Computer Networks Study Guide
Computer Networks Study Guide ITCS 3166
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This 31 page Study Guide was uploaded by Jakeya Flood on Thursday September 24, 2015. The Study Guide belongs to ITCS 3166 at University of North Carolina - Charlotte taught by Dr. Angelina Tzacheva in Fall 2015. Since its upload, it has received 426 views. For similar materials see Intro to Computer Networks in ComputerScienence at University of North Carolina - Charlotte.
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Date Created: 09/24/15
ITCS 3166 Exam Study Guide User Model Unit 5 Application Layer 4 Transport Layer Segments 3 Network Layer Packets 2 Data Link Layer Frames 7 1 Physical Layer Bits 2 2124 The Message we re trying to send comes from the application layer H4M H3H4M H2 H3 H4 M The Physical Layer Chapter 1 Application Layer contains a variety of protocols that are commonly needed by users 2 Example HTTP HyperText Transfer Protocol 3 Transport Layer accept data from above it split it up into smaller units if need be pass these to the network layer and ensure that the pieces all arrive correctly at the other end 4 Network Layer controls the operation of the subnet 5 Data Link Layer transforms raw transmission facility into a line that appears free of undetected transmission errors 6 Physical Layer concerned with transmitting raw bits over a communication channel 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 root mean square Fourier amplitudes 0 Bandwidth limited 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 log2 1SN Guided Transmission Media 0 Magnetic Media 0 Twisted Pairs 0 Twisted Pairs cat 5 UTP I Category 5 UTP cables with four twisted pairs I Twisted to prevent interference with other cables I Coaxial Cables 0 Used for TV cables 0 Parts 1 Copper Core 2 Insulating material 3 Braided outer conductor 4 Protective plastic covering 0 Power Lines 0 A network that uses household electrical wiring O 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 sphere K O i 5 xx 039quot39 xx quot39 z 2quot s x if H 5quot I f x f x x x x x N x x x f aquot Earth39s surface 390 The Politics of the Electromagnetic Spectrum 0 ISM and U NII bands used in the United States by wireless devices ISM lbandl ISM band ISM lbandl r25 55quot mu 255 rml Mle MHz MHz 7 MHz Mle 552 52 24 24555 525 555 54 525 525 MI39IIE MllIz El llz Ellll 3le EllI2 GHE Ellll Ellll LJlNll lbandls Communication Satellites 0 Geostationary Satellites GEO TV 0 VSATS using a hub 0 Used for satellite TV 0 Medium Earth Orbit Satellites MEO 0 Low Earth 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 Type me 3315 needle HEIDI GEE 2 3 l 25TH E l ump r lfan Allen belt 151 mane 35 1315 1 also quot 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 44 39 y r 4 39 0 Finite limited data rate bandwidth quot 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 connection oriented 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 flag 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 1 11 11 1 0 4 Physical layer coding violations Error Correcting Codes find and fiX the error 1 Hamming codes 2 Binary convolutional codes 3 Reed Solomon codes 4 Low Density 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 Stop and Wait Protocol Error Free Channel 0 SimpleX Stop and Wait Protocol Noisy Channel Pre Notes 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 0 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 0 Carrier Sense Multiple Access Protocols 0 Allows LANS to achieve improved utilization compared to ALOHA 0 Persistent CSMA Carrier Sense Multiple Access 0 No persistent CSMA O P persistent CSMA I 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 0 Limited Contention Protocols 0 Contentions slotted or pure ALOHA is more preferable if the conditions are light load The light load 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 I Collision Free Protocols I Bit Map I Contention period starts and stations are divided into slots The Medium Access Control Sublayer Chapter 4 41 42 I How to allocate single broadcast channel among competing users 0 Static Channel Allocation 0 Dynamic Channel Allocation 0 Channel Allocation Problem 0 Static channel allocation I How 0 For N users the Bandwidth is divided equally by N I Works well with 0 Works well with small constant number of users with steady or heavy traffic I Does not work well with 0 Doesn t work well with large and varied number of users with bursty traffic If