COM-405 Mobile Networks. Module A (Part A2) Introduction

COM-405 Mobile Networks Module A (Part A2) Introduction Prof. JP Hubaux http://mobnet.epfl.ch Note: some of the slides of this and other modules and derived from Schiller s book 1

Modulation and demodulation (reminder) analog baseband digital data signal digital analog 101101001 modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier 2

About CSMA/CD Can we borrow media access methods from fixed networks? Example of CSMA/CD q Carrier Sense Multiple Access with Collision Detection q send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks q a radio can usually not transmit and receive at the same time q signal strength decreases proportionally to the square of the distance or even more q the sender would apply CS and CD, but the collisions happen at the receiver q it might be the case that a sender cannot hear the collision, i.e., CD does not work q furthermore, CS might not work if, e.g., a terminal is hidden 3

Hidden and exposed terminals Hidden terminals q A sends to B, C cannot receive A q C wants to send to B, C senses a free medium (CS fails) q collision at B, A cannot receive the collision (CD fails) q A is hidden for C Exposed terminals q B sends to A, C wants to send to another terminal (not A or B) q C has to wait, CS signals a medium in use q but A is outside the radio range of C, therefore waiting is not necessary q C is exposed to B A B C 4

Motivation - near and far terminals Terminals A and B send, C receives q signal strength decreases (at least) proportionally to the square of the distance q the signal of terminal B therefore drowns out A s signal q è C cannot receive A A B C If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! 5

Access methods SDMA/TDMA/FDMA/CDMA SDMA (Space Division Multiple Access) q segment space into sectors, use directed antennas q cell structure TDMA (Time Division Multiple Access) q assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time FDMA (Frequency Division Multiple Access) q assign a certain frequency to a transmission channel between a sender and a receiver q permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) CDMA (Code Division Multiple Access) q assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum) q frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum) 6

Some medium access control mechanisms for wireless SDMA TDMA FDMA CDMA Fixed Used in GSM FHSS Used in Bluetooth DSSS Used in UMTS Fixed Used in Aloha CSMA Reservations DAMA GSM Pure Slotted Multiple Access with Collision Avoidance Polling Used in 802.11 (optional) Non-persistent p-persistent CSMA/CA Used in 802.11 (mandatory) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional) MACAW MACA-BI FAMA CARMA 7

Some medium access control mechanisms for wireless SDMA TDMA FDMA CDMA Fixed Used in GSM FHSS Used in Bluetooth DSSS Fixed Used in Aloha CSMA Reservations DAMA GSM Pure Slotted Multiple Access with Collision Avoidance Polling Used in 802.11 (optional) Non-persistent p-persistent CSMA/CA Used in 802.11 (mandatory) FHSS: Frequency-Hopping Spread Spectrum DSSS: Direct Sequence Spread Spectrum CSMA: Carrier Sense Multiple Access CA: Collision Avoidance DAMA: Demand-Assigned Multiple Access MACA-BI: MACA by invitation FAMA: Floor Acquisition Multiple Access CARMA: Collision Avoidance and Resolution Multiple Access Copes with hidden and exposed terminal RTS/CTS Used in 802.11 (optional) MACAW MACA-BI FAMA CARMA 8

Time multiplex A channel gets the whole spectrum for a certain amount of time. Advantages: q only one carrier in the medium at any time Disadvantages: q precise synchronization required c k 1 k 2 k 3 k 4 k 5 k 6 f t 9

Frequency multiplex Separation of the whole spectrum into smaller frequency bands. A channel gets a certain band of the spectrum for the whole time. Advantages: q looser coordination q works also for analog signals k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: q wastage of bandwidth if the traffic is distributed unevenly q inflexible q guard spaces t c f 10

Time and frequency multiplex Combination of both methods. A channel gets a certain frequency band for a certain amount of time. Example: GSM Advantages: q more flexibility But: precise coordination required c k 1 k 2 k 3 k 4 k 5 k 6 f t 11

Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages: q bandwidth efficient q good protection against interference and eavesdropping Disadvantage: q more complex signal regeneration Implemented using spread spectrum technology k 1 k 2 k 3 k 4 k 5 k 6 c f t 12

