SlideShare a Scribd company logo

note9 (1).pptx on medium access control protocols

Download as PPTX, PDF
S
sadiariasat10

The document describes various medium access control (MAC) protocols, focusing on random access methods such as ALOHA and CSMA, including their throughput analysis and collision management strategies. It discusses scheduling approaches like reservation and polling systems for efficient channel utilization and reduced delay. Additionally, it examines token passing mechanisms in ring networks for frame transmission management.

Engineering
1 of 60
Download to read offline
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1 Note 6: Medium Access Control Protocols Random Access
Quiz #3 Describe in your own words bit stuffing and byte stuffing. 2
3 ALOHA • Wireless link to provide data transfer between main campus & remote campuses of University of Hawaii • Simplest solution: just do it – A station transmits whenever it has data to transmit – If more than one frames are transmitted, they interfere with each other (collide) and are lost – If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision) – Station retransmits frame after backoff time t t0 t0-X t0+X t0+X+2tprop t0+X+2tprop + B Vulnerable period Time-out Backoff period B First transmission Retransmission
4 ALOHA Model • Definitions and assumptions – X: frame transmission time (assumed to be constant) – S: throughput (average # of successful frame transmissions per X seconds) – G: load (average # of transmission attempts per X sec.) – Psuccess : probability a frame transmission is successful success GP S  X X frame transmission Prior interval • Any transmission that begins during vulnerable period leads to collision • Success if no arrivals during 2X seconds
5 Abramson’s Assumption • What is probability of no arrivals in vulnerable period? • Abramson assumption: The backoff algorithm spreads the retransmissions so that frame transmissions, new and repeated, are equally likely to occur at any instant • This implies that the number of frames transmitted in a time interval has a Poisson distribution e e e G X success t k X P P k t t k P 2 2 0 ! 0 ) 2 ( seconds] 2X in arrivals 0 [ ! ) ( seconds] in arrivals [           
6 Throughput of ALOHA G success Ge GP S 2    0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0 . 0 0 7 8 1 2 5 0 . 0 1 5 6 2 5 0 . 0 3 1 2 5 0 . 0 6 2 5 0 . 1 2 5 0 . 2 5 0 . 5 1 2 4 G S • Collisions are means for coordinating access • Max throughput is rmax= 1/2e (18.4%) • Bimodal behavior: Small G, S≈G Large G, S↓0 • Collisions can snowball and drop throughput to zero e-2 = 0.184
7 Slotted ALOHA • Time is slotted in X seconds slots • Stations synchronized to frame times • Stations transmit frames in first slot after frame arrival • Backoff intervals in multiples of slots t (k+1)X kX t0 +X+2tprop+ B Vulnerable period Time-out Backoff period B t0 +X+2tprop Only frames that arrive during prior X seconds collide
8 Throughput of Slotted ALOHA 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.01563 0.03125 0.0625 0.125 0.25 0.5 1 2 4 8 Ge-G Ge-2G G S 0.184 0.368 e e e e G success G X success t k G GP S X P P k t t k P               ! 0 ) ( seconds] X in arrivals 0 [ ! ) ( seconds] in arrivals [ 0
9 Application of Slotted Aloha • Reservation protocol allows a large number of stations with infrequent traffic to reserve slots to transmit their frames in future cycles • Each cycle has mini-slots allocated for making reservations • Stations use slotted Aloha during mini-slots to request slots cycle X-second slot Reservation mini-slots . . . . . .
10 Carrier Sensing Multiple Access (CSMA) A Station A begins transmission at t = 0 A Station A captures channel at t = tprop • A station senses the channel before it starts transmission – If busy, either wait or schedule backoff (different options) – If idle, start transmission – Vulnerable period is reduced to tprop (due to channel capture effect) – Collisions in ALOHA or Slotted ALOHA involve 2 or 1 frame transmission times X – If tprop >X (or if a>1), no gain compared to ALOHA or slotted ALOHA
11 • Transmitter behavior when busy channel is sensed – 1-persistent CSMA (most greedy)  Start transmission as soon as the channel becomes idle  Low delay and high collision rates – Non-persistent CSMA (least greedy)  Wait a backoff period, then sense carrier again  High delay and low collision rates – p-persistent CSMA (adjustable greedy)  Wait till channel becomes idle, transmit with prob. p; or wait another tprop & re-sense with probability 1-p  Delay and collisions rates can be balanced CSMA Options Sensing
12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.02 0.03 0.06 0.13 0.25 0.5 1 2 4 8 16 32 64 0.53 0.45 0.16 S G a  0.01 a =0.1 a = 1 1-Persistent CSMA Throughput • Better than Aloha & slotted Aloha for small a • Worse than Aloha for a > 1
13 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.02 0.03 0.06 0.13 0.25 0.5 1 2 4 8 16 32 64 0.81 0.51 0.14 S G a = 0.01 Non-Persistent CSMA Throughput a = 0.1 a = 1 • Higher maximum throughput than 1- persistent for small a • Worse than Aloha for a > 1
14 CSMA with Collision Detection (CSMA/CD) • Monitor for collisions & abort transmission – Stations with frames to send, first do carrier sensing – After beginning transmissions, stations continue listening to the medium to detect collisions – If collisions detected, all stations involved stop transmission, reschedule random backoff times, and try again at scheduled times • In CSMA collisions result in wastage of X seconds spent transmitting an entire frame • CSMA-CD reduces wastage to time to detect collision and abort transmission