user is dormant then bandwidth is lost 0 Assumptions for dynamic 1 Independent traffic N independent stations each generating frames for transmission Once a frame is generated the station is blocked and does nothing until the frame has been transmitted Single channel One channel is available for all communication Observable Collisions Two frames may be overlapped and signal garbled or result in a collision Collisions must be transmitted again later Continuous or slotted time If time is continuous transmission can begin at any time If it s divided into discrete intervals transmission must begin at the beginning Carrier sense or no carrier Can tell if channel is in use before trying to use it Without carrier sense stations transmit blindly and then check back later to see if transmission was successful 0 Multiple Access Protocols 0 ALOHA I The ALOHA system was established in 1970 s by Norman Abramson It uses group based radio broadcasting where the users could send frames to the central unit I Collisions is one problem that cannot be avoided but can be reduced I Contention period The time interval in which frames can overlap I There are two versions of ALOHA 0 Pure ALOHA that has continuous time 0 Slotted ALOHA that divides time into slots and it was made to double the capacity of ALOHA USE e I j a a I a a 2 a l 1 I Ii D 3 g II L E i i 3l I I II IJ I I II I Callistmm vE Time 7 Willisi m 0 Users transmit whenever they have data to be sent O Stations listens to the channel to find out if the frame was broadcasted 0 If frame was destroyed sender must wait a random amount of time and send it again 0 Carrier Sense Multiple Access ALOHA simply means goodbye in this case Allows LANs achieve a much better utilization than ALOHA Types of Carrier Sense Multiple Access CSMA Persistent CSMA Station listens to channel first then transmits only if the channel is idle O Collisions still an issue 0 BandwidthDelay Product The number of frames that fit on a channel 0 Outperforms pure ALOHA Nonpersistent CSMA Similar to l persistent CSMA but less greedy O Sends a packet only when channel is idle 0 If the channel is busy then the station will check back later 0 Higher channel utilization but longer delays than 1 persistent CSMA Ppersistent CSMA A little bit of both 0 Slotted channels CSMA with Collision Detection O CSMACD Like other CSMA s listens to signal if it is different than the signal it is putting out it knows a collision is occurring 0 Is the basis of the classic Ethernet LAN 0 Collision detection is an analog process 0 If a collision is detected transmission is aborted Examination quotin slots iI 5 Frame U H I Frame D Frame T M Y JV 39 v J Transmission Grizmention l llE p l period Elr i d Time p O Collision free protocols There are no collisions in CSMACD but Collisions still occur in contention period Collisions downgrade performance Solutions include 0 Bit Map Protocol waiting list 0 O O 0 During contention period stations are divided into slots Each station can say it has frames to send during its slot only Once all slots are known they transmit is numerical order After all transmissions another contention period occurs This is a Reservation Protocol reserving the ability to transmit before doing so 0 Token Ring Protocol form a circle 0 0 Basic Token Ring Method Form a Circle I Pass a small message called a token from one station to the next in a predefined order I Token represents permission to send I Order is determined by topology of a network I Frames sent in the same direction of the token I Physical Ring not necessary Can be in a line or bus known as a Token Bus What s Different from BitMap I All positions are equal no bias on high or low numbered stations I Each token does not need to propagate to all stations before the protocol advances to the next step 0 Binary Countdown Protocol top dog takes priority 0 O A station wanting to transmit broadcasts its address as a binary string Implicitly assumes that the transmission delays are negligible All stations see bits essentially instantaneously Bidding for the channel higher addressed stations have priority over the lower numbered stations Simple and very efficient O Limited contention protocols Contention pure or slotted ALOHA is preferable under conditions of light load due to its low delay since collisions are rare Collision free protocols are favorable at high load because of the improved channel efficiency since overheads are fixed Limited contention protocols attempt to combine