TDMA/TDD example: DECT 417 µs 1 2 3 11 12 1 2 3 11 12 downlink uplink t DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex 13

FDMA/FDD example: GSM 960 MHz f downlink 124 935.2 MHz 915 MHz 1 124 20 MHz 200 khz 890.2 MHz 1 uplink t FDD: Frequency Division Duplex 14

Mechanism Aloha Aloha/slotted aloha q random, distributed (no central arbiter), time-multiplex q Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries collision sender A sender B sender C Slotted Aloha collision t sender A sender B sender C t 16

Performance of Aloha (1/4) First transmission Retransmission (if necessary) t 0 -X t 0 t 0 +X t 0 +X+2t prop t 0 +X+2t prop +B Vulnerable period Time-out Backoff period B t prop : maximum one-way propagation time between 2 stations Information about the outcome of the transmission is obtained after the reaction time 2 t prop B: backoff time 17

G : total load S Performance of Aloha (2/4) S: new packets S: throughput of the system { G : arrival rate of new packets Assumption: Poisson distribution of the aggregate arrival process, with an average number of arrivals of 2G arrivals/2x seconds ( ) 2G 2 Pr [ k transmissions in 2 X seconds ] e G =, k = 0,1,2,... k! Throughput S: total arrival rate G times the prob. of a successful transmission: ( 2G) 2G [ ] [ ] S = G.Pr no collision = G.Pr 0 transmissions in 2 X seconds = = G Ge 0! 0 e 2G Peakvalue at G = 0.5 : S = 1 0.184 2e k 18

Performance of Aloha (3/4) Detail of computation of throughput of previous slide: Define: T : Transmission by a given station A: Absence of transmission by any other station Throughput: N 1.Pr(T, A) = N. Pr(T, A) i=1 = N.Pr(A T ).Pr(T ) = N.Pr(T ).Poisson(0,2G) = G.Poisson(0,2G) = G.e 2G 19

Performance of Aloha (4/4) Computation of the average packet transmission time Average number of transmission attempts/packet: G 2G = e attempts per packet S Average number of unsuccessful attempts per packet: 2G ε = G 1= e 1 S The first transmission requires X + t seconds, and each subsequent retransmission requires 2t + X + B [ ] [ ] prop Thus the average packet transmission time is approx: ET X t e X t B 2G aloha = + prop + ( 1)( + 2 prop + ) expressed relatively to X: 2G ETaloha / X= 1 + a+ ( e 1)(1+ 2 a+ B ) X t prop where a = is the normalized one-way propagation delay X prop 20

Performance of Slotted Aloha First transmission Retransmission (if necessary) t 0 =kx (k+1)x t 0 +X+2t prop t 0 +X+2t prop +B Vulnerable period Time-out Backoff period S = Ge -G Peakvalue at G = 1 : S = 1 0.368 e Average packet delay: [ ] G ETslotaloha / X= 1 + a+ ( e 1)(1+ 2 a+ B ) X 21

Carrier Sense Multiple Access (CSMA) q Goal: reduce the wastage of bandwidth due to packet collisions q Principle: sensing the channel before transmitting (never transmit when the channel is busy) q Many variants: q Collision detection (CSMA/CD) or collision avoidance(csma/ca) q Persistency (in sensing and transmitting) Station A begins transmission at t=0 A Station A captures the channel at t=t prop A 23

1-Persistent CSMA q Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle. q As soon as the channel is sensed idle, they transmit their packet. q If more than one station is waiting, a collision occurs. q Stations involved in a collision perform a the backoff algorithm to schedule a future time for resensing the channel q Optional backoff algorithm may be used in addition for fairness Consequence : The channel is highly used (greedy algorithm). 24

Non-Persistent CSMA q Attempts to reduce the incidence of collisions q Stations with a packet to transmit sense the channel q If the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing time q If the channel is idle, then the station transmits Consequence : channel may be free even though some users have packets to transmit. 25

p-persistent CSMA q Combines elements of the above two schemes q Stations with a packet to transmit sense the channel q If it is busy, they persist with sensing until the channel becomes idle q If it is idle: l With probability p, the station transmits its packet l With probability 1-p, the station waits for a random time and senses again 26