15 CSMA/CD reaction time It takes 2 tprop to find out if channel has been captured A begins to transmit at t = 0 A B B begins to transmit at t = tprop- ; B detects collision at t = tprop A B A B A detects collision at t= 2 tprop- 
16 CSMA-CD Model • Assumptions – Collisions can be detected and resolved in 2tprop – Time slotted in 2tprop slots during contention periods – Assume n busy stations, and each may transmit with probability p in each contention time slot – Once the contention period is over (a station successfully occupies the channel), it takes X seconds for a frame to be transmitted – It takes tprop before the next contention period starts. Busy Contention Busy (a) Time Idle Contention Busy
17 Contention Resolution • How long does it take to resolve contention? • Contention is resolved (“success’) if exactly 1 station transmits in a slot: 1 ) 1 (    n success p np P • By taking derivative of Psuccess we find max occurs at p=1/n e n n n n P n n success 1 ) 1 1 ( ) 1 1 ( 1 1 1 max        • On average, 1/Pmax = e = 2.718 time slots to resolve contention seconds Period Contention Average 2 e tprop 
18 CSMA/CD Throughput • At maximum throughput, systems alternates between contention periods and frame transmission times     L Rd e a e et t X X prop prop / 1 2 1 1 1 2 1 1 2 max  r          Time Busy Contention Busy Contention Busy Contention Busy • where: R bits/sec, L bits/frame, X=L/R seconds/frame a = tprop/X  meters/sec. speed of light in medium d meters is diameter of system 2e+1 = 6.44 Rd/(vL)
19 CSMA-CD Application: Ethernet • First Ethernet LAN standard used CSMA-CD – 1-persistent Carrier Sensing – R = 10 Mbps – tprop = 51.2 microseconds  512 bits = 64 byte slot  accommodates 2.5 km + 4 repeaters – Truncated Binary Exponential Backoff  After the nth collision, select backoff from {0, 1,…, 2k – 1}, where k=min(n, 10)
20 Throughput for Random Access MACs 0 0.2 0.4 0.6 0.8 1 0.01 0.1 1 ALOHA Slotted ALOHA 1-P CSMA Non-P CSMA CSMA/CD a rmax • For small a: CSMA-CD has best throughput • For larger a: Aloha & slotted Aloha better throughput
21 Carrier Sensing and Priority Transmission • Certain applications require faster response than others, e.g. ACK messages • Impose different interframe times – High priority traffic sense channel for time t1 – Low priority traffic sense channel for time t2>t1 – High priority traffic, if present, seizes channel first • This priority mechanism is used in IEEE 802.11 wireless LAN
22 Note 6: Medium Access Control Protocols Scheduling
23 Scheduling for Medium Access Control • Schedule frame transmissions to avoid collision in shared medium  More efficient channel utilization  Less variability in delays  Can provide fairness to stations  Increased computational or procedural complexity • Two main approaches – Reservation – Polling
24 Reservation Systems Time Cycle n Reservation interval Frame transmissions r d d d r d d d Cycle (n + 1) r = 1 2 3 M • Transmissions organized into cycles • Cycle: reservation interval + frame transmissions • The reservation intervals has a minislot for each station to request reservations for frame transmissions
25 Reservation System Options • Centralized or distributed system – Centralized systems: A central controller listens to reservation information, decides order of transmission, issues grants – Distributed systems: Each station determines its slot for transmission from the reservation information • Single or Multiple Frames – Single frame reservation: Only one frame transmission can be reserved within a reservation cycle – Multiple frame reservation: More than one frame transmission can be reserved within a frame • Channelized or Random Access Reservations – Channelized (typically TDMA) reservation: Reservation messages from different stations are multiplexed without any risk of collision – Random access reservation: Each station transmits its reservation message randomly until the message goes through
26 t r 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3 (a) t r 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3 8 (b) Example • Initially stations 3 & 5 have reservations to transmit frames • Station 8 becomes active and makes reservation • Cycle now also includes frame transmissions from station 8
27 Efficiency of Reservation Systems • Assume minislot duration = vX • TDM single frame reservation scheme – If propagation delay is negligible, a single frame transmission requires (1+v)X seconds – Link is fully loaded when all stations transmit, maximum efficiency is: • TDM k frame reservation scheme – If k frame transmissions can be reserved with a reservation message and if there are M stations, as many as Mk frames can be transmitted in XM(k+v) seconds – Maximum efficiency is: max 1 1 MX M MX r     vX v max 1 1 MkX M MkX k r     v vX
28 Random Access Reservation Systems • Large number of light traffic stations – Dedicating a minislot to each station is inefficient • Slotted ALOHA reservation scheme – Stations use slotted Aloha on reservation minislots – On average, each reservation takes at least e minislot attempts – Effective time required for the reservation is 2.71vX max 1 (1 ) 1 2.71 X X ev r     v
29 Example: GPRS • General Packet Radio Service – Packet data service in GSM cellular radio – GPRS devices, e.g. cellphones or laptops, send packet data over radio and then to Internet – Slotted Aloha MAC used for reservations – Single & multi-slot reservations supported
