakening) Attenuation causes the weakening of the amplitude of a signal with increasing distance on all transmission media If the amplitude of a data signal has dropped below a certain value, it can no longer be clearly detected Thus, the attenuation limits the maximum bridgeable distance for all transmission media The higher the frequency, the higher is the attenuation

42 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Fourier Series Image source: Jörg Rech. Ethernet. Heise According to the fourier series, which is named in honour of Jean Baptiste Joseph Fourier ( ), a square-wave signal (e.g. a binary signal), consists of the sum of a set of oscillating functions A square wave signal consists of a fundamental frequency and harmonics Harmonics are integer multiples of the fundamental frequency They are often referred to as harmonics of the 3rd, 5th, 7th, etc. order The more harmonics are taken into account, the closer to get a perfect square wave

43 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Fourier Series and Bandwidth To transmit a square-wave signal clearly, at least the fundamental frequency and the harmonics of the 3rd and 5th order need to be transmitted The harmonics of the 3rd and 5th order are necessary for keeping the square wave its rectangular shape and preventing that it looks rounded Thus, the transmission medium must not only transmit the fundamental frequency bug-free, but also the 3rt and 5th harmonic In practice, the harmonics are more attenuated than the fundamental frequency The bandwidth, from the viewpoint of the transmission medium, is the range of frequencies which can be transmitted via the transmission medium without interferences The attenuation of the signal increases with the frequency

44 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Fourier Synthesis of a square-wave Signal Source: Wikipedia The graphs in the first column show the oscillation, which is added in the respective row. The graphs in the second column show all so far recognized oscillations, which are then added to the diagrams of the third column, to reach as close as possible the signal which shall be generated. The more harmonics (multiples of the fundamental frequency) are taken into account, the more we get an ideal square-wave signal. The fourth column shows the amplitude spectrum, normalized to the fundamental frequency

45 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Bitrate and Baud Rate Image source: Bitrate: Transferred payload bits per time unit Typically measured as bits per second (bit/s or bps) Baud Rate: Transferred symbols per second Unit of the symbol rate 1 baud means 1 symbol is transferred per second Originally the baud rate described the number of distinct symbol changes (signaling events) when using the telegraph Number of morse characters per second The ratio between bitrate and baud rate depends of the line encoding scheme used The line code... The line code specifies in computer networks the maximum number of signals that can be transmitted via the transmission midia used The line code of a network technology is specified by the layer protocol protocol used More information about line codes provides slide set 3

46 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Bandwidth and Latency (1/3) Main factors, influencing the performance of a computer network: Bandwidth (throughput) Latency (delay) The bandwidth specifies how many bits can be transmitted within a period via the network In a network has a bandwidth (throughput) of 1 Mbit/s, one million bits can be transmitted per second Thus, a bit has a width of 1 µs If the bandwidth is doubled, the number of bits that can be transmitted per second doubles too

47 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Bandwidth and Latency (2/3) The latency of a network is the time, a message needs to travel from one end of the network to the most distant end Latency = Propagation delay + Transmission delay + Waiting time Propagation delay = Distance Speed of light Velocity factor Distance: Length of the network connection Speed of light: 299, 792, 458 m/s Velocity factor: 1 for vacuum, 0.6 for twisted pair cables, 0.67 for optical fiber and 0.77 for coaxial cables Source: Larry L. Peterson, Bruce S. Davie. Computernetzwerke. dpunkt (2008)

48 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Bandwidth and Latency (3/3) Latency = Propagation delay + Transmission delay + Waiting time Transmission delay = Message size Bandwidth Waiting times are caused by network devices (e.g. Switches) They need to cache received data first before forwarding it Transmission delay = 0, if... the message consists only of a single bit Waiting time = 0, if... the network connection between sender and destination is just a single line or a single channel Source: Larry L. Peterson, Bruce S. Davie. Computernetzwerke. dpunkt (2008)

49 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Bandwidth-Delay Product Calculates the volume of a network connection Signals cannot be transmitted with infinite speed via the transmission media The propagation speed is in any event limited by the speed of light and it depends on the velocity factor of the transmission medium The product of bandwidth and delay (latency) corresponds to the maximum number of bits that can reside inside the line between sender and receiver Example: A network with 100 Mbit/s bandwidth, and 10 ms latency 100, 000, 000 Bits/s 0.01 s = 1, 000, 000 Bits There are a maximum number of 1, 000, 000 Bits inside the network line This is equivalent to 125, 000 Bytes (approx. 123 kb)

50 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Media Access Control When using wired networks, such as Thin Ethernet (10BASE2) or Token Ring, or wireless networks, all participants access a common transmission medium This is called Shared Media A media access control method must ensure, that at any time, only a single participant sends data Only if this condition is satisfied, data can be transmitted error-free For shard media, 2 media access control methods exist: 1 Deterministic media access control 2 Non-deterministic media access control