the best properties of contention and collision free protocols Stations get divided into groups I Only members of group O can compete for slot 0 I If successful the member acquires the channel and transmits its frame I If not the members of group 1 compete for slot 1 0 Wireless LAN protocols Wireless LAN protocol are the rules that govern the interactions between wireless devices I When dealing with wireless LAN the distance between access points has to be taken into consideration I Wireless communications can overlap and cause collisions between packets being sent I Problems such as these are resolved by using various protocols to regulate these interactions 0 How to Assign Stations to Slots 0 Special Cases I Each group has one member No chance for collision binary countdown I Two stations per group Little chance of collision 0 As more stations are assigned to the same slot the probability of collisions grows but the length of the bit map scan needed to give everyone a chance shrinks Limiting Case Single group containing all stations slotted ALOHA 0 What we need I A method to assign stations to slots dynamically with many stations per slot when the load is low and few or even just one station per slot when the load is high 0 Chapter 4 43 45 0 Ethernet a Physical layer b MAC sublayer protocol c Ethernet performance 21 Efficiency of Ethernet at 10 Mbps with 512 bit slot times 1H tumbyte ames 393 39 7 1 LEEhth frame I H ESEbyte frames t B hy te 1m mes m4 Channel efficiency Brillbyte ames m2 v1 l El 1 4 m 1 E I54 1 Number mi sta i ns trying in send 1 Switched Ethernet ijt Line a Hub b Switch C An Ethernet Switch I III e Fast Ethernet a The original fast Ethernet cabling Marne Callble Max segment i Advantages 1008aeeT4 twisted pair 10 m i Uses categoryS UTP 1 1noeaseTx twisted pair 9 10 m Full duplex at 100 Mbpe Cat 5 UTP 1 0039BaeelFX Fiber optics 20039 m Full duplex at 10 Millaps long runs f Gigabit Ethernet a A two station Ethernet Ethernet C emuter b Gigabit Ethernet Cabling Name Cable I Max segment I Advantages i1U UKOBa5eSX i Fiber optics i 550m i Wilultimede fiber 50 625 microns 10 UIUBa5eLX 9 Fiber optics 9 5000 m 9 Single 10 In ormultimode 50 625 p 10 UIOBaseCX 2 Pairs of STP 25 m Shielded twisted pair 39 10 0IOBaseT i 4 Pairs of UliP i 100 m i Standard category 5 UTP g 10 Gigabit Ethernet a Gigabit Ethernet cabling Name Cable Max segment Advantages 1i390GBassSH Fiber optics Up to 300 m Multimon fiber 085m 1i 0G BasseLiFi Fiber optics 10 km Siniglsmooo fiber 13m 1i390GBasseEiFi Fiber optics 40 km Stinlglsemooe fiber 15m 1i390GBasseCX4 4 Pairs oili twinax 15 m Twiniaxiisl copper 1i GlBassT 4 Pairs of lLlill39P 1i 3930 on Category Ba lLlill39iP h IEEE 8022 Logical Link Control i Retrospective on Ethernet 0 Wireless Lans a 80211 architecture and protocol stack 80211 architecture infrastructure mode a ossss Pint L quot 7 is r i 39539 F I E J39 n r quot I II In as L r r r r a 4 a gt i 5 a E a I I 5 Eliont 1 o l If To Moscow C b 80211 architecture ad hoc mode Part of the 80211 protocol stack IMAG euhleyrer Release date Legiieel Ilinrk Meyer 812121 391 legacy J V Upper gt1 Meyers Data llinrk Meyer IFreeluerue1r 30231 1a 1 1 3021 19 hreppiing FIDM E pggf lm 0mm 0mm and infrared 39 39 1 EST 1999 1 999 391 999 20113 2009 Physical Meyer b 80211 physical layer C 80211 MAC sublayer protocol a Sending a frame with CSMNCA Siteiiien eende ie D E eeke A A Date Ace e i I 1 Date i Hell T Weii ifer idile Beekeiff Wail ifer idile Fiveei ef haeke i C ready m send eende ie D D eelire C C i i Date Heir Wail ifer idle Eeeke i I I E reedyiie eeiridi 39 v E eendieie i D eelirs B i r f Time b The hidden terminal problem wenie te semi quotii but eennet heer thet is busy trenemtiingf C The exposed terminal problem wants tn send tn but mistakenly thinks the iransmiss nn will fail d The use of Virtual Channel sensing using CSMNCA A HTS Data B GTE WE I MM I In NM f Time I C all SIFSF Interframe spacing in 80211 Cantrell reme er next ifregImem may be sent lhere 11 else inghplieriw I frerne here 11 DIFS h Regular DEF reme here AIFS4 Lewprieriliy frame here Bed trerIIIe reeeuery dene ax AGIK EIFS F d 80211 frame structure a Format of the 80211 data frame Time E Bytes 2 2 B E E 2 1 2312 4 Frame I Address 1 Address 2 a a I Cheek mmml Dura lilen regiment transmmer Address 31 Sequence Data sequence I H I mseehr I ewee I Hrewee Versien Type Subwpe Te FreIIrII Mere I H Pwr Mere I v as 111 2 as 15 11mg R9quot 11191 ears Pm me i Order Bite 2 2 4 1 1 1 1 1 1 1 1 e Services