30 Polling Systems • Centralized polling systems: A central controller transmits polling messages to stations according to a certain order • Distributed polling systems: A permit for frame transmission is passed from station to station according to a certain order • A signaling procedure exists for setting up order Central Controller
31 Polling System Options • Service Limits: How much is a station allowed to transmit per poll? – Exhaustive: until station’s data buffer is empty (including new frame arrivals) – Gated: all data in buffer when poll arrives – Frame-Limited: one frame per poll – Time-Limited: up to some maximum time • Priority mechanisms – More bandwidth & lower delay for stations that appear multiple times in the polling list – Issue polls for stations with message of priority k or higher
33 Average Cycle Time • Assume walk times all equal to t’ • Exhaustive Service: stations empty their buffers • Cycle time = Mt’ + time to empty M station buffers • /M be frame arrival rate at a station • NC average number of frames transmitted from a station • Time to empty one station buffer: t 1 3 2 4 5 1 … M t’ t’ t’ t’ t’ t’ Tc Tstation = Nc X = ( l M Tc )X = rTc M • Average Cycle Time: r r           1 t M T T t M MT t M T c c station c X  r 
34 Efficiency of Polling Systems • Exhaustive Service – Cycle time increases as traffic increases, so delays become very large – Walk time per cycle becomes negligible compared to cycle time: c c T Mt Efficiency T r     Can approach 100% • Limited Service – Many applications cannot tolerate extremely long delays – Time or transmissions per station are limited – This limits the cycle time and hence delay – Efficiency of 100% is not possible X t t M MX MX Efficiency / 1 1       Single frame per poll
35 Application: Token-Passing Rings Free Token = Poll Listen mode Delay Input from ring Output to ring Ready station looks for free token Flips bit to change free token to busy Transmit mode Delay To device From device Ready station inserts its frames Reinserts free token when done token Frame Delimiter is Token Free = 01111110 Busy = 01111111
36 Methods of Token Reinsertion • Multi-token operation – Free token transmitted immediately after last bit of data frame • Single-token operation – Free token inserted after last bit of the busy token is received back – Transmission time at least ring latency – If frame transmission time is longer than ring latency, equivalent to multi-token operation • Single-Frame operation – Free token inserted after transmitting station has received last bit of its frame – Equivalent to attaching trailer equal to ring latency Busy token Free token Frame Idle Fill
37 Token Ring Throughput • Definition – t’: ring latency (time required for bit to circulate ring) – X: maximum frame transmission time allowed per station • Multi-token operation – Assume network is fully loaded, and all M stations transmit for X seconds upon the reception of a free token – This is a polling system with limited service time: M a MX MX MX / 1 1 / 1 1 max          t t r latency ring normalized the is X a t  
38 • Single-token operation – Effective frame transmission time is maximum of X and t’ , therefore Token Ring Throughput ρmax = = MX t΄+ M(X+ t΄) 1 1+a΄(1 + 1/M) ρmax = = MX t΄+ M max{(X,t΄} 1 max{1, a΄} + a΄/M • Single-frame operation – Effective frame transmission time is X+ t’ ,therefore
39 Token Reinsertion Efficiency Comparison Maximum throughput a  Multiple token operation 0 0.2 0.4 0.6 0.8 1 1.2 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 Single frame operation M = 50 M = 10 Single token operation M = 50 M = 10 • a <<1, any token reinsertion strategy acceptable • a ≈1, single token reinsertion strategy acceptable • a >1, multitoken reinsertion strategy necessary
40 Application Examples • Single-frame reinsertion – IEEE 802.5 Token Ring LAN @ 4 Mbps • Single token reinsertion – IBM Token Ring @ 4 Mbps • Multitoken reinsertion – IEEE 802.5 and IBM Ring LANs @ 16 Mbps – FDDI Ring @ 50 Mbps • All of these LANs incorporate token priority mechanisms
41 Comparison of MAC approaches • Aloha & Slotted Aloha – Simple & quick transfer at very low load – Accommodates a large number of low-traffic bursty users – Highly variable delay at moderate loads – Efficiency does not depend on a • CSMA-CD – Quick transfer and high efficiency for low delay-bandwidth product – Can accommodate a large number of bursty users – Variable and unpredictable delay
42 Comparison of MAC approaches • Reservation – On-demand transmission of bursty or steady streams – Accommodates large number of low-traffic users with slotted Aloha reservations – Can incorporate QoS – Handles large delay-bandwidth product via delayed grants • Polling – Generalization of time-division multiplexing – Provides fairness through regular access opportunities – Can provide bounds on access delay – Performance deteriorates with large delay-bandwidth product
43 Note 6: Medium Access Control Protocols Channelization
44 Why Channelization? • Channelization – Semi-static bandwidth allocation of portion of shared medium to a given user • Highly efficient for constant-bit rate traffic • Preferred approach in – Cellular telephone networks – Terrestrial & satellite broadcast radio & TV
