WIXLIB

Propagation modesTransmissionAntennaa) Ground Wave gnalb) Sky Wave PropagationEarthSignalc) Line of Sight PropagationEarthSridhar IyerIIT Bombay37

Modulation Digital modulation– digital data is translated into an analog signal (baseband)– ASK, FSK, PSK– differences in spectral efficiency, power efficiency, robustness Analog modulation– shifts center frequency of baseband signal up to the radiocarrier Motivation– smaller antennas (e.g., λ /4)– Frequency Division Multiplexing– medium characteristics Basic schemes– Amplitude Modulation (AM)– Frequency Modulation (FM)– Phase Modulation (PM)Sridhar IyerIIT Bombay38

Modulation and nalogbasebandsignalanalogmodulationradio 1001radio receiverradiocarrierSridhar IyerIIT Bombay39

Digital modulation Modulation of digital signals known as Shift Keying1 Amplitude Shift Keying (ASK):01– very simple– low bandwidth requirements– very susceptible to interference Frequency Shift Keying (FSK):– needs larger bandwidtht101t Phase Shift Keying (PSK):– more complex– robust against interference101 Many advanced variantstSridhar IyerIIT Bombay40

Multiplexing Mechanisms

Multiplexingchannels kik1k2k3k4k5k6c Multiplexing in 4 dimensionst– space (si)ct– time (t)– frequency (f)– code (c)s1fs2fct Goal: multiple useof a shared mediums3f Important: guard spaces needed!Sridhar IyerIIT Bombay42

Frequency multiplexSeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordinationnecessaryk1k2k3k4k5k6 works also for analog signals c Disadvantages: waste of bandwidthif the traffic isdistributed unevenly inflexible guard spacesftSridhar IyerIIT Bombay43

Time multiplex A channel gets the whole spectrum for a certainamount of time Advantages: only one carrier in themedium at any time throughput high evenfor many usersk1k2k3k4k5k6cf Disadvantages: precisesynchronizationnecessary tSridhar IyerIIT Bombay44

Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amountof timek1k2k3k4k5k6 Example: GSM Advantages:c– better protection againsttapping– protection against frequencyselective interference– higher data rates compared tocode multiplexf but: precise coordinationtrequiredSridhar IyerIIT Bombay45

Code multiplex Each channel has a unique codek All channels use the samespectrum at the same time Advantages:1k2k3k4k5k6c– bandwidth efficient– no coordination and synchronizationnecessary– good protection against interferenceand tappingf Disadvantages:– lower user data rates– more complex signal regeneration Implemented using spreadspectrum technologySridhar IyerIIT Bombayt46

CDMA Example– D rate of data signal– Break each bit into k chips Chips are a user specific fixed pattern– Chip data rate of new channel kD If k 6 and code is a sequence of 1s and 1s– For a ‘1’ bit, A sends code as chip pattern c1, c2, c3, c4, c5, c6 – For a ‘0’ bit, A sends complement of code c1, c2, c3, c4, c5, c6 Receiver knows sender’s code and performs electronicdecode functionSu ( d ) d1 c1 d 2 c 2 d 3 c3 d 4 c 4 d 5 c5 d 6 c6 d1, d2, d3, d4, d5, d6 received chip pattern c1, c2, c3, c4, c5, c6 sender’s codeSridhar IyerIIT Bombay47

CDMA Example User A code 1, –1, –1, 1, –1, 1 – To send a 1 bit 1, –1, –1, 1, –1, 1 – To send a 0 bit –1, 1, 1, –1, 1, –1 User B code 1, 1, –1, – 1, 1, 1 – To send a 1 bit 1, 1, –1, –1, 1, 1 Receiver receiving with A’s code– (A’s code) x (received chip pattern) User A ‘1’ bit: 6 1 User A ‘0’ bit: 6 0 User B ‘1’ bit: 0 unwanted signal ignoredSridhar IyerIIT Bombay48

Spread spectrum technology Problem of radio transmission: frequency dependentfading can wipe out narrow band signals for duration ofthe interference Solution: spread the narrow band signal into a broadband signal using a special code protection againstnarrow band ction atreceiverfsignalspreadinterferencef Side effects:– coexistence of several signals without dynamic coordination– tap proof Alternatives: Direct Sequence, Frequency HoppingSridhar IyerIIT Bombay49

