Week 4 Notes - Computer Networks
Week 4 Notes - Computer Networks ITCS 3166
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This 6 page Class Notes was uploaded by Jakeya Flood on Friday September 18, 2015. The Class Notes belongs to ITCS 3166 at University of North Carolina - Charlotte taught by Dr. Angelina Tzacheva in Fall 2015. Since its upload, it has received 51 views. For similar materials see Intro to Computer Networks in ComputerScienence at University of North Carolina - Charlotte.
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Date Created: 09/18/15
User Model Unit The Physical 5 Application Layer Layer 4 Transport Layer Segments 7 3 Network Layer Packets 2 Data Link Layer Frames Chapter 2 7 1 Physical Layer Bits 214 4 The Message we re trying to send comes from the application layer H4M H3H4M H2 H3 H4 M Theoretical Basis for Communication 0 Fourier Analysistransport 0 We model the behavior of variation of voltage of current with mathematical functions 0 Fourier series is used Signal sum of its components 0 A binary signal and its rootmeansquare Fourier amplitudes 0 Bandwidthlimited signals 0 Maximum data rate of a channel 0 Nyquists theorem I Maximum data rule 28 log bitssec O Shannon s formula for capacity of a noisy channel maximum number od bitssec B logz 1SN Guided Transmission Media 0 Magnetic Media 0 Twisted Pairs 0 Twisted Pairs cat 5 UTP 39 Category 5 UTP cables with four twisted pairs I Twisted to prevent interference with other cables 0 Coaxial Cables 0 Used for TV cables 0 Parts Copper Core Insulating material Braided outer conductor Protective plastic covering PP N 0 Power Lines 0 A network that uses household electrical wiring 0 The problem There is a lot of interference 0 Parts 1 Electrical cable 2 Power Signal 3 Data Signal 0 Fiber Optics 0 Similar to coaxial cable in that it is a tube except it s made of glass 0 Light trapped by total internal re ection The Electromagnetic Spectrum 0 The electromagnetic spectrum and its uses for communication Visible Light Audible Sound Low frequency Radio Transmission 0 In the VLF LF and MF bands radio waves follow the curvature of the earth Ground Earth39s surface a 0 In the HF band they bounce off the ionosphere ere onOED h K39s I t vquot a f quotquot39quot a f i f f h H a I 5 x I f x a x f N Oquot x x x x x f f x x x x f I wk 7 A I Earth 5 surface The Politics of the Electromagnetic Spectrum 0 ISM and UNII bands used in the United States by Wireless deVices ISM band ISM band ISM band VilaW 353 mo 255 quot T1iIiIll M H 1 Hz Hz MHz MHJE 552 925 24 24535 525 535 54 525 525 MHIE MilI2 Elll Ellll EHIE GilI2 SHE EllII Elll UNll bands Communication Satellites 0 Geostationary Satellites GEO TV 0 VSATS using a hub 0 Used for satellite TV 0 MediumEarth Orbit Satellites MEO 0 LowEarth Orbit Satellites LEO 0 The Iridium satellites form six necklaces around the 0 Relaying in space 0 Relaying on the ground 0 Satellites Versus Fiber Each satellite has 4 neighbors Communication Satellites some properties including altitude above earth round trip delay time and number of satellites for global coverage Altitude km Tye L t y me Sets needed E ai i V are 3 Eai E ai i E ai 153D 1li 3 35 5315 1 SIDED 391 1 The Data Link Layer Chapter 3 3133 Data Link Layer Design Issues 0 Network layer services 0 Framing 0 Error control 0 Flow control Packets and Frames 0 Algorithm for achieving reliable efficient communication of whole units of info called frames between 2 adjacent matrices Efficiency of Data Transfer 0 Communication channels make errors 0 Finite limited data rate bandwidth 0 Propagation delay time time bit sent I time bit received Frames 0 Provide service to network layer 0 Transferring data from network layer from source machine to network layer on destination machine 0 Error Control 0 Flow Control Possible SerVice Offered 1 Unacknowledged connectionless serVice 2 Acknowledged connectionless serVice 3 Acknowledged connectionoriented serVice Framing Methods 1 Byte count 0 Uses a field in the header to specify the number of bytes in the frame receiVing machine knows where frame ends 01111110 00100100101111110 ag ag 2 Flag bytes with byte stuffing 0 Each frame starts and ends with special bytes often byte same called 3 Flag bits with bit stuffing 0 Can be done at the bit level to avoid using the entire byte 68 bits 0 w 0 4 Physical layer coding Violations Error Correcting Codes find and fix the error 1 Hamming codes 2 Binary convolutional codes 3 ReedSolomon codes 4 LowDensity Parity Check codes Error Detecting Codes 0 Linear systematic block codes 1 Parity 2 Checksums 3 Cyclic Redundancy Check CRC Elementary Data Link Protocols 0 Utopian Simplex Protocol 0 Simplex StopandWait Protocol 0 ErrorFree Channel 0 Simplex StopandWait Protocol Noisy Channel PreNotes Next Week CSMA 0 The central theme of this chapter is how to allocate a single broadcast channel among competing users The channel might be a portion of the wireless spectrum in a geographic region or a single wire or optical fiber to which multiple nodes are connected It does not matter In both cases the channel connects each user to all other users and any user who makes full use of the channel interferes with other users who also wish to use the channel Pg258 Pure ALOHA VS Slotted ALOHA 0 Pure ALOHA I Simple I Allows transmission to start immediately I At low load no collisions are expected so the transmission is likely to be successful I Time Continuous I Broadcasting At any time 0 Slotted ALOHA I Has to wait for the next slot I Introduces half a slot time of delay I Time Intervals I Broadcasting Beginning of slot Carrier Sense Multiple Access Protocols 0 Allows LANS to achieve improved utilization compared to ALOHA 0 Persistent CSMA Carrier Sense Multiple Access 0 Nonpersistent CSMA O Ppersistent CSMA CSMACD CSMA with Collision Detection is the basis of the classic Ethernet LAN so it is worth devoting some time to looking at it in detail LimitedContention Protocols 0 Cotentions slotted or pure ALOHA is more preferable if the conditions are light load The light lod conditions produces low decay making collisions rare 0 At high load collision free protocols are favorable since there is an improved channel frequency 0 Limited contention protocols make an attempt to combine the most favorable properties of contention and collision free protocols CollisionFree Protocols I Bit Map I Contention period starts and stations are divided into slote
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