45 Why not Channelization? • Inflexible in allocation of bandwidth to users with different requirements • Inefficient for bursty traffic • Does not scale well to large numbers of users – Average transfer delay increases with number of users M • Dynamic MAC much better at handling bursty traffic 0 5 10 15 20 25 30 0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 r E[T]/X M=16 M=8 M=4 M=2 M=1
46 Channelization Approaches • Frequency Division Multiple Access (FDMA) – Frequency band allocated to users – Broadcast radio & TV, analog cellular phone • Time Division Multiple Access (TDMA) – Periodic time slots allocated to users – Telephone backbone, GSM digital cellular phone • Code Division Multiple Access (CDMA) – Code allocated to users – Cellular phones, 3G cellular
47 Channelization: FDMA • Divide channel into M frequency bands • Each station transmits and listens on assigned bands • Each station transmits at most R/M bps • Good for stream traffic; Used in connection-oriented systems • Inefficient for bursty traffic Frequency Guard bands Time W 1 2 M M–1 …
48 Channelization: TDMA • Dedicate 1 slot per station in transmission cycles • Stations transmit data burst at full channel bandwidth • Each station transmits at R bps 1/M of the time • Excellent for stream traffic; Used in connection-oriented systems • Inefficient for bursty traffic due to unused dedicated slots 1 Time Guard time One cycle 1 2 3 M W Frequency ...
49 Guardbands • FDMA – Frequency bands must be non-overlapping to prevent interference – Guardbands ensure separation; form of overhead • TDMA – Stations must be synchronized to common clock – Time gaps between transmission bursts from different stations to prevent collisions; form of overhead – Must take into account propagation delays
50 Channelization: CDMA • Code Division Multiple Access – Channels determined by a code used in modulation and demodulation • Stations transmit over entire frequency band all of the time! Time W Frequency 1 2 3
51  Binary information R1 bps W1 Hz Unique user binary random sequence Digital modulation Radio antenna Transmitter from one user R >> R1bps W >> W1 Hz  CDMA Spread Spectrum Signal • User information mapped into: +1 or -1 for T sec. • Multiply user information by pseudo- random binary pattern of G “chips” of +1’s and -1’s • Resulting spread spectrum signal occupies G times more bandwidth: W = GW1 • Modulate the spread signal by sinusoid at appropriate fc
52 Signal and residual interference Correlate to user binary random sequence Signals from all transmitters Digital demodulation Binary information   CDMA Demodulation • Recover spread spectrum signal • Synchronize to and multiply spread signal by same pseudo-random binary pattern used at the transmitter • In absence of other transmitters & noise, we should recover the original +1 or -1 of user information • Other transmitters using different codes appear as residual noise
53 R0 R1 R2 g(x) = x3 + x2 + 1 g0 g2 g3 The coefficients of a primitive generator polynomial determine the feedback taps Time R0 R1 R2 0 1 0 0 1 0 1 0 2 1 0 1 3 1 1 0 4 1 1 1 5 0 1 1 6 0 0 1 7 1 0 0 Sequence repeats from here onwards output Pseudorandom pattern generator
54 Channelization in Code Space • Each channel uses a different pseudorandom code • Codes should have low cross-correlation – If they differ in approximately half the bits the correlation between codes is close to zero and the effect at the output of each other’s receiver is small • As number of users increases, effect of other users on a given receiver increases as additive noise • CDMA has gradual increase in BER due to noise as number of users is increased • Interference between channels can be eliminated if codes are selected so they are orthogonal and if receivers and transmitters are synchronized – Shown in next example
55 Example: CDMA with 3 users • Assume three users share same medium • Users are synchronized & use different 4-bit orthogonal codes: {-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1} +1 -1 +1 User 1 x -1 -1 +1 User 2 x User 3 x +1 +1 -1 Shared Medium + Receiver
56 Channel 1: 110 -> +1+1-1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1) Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1) Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1) Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 1 Channel 2 Channel 3 Sum Signal Sum signal is input to receiver
57 Example: Receiver for Station 2 • Each receiver takes sum signal and integrates by code sequence of desired transmitter • Integrate over T seconds to smooth out noise x Shared Medium + Decoding signal from station 2 Integrate over T sec
58 Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1) Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1) Integrated Output: -4, +4, -4 Binary Output: 0, 1, 0 Sum Signal Channel 2 Sequence Correlator Output Integrator Output -4 +4 -4 Decoding at Receiver 2 X =
59 W1= 0 W2= 0 0 0 1 W4= 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 W8= 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 0 0 0 1 Walsh Functions • Walsh functions provide orthogonal code sequences by mapping 0 to -1 and 1 to +1 • Walsh matrices constructed recursively as follows: W2n= Wn Wn Wn Wn c
60 Channelization in Cellular Telephone Networks • Cellular networks use frequency reuse – Band of frequencies reused in other cells that are sufficiently far that interference is not a problem – Cellular networks provide voice connections that are steady streams • FDMA used in AMPS • TDMA used in IS-54 and GSM • CDMA used in IS-95
61 Further Reading • Textbook: 6.1, 6.2, 6.3, 6.4

