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Computer Networks Ch.2 Direct Link Networks Lecturer: Tae-Hyong Kim (D132) [email protected] Problem: Physically Connecting Hosts How to connect hosts physically? Suitable medium Framing Encoding Media access control Error Detection Reliable Delivery 2 Contents Signals Sine Wave Ti me & Frequency Domain Composite Signals Digital Signals Shannon Capacity Hardware Building Blocks Encoding Framing Error Detection Reliable Transmission Project #1 Ethernet Wireless Assignment #2 3 Signals Analog and digital signals Analog signal: infinitely many levels of intensity continuous Digital signal: only a limited number of defined values discrete Periodic and Nonperiodic Signals Period? What signals can be used for data transmission? 4 Sine Wave Sine wave s(t) = Asin(2ft + ) s : instantaneous amplitude A : peak amplitude f : frequency : phase Period = ? 5 Sine Wave Period and Frequency Period: the amount of time a signal needs to complete one cycle Frequency: the number of periods in one second they are inverses of each other T = 1/f Ex. 100 ms = 10-1 s 1/(10-1) Hz = 10Hz = 10-2KHz 6 Sine Wave More about Frequency Frequency: the rate of change wrt. time. Two Extremes Change in a short span of time high frequency Change over a long span of time low frequency A signal does not change at all frequency = 0 (DC) A signal changes instantaneously frequency = infinite Phase Position of the waveform relative to time zero 7 Wavelength Distance a simple signal can travel in one period Wavelength() = propagation speed(c) period(T) = propagation speed(c) / frequency(f) Usually used for the transmission of light in an optical fiber Depend on both the frequency of a signal and the medium = cT = c/f Ex.1 the wavelength of red light (f = 41014 Hz) in air: = c/f = (3108)/(41014) = 0.75m 8 Time and Frequency Domain Time-domain plot Frequency-domain plot instantaneous amplitude with respect to time. maximum amplitude with respect to frequency Analog signals are best represented in the freq. domain Relation 9 Time and Frequency Domain Relation The frequency domain is more compact and useful when we are dealing with more than one sine wave 10 Composite Signals Usage of a single sine wave Carry of electric energy (power) single tone not useful in data communications to make signals that can carry information, we have to add several different sine waves (composite signals) Composite Signals A periodic signal decomposed into two or more sine waves. Fourier Analysis (Transform) is used to decompose a composite signal into its components 11 Composite Signals Fourier Analysis Any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes An example: a square wave 12 Composite Signals Fourier Series Periodic time domain signals discrete frequency domain signals 13 Composite Signals Fourier Transform Nonperiodic time domain signals continuous frequency domain signals 14 Composite Signals Fourier Analysis An example: a square wave First three harmonics : f, 3f, 5f Adding first three harmonics 15 Composite Signals Fourier Analysis An example: a square wave Frequency spectrum comparison 16 Composite Signals Fourier Analysis An example: nonperiodic composite signal e.g., voice level (microphone) 17 Bandwidth Definition: The range of frequencies contained in a composite signal the difference between the highest and the lowest frequencies 18 Bandwidth Example: Which signal has a wider bandwidth, a sine wave with a frequency of 100Hz or a sine wave with a frequency of 200Hz? 19 Digital Signals Digital signals with different signal levels if a signal has L levels each level needs log2L bits e.g., 8 levels no. of bits per level = log28 = 3 20 Bit Rate the number of bit sent in 1 second (bps) Ex. 100 page (24line*80col) text per minute 100*24*80*8 = 1,636,000 bps = 1.636 Mbps Ex. HDTV : 1920*1080, refresh rate : 30/s, 24 bit color depth 1920*1080*30*24 = 1,492,992,000 ≈ 1.5Gbps 21 Bandwidth of Digital Signals A digital signal is a composite analog signal Bandwidth = ? 22 Digital Signal Transmission Two Approaches Baseband transmission: digital ( digital) Broadband transmission: digital analog BW of Physical Medium The frequency BW that medium can pass 23 Bit Rate and BW The required BW for the given bit rate Nyquist theorem (Noiseless assumption) Bit Rate (n) 2BW log2L (L = # of signal levels) Why? Ex.1 Consider the same noiseless channel, transmitting a signal with four signal levels. The maximum bit rate is: BitRate = 2 3000 log24 = 12,000bps Ex.2 We need to send 256 kbps over a noiseless channel with a BW of 20kHz. how many signal levels do we need? 256,000 = 2 20,000 log2L log2L = 6.625, L = 26.625 = 98.7 levels 128 levels 24 Shannon Capacity Theoretical highest data rate for a noisy channel: Capacity = BW log2(1+SNR) (bps) SNR (Signal-to-Noise Ratio) = Signal Power/Noise Power Ex.2 What is the theoretical highest bit rate of a regular telephone line? (BW:3000 hz, SNR : 3162 (35 dB)) C=3000log2(1+3162)=3000log23163=34,680 bps How are 56kbps modems possible? 