Spread spectrum communicationsSridhar IyerIIT Bombay50Source: Intersil

Effects of spreading and interferencedP/dfdP/dfi)user signalbroadband interferencenarrowband ar Iyerfv)freceiverIIT Bombayf51

DSSS propertiesSridhar IyerIIT Bombay52Source: Intersil

DSSS (Direct Sequence) XOR of the signal with pseudo random number(chipping sequence)– many chips per bit (e.g., 128) result in higher bandwidth of thesignaltb Advantages– reduces frequency selectivefading– in cellular networks base stations can use thesame frequency range several base stations candetect and recover the signal soft handover Disadvantages– precise power control necessarySridhar IyerIIT Bombayuser data01XORtcchippingsequence01101010110101 resultingsignal01101011001010tb: bit periodtc: chip period53

DSSS Transmit/Receivespreadspectrumsignaluser rSridhar IyerIIT Bombay54

Frequency Hopping SpreadSpectrum (FHSS) Signal is broadcast over seemingly random series of radiofrequencies Signal hops from frequency to frequency at fixed intervals Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronizationwith transmitter, picks up message Advantages– Eavesdroppers hear only unintelligible blips– Attempts to jam signal on one frequency succeed only at knockingout a few bitsSridhar IyerIIT Bombay55

FHSS (Frequency Hopping) Discrete changes of carrier frequency– sequence of frequency changes determined via pseudo randomnumber sequence Two versions– Fast Hopping: several frequencies per user bit– Slow Hopping: several user bits per frequency Advantages– frequency selective fading and interference limited to shortperiod– simple implementation– uses only small portion of spectrum at any time Disadvantages– not as robust as DSSS– simpler to detectSridhar IyerIIT Bombay56

Slow and Fast FHSStbuser data01f011ttdf3slowhopping(3 bits/hop)f2f1fttdf3fasthopping(3 hops/bit)f2f1ttb: bit periodSridhar Iyertd: dwell timeIIT Bombay57

FHSS Transmit/Receivenarrowbandsignaluser terreceivedsignalhoppingsequenceSridhar rreceiverIIT Bombay58

OFDM (Orthogonal Frequency Division) Parallel data transmission on severalorthogonal subcarriers with lower ratecfk3tMaximum of one subcarrier frequency appears exactly at a frequencywhere all other subcarriers equal zero superposition of frequencies in the same frequency rangeAmplitudesubcarrier: sin(x)SI function xfSridhar IyerIIT Bombay59

OFDM Properties– Lower data rate on each subcarrier less ISI– interference on one frequency results in interference of onesubcarrier only– no guard space necessary– orthogonality allows for signal separation via inverse FFT onreceiver side– precise synchronization necessary (sender/receiver) Advantages– no equalizer necessary– no expensive filters with sharp edges necessary– better spectral efficiency (compared to CDM) Application– 802.11a, HiperLAN2, ADSLSridhar IyerIIT Bombay60

ALOHAStations transmit whenever they have data to send Detect collision or wait for acknowledgment If no acknowledgment (or collision), try again after arandom waiting timeCollision: If more than one node transmits at thesame timeIf there is a collision, all nodes have to re transmitpacketsSridhar IyerIIT Bombay61

Aloha/slotted Aloha Mechanism– random, distributed (no central arbiter), time multiplex– Slotted Aloha additionally uses time slots, sending mustalways start at slot boundaries Alohacollisionsender Asender Bsender C Slotted Alohatcollisionsender Asender Bsender CSridhar IyertIIT Bombay62

Slotted Aloha Time is divided into slots– slot one packet transmission time at least Master station generates synchronizationpulses for time slots Station waits till beginning of slot to transmit Vulnerability Window reduced from 2T to T;goodput doublesSridhar IyerIIT Bombay63

Error control

Bit level error detection/correctionSingle bit, multi bit or burst errors introduceddue to channel noise– Detected using redundant information sent alongwith data Full Redundancy:– Send everything twice– Simple but inefficient Common Schemes:– Parity– Cyclic Redundancy Check (CRC)– ChecksumSridhar IyerIIT Bombay65

Error detection process Transmitter– For a given frame, an error detecting code (check bits)is calculated from data bits– Check bits are appended to data bits Receiver– Separates incoming frame into data bits and checkbits– Calculates check bits from received data bits– Compares calculated check bits against receivedcheck bits– Detected error occurs if mismatchSridhar IyerIIT Bombay66