More Related Content

PPT
aloha
DeepikaT13
 
PDF
Medium range protocol and multiple access
sushmitjivtode4
 
PPT
Multiple Access Techniques
inayat khan
 
PPT
Jaimin chp-4 - media access sub-layer- 2011 batch
Jaimin Jani
 
PDF
CN R16 -UNIT-4.pdf
Joshuaeeda1
 
PPTX
Media Access Control
VijayaLakshmi514
 
PPT
Unit 1 mac vsd
SHUBHAM MORGAONKAR
 
PPTX
computer__networks .pptx
chmanvitha2
 
aloha
DeepikaT13
 
Medium range protocol and multiple access
sushmitjivtode4
 
Multiple Access Techniques
inayat khan
 
Jaimin chp-4 - media access sub-layer- 2011 batch
Jaimin Jani
 
CN R16 -UNIT-4.pdf
Joshuaeeda1
 
Media Access Control
VijayaLakshmi514
 
Unit 1 mac vsd
SHUBHAM MORGAONKAR
 
computer__networks .pptx
chmanvitha2
 

Similar to note9 (1).pptx on medium access control protocols (20)

PPTX
Media Access Control (MAC Layer)
Meenakshi Paul
 
PDF
Mac
Nitesh Singh
 
PPTX
Lecture 10
Anwal Mirza
 
PDF
Ch12
Mohammed Romi
 
PPT
Computer communication multiple access.ppt
rohinitalekar1
 
PPTX
Multipleaccessprotocol 140213082552-phpapp02
arifur rahman
 
PPTX
module 2 ppt.pptx
Divya Mani
 
PDF
multipleaccessprotocol-140213082552-phpapp02.pdf
GauravWankar2
 
PPT
MultipleAccess.ppt
Jayaprasanna4
 
PPTX
Multiple access protocol
Merlin Florrence
 
PPTX
tow
م.وائل الزعبي
 
PPTX
bcnunit34645788666666666666666675666666667
akankshachouhan14
 
PPT
kamalaveni.ppt
PoituErmawantia1
 
PPT
12 Multiple Access
Ahmar Hashmi
 
PPTX
Multiple access protocols in data communication networks
Nt Arvind
 
PPT
Chapt-06.ppt
SureshChavhan9
 
PPT
multiple access techniquesch12-multiple-access.ppt
bandiprathyu
 
PPT
Mac sub layer
DIKSHA_LAHRANI
 
PPT
mux aloha csma schemes in networking world
circularsuom
 
PDF
Lecture Notes Unit III PArt 2 - The DataLink Layer Medium Access Control Subl...
Murugan146644
 
Media Access Control (MAC Layer)
Meenakshi Paul
 
Mac
Nitesh Singh
 
Lecture 10
Anwal Mirza
 
Ch12
Mohammed Romi
 
Computer communication multiple access.ppt
rohinitalekar1
 
Multipleaccessprotocol 140213082552-phpapp02
arifur rahman
 
module 2 ppt.pptx
Divya Mani
 
multipleaccessprotocol-140213082552-phpapp02.pdf
GauravWankar2
 
MultipleAccess.ppt
Jayaprasanna4
 
Multiple access protocol
Merlin Florrence
 
tow
م.وائل الزعبي
 
bcnunit34645788666666666666666675666666667
akankshachouhan14
 
kamalaveni.ppt
PoituErmawantia1
 
12 Multiple Access
Ahmar Hashmi
 
Multiple access protocols in data communication networks
Nt Arvind
 
Chapt-06.ppt
SureshChavhan9
 
multiple access techniquesch12-multiple-access.ppt
bandiprathyu
 
Mac sub layer
DIKSHA_LAHRANI
 
mux aloha csma schemes in networking world
circularsuom
 
Lecture Notes Unit III PArt 2 - The DataLink Layer Medium Access Control Subl...
Murugan146644
 
Ad

More from sadiariasat10 (20)

PPTX
transactionprocessingppt-171212112608.pptx
sadiariasat10
 
PPTX
BrownbagIntrotosqltuning.pptx SQL tunning
sadiariasat10
 
PPTX
BrownbagIntrotosqltuning.pptx SQL tunning
sadiariasat10
 
PPTX
48131.pptx researching methods research methods
sadiariasat10
 
PPTX
6._Research_methods_quan_qual_and_mixed_Paul_B_and_Anne_J.ppt methods x
sadiariasat10
 