Protocol Throughput expression Throughput Pure ALOHA Slotted ALOHA Unslotted 1-persistent CSMA Slotted 1-persistent CSMA Unslotted nonpersistent CSMA S = S = Ge 2G S = Ge G ( ) G"# 1+ G + ag 1+ G + ag / 2 $ % e ( ) 1 e ag G 1+ 2a S = ( ) + 1+ ag G" # 1+ a e ag ( 1+ a) 1 e ag S = G( 1+2a) G 1+a ( )e $ ( 1+a) % e G ( ) + ae Ge ag G( 1+ 2a) + e ag G( 1+a) ( ) Slotted nonpersistent CSMA S = age ag 1 e ag + a 27

Throughput plot Normalized propagation delay is a =0.01 28

CSMA/CD (reminder) Repeater Terminator Station CS: Carrier Sense (Is someone already talking?) MA: Multiple Access (I hear what you hear!) CD: Collision Detection (We are both talking!!) Three states for the channel : contention, transmission, idle Operating principle q Check whether the channel is idle before transmitting q Listen while transmitting, stop transmission when collision q If collision, one of the 3 schemes above (1-persistent, nonpersistent or p-persistent) 29

Why CSMA/CD is unfit for WLANs q Collision Detection requires simultaneous transmission and reception operations (which a radio transceiver is usually unable to do) è detecting a collision is difficult q Carrier Sensing may be suitable to reduce interference at sender, but Collision Avoidance is needed at receiver q CSMA/CD does not address the hidden terminal problem 30

CSMA/CA Is described in the module B devoted to IEEE 802-11 (we ll see it next week) 31

DAMA - Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha Reservation can increase efficiency to 80% q a sender reserves a future time-slot q sending within this reserved time-slot is possible without collision q reservation also causes higher delays q typical scheme for satellite links Examples for reservation algorithms: q Explicit Reservation (Reservation-ALOHA) q Implicit Reservation (PRMA) q Reservation-TDMA 33

DAMA / Explicit Reservation Explicit Reservation (Reservation Aloha): q two modes: l ALOHA mode for reservation: competition for small reservation slots, collisions possible l reserved mode for data transmission within successful reserved slots (no collisions possible) q it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision Aloha reserved Aloha reserved Aloha reserved Aloha t 34

DAMA / Packet reservation (PRMA) Implicit reservation q based on slotted Aloha q a certain number of slots form a frame, frames are repeated q stations compete for empty slots according to the slotted aloha principle q once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send q competition for a slot starts again as soon as the slot was empty in the last frame reservation ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD frame 1 frame 2 frame 3 frame 4 frame 5 1 2 3 4 5 6 7 8 time-slot A C D A B A A C A A A B A B A F F B A F D A C E E B A F D t collision at reservation attempts 35

DAMA / Reservation-TDMA Reservation Time Division Multiple Access q every frame consists of N mini-slots and x data-slots q every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). q other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) N mini-slots N * k data-slots e.g. N=6, k=2 reservations for data-slots other stations can use free data-slots based on a round-robin scheme 36

MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance q Designed especially for packet radio networks (Phil Karn, 1990) q Principle: l RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet l CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain q sender address q receiver address q packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) 38

MACA principle MACA mitigates the problem of hidden terminals q A and C want to send to B q A sends RTS first q C waits after receiving CTS from B RTS CTS CTS A B C The hidden terminal problem might still arise, especially in case of mobility of the nodes 39

MACA example A RTS B D A CTS B D C E C 1 2 E A DATA B D : blocked from Tx C 3 E 40

MACA variant: application in IEEE 802.11 sender receiver idle idle ACK RxBusy time-out NAK; RTS packet ready to send; RTS wait for the right to send CTS; data time-out; RTS data; ACK time-out Data with errors; NAK RTS; CTS wait for ACK wait for data RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy 41

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Φ s ( f ) Synchronization n c(t) S s f Φs ( f ) Φ j ( f ) S B s s power density spectrum of the original signal power density spectrum of the jamming signal power density of the original signal bandwidth of the original signal 43 B s