Frame level error correction Problems in transmitting a sequence of framesover a lossy link– frame damage, loss, reordering, duplication, insertion Solutions:– Forward Error Correction (FEC) Use of redundancy for packet level error correction Block Codes, Turbo Codes– Automatic Repeat Request (ARQ) Use of acknowledgements and retransmission Stop and Wait; Sliding WindowSridhar IyerIIT Bombay67

Block Code (Error Correction) Hamming distance – for 2 n bit binary sequences, the number ofdifferent bits– E.g., v1 011011; v2 110001; d(v1, v2) 3 For each data block, create a codeword Send the codeword If the code is invalid, look for data with shortest hamming distance(possibly correct code)Datablock (k 2)Codeword (n 5)00 0000001 0011110 1100111 11110Suppose you receive codeword 00100 (error)Closest is 00000 (only one bit different) Efficient version: Turbo CodesSridhar IyerIIT Bombay68

Stop and Wait ARQ Sender waits for ACK(acknowledgement) aftertransmitting each frame; keepscopy of last frame. Receiver sends ACK if receivedframe is error free. Sender retransmits frame ifACKnot received before timerexpires. Simple to implement but maywaste bandwidth. Efficient Version: Sliding WindowSridhar IyerIIT Bombay69

Bandwidth and Delay

Bandwidth Amount of data that can be transmitted per unit time– expressed in cycles per second, or Hertz (Hz) for analogdevices– expressed in bits per second (bps) for digital devices– KB 2 10 bytes; Mbps 10 6 bps Link v/s End to EndSridhar IyerIIT Bombay71

Bandwidth v/s bit widthSridhar IyerIIT Bombay72

Latency (delay) Time it takes to send message from point A to point B– Latency Propagation Transmit Queue– Propagation Distance /SpeedOfLight– Transmit Size / Bandwidth Queueing not relevant for direct links Bandwidth not relevant if Size 1 bit Software overhead can dominate when Distance is small RTT: round trip timeSridhar IyerIIT Bombay73

Delay X Bandwidth product Relative importance of bandwidth and delay Small message: 1ms vs 100ms dominates1Mbps vs 100Mbps Large message: 1Mbps vs 100Mbps dominates1ms vs 100msSridhar IyerIIT Bombay74

Delay X Bandwidth product100ms RTT and 45Mbps Bandwidth 560 KB of dataSridhar IyerIIT Bombay75

TCP/IP Basics

Interconnection devicesBasic Idea: Transfer data from input to output Repeater– Amplifies the signal received on input and transmits it on output Switch– Reads destination address of each packet and forwardsappropriately to specific port– Layer 3 switches (IP switches) also perform routing functions Router– decides routes for packets, based on destination address andnetwork topology– Exchanges information with other routers to learn network topologySridhar IyerIIT Bombay77

Switched networksSridhar IyerIIT Bombay78

TCP/IP layers Physical Layer:– Transmitting bits over a channel.– Deals with electrical and procedural interface to thetransmission medium. Data Link Layer:– Transform the raw physical layer into a link' for thehigher layer.– Deals with framing, error detection, correction andmultiple access.Sridhar IyerIIT Bombay79

TCP/IP layers (contd.) Network Layer:– Addressing and routing of packets.– Deals with subnetting, route determination. Transport Layer:– end to end connection characteristics.– Deals with retransmissions, sequencing andcongestion control.Sridhar IyerIIT Bombay80

TCP/IP layers (contd.) Application Layer:– application'' protocols.– Deals with providing services to users and applicationdevelopers. Protocols are the building blocks of a networkarchitecture.Sridhar IyerIIT Bombay81

Lower layer services Unacknowledged connectionless service– No acknowledgements, no connection– Error recovery up to higher layers– For low error rate links or voice traffic Acknowledged connectionless service– Acknowledgements improve reliability– For unreliable channels. e.g.: wireless systems Acknowledged connection oriented service– Equivalent of reliable bit stream; in order delivery– Connection establishment and release– Inter router trafficSridhar IyerIIT Bombay82

Generic router architectureData HdrHeader ProcessingLookupIP AddressUpdateHeader11BufferMemoryAddressTableData HdrHeader ProcessingLookupIP AddressQueuePacket22UpdateHeaderNQueuetimes lineratePacketBufferMemoryAddressTableN times line rateData HdrHeader ProcessingLookupIP ar IyerQueuePacketIIT Bombay83