PPTX
IS-Overview.pptx information system in business
sadiariasat10
 
PPTX
aggregatefunction-220420051702.pptx aggregate
sadiariasat10
 
PPTX
DDL and DML.pptx ddl vs dml ddl and dml ddl and dml
sadiariasat10
 
PPTX
me-introduction-may-2.pptx intro to basic
sadiariasat10
 
PPTX
module-5-session-1.pptx module 5 session
sadiariasat10
 
PPTX
hrm301-10.pptx HRm HRM HRM HRM HRM HRM HRM HRM
sadiariasat10
 
PPTX
EquityReinforcement.pptx Equity theory in
sadiariasat10
 
PPTX
Normalization.pptx Functional dependence
sadiariasat10
 
PPTX
ch13.pptx Database effefiency and tuning
sadiariasat10
 
PPTX
ieee802-11.pptx wireless lan IEEE 802 11
sadiariasat10
 
PPTX
IEEE-802.11overview.pptx ieee 802 11 overview
sadiariasat10
 
PPTX
lecture22.pptx relational Algebra basics types
sadiariasat10
 
PPTX
signal propagation.pptx basics of signal propagation
sadiariasat10
 
PPTX
lan-141028090025-conversion-gate02.pptx LAN MAN WAN
sadiariasat10
 
PPTX
DOC-20240829-WA0013..pptx introduction to LAN Man and wan
sadiariasat10
 
transactionprocessingppt-171212112608.pptx
sadiariasat10
 
BrownbagIntrotosqltuning.pptx SQL tunning
sadiariasat10
 
BrownbagIntrotosqltuning.pptx SQL tunning
sadiariasat10
 
48131.pptx researching methods research methods
sadiariasat10
 
6._Research_methods_quan_qual_and_mixed_Paul_B_and_Anne_J.ppt methods x
sadiariasat10
 
IS-Overview.pptx information system in business
sadiariasat10
 
aggregatefunction-220420051702.pptx aggregate
sadiariasat10
 
DDL and DML.pptx ddl vs dml ddl and dml ddl and dml
sadiariasat10
 
me-introduction-may-2.pptx intro to basic
sadiariasat10
 
module-5-session-1.pptx module 5 session
sadiariasat10
 
hrm301-10.pptx HRm HRM HRM HRM HRM HRM HRM HRM
sadiariasat10
 
EquityReinforcement.pptx Equity theory in
sadiariasat10
 
Normalization.pptx Functional dependence
sadiariasat10
 
ch13.pptx Database effefiency and tuning
sadiariasat10
 
ieee802-11.pptx wireless lan IEEE 802 11
sadiariasat10
 
IEEE-802.11overview.pptx ieee 802 11 overview
sadiariasat10
 
lecture22.pptx relational Algebra basics types
sadiariasat10
 
signal propagation.pptx basics of signal propagation
sadiariasat10
 
lan-141028090025-conversion-gate02.pptx LAN MAN WAN
sadiariasat10
 
DOC-20240829-WA0013..pptx introduction to LAN Man and wan
sadiariasat10
 
Ad

Recently uploaded (20)

DOCX
573137875-Attendance-Management-System-original
AYUSHMANAV
 
PPTX
CYBER-CRIMES AND SECURITY A guide to understanding
Davies Chacko
 
PPTX
Sustainable Sites - Green Building Construction
mhdumerali
 
PDF
III.4.1.2_The_Space_Environment.p pdffdf
sittiporn2
 
PPTX
Fundamentals of safety and accident prevention -final (1).pptx
Palani kumar
 
PDF
BMEC211 - INTRODUCTION TO MECHATRONICS-1.pdf
ryankakungu
 
DOCX
ASol_English-Language-Literature-Set-1-27-02-2023-converted.docx
AYUSHMANAV
 
PDF
keyrequirementskkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk
mojeeburahmanwardak
 
PPTX
FINAL REVIEW FOR COPD DIANOSIS FOR PULMONARY DISEASE.pptx
rubeshbme
 
PPTX
bas. eng. economics group 4 presentation 1.pptx
kiribakimoses1993
 
PDF
TFEC-4-2020-Design-Guide-for-Timber-Roof-Trusses.pdf
AinieButt1
 
PPTX
web development for engineering and engineering
keshriji700
 
PDF
composite construction of structures.pdf
AinieButt1
 
PDF
Evaluating the Democratization of the Turkish Armed Forces from a Normative P...
jpsjournal1
 
PDF
Model Code of Practice - Construction Work - 21102022 .pdf
AinieButt1
 
PPTX
UNIT-1 - COAL BASED THERMAL POWER PLANTS
Chandra Kumar S
 
PDF
Embodied AI: Ushering in the Next Era of Intelligent Systems
Yu Huang
 
PPTX
UNIT 4 Total Quality Management .pptx
gokuld13012005
 
PPTX
Current and future trends in Computer Vision.pptx
Subramanyam Natarajan
 
PDF
Human-AI Collaboration: Balancing Agentic AI and Autonomy in Hybrid Systems
ijccsa
 