51 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Deterministic Media Access Control Access to the transmission medium is carried out at a certain time in coordination with with the other participants Example: Token passing scheme of Token Ring and FDDI The permission to send is realized via a token The participant who has the token is allowed to send data for a specific (limited!) period of time If a participant finished its sending procedure, he hands over the token to the next participant If a participant does not want to send data, he hands over the token to the next participant immediately No participant is skipped = fair media access control method It is guaranteed that each participant after a specific waiting period with a foreseeable maximum duration is allowed to transmit data

52 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Non-deterministic Media Access Control All participants are in terms of access to the transmission medium in direct competition with each other Example: CSMA/CD when using Thin Ethernet (10BASE2) If a participant wants to transmit data, it previously checks if the transmission medium is idle If it is idle, the participant starts to send data If 2 participants transmit data at the same time, both assume the transmission medium is idle = a collision occurs By using CSMA/CD, collisions are detected by all participants If a collision is detected, the sender stop their sending procedures After a random waiting period, the participants try again to send Impossible to predict, are... the waiting period, before the transmission medium can be accessed the amount of data, that can be transferred the probability of collisions CSMA/CD is part of the slide sets that cover the data link layer

53 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Collision Domain When multiple network devices share a transmission medium and try to transmit data at the same time, unwanted collisions occur The area in which collisions can expand is called collision domain It is a network, or a part of a network, where multiple network devices share a transmission medium Repeaters and Hubs increase the collision domain Bridges, Switches and Routers divide the collision domain For collision detection, the wired network standard Ethernet uses the media access control method CSMA/CD For wireless networks, a reliable collision detection is impossible For this reason, WLAN uses for collision avoidance the media access control method CSMA/CA

54 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Polybius Square (2nd Century BC) The search for efficient forms of communication is not a modern challenge Example: The signaling method of Polybius The 24 letters of the Greek alphabet are placed in a 5x5 matrix The encoding results from the coordinates of the letters in the matrix α β γ δ ɛ 2 ζ η θ ι κ 3 λ µ ν ξ o 4 π ρ σ τ υ 5 φ χ ψ ω Torches are used for the optical transmission of encoded messages In the most simple case, the sender holds up in one hand the torches to announce the line number (y-coordinate) and in his other hand the torches to indicate the column number (x-coordinate)

55 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Polybius Square Torches of Polybius Proceeding In order to improve the communication, several strategies exist Potential way to establish the connection: The sender raises 2 torches to transmit his intention for transmission The receiver raises 2 torches to signal readiness to receive Lowering the torches on both sides closes the connection establishment Potential way to transmit data: The sender raises torches to announce the line number The sender lowers the torches The sender raises torches to announce the column number The sender lowers the torches These strategies to establish connections and to transmit data are examples for protocols

56 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Protocols A protocol is the set of all previously made agreements between communication partners These agreements include: Rules for connection establishment and clearing Method of synchronization between sender and receiver Measures for the detection and treatment of transmission errors Definition of valid messages (vocabulary) Format and encoding of messages Protocols specify... the syntax (= format of valid messages) the semantics (= vocabulary and meaning of valid messages)

57 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Reference Models Communication in computer networks is subdivided into reference models Each layer of a reference model handles a particular aspect of communication and offers interfaces to the overlying layer and underlying layer Each interface consists of a set of operations, which together define a service In the layers, the data is encapsulated (= encapsulation) Because each layer is complete in itself, single protocols can be modified or replaced without affecting all aspects of communication The most popular reference models are the TCP/IP reference model, the OSI reference model and the hybrid reference model

58 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 TCP/IP Reference Model or DoD Model Developed from 1970 onwards by the Department of Defense (DoD) in the Arpanet project Divides the required functionality to realize communication into 4 layers For each layer, it is specified, what functionality it has to provide These requirements are implemented by communication protocols Concrete implementation is not specified and can be realized in different ways Therefore, for each of the 4 layers, multiple protocols exist Nummber Layer Protocols (Examples) 4 Application Layer HTTP, FTP, SMTP, POP3, DNS, SSH, Telnet 3 Transport Layer TCP, UDP 2 Internet Layer IP (IPv4, IPv6), ICMP, IPsec, IPX 1 Link Layer Ethernet, WLAN, ATM, FDDI, PPP, Token Ring

59 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 TCP/IP Reference Model Message Structure Each layer adds additional information as header to the message Some protocols (e.g. Ethernet) add in the link layer not only a header but also a trailer at the end of the message The receiver analyzes the header (and trailer) on the same layer

60 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Hybrid Reference Model The TCP/IP reference model is represented frequently in the literature (e.g. by Andrew S. Tanenbaum) as a 5-layer model Reason: It makes sense to split the link layer into 2 layers, because they cover completely different responsibilities This model is an extension of the TCP/IP model and is called hybrid reference model The objects of the individual layers will be discussed on the basis of the hybrid reference model

61 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Physical Layer Transmits the ones and zeros Physical connection to the network Conversion of data in signals Protocol and transmission medium specify among others: How many bits can be transmitted per second? Can transmission take place simultaneously in both directions? Devices: Repeater, Hub (Multiport Repeater), Modem