Typical TCP behaviourCongestion Window size(segments)14Congestionavoidance1210Slow startthreshold864Slow start20012345678Time (round trips)Sridhar IyerIIT Bombay84

Slow start phase initialize: Cwnd 1 for (each ACK) Cwnd until loss detection OR Cwnd ssthreshRTTHost AHost Bone segmenttwo segmentsfour segmentstimeSridhar IyerIIT Bombay85

Congestion avoidance phase1Sridhar IyerHost Bfour segmRTT/* Cwnd threshold */ Until (loss detection) {every w ACKs:Cwnd } ssthresh Cwnd/2 Cwnd 1 perform slow startHost Aentsfive segmentstimeIIT Bombay86

TCP: Fast retransmit and Fast recoveryWindow size (segments)10advertised window864After fast recovery200246810 12 14Time (round trips)Sridhar IyerIIT Bombay87

802.11 (WiFi) Overview

Wireless LANs Infrared (IrDA) or radio links (Wavelan) Advantages– very flexible within the reception area– Ad hoc networks possible– (almost) no wiring difficulties Disadvantages– low bandwidth compared to wired networks– many proprietary solutions Infrastructure v/s ad hoc networks (802.11)Sridhar IyerIIT Bombay89

Infrastructure vs. Ad hoc networksinfrastructurenetworkAP: Access PointAPAPwired networkAPad hoc networkSridhar IyerIIT Bombay90Source: Schiller

Difference between wired and wirelessEthernet LANWireless LANBABCAC If both A and C sense the channel to be idle at thesame time, they send at the same time. Collision can be detected at sender in Ethernet. Half duplex radios in wireless cannot detect collisionat sender.Sridhar IyerIIT Bombay91

Carrier Sense Multiple Access (CSMA) Listen before you speak Check whether the medium is active before sending apacket (i.e carrier sensing) If medium idle, then transmit If collision happens, then detect and resolve If medium is found busy, transmission follows:– 1 persistent– P persistent– Non persistentSridhar IyerIIT Bombay92

Collision detection (CSMA/CD) All aforementioned scheme can suffer from collision Device can detect collision– Listen while transmitting– Wait for 2 * propagation delay On collision detection wait for random time beforeretrying Binary Exponential Backoff Algorithm– Reduces the chances of two waiting stations picking thesame random timeSridhar IyerIIT Bombay93

Binary Exponential Backoff1.On detecting 1st collision for packet xstation A chooses a number r between 0 and 1.wait for r * slot time and transmit.Slot time is taken as 2 * propagation delayk. On detecting kth collision for packet xchoose r between 0,1,.,(2k –1) When value of k becomes high (10), give up. Randomization increase with larger window, but delayincreases.Sridhar IyerIIT Bombay94

Hidden Terminal ProblemABC– A and C cannot hear each other.– A sends to B, C cannot receive A.– C wants to send to B, C senses a “free” medium(CS fails)– Collision occurs at B.– A cannot receive the collision (CD fails).– A is “hidden” for C.Sridhar IyerIIT Bombay95

Effect of interference rangeTransmission from 1 2 will fail

Solution for Hidden Terminals A first sends a Request to Send (RTS) to B On receiving RTS, B responds Clear to Send (CTS) Hidden node C overhears CTS and keeps quiet– Transfer duration is included in both RTS and CTS Exposed node overhears a RTS but not the CTS– D’s transmission cannot interfere at BRTSDRTSACTSBCTSCDATASridhar IyerIIT Bombay97

Components of IEEE 802.11architecture The basic service set (BSS) is the basic buildingblock of an IEEE 802.11 LAN The ovals can be thought of as the coverage areawithin which member stations can directlycommunicate The Independent BSS (IBSS) is the simplest LAN. Itmay consist of as few as two stationsad hoc networkSridhar IyerBSS1IIT BombayBSS298

802.11 ad hoc network (DCF)802.11 LANSTA1 Di

Wireless networks Access computing/communication services, on the move Wireless WANs – Cellular Networks: GSM, GPRS, CDMA – Satellite Networks: Iridium Wireless LANs – WiFi Networks: 802.11 – Personal Area Networks: Bluetooth Wireless MANs