25 Using Both Limits Shannon capacity the upper limit Nyquist formula no. of signal levels Ex.1 We have a channel with a 1MHz BW. The SNR for this channel is 63; what is the appropriate bit rate and signal level? Upper limit: by Shannon formula Capacity = B log2(1+SNR) = 106 log2 (1+63) = 106 log2 64 = 6Mbps Let’s choose 4Mbps for better performance Then, the number of signal levels: by Nyquist formula 4M bps = 2 1 MHz log2L L=4 26 Broadband Transmission Modulation : digital signal analog signal Why? Examples? 27 Electromagnetic Spectrum Speed of Electromagnetic waves = Speed of light 3108m/s 28 Contents Signals Hardware Building Blocks Nodes Links Encoding Framing Error Detection Reliable Transmission Project #1 Ethernet Wireless Assignment #2 29 Nodes Workstations Hosts, switches, routers 30 Nodes Network adaptor 31 Nodes Why throughput < BW? Nodes Sender Network Receiver Node design Memory, bus, … Congestion Where? 32 Cables Common types of cables 33 Leased Lines Common BW (service) available from the carriers Line T1 T3 OC-1 OC-3 OC-12 OC-24 OC-48 34 Last-Mile Links Common services available to connect your home Service BW ADSL ~12Mbps / ~1.3Mbps VDSL ~55Mbps / ~2.3Mbps Cable Modem 10Mbps Metro Optical Ethernet 100Mbps, 1Gbps VDSL 35 Wireless Links Radio band should be licensed ISM(Industrial, Scientific, Medical) Bands Ex. 900MHs, 2.4GHz, 5.8GHz Service BW Wi-Fi (802.11G/A) 54Mbps Wi-Fi(802.11N) 150, 300, 600Mbps Bluetooth (2.0EDR) 2.1Mbps HSDPA (3.5G) 14Mbps WiMax(802.16) 75Mbps LTE(3.9G) 100Mbps LTE-Advanced(4G) 1Gbps 36 Contents Signals Hardware Building Blocks Encoding NRZI, RZ, Manchester, Block coding Decoding Problems Framing Error Detection Reliable Transmission Project #1 Ethernet Wireless Assignment #2 37 What is Encoding? Encoding Data Code (Bits? Signals?) Signal encoding is required for physical transmission cf. Modulation Digital (data) to digital (signal) encoding Line coding Bit-by-bit encoding NRZ, NRZI, Manchester, … block coding (Prior to line coding) Block-by-block encoding 4B/5B, 8B/10B, … 38 Basic Encoding: NRZ NRZ(NonReturn to Zero) Decoding problems? (consecutive 1’s or 0’s) Baseline wander Average level of the encoded signal? Clock recovery How to decode correctly when clock difference or signal delay change? 39 How to Solve Decoding Problems? Baseline Wander Clock Recovery How to make average = 0? Separate clock transmission? Encoding Design Problem No Baseline Wander Self Clock Recovery Signal BW , bit rate Reliability (special features) 40 Bit Rates and Baud Rates Signal element vs. Data Element r = no. of data elements carried by each signal element 41 Bit Rates and Baud Rates Data rate vs. Signal rate data rate (N): no. of data elements (bits) sent in 1s (bit rate) signal rate (S): no. of signal elements sent in 1s (baud rate, symbol rate) BW is related to Bit rate? or Baud Rate? How to increase bit rate while decreasing baud rate? S = c ×N × 1/r (baud) c: case factor (0≤c≤1), depends on no. of 0's and 1's usually BW S Ex.1 A signal is carrying data in which one data element is encoded as one signal element (r=1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? caverage = 1/2, S = c×N×1/r = 1/2×100,000×1=50,000=50k baud 42 NRZI NRZI(NRZ Inverted) r =? Consecutive 1’s? Consecutive 0’s? Signal BW (baud rate)? 43 RZ RZ (Return-to-Zero) uses three values (positive, negative, zero) 1 : positive-to-zero 0 : negative-to-zero Two signal changes for a bit BW? Baseline Wander? Clock Recovery? r = 1/2, BW(RZ) = 2 BW(NRZ) 44 Manchester Manchester & Differential Manchester Baseline wander? Clock Recovery? Signal BW (baud rate)? (r=?) 