62 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Data Link Layer Ensures error-free data exchange of frames between devices in physical networks Detects transmission errors with checksums Controls the access to the transmission medium (e.g. via CSMA/CD or CSMA/CA) Bridges and Layer-2-Switches (multiport Bridges) connect physical networks Specifies physical network addresses (MAC addresses) At sender site: Packs the Network Layer packets into frames and transmits them with the desired reliability via a physical network from one device to another At receiver site: Identifies frames in the bit stream from the Physical Layer

63 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Network Layer Forwards (routes) packets between logical networks (over physical networks) For this internetworking, the network layer defines logical addresses (IP address) Each IP packet is routet independently to its destination and the path is not recorded At sender site: Packs the segments of the Transport Layer in packets At receiver site: Unpacks the packets in the frames from the Data Link Layer Router and Layer-3-Switches connect logical networks Usually the connectionless Internet Protocol (IP) is used Other protocols (e.g. IPX) have been replaced by IP

64 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Transport Layer Transports segments between processes on different devices via so-called end-to-end protocols At sender site: Packs the data of the Application Layer into segments At receiver site: Unpacks the segments in the packets from the network link layer Addresses processes with port numbers Data Link Layer and Network Layer implement physical and logical addressing of the network devices Different transport protocols implement different ways to communicate UDP (User Datagram Protocol): Connectionless communication TCP (Transport Control Protocol): Connection-oriented communication Combination of TCP/IP = de facto standard for computer networks

65 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Different Forms of Communication Connectionless communication Analogous to a mailbox Sender transmits messages without prior connection establishment Disadvantage: No validation that a segment arrives at the destination If validation is wanted, it must be implemented in the Application Layer Benefit: Better throughput, because of lesser overhead Connection-oriented communication Analogous to a telephone Prior data exchange, a connection is established between sender and receiver The connection is not terminated, even if no data is transmitted After all data is exchanged, the connection becomes terminated Implements flow control and congestion control Ensures lossless segment delivery in the correct order = Successful delivery is guaranteed

66 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Application Layer Contains all protocols that work together with the application programs (e.g. browser or program) Here are the messages (e.g. HTML pages or s), formated according to the used application protocol Some Application Layer protocols: HTTP, FTP, SMTP, POP3, DNS, SSH, Telnet

67 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 How Communication works (1/2) Vertical communication Messages are packed from top to bottom layer by layer and extracted at the receiver in the reverse layer sequence Data encapsulation and de-encapsulation

68 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 How Communication works (2/2) Horizontal communication Equal protocol functions are used in the equivalent layers by sender and receiver

69 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 OSI Reference Model Some years after the TCP/IP reference model (1970s), the OSI reference model was developed from 1979 onwards 1983: Standardized by the Intern. Organization for Standardization (ISO) OSI = Open Systems Interconnection The structure is similar to the TCP/IP reference model The OSI model implements 7 layers In contrast to the hybrid reference model, the Application Layer functionality is distributed across 3 layers in the OSI reference model

70 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Session Layer (1/2) Tasks: Establishment, monitoring and termination of sessions A session is the basis for a virtual connection between 2 applications, running on physically independent computers Controls the dialogues (connections) between processes Controls which participant is allowed to send next Provides checkpointing which is useful for longer data transmissions to enable synchronization If the connection fails, returning to a checkpoint avoids starting the transmission from the beginning

71 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Session Layer (2/2) Protocols that meet the required capabilities of the Session Layer are Telnet for remote controlling computers and FTP for files transmission These protocols can be assigned to the Application Layer too The Application Layer includes the protocols, used by the users applications FTP and Telnet are used directly by the relevant programs and not by abstract protocols of upper levels Thus, it makes sense to assign these Session Layer protocols to the Application Layer The Session Layer is seldom used in practice, because all tasks intended to this layer are fulfilled by Application Layer protocols today

72 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Presentation Layer Contains rules for changing the format (presentation) of messages The sender can notify the receiver that a message has a specific format (e.g. ASCII) to make a perhaps necessary conversion possible Data records can be defined here with fields (e.g. name, student ID number... ) Data types and their length can be defined here Compression and encryption could be covered by this layer The Presentation Layer is seldom used in practice, because all tasks intended to this layer are fulfilled by Application Layer protocols today

73 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Reference Models Summary (1/2) The TCP/IP reference model specifies the functions of the layers but how they are implemented The OSI reference model distributes the tasks of the... Link Layer between Physical Layer and Data Link Layer This is useful, because the objectives of Physical Layer and Data Link Layer differ a lot Application Layer between Session, Presentation and Application Layer This is not useful and does not take place in practice Functionalities, which are intended for Session Layer and Presentation Layer, are provided by Application Layer protocols and services

74 Prof. Dr. Christian Baun 1st Slide Set Computer Networks Frankfurt University of Applied Sciences WS /74 Reference Models Summary (2/2) Conclusion: The hybrid reference model illustrates the functioning of computer networks in a realistic way It distinguishes between the Physical Layer and Data Link Layer It does not subdivide the Application Layer It combines the advantages of the TCP/IP reference model and the OSI reference model, without taking over their drawbacks