Broadband Wireless a Comparison of 80216 with 80211 3G a The 80216 architecture Mebille sI39te iens Subscriber stet ens Air inter eee b 80216 architecture and protocol stack s1e1iien Eeelkbene IrIeI39Iwerlk 1e IlIrIIIemet a The 80216 protocol stack Ll v sr HFquot fsr sssmplls 3 332 Bsnr ss spssi s ssnusrgsmss suslsy39sr 3 Data link quot llsyvsr Security subllsyrsr Fixed WiMAK Msbilis WiMAX Physics FDM 21163 Sss strls PUMA 2116s llsyvsr Flsllssss date B i 2005 C 80216 physical layer a Frames structure for OFDMA with time division duplexing Bswnllirik Upllirik I Burst l Burst I I l Burst T m I H E Burst I m E E3 39 E s s I E Em l E E3 I B l m e E l Burst I I Hanging Guard Turns 39s 1 80216 frame structure a A generic frame b A bandwidth request frame Bits1 1 is 11 2 1 11 15 s 4 I J 3 NE Type C EK Length Ennnec ion Headquot Data CHI 3 I CHE Lil3 Bits1 1 E 15 15 s b 1 E1 Type ytes neede Connection H2333quot 0 80216 MAC Sublayer Protocol Classes of service Constant bit rate service Real time variable bit rate service Non real time variable bit rate service Best effort service a b C d 6 Sample questions and Answers 1 In the 21St century as a result of rapid technological progress We have great mechanical systems We have installed worldwide telephone networks C Telephone networks radio and television and the computer industry are quickly converging D Collecting transporting storing and processing information have become distinct and separate tasks 2 Computer networks like the printing press 500 years ago allow ordinary citizens to distribute and view content in ways that were not previously possible What kind of social issues does this new found freedom bring i Network operators may treat different customers differently big well paying companies get good service small time players get poor service ii Peer to peer networks being sued for conducing to phishing messages iii Government versus citizen s rights and FBI snooping on emails at Internet Service Providers iv The private sector profiling users by tracking their cyberspace activities and maintaining large amounts of personal information A i and ii B i and iii C i iii and iv D all of the above 3 Which of the following statements is incorrect in relation to network software A To reduce design complexity most networks are organized as a stack of layers or levels each one built upon the one below it B Each layer below the application layer 5 puts a header in front of the message The header includes control information C A protocol is another name for a connection with associated resources such as a fixed bandwidth D A connectionless service is used for example with electronic junk mail and a database query 4 With the Open Systems Interconnection OSI reference model A The session and presentation layers are not present in the model B The data link layer determines how packets are routed from source to destination C The application layer splits the data into smaller units if need be and ensures the pieces arrive correctly D The function of each layer should be chosen with an eye toward defining internationally standardized protocols 5 The subject of computer networking covers many kinds of networks large and small well known and less well known Which of the following are examples of computer networks i Global System for Mobile communications ii Cable Modem Termination System iii Wireless LANs 80211 iV Radio Frequency Identification and Sensor Networks A i iii and iV B ii iii and iV C i and iV D ii and iii 6 Which of the following statements is incorrect in relation to metric units A 103 1000 is denoted with the prefiX Kilo B 109 1000000000 is denoted with prefiX Tera C A 1MB memory contains 220 1048576 bytes D A 1GB memory contains 230 1073741824 bytes 7 With data communication in the physical layer Fourier analysis is used to A Decompose a signal into its frequency components also called harmonics B Make magnetic media work more efficiently C D Amplify a signal after it travels long distances Shift a signal into occupying a higher range of frequencies 8 Which of the following are examples of guided transmission media 1 ii iii iv A Twisted pair cable of category 5 cat5 A multimode fiber optic cable Frequency hopping spread spectrum Free space optics i and ii B i and iii C D i ii and iv ii iii and iv 9 Communication satellites have properties that make them attractive for many applications Which of the statements below is incorrect in relation to satellites A C D A satellite is like a