45 Block Coding 4B/5B 1 Redundant bit (cost), why? No more than 3 consecutive 0’s are encountered How to solve consecutive 1’s? NRZI Unused code(25-24)? cf. 8B/10B Data 0000 0001 0010 0011 0100 0101 0110 0111 Code 11110 01001 10100 10101 01010 01011 01110 01111 Data 1000 1001 1010 1011 1100 1101 1110 1111 Code 10010 10011 10110 10111 11010 11011 11100 11101 46 Contents Signals Hardware Building Blocks Encoding Framing Byte-Oriented vs. Bit-Oriented Framing Approaches Error Detection Reliable Transmission Project #1 Ethernet Wireless Assignment #2 47 What is Framing? Break bitstream into a frame Typically implemented by network adaptor Design problem in framing Point-to-point links Multipoint links 48 Byte-Oriented Protocols Frame a collection of bytes (characters) Examples BISYNC(Binary Synchronous Comm.) (IBM, 1967) DDCMP(Digital Data Comm. Message Prot.)(DECNET) PPP(Point-to-Point Protocol) (RFC1661) 49 Bit-Oriented Protocols Frame a collection of bits Examples HDLC(High-level Data Link Control)(ISO) Ethernet, LLC, … How to construct a frame? Frame starting? Header? Trailer? 50 Framing Approaches Sentinel-based Delineate frame with special character or pattern BISYNC, DDCMP: SYN character PPP, HDLC: Flag(01111110) Problem: special char. or pattern appears in the payload Charater stuffing (Byte-Oriented Protocols) Extra characters are inserted (ESC character) Bit stuffing (Bit-Oriented Protocols) Sender: insert 0 after five consecutive 1’s (why?) Receiver: delete 0 that follows five consecutive 1’s 51 Framing Approaches Counter-based (Byte-Counting) When payload length is variable Include payload length in header E.g., DDCMP Problem: count field corrupted Solution: catch when CRC fails Cf. CRC(Cyclic Redundancy Check) 52 Framing Approaches Clock-based Each frame is a specific time long E.g., SONET(Synchronous Optical Network) (125s) STS-n multiplexing (STS-1 = 51.84Mbps) concatenated 53 Contents Signals Hardware Building Blocks Encoding Framing Error Detection Error Detection and Error Correction 2D parity, Checksum, CRC Reliable Transmission Project #1 Ethernet Wireless Assignment #2 54 Type of Errors Single-bit error The least likely type of error in serial data transmission Burst error two or more bits in the data unit have changed Does not necessary mean that the errors occur in consecutive bits 55 Detection Vs. Correction Redundancy Adding extra bits and Sending Error Detection Has any error occurred? Yes, or No Error Correction the number of bits corrupted the location of corrupted bits ex. the number of bits: 10 bits, 1000 bit message # of possibility: 1000C10 = 1000×999×…×991/(10×9×…×1)=? Error correcting code (FEC) vs. Retransmission 56 Error Detecting Code How do we create redundant bits? Cost? Performance? Parity Check One bit is added (even-parity bit, odd-parity bit) Performance? Two-Dimensional Parity CRC (Cyclic Redundancy Check) Checksum 57 Modular Arithmetic Features a limited range of integers are used : 0 ~ N-1 an upper limit: N Modulus Modulo-N arithmetic No carry when you add two digits in a column No carry when you subtract one digit from another in a column Modulo-2 arithmetic: N = 2 (0~1) Adding: 0+0 = 0, 0+1=1, 1+0=1, 1+1=0 Subtracting: 0−0 = 0, 0−1=1, 1−0=1, 1−1=0 same result same as XOR operation 58 Two-Dimensional Parity Cost? Added bits? Performance? One-bit error? 2-bit, 3-bit error? 4-bit error? 59 Checksum Sum (1's complement arithmetic wrapped sum) If the number has more than n bits, the extra leftmost bits is added to the n rightmost bits (wrapping) Checksum Inverting all bits of sum (negative value) 60 Checksum Routine in C (16-bit checksum) u_short cksum(u_short *buf, int count) { register u_long sum = 0; while (count--) { sum += *buf++; if (sum & 0xFFFF0000) { /* carry occurred, so wrap around */ sum &= 0xFFFF; sum++; } } return ~(sum & 0xFFFF); } Cost & Performance? 61 CRC(Cyclic Redundancy Check) Cyclic code If a codeword is cyclically shifted (rotated), the result is another codeword E.g., CRC code with C(7,4) 62 CRC(Cyclic Redundancy Check) Encoding concept 63 CRC(Cyclic Redundancy Check) Decoding concept 64 CRC Polynomial Polynomial representation CRC divisor using polynomial 65 CRC Performance Some definitions Message: M(x) Transmitted Message: P(x) Divisor: C(x) Error: E(x) In a cyclic code: If P(x)/C(x) ≠ 0, one or more bits is corrupted If P(x)/C(x) = 0, either No bit is corrupted, or Some bits are corrupted, but the decoder failed to detect them Received codeword = P(x) + E(x) Received codeword/g(x) = P(x)/C(x) + E(x)/C(x) E(x) errors that are divisible by C(x) are not caught 66 CRC Performance Single-bit error E(x) = xi (i: the position of the bit) If a single-bit error is caught, xi is not divisible by C(x) C(x) has more than one term and the coefficient of x0 is 1 all single-bit errors can be caught e.g., C(x) = x3+1 Two isolated single-bit error E(x) = xj + xi (j – i = the distance between the two errors) E(x) = xi (xj-i + 1), j-i>1, i≥0 If C(x) has more than one term and one term is x0 it cannot divide xj. And if C(x) cannot divide xt+1 (2≤t≤n-1) (at least 3 terms) all isolated double errors can be detected (n=degree of P(x)+1) 67 CRC Performance Odd number of errors C(x) containing (x+1) can detect all odd numbers of errors Proof? e.g., x4+x2+x+1 = (x+1)(x3+x2+1) Burst errors E(x) = xj+…+xi (two terms or more) E(x) = xi(xj-i+…+1) If C(x) can detect a single error, it cannot divide xi the remainder of (xj-i+…+1)/(xr+…+1) must not be zero (C(x)=xr+…+1) If (j – i < r), the remainder can never be zero j – i = L – 1 (L=error length) (L – 1 < r) (L < r+1) (L ≤ r) All burst errors with L≤r will be detected 68 CRC Summary (1) Characteristics of Good Polynomial Divisor 1. It should have at least three terms 2. The coefficient of the term x0 should be 1 3. It should have the factor x+1 Standard Polynomials 69 CRC Summary (2) Advantages of CRC Very good performance in detecting single-bit errors, double errors, an odd number of errors, and burst errors can be easily implemented in hardware 70 Contents Signals Hardware Building Blocks Encoding Framing Error Detection Reliable Transmission Stop-and Wait Sliding Windows Go-Back-N, Selective Repeat Project #1 Ethernet Wireless Assignment #2 71 Flow and Error Control Flow control Error Control Restrict the amount of data that the sender can send before waiting for acknowledgment Based on Automatic Repeat reQuest (ARQ): the retransmission of data Protocols Stop-and-Wait ARQ Sliding Window Go-Back-N ARQ Selective Repeat ARQ 72 Stop-and-Wait Basic Stop-and-Wait Scenarios (1) No error and frame lost 73 Stop-and-Wait Basic Stop-and-Wait Scenarios (2) How to solve the scenario (d)? 74 Stop-and-Wait SAW with sequence number (1-bit) Can this solve scenario (d)? Cost? Ack number rule? Link Utilization (U) ttrans = x, tprop = y, a=y/x Perceived latency 2y + x U = x/(2y+x)=1/(2a+1) U L/(RTTBW)=x/2y=1/(2a) E.g. 1.5Mbps link, RTT=45ms , frame size = 1KB, U=? How to increase U? 75 Stop-and-Wait Link Utilization (at error condition) The time for transmission of the frame in the case that a frame lost or that its ACK is lost. Two transmission attempts are required for succesful transmission. T Tframe Timeout Tframe 2Tprop assume that Timeout value is equal to twice Tprop (In fact, slightly longer) T 2(Tframe 2Tprop ) N r : the average of times each frame must be transmitted T N r (Tframe 2Tprop ) normalized throughput (link utilization) U Tframe 1 N r (Tframe 2Tprop ) N r (1 2a) 76 Stop-and-Wait Link Utilization (at error condition) the probability p: probability that a single frame is in error Pr[exactly k attempts] Pr[(k - 1) unsuccessful attempts] Pr[successful attempts] p k 1 (1 p) N r E[transmissions] (i Pr[i transmissions]) i 1 1 1 p i 1 normalized throughput (link utilization) (ip i 1 (1 p)) U 1 1 p N r (1 2a) 1 2a 77 Stop-and-Wait Link Utilization (at error condition) Performance of stop-and-wait protocol (p=10-3) 1 1 p U N r (1 2a ) 1 2a 78 Sliding Window How to improve the efficiency of SAW? Concept in timeline Sequence number modulo-2m (m: the size of the sequence number field in bits) e.g., m=3 0,1,2,3,4,5,6,7,0,1,2, ... How to manage frames to be sent? 79 Sliding Window Sliding window on sender LFS – LAR SWS Sliding window on receiver LAF-LFR RWS Notations SWS: Send Window Size, RWS: Receive Window Size LAR: Last Ack Received, LFS: Last Frame Sent LFR: Last Frame Received, LAF: Last Acceptable Frame 80 Sliding Window The receiver, when receiving a frame with SeqNum if (SeqNum ≤ LFR) or (SeqNum > LAF) (out of valid range) Discard the frame? Let’s consider in more detail later If (LFR < SeqNum ≤ LAF) (within range) Accept the frame! How to send an ACK? Ack Number? SeqNumToAck all frames upto SeqNumToAck have been received ACK may be sent for each frame or cumulative frames Set LFR SeqNumToAck and LAF LFR+RWS 81 Sliding Window 82 Sliding Window 83 Sliding Window Design Issues When the receiver receives the frame with (LFR<SeqNum≤LAF) but (SeqNum ≠ LFR+1) ACK? AckNum? Negative ACK(NAK)? Selective ACK(SACK)? Pros and Cons? When the timer for the lost frame expires, Send the frame only? Send the frame and the subsequent frames that have been sent together? Pros and Cons? 84 Sliding Window Well-known sliding window ARQ protocols Go-Back-N Retransmit all subsequent frames sent if frame error RWS = 1 Motivation? Effect? Selective Repeat (Selective Reject) Retransmit the error frame only RWS = SWS Motivation? Effect? What if RWS > SWS? 85 Sequence Numbers and Sliding Window Sequence numbers m bits in header 0 … 2m-1 Sequence numbers and sliding window size If sequence numbers are 0 … MaxSeqNum-1, what is the maximum size of SWS (to increase channel utilization)? SWS > MaxSeqNum, possible? SWS = MaxSeqNum, possible? SWS ≤ MaxSeqNum – 1, sufficient? When Go-Back-N ARQ (RWS=1)? When Selective Repeat ARQ (RWS=SWS)? 