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Φ t ( f ) Synchronization c(t) S t = SB s B t s f 44 B t

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Synchronization Φ j ( f ) c(t) S j f 45 B j

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Synchronization Φ r c(t) ( f ) S j B j S t f 46 B t

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Synchronization c(t) ~ ( ) Φ f P signal P jamming = S s B s S j B j B t B s =! # " S s B s S j B! j $ & % P signal P jamming original B t B s! Processing gain B s B j S j B t S s Processing gain: Increase in received signal power thanks to spreading f 47 B t

Spread Spectrum principle Φ j ( f ) Φ s ( f ) Coder Φ t ( f ) Φ r ( f ) Decoder ~ ( ) Φ f Filter! ( ) Φ f c(t) Pseudo-random code Synchronization c(t)! ( ) Φ f B j S j B t S s f 48 B s

Frequency Hopping Spread Spectrum (FHSS) (1/2) q Signal broadcast over seemingly random series of frequencies q Receiver hops between frequencies in sync with transmitter q Eavesdroppers hear unintelligible blips q Jamming on one frequency affects only a few bits q Rate of hopping versus Symbol rate q Fast Frequency Hopping: One bit transmitted in multiple hops. q Slow Frequency Hopping: Multiple bits are transmitted in a hopping period q Example: Bluetooth (79 channels, 1600 hops/s) 50

Frequency Hopping Spread Spectrum (FHSS) (2/2) t c t b t b : duration of one bit Fast Frequency Hopping: t c : duration of one chip Chip: name of the sample period in spread-spectrum jargon t b > t c 51

Direct Sequence Spread Spectrum (DSSS) (1/2) XOR of the signal with pseudo-random number (chipping sequence) q many chips per bit (e.g., 128) result in higher bandwidth of the signal Advantages q reduces frequency selective fading q in cellular networks Disadvantages l neighboring base stations can use the same frequency range l neighboring base stations can detect and recover the signal l è enables soft handover q precise power control necessary q complexity of the receiver t b 0 1 t c 0 1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 1 1 0 0 1 0 1 0 t b : bit period t c : chip period user data XOR chipping sequence = resulting signal 53

Direct Sequence Spread Spectrum (DSSS) (2/2) user data X spread spectrum signal modulator transmit signal chipping sequence radio carrier transmitter correlator received signal demodulator lowpass filtered signal products X integrator sampled sums decision data radio carrier chipping sequence receiver 54

Categories of spreading (chipping) sequences q Spreading Sequence Categories q Pseudo-random Noise (PN) sequences q Orthogonal codes q For FHSS systems q PN sequences most common q For DSSS beside multiple access q PN sequences most common q For DSSS CDMA systems q PN sequences q Orthogonal codes 55

Generating a Pseudo-random Noise chip sequence with a linear feedback shift-register (LFSR) number of registers: n n period: N = 2 1 Properties of PN sequences: 1 q Property 1: In a PN sequence we have: Pr{ 0} = Pr{ 1} = 1 2 1 N { } { } 1 1 3 Pr 0 Pr 1 for n 10 10 2 N q Property 2: For a window of length n slid along output for N (=2 n -1) shifts, each n-tuple appears once, except for the all zeros sequence q Property 3: The periodic autocorrelation of a PN sequence is: 1 τ = 0,N, 2N,... R( τ ) = 1 otherwise N 1 1 2 1+ N 56

Orthogonal Codes q Orthogonal codes q All pairwise cross correlations are zero q Fixed- and variable-length codes used in CDMA systems q For CDMA application, each mobile user uses one sequence in the set as a spreading code l Provides zero cross correlation among all users q Types q Walsh codes q Variable-Length Orthogonal codes 57

Walsh Codes q Set of Walsh codes of length n consists of the n rows of an n x n Hadamard matrix: 1 1 Hk 1 Hk 1 H1 = Hk = 1 0 Hk 1 H k 1 q Sylvester's construction: 1 1 1 1 1 1 1 0 1 0 H1 = H2 =... 1 0 1 1 0 0 1 0 0 1 q Every row is orthogonal to every other row and to the logical not of every other row q Requires tight synchronization q Cross correlation between different shifts of Walsh sequences is not zero 58