573137875-Attendance-Management-System-original
AYUSHMANAV
 
CYBER-CRIMES AND SECURITY A guide to understanding
Davies Chacko
 
Sustainable Sites - Green Building Construction
mhdumerali
 
III.4.1.2_The_Space_Environment.p pdffdf
sittiporn2
 
Fundamentals of safety and accident prevention -final (1).pptx
Palani kumar
 
BMEC211 - INTRODUCTION TO MECHATRONICS-1.pdf
ryankakungu
 
ASol_English-Language-Literature-Set-1-27-02-2023-converted.docx
AYUSHMANAV
 
keyrequirementskkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk
mojeeburahmanwardak
 
FINAL REVIEW FOR COPD DIANOSIS FOR PULMONARY DISEASE.pptx
rubeshbme
 
bas. eng. economics group 4 presentation 1.pptx
kiribakimoses1993
 
TFEC-4-2020-Design-Guide-for-Timber-Roof-Trusses.pdf
AinieButt1
 
web development for engineering and engineering
keshriji700
 
composite construction of structures.pdf
AinieButt1
 
Evaluating the Democratization of the Turkish Armed Forces from a Normative P...
jpsjournal1
 
Model Code of Practice - Construction Work - 21102022 .pdf
AinieButt1
 
UNIT-1 - COAL BASED THERMAL POWER PLANTS
Chandra Kumar S
 
Embodied AI: Ushering in the Next Era of Intelligent Systems
Yu Huang
 
UNIT 4 Total Quality Management .pptx
gokuld13012005
 
Current and future trends in Computer Vision.pptx
Subramanyam Natarajan
 
Human-AI Collaboration: Balancing Agentic AI and Autonomy in Hybrid Systems
ijccsa
 