86 Sequence Numbers and Sliding Window Go-Back-N ARQ (MaxSeqNum = 4) 87 Sequence Numbers and Sliding Window Selective Repeat ARQ (MaxSeqNum = 4) SWS < (MaxSeqNum+1)/2 88 Go-Back-N ARQ Performance Timing of sliding window mechanism (error-free) ≥ 89 Go-Back-N ARQ Performance Timing of sliding window mechanism (error-free) 90 Go-Back-N ARQ Performance The utilization of error-free sliding window mechanism W 2a 1 1 U W 2a 1 W 2a 1 W=2n-1 W=1: stop and wait W=7: many case W=127: high speed WANs 91 Go-Back-N ARQ Performance The utilization of Go-back-N ARQ N r E[number of transmitted frames to succesfully transmit one frame] f (i ) p i 1 (1 p) where f (i) is the number of frames transmitted i 1 if the original frame must be transmitted i times. f (i ) 1 (i 1) K (1 K ) Ki (K : number of frames to be retransmitted) K 1 p Kp 1 p 1 p i 1 i 1 K is approximately equal to (2a 1) for w (2a 1), and K W for W (2a 1) 1 p U U No error / N r W 2a 1 1 2ap W (1 p) U U No error / N r W 2a 1 (2a 1)(1 p Wp) N r (1 K ) p (1 p ) K ip i 1 (1 p ) 1 K i 1 92 Selective Repeat ARQ Performance The utilization of selective repeat ARQ N r average number of transmitted frames to succesfully transmit one frame 1 (p: probability that a single frame is in error) 1 p U' U (U ' error-free utilization of sliding window mechanism) Nr Nr W 1 U' W 2a 1 W 1 p U W (1 p ) 2a 1 2a 1 2a 1 W 2a 1 W 2a 1 93 Selective Repeat ARQ Performance The utilization of selective repeat ARQ 94 Selective Repeat ARQ Performance The utilization of selective repeat ARQ 95 Implementation of Sliding Window Protocol stack (assumption) HLP HLP SWP SWP LINK LINK Frame header 96 Implementation of Sliding Window State of SW protocol 97 Implementation of Sliding Window sendSWP() send(SWP, packet) 98 Implementation of Sliding Window deliverSWP() 99 Implementation of Sliding Window 100 Implementation of Sliding Window swpInWindow() 101 Implementation of Sliding Window Piggybacking In full-duplex transmission Frame can carry both user data with SeqNum and AckNum Example: HDLC 102 The Role of Sliding Window Algorithm To reliably deliver frames across an unreliable link To preserve the order in which frames are transmitted To support flow control A feedback mechanism by which the receiver is able to throttle the sender 103 Contents Signals Hardware Building Blocks Encoding Framing Error Detection Reliable Transmission Project #1 Ethernet Wireless Assignment #2 104 Project #1 Mandatory Implement Selective Repeat ARQ (SWS=RWS) algorithm in C Refer to Code in Textbook Selective (1) Implement real-time Simulator of Selective Repeat ARQ (2) Implement visual trace-based Simulator of Selective Repeat ARQ (3) Implement performance-based Simulator of Selective Repeat ARQ 105 Project #1 Requirements and Materials Discrete Event-based Simulator Engine will be provided Study Discrete Event-Driven Simulation With Slides Lecture movie Configure Simulation Environment Design Simulator UI Show simulation information and/or performance result Detailed guideline will be provided 106 Contents Signals Hardware Building Blocks Encoding Framing Error Detection Reliable Transmission Project #1 Ethernet IEEE Standards Physical Properties Frame Format Transmitter Algorithm Evolution of Ethernet Wireless Assignment #2 107 IEEE Standards IEEE Standard for LAN: Project 802 108 IEEE Standards Data Link Layer Logical Link Control (LLC) in charge of flow control, error control, framing (partly) 109 IEEE Standards Data Link Layer Physical Layer Media Access Control (MAC) dependent on the implementation and type of physical media used Ethernet evolution through four generations 110 Physical Properties Categories of standard Ethernet 111 Physical Properties 10Base5 (Thick Ethernet) 10Base2 (Thin Ethernet) 112 Physical Properties 10Base-T: Twisted Pair Ethernet 113 Physical Properties Maximum link length 2500m with repeaters Collision domain Hosts are competing for access 114 Frame Format 802.3 MAC frame ~1500B Ethernet V2 frame PDU Type ~1500B 115 Frame Format Preamble (7B) SFD (1B: 10101011) indicates a last chance for synchronization Destination/Source address (6B) serial number on the NIC (unique) Broadcast address (only for DA) : all 6 bytes set to 1 Length/Type (2B) for synchronization of receiver’s H/W with the incoming signal bit pattern : 10101010….. Added at the physical layer (not formal part of the frame) < 1518 (802.3): length field length of data field > 1536 (V2): the type of the PDU packet encapsulated in the frame Data (46-1500B) CRC (4B): CRC-32 116 Frame Format Frame length: minimum and maximum The maximum length (1518B): historical reasons by reducing size of buffer, for preventing monopolizing the medium The minimum length (64B): for CSMA/CD If there is a collision before the physical layer sends a whole frame, it must be heard by all stations 64 bytes for 10Mbps Ethernet (collision domain = 2500m) If the upper-layer packet is less than 46 bytes, padding is added to make up the difference 117 Frame Format Addressing 6-byte physical address on network interface card (NIC) Source address unicast (only one station) Destination address Unicast: 1-to-1 Multicast: 1-to-many Broadcast: 1-to-all (FFFFFFFFFFFF) 118 Access Control Methods Who, When can send data? Random Access Controlled Access CSMA/CD, CSMA/CA Token Bus, Token Ring Reservation-based Polling-based Channelization (Data Link Layer Techniques) FDMA, TDMA, CDMA 119 CSMA(Carrier Sense Multiple Access) Approach Minimize the chance of collision increase the performance Each station first listen to the medium before sending (CS) Reduces the possibility of collision but not eliminate due to propagation delay Collision in CSMA 120 CSMA Persistence Method The procedure for a station that senses a busy medium 1-persistent, nonpersistent, p-persistent 121 CSMA Persistence Method 1-persistent, nonpersistent, p-persistent (cont.) if p=1 122 CSMA/CD (Collision Detection) Approach A station monitors the medium after it sends a frame to see if the transmission was successful, If so, the station is finished, otherwise, the frame is sent again 123 CSMA/CD Procedure 124 CSMA/CD Minimum frame size Before sending the last bit of the frame, the sending station must detect a collision frame transmission time ≥ 2 × max. propagation time L/R ≥ 2dmax/V L ≥ 2Rdmax/V 125 Evolution of Ethernet Four Generation Ethernet 126 Fast Ethernet Goals Upgrade the data rate to 100 Mbps Make it compatible with Standard Ethernet Keep the same 48-bit address Keep the same frame format Keep the same minimum and maximum frame length Access Method Half duplex: CSMA/CD (collision domain = 250m WHY?) Full duplex: no CSMA/CD The implementations keep CSMA/CD for backward compatibility Minimum and maximum frame size: same as those of Ethernet 127 Fast Ethernet Autonegotiation Allows two devices to negotiate the mode or data rate of operation In order to allow incompatible devices to connect one another In order to allow one device to have multiple capabilities In order to allow a station to check a hub’s capabilities Implementations 128 Gigabit Ethernet Goals Upgrade the data rate to 1Gbps Make it compatible with Standard or Fast Ethernet Keep the same 48-bit address Keep the same frame format Keep the same minimum and maximum frame length To support autonegotiation as defined in Fast Ethernet Usage Backbone, high-speed links Implementation 129 Gigabit Ethernet Access method Full-duplex Mode No collision CSMA/CD is not used the maximum length of the cable is determined by the signal attenuation in the cable Half-duplex Mode (1000BaseT) Traditional Slot time for Gigabit Ethernet: 5.12 bit = 0.512μs Collision domain = 25m too short Frame Bursting not efficient Jumbo frame: up to 9Kbytes 130 10-Gigabit Ethernet Goals Upgrade the data rate to 10Gbps Make it compatible with Standard, Fast, and Gigabit Ethernet Keep the same 48-bit address Keep the same frame format Keep the same minimum and maximum frame length Allow the interconnection of existing LANs into a metropolitan are network (MAN) or a wide are network (WAN) Make Ethernet compatible with technologies such as Frame Relay and ATM Usage Backbone, high-speed links Only Full-duplex mode with optical fiber 131 Packet Capture with Wireshark Capture packets in Ethernet LAN Check the version of Ethernet Check the fields of Ethernet frame Framing sequences Addresses Size/Type CRC 132 Contents Signals Hardware Building Blocks Encoding Framing Error Detection Reliable Transmission Project #1 Ethernet Wireless Overview Bluetooth (802.15.1) Wi-Fi (802.11): Physical, CSMA/CA, Architecture, Frame Format WiMax Cell Phone Technologies Assignment #2 133 Overview Leading wireless technologies Bluetooth 802.15.1 Wi-Fi 