big microwave repeater in the sky it amplifies the incoming signal and then rebroadcasts it in another frequency to avoid interference with the incoming signal Geostationary satellites move with about the same speed as the earth rotates so they appear motionless in the sky If we use low earth orbit satellites LEO we would need about 10 of them to achieve global coverage A satellite placed between 15000 and 20000 km above earth would be destroyed by cosmic particles 10 Wires and wireless channels carry analog signals such as continuously varying voltage light intensity or sound intensity How do we send digital information A We must devise analog signals to represent bits we use a process of converting between bits and signals called digital modulation By using more than two signaling levels C By using the NRZI non retum to zero inverted coding By balancing the signals so they have as much positive voltage as negative voltage 11 The last mile of the telephone system is the two wire local loop coming from a telephone company end office into houses It has carried analog information for over 100 years and is likely to continue doing so for some years to come due to the high cost of converting to digital What methods are used for Internet connection over the local loop GDP 1 To send bits over the local loop they must be converted to analog signals that can be transmitted over the channel A device that makes this conversion is called a modem modulator demodulator ii The cable TV is offering speeds up to lOMbps on shared cables making it possible to offer another digital service over the local loop called digital subscriber line DSL Telephone companies can upgrade the local loop from copper to fiber to provide faster and better network service called Fiber To The Home FttH iii i and ii i and iii ii and iii All of the above 12 Which of the following statements is incorrect in relation to the mobile telephone system A In all mobile phone systems a geographic region is divided up into cells which is why the devices are sometimes called cell phones Mobile phones have always transmitted digital voice C The third generation of mobile phones or 3G as it is called is about digital voice and data D Wireless networks with 4G levels of performance are already available such as WiMAX ANSWERS l C Telephone networks radio and television and the computer industry are quickly converging 2 C i iii and iv 3 C a protocol is another name for a connection with associated resources such as a fixed bandwidth 4 D The function of each layer should be chosen with an eye toward defining 5 internationally standardized protocols A i iii and iv 0 B 109 1000000000 is denoted with prefix Tera 7 A Decompose a signal into its frequency components also called harmonics 8 A i and ii 9 C if we use low earth orbit satellites LEO we would need about 10 of them to achieve global coverage 10 A we must devise analog signals to represent bits we use a process of converting between bits and signals called digital modulation 11 D all of the above 12 B mobile phones have always transmitted digital voice 1 2 3 4 5 Homework Questions and Answers How a cell is defined a The coverage area of a base transceiver station BTS defines a cell How many subsystems are there in GSM network a A GSM network can be divided into subsystems They are i The MS Mobile Station and a SIM card ii The BSS Base Station Subsystem iii The NSS Network Subsystem iv The OMSS Operation amp Maintenance Subsystem How does cell ensure continuous coverage a To ensure continuous coverage each cell overlaps at the borders slighth into the other cells therefore the coverage area for a single base transceiver station is simplified as a hexagon And many of these hexagons side by side create a cellular structured network that looks like a honeycomb As the number of subscribers steadily increases the density within the network will also increase What are the advantages of fiber optics over copper as a transmission medium Is there any downside of using fiber optics over copper a Fiber Optic cables are lightweight while copper wire cables are weighted b Fiber optic cables are less expensive than copper wire cables A number of fiber optic cables can be made cheaper than the equivalent number of copper wire c Fiber optic cables are thinner than copper wire Fiber optic cables have a smaller diameter Copper wires are thicker d Fiber optic cables are light signals and don t disturb one fiber to another if they were in the same cable