Typical Multiple Spreading Approach q Spread data rate by an orthogonal code (channelization code) q Provides mutual orthogonality among all users in the same cell q Further spread result by a PN sequence (scrambling code) q Provides mutual randomness (low cross correlation) between users in different cells 59

CDMA (Code Division Multiple Access) Principles q all terminals send on the same frequency and can use the whole bandwidth of the transmission channel q each sender has a unique code q The sender XORs the signal with this code q the receiver can tune into this signal if it knows the code of the sender q tuning is done via a correlation function Disadvantages: q higher complexity of the receiver (receiver cannot just listen into the medium and start receiving if there is a signal) q all signals should have approximately the same strength at the receiver Advantages: q all terminals can use the same frequency, no planning needed q huge code space (e.g., 2 32 ) compared to frequency space q more robust to eavesdropping and jamming (military applications ) q forward error correction and encryption can be easily integrated 60

CDMA: principle (very simplified) Spreading Despreading A k A k A d X A s X C+D A d A s + B s B k B k B d X B s X C+D B d C+D: Correlation and Decision 61

CDMA: example Sender A q sends A d = 1, key A k = 010011 (assign: 0 = -1, 1 = +1) q sending signal A s = A d * A k = (-1, +1, -1, -1, +1, +1) Sender B q sends B d = 0, key B k = 110101 (assign: 0 = -1, 1 = +1) q sending signal B s = B d * B k = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space q interference neglected (noise etc.) q A s + B s = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A q apply key A k bitwise (inner product) l A e = (-2, 0, 0, -2, +2, 0) A k = 2 + 0 + 0 + 2 + 2 + 0 = 6 l result greater than 0, therefore, original bit was 1 q receiving B l B e = (-2, 0, 0, -2, +2, 0) B k = -2 + 0 + 0-2 - 2 + 0 = -6, i.e. 0 62

Spreading of signal A data A d 1 0 1 key sequence A k A d +A k 0 1 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 1 0 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0 signal A s 1-1 Real systems use much longer keys resulting in a larger distance between single code words in code space. 63

Spreading of signal B 1 signal A s -1 data B d 1 0 0 key sequence B k B d +B k 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1 signal B s 1-1 2 A s + B s 0-2 64

Despreading of signal A data A d 1 0 1 2 A s + B s 0-2 1 A k (A s + B s ) * A k correlator output decision output 0 1 0 0-1 2 0-2 Note: the received signal is inverted 65

Despreading of signal B data B d 1 0 0 2 A s + B s 0-2 1 B k (A s + B s ) * B k correlator output decision output 0 1 1 0-1 2 0-2 Note: the received signal is inverted 66

Despreading with a wrong key 2 A s + B s wrong key K (A s + B s ) * K 0-2 1 0-1 2 0-2 correlator output decision output (1) (1)? 67

Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation segment space into cells/sectors only one terminal can be active in one cell/one sector cell structure, directed antennas segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain spread the spectrum using orthogonal codes all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers Advantages very simple, increases capacity per km² Disadvantages Comment inflexible, antennas typically fixed used in all cellular systems established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders higher complexity In practice, several access methods are used in combination Example: SDMA/TDMA/FDMA for GSM 68

Orthogonal Frequency Division Modulation (ODFM) (In a nutshell) 69

70 Credit: http://www.csie.ntu.edu.tw

For Next Week - Read the Web site Mobnet.epfl.ch - Review the lecture; get prepared for the quiz!! (Starts at 13:15) - Get your clicker - Visit the Library at the Rolex Learning Center, room RLC D1 210, Monday to Friday 8:00 18:00 - The app on smartphone or tablet is not allowed - For all clicker-related issues, please contact Alexandra: alexandramihaela.olteanu@epfl.ch - Try to solve the homework (will be online very soon) We help you as of 15:00 on it 75

References q T. Rappaport: Wireless Communications, Principles and Practice (2 nd edition), Prentice Hall, 2002 q M. Schwartz: Mobile Wireless Communications, Cambridge University Press, 2005 q J. Schiller: Mobile Communications (2 nd edition), Addison- Wesley, 2004 q Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000 76