802.11 WiMax 802.16 3G Cellular Typical Link Length 10m 100m 10km Tens of km Typical BW 21.Mbps (shared) 54Mbps (shared) 70Mbps (shared) 384+Kbps (per conn) Typical Use Link a peripheral to a notebook computer Link a notebook computer to a wired base Link a building to a wired tower Link a cell phone to a wired tower Wired Analogy USB Ethernet Coaxial cable DSL 134 Overview Wireless network using a base station 135 Overview Wireless ad hoc or mesh network 136 Bluetooth Features Applications Peripheral devices: wireless mouse or keyboard Monitoring devices: sensor devices, home security devices Origin of name WLAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network formed spontaneously Harald Blaatand, king of Denmark (a project by the Ericsson Co.) Standard IEEE 802.15.1 137 Bluetooth Two types of networks piconet (small net) can have up to eight active stations (1 primary, the rest secondaries) the communication between the primary and the secondary can be oneto-one or one-to-many communication is only between the primary and a secondary/secondaries an additional 8 secondaries can be in parked state cannot take part in communication 138 Architecture Two types of networks scatternet Piconets can be combined to form a scatternet A secondary station in one piconet can be the primary in another piconet This station can receive messages from the primary in the first piconet and deliver to the secondaries in the second piconet A station can be a member of two piconets 139 Bluetooth Bluetooth devices has built-in short range radio transmitter data rate: 1Mbps with a 2.4-GHz band possible interference between IEEE 802.11b WLAN and Bluetooth LANs 140 Wi-Fi Two sublayers 141 Wi-Fi Physical Properties ISM (Industrial, Scientific, and Medical) band 142 Wi-Fi Wireless Problem In wireless applications, stations must be able to share air medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder Solution approach : Spread spectrum spread the original spectrum needed for each station BSS >> B (the required BW) Frequency hopping spread spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) 143 Wi-Fi FHSS concept 144 Wi-Fi DSSS Concept 145 Wi-Fi OFDM? Orthogonal Frequency Division Multiplexing FDM and OFDM 146 Wi-Fi MAC DCF (Distributed Coordination Function) Access method: CSMA/CA The reasons WLAN cannot implement CSMA/CD For collision detection, a station must be able to send data and receive collision signals at the same time costly station and increased BW requirements The distance between stations can be great. Signal fading could prevent a station at one end from hearing a collision at the other end Even Carrier Sense may not be possible because of the hidden station problem 147 Wi-Fi MAC CSMA/CA 148 Wi-Fi MAC CSMA/CA and NAV 149 Wi-Fi MAC Point Coordination Function (PCF) Optional access method for an infrastructure network implemented on top of the DCF used mostly for time-sensitive transmission centralized, contention-free polling access method AP performs polling for stations that are capable of being polled The stations are polled one after another, sending any data they have to the AP To give priority to PCF over DCF, another set of interframe spaces has neen defined: PIFS (PCF IFS) < DIFS 150 Wi-Fi MAC Hidden Node Problem Solution: RTS/CTS handshaking 151 Wi-Fi MAC Exposed Node Problem a station refrains from using a channel when it is available(BA (no i/f) CD) waste the capacity of the channel CTS/RTS cannot help in this case: half-duplex cannot hear when sending 152 Wi-Fi MAC CTS/RTS shortcoming Case 1) (AB (no interference) CD) A B C D RTS CTS CTS packet transmission time RTS Collision RTS CTS CTS Collision 153 Wi-Fi MAC CTS/RTS shortcoming Case 2) A B C D RTS CTS CTS RTS C did not hear B’s CTS since it was transmitting its own RTS to D CTS packet transmission AB time packet transmission CD Collision packet transmission CD 154 Wi-Fi Architecture Basic Service Set (BSS) Made of stationary or mobile wireless stations and an optional central base station (AP) Adhoc network: a BSS without an AP Intrastructure network: a BSS with an AP 155 Wi-Fi Architecture Extended Service Set (ESS) Made of two or more BSSs with Aps BSSs are connected through a distribution system Distribution system connects the APs in the BSSs A mobile station can belong to more than one BSS at the same time 156 Wi-Fi Architecture Station Types Based on mobility No-transition mobility Stationary : not moving Moving only inside a BSS BSS-transition mobility Moving from one BSS to another