Copper wire cables disturb fibers in the same cable 6 Fiber optic cables can carry higher capacity and they are thinner which allows more fibers to be bundled together Copper wire cables don t carry a high capacity and they are thicker than fiber optic cables f Fiber optic cables consume less power than copper wire cables As a result fiber optic cables degrade less and save money and power Copper wire cables consume more power g Fiber optic cables have less signal degradation Copper wire cables result in more lost signals The cost of a fast microprocessor has dropped to the point where it is now possible to put one in each modem How does that affect the handling of telephone line errors Does it negate the need for error checkingcorrection in layer 2 a Handling of Telephone Line Errors b Usually in the physical layer bits are been sent over the line without any error correcting scheme i ii In each modem the presence of a CPU makes it possible to include an error correcting code in layer to greatly reduce the effective error rate seen by layer 2 The error handling by the modems can be done totally transparently to layer 2 Nowadays many modems now have built in error correction this significantly reduces the effective error rate seen at layer 2 but errors at layer 2 are still possible c Error CheckingCorrection in Layer 2 i As stated brie y in part a there is still a possibility of errors in layer 2 A good example is when data is transferred from layer 1 to layer 2 During this transfer data is lot due to the lack of buffer space in layer 2 During this occurrence the error checkingcorrection mechanism is not implemented which results in the error going undetected This means that the Error checkingcorrection mechanism is needed in layer 2 6 The following data fragment occurs in the middle of a data stream for which the byte 7 stuffing algorithm described in the text is used A B ESC C ESC FLAG FLAG D What is the output after stuffing i The output after stuffing is A B ESC ESC C ESC ESC ESC FLAG ESC FLAG D One of your classmates Scrooge has pointed out that it is wasteful to end each frame with a ag byte and then begin the next one with a second flag byte One flag byte could do the job as well and a byte saved is a byte earned Do you agree i No If you could always have an endless stream of frames where there are no errors one flag byte might be enough and that would be an ideal situation The problem is that ideal doesn t always present itself so what if there is a gap between the first and second frame The receiver wouldn t be able to tell when the first frame has finished or if it is even finished or not The process is simpler when you have both ags It also makes it more stable 8 What is the use of Data Link Layer a The role of the data link layer is to prepare the network layer packets for transmission and to control access to the phvsical media It also exists as a connecting layer between the software processes of the layers above it and the physical layers below it It relieves the upper layers of the responsibility of putting data on the network and receiving data from the network Another use of the data link layer is to prepare a network packet for transport across the local media by encapsulating it with a header in a trailer to create its own protocol data unit PDU 9 What are the 2 sub layers of data link layer and brief their purpose a The Upper Sub Layer Logical Link Control i Places information in the frame that identifies which network layer protocol is being used for the frame and this information allows multiple layer 3 protocols to utilize the same network interface and media b The Lower Sub Layer Media Access Control i Regulates the placement of data frames on to the media It also provides addressing and delimiting of data according to the physical signaling requirements of the medium Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway 10 What information does the frame header and trailer contain a The frame header contains the control information for the specific logical topology and media used Frame control information is unique to each type of protocol but typical frame header fields begin with a start frame which tells devices along the way that a frame is coming 11 Consider the delay of pure ALOHA versus slotted ALOHA at low load Which one is less Explain your answer i Pure ALOHA is a simple system It allows transmission to start immediately With it some collisions will happen and the colliding frames will be