inside one BSS Inter-BSS Handover ESS-transition mobility Moving from one ESS to another Inter-ESS Handover cf. Handover (Handoff) Issues Seamless HO, Smooth HO, Fast HO Soft HO, Hard HO Vertical HO 157 Wi-Fi Frame Format Frame format FC: Frame Control type of frame and some control information D: NAV, or ID of the frame SC: Sequence Control Seq. no for flow control 158 Wi-Fi Frame Format Frame format Subfields in FC field 159 Wi-Fi Frame Format Frame Types Management frames for the initial communication between stations and APs Control frames for accessing the channel and acknowledging frames values of subfields in control frames Data frames for carrying data and control information 160 Wi-Fi Frame Format Four cases in addresses use 'To DS' and 'From DS' flags in the FC field Address 1: the address of the next device Address 2: the address of the previous device Address 3: the address of the final station if it is not defined by address 1 Address 4: the address of the original source if it is not the same as address 2 161 Wi-Fi Frame Format Four cases in addresses 162 Packet Capture with Wireshark Capture packets in Wireless LAN Check Wireless LAN technology Check the fields of WLAN frame Non-security frame Security frame Consider WLAN Security Issues Eavesdropping Hacking 163 WiMax What is WiMax(Worldwide Interoperability for Microwave Access)? For the delivery of last mile wireless broadband access as an alternative to cable and DSL. Provides fixed, nomadic, portable and, eventually, mobile wireless broadband connectivity without the need for direct line-of-sight (LOS) with a base station. In a typical cell radius deployment of three to ten kilometers, WiMAX Forum Certified™ systems can be expected to deliver capacity of up to 40 Mbps per channel, for fixed and portable access applications. Mobile network deployments are expected to provide up to 15 Mbps of capacity within a typical cell radius deployment of up to three kilometers. 164 WiMax WiMax Standard 802.16 802.16a 802.162004 802.16e-2005 Date Completed December 2001 January 2003 June 2004 December 2005 Spectrum 10-66 GHz < 11 GHz < 11 GHz < 6 GHz Operation LOS Non-LOS Non-LOS Non-LOS and Mobile Bit Rate 32-134 Mbps Up to 75 Mbps Up to 75 Mbps Up to 15 Mbps Cell Radius 1-3 miles 3-5 miles 3-5 miles 1-3 miles 165 WiMax Usage 166 Cell Phone Technologies Overview 167 Cell Phone Technologies Provides communications between two moving units (Mobile Stations) or between one mobile unit and one stationary unit A service provider must be able to: Locate and track a caller Assign a channel to the call Transfer the channel from BS (Base Station) to BS as caller moves out of range Cells: small regions each cellular service area is divided into Contains an antenna (uses own range of frequency) Controlled by a small office (BS) controlled by a switching office (Mobile Switching Center) 168 Cell Phone Technologies Message Switching Center (MSC) Telephone central office Connects calls, records call information, and bills Shape of cells Coordinates communication between all BSs and the telephone central office Square Hexagon: for equidistance antennas Cell size Depends on the population of the area Typically 1~12 miles in radius The transmission power of each cell is kept low to prevent its signal from interfering with those of other signals 169 Cell Phone Technologies Handoff(Handover) Hard handoff (using one BS) : early model First communication must be broken with the previous BS and then communication can be established with the new one Soft handoff (using two BSs; Seamless handoff) During handoff MS may continue with the new BS before breaking from the old one Signal strength due to BSi Signal strength due to BSj Pj(x) Pi(x) E Pmin BS i X X 1 3 MS X 5 Xth X BS Xj 4 2 170 Cell Phone Technologies Roaming Problem A user can have access to communication or can be reached where there is coverage But a service provider usually has limited coverage Solution approach: roaming Neighboring service providers can provide extended coverage through a roaming contract 171 Cell Phone Technologies Satellite Network telephony 172 Assignment #2 Exercises Calculate: 1, 3, 7, 19, 31, 32, 33, 34, 41, 43, 44, 47, 48 Analyze: 13, 24, 27, 28, 35, 52, 66 Experiments Wireshark experiments 5-Slide Survey Capture HTTP traffic from your PC with capture filter Screenshots: capture filter, captured traffic and information Wireless mesh networks Motivation (why?), Problem (what?), Technique (how?) Use PPT slides and upload at the Report board 173