damaged but at low load no collisions are projected This allows the transmission be sent successfully Slotted ALOHA has to wait for the next slot which presents half a slot time of delay 12 In the binary countdown protocol explain how a lower numbered station may be starved from sending a packet i In a situation where a higher numbered station and a lower numbered station have to send packets simultaneously the higher numbered station will always come out on top Because of this a lower numbered station will be starved from sending its packets if there are higher numbered stations sending packets at the same time and especially If there are multiple higher numbered packets 13 What are the 7 Layers of OSI Reference Model and their purpose 1 Application Layer This layer supports application and end user processes 2 Presentation Layer The presentation layer works to transform data into the form that the application layer can accept 3 Session Layer This layer establishes manages and terminates connections between applications 4 Transport Layer This layer provides transparent transfer of data between end systems or m and is responsible for end to end error recovery and flow control It ensures complete data transfer 5 Network Layer This layer provides switching and routing technologies creating logical paths known as virtual circuits for transmitting data from m to node 6 Data Link Layer At this layer data packets are encoded and decoded into bits It furnishes transmission protocol knowledge and management and handles errors in the physical layer ow control and frame synchronization 7 Physical Link Layer This layer conveys the bit stream electrical impulse light or radio signal through the network at the electrical and mechanical level 14 Brie y explain the process that takes place while passing the message from one host to another host a When the message has to be sent from one host to the another host the host which is sending the message communicates with the Application layer this layer communicates with the presentation layer below it and the communication goes all the way down to the Physical layer as the data goes down the layers more information is added such as packet headers and checksums the data is then communicated across the network over the physical layer The Physical layer on the other side will check the checksum and passes the data to the layers above as the data passes to the layers above the all the headers and checksums are removed and the data is presented to the application in the way it can understand 15 What is the purpose of Layer 2 and Layer 3 devices a Layer 2 supports local network switching Ex Switches operating at Layer 2 b Layer 3 supports routing between networks Ex Routers operate at Layer 3 16 What are the various protocolsservices in the each layer of TCPIP a Application layer HTTP SMTP DNS RIP b Transport layer TCP Reliable UDP Unreliable c Internet layer IPv4 IPv6 17 Difference between 081 Model and TCPIP Model a Application layer b Transport layer c Internet layer d Network Interface 18 The performance of a client server system is strongly in uenced by two major network characteristics the bandwidth of the network that is how many bitssec it can transport and the latency that is how many seconds it takes for the first bit to get from the client to the server Give an example of a network that exhibits high bandwidth but also high latency Then give an example of one that has both low bandwidth and low latency a A transcontinental fiber link might have many gigabitssec of bandwidth but the latency will also be high due to the speed of light propagation over thousands of kilometers In contrast a 56 kbps modem calling a computer in the same building has low bandwidth and low latency 19 Mobile phone network operators need to know where their subscribers mobile phones hence their users are located EXplain why this is bad for users Now give reasons why this is good for users a Having mobile phone operators know the location of user s lets the operators learn much personal information about users such as where they sleep work travel and shop This information might be sold to others or stolen it could let the government monitor citizens On the other hand knowing the location of the user lets the operator send help to the right place in an emergency It might also be used to deter fraud since a person who claims to be you will usually be near your mobile phone
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