note9 (1).pptx on medium access control protocols

  • 1. 1 Note 6: Medium Access Control Protocols Random Access
  • 2. Quiz #3 Describe in your own words bit stuffing and byte stuffing. 2
  • 3. 3 ALOHA • Wireless link to provide data transfer between main campus & remote campuses of University of Hawaii • Simplest solution: just do it – A station transmits whenever it has data to transmit – If more than one frames are transmitted, they interfere with each other (collide) and are lost – If ACK not received within timeout, then a station picks random backoff time (to avoid repeated collision) – Station retransmits frame after backoff time t t0 t0-X t0+X t0+X+2tprop t0+X+2tprop + B Vulnerable period Time-out Backoff period B First transmission Retransmission
  • 4. 4 ALOHA Model • Definitions and assumptions – X: frame transmission time (assumed to be constant) – S: throughput (average # of successful frame transmissions per X seconds) – G: load (average # of transmission attempts per X sec.) – Psuccess : probability a frame transmission is successful success GP S  X X frame transmission Prior interval • Any transmission that begins during vulnerable period leads to collision • Success if no arrivals during 2X seconds
  • 5. 5 Abramson’s Assumption • What is probability of no arrivals in vulnerable period? • Abramson assumption: The backoff algorithm spreads the retransmissions so that frame transmissions, new and repeated, are equally likely to occur at any instant • This implies that the number of frames transmitted in a time interval has a Poisson distribution e e e G X success t k X P P k t t k P 2 2 0 ! 0 ) 2 ( seconds] 2X in arrivals 0 [ ! ) ( seconds] in arrivals [           
  • 6. 6 Throughput of ALOHA G success Ge GP S 2    0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0 . 0 0 7 8 1 2 5 0 . 0 1 5 6 2 5 0 . 0 3 1 2 5 0 . 0 6 2 5 0 . 1 2 5 0 . 2 5 0 . 5 1 2 4 G S • Collisions are means for coordinating access • Max throughput is rmax= 1/2e (18.4%) • Bimodal behavior: Small G, S≈G Large G, S↓0 • Collisions can snowball and drop throughput to zero e-2 = 0.184
  • 7. 7 Slotted ALOHA • Time is slotted in X seconds slots • Stations synchronized to frame times • Stations transmit frames in first slot after frame arrival • Backoff intervals in multiples of slots t (k+1)X kX t0 +X+2tprop+ B Vulnerable period Time-out Backoff period B t0 +X+2tprop Only frames that arrive during prior X seconds collide
  • 8. 8 Throughput of Slotted ALOHA 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.01563 0.03125 0.0625 0.125 0.25 0.5 1 2 4 8 Ge-G Ge-2G G S 0.184 0.368 e e e e G success G X success t k G GP S X P P k t t k P               ! 0 ) ( seconds] X in arrivals 0 [ ! ) ( seconds] in arrivals [ 0
  • 9. 9 Application of Slotted Aloha • Reservation protocol allows a large number of stations with infrequent traffic to reserve slots to transmit their frames in future cycles • Each cycle has mini-slots allocated for making reservations • Stations use slotted Aloha during mini-slots to request slots cycle X-second slot Reservation mini-slots . . . . . .
  • 10. 10 Carrier Sensing Multiple Access (CSMA) A Station A begins transmission at t = 0 A Station A captures channel at t = tprop • A station senses the channel before it starts transmission – If busy, either wait or schedule backoff (different options) – If idle, start transmission – Vulnerable period is reduced to tprop (due to channel capture effect) – Collisions in ALOHA or Slotted ALOHA involve 2 or 1 frame transmission times X – If tprop >X (or if a>1), no gain compared to ALOHA or slotted ALOHA
  • 11. 11 • Transmitter behavior when busy channel is sensed – 1-persistent CSMA (most greedy)  Start transmission as soon as the channel becomes idle  Low delay and high collision rates – Non-persistent CSMA (least greedy)  Wait a backoff period, then sense carrier again  High delay and low collision rates – p-persistent CSMA (adjustable greedy)  Wait till channel becomes idle, transmit with prob. p; or wait another tprop & re-sense with probability 1-p  Delay and collisions rates can be balanced CSMA Options Sensing
  • 12. 12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.02 0.03 0.06 0.13 0.25 0.5 1 2 4 8 16 32 64 0.53 0.45 0.16 S G a  0.01 a =0.1 a = 1 1-Persistent CSMA Throughput • Better than Aloha & slotted Aloha for small a • Worse than Aloha for a > 1
  • 13. 13 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.02 0.03 0.06 0.13 0.25 0.5 1 2 4 8 16 32 64 0.81 0.51 0.14 S G a = 0.01 Non-Persistent CSMA Throughput a = 0.1 a = 1 • Higher maximum throughput than 1- persistent for small a • Worse than Aloha for a > 1
  • 14. 14 CSMA with Collision Detection (CSMA/CD) • Monitor for collisions & abort transmission – Stations with frames to send, first do carrier sensing – After beginning transmissions, stations continue listening to the medium to detect collisions – If collisions detected, all stations involved stop transmission, reschedule random backoff times, and try again at scheduled times • In CSMA collisions result in wastage of X seconds spent transmitting an entire frame • CSMA-CD reduces wastage to time to detect collision and abort transmission
  • 15. 15 CSMA/CD reaction time It takes 2 tprop to find out if channel has been captured A begins to transmit at t = 0 A B B begins to transmit at t = tprop- ; B detects collision at t = tprop A B A B A detects collision at t= 2 tprop- 
  • 16. 16 CSMA-CD Model • Assumptions – Collisions can be detected and resolved in 2tprop – Time slotted in 2tprop slots during contention periods – Assume n busy stations, and each may transmit with probability p in each contention time slot – Once the contention period is over (a station successfully occupies the channel), it takes X seconds for a frame to be transmitted – It takes tprop before the next contention period starts. Busy Contention Busy (a) Time Idle Contention Busy
  • 17. 17 Contention Resolution • How long does it take to resolve contention? • Contention is resolved (“success’) if exactly 1 station transmits in a slot: 1 ) 1 (    n success p np P • By taking derivative of Psuccess we find max occurs at p=1/n e n n n n P n n success 1 ) 1 1 ( ) 1 1 ( 1 1 1 max        • On average, 1/Pmax = e = 2.718 time slots to resolve contention seconds Period Contention Average 2 e tprop 
  • 18. 18 CSMA/CD Throughput • At maximum throughput, systems alternates between contention periods and frame transmission times     L Rd e a e et t X X prop prop / 1 2 1 1 1 2 1 1 2 max  r          Time Busy Contention Busy Contention Busy Contention Busy • where: R bits/sec, L bits/frame, X=L/R seconds/frame a = tprop/X  meters/sec. speed of light in medium d meters is diameter of system 2e+1 = 6.44 Rd/(vL)
  • 19. 19 CSMA-CD Application: Ethernet • First Ethernet LAN standard used CSMA-CD – 1-persistent Carrier Sensing – R = 10 Mbps – tprop = 51.2 microseconds  512 bits = 64 byte slot  accommodates 2.5 km + 4 repeaters – Truncated Binary Exponential Backoff  After the nth collision, select backoff from {0, 1,…, 2k – 1}, where k=min(n, 10)
  • 20. 20 Throughput for Random Access MACs 0 0.2 0.4 0.6 0.8 1 0.01 0.1 1 ALOHA Slotted ALOHA 1-P CSMA Non-P CSMA CSMA/CD a rmax • For small a: CSMA-CD has best throughput • For larger a: Aloha & slotted Aloha better throughput
  • 21. 21 Carrier Sensing and Priority Transmission • Certain applications require faster response than others, e.g. ACK messages • Impose different interframe times – High priority traffic sense channel for time t1 – Low priority traffic sense channel for time t2>t1 – High priority traffic, if present, seizes channel first • This priority mechanism is used in IEEE 802.11 wireless LAN
  • 22. 22 Note 6: Medium Access Control Protocols Scheduling
  • 23. 23 Scheduling for Medium Access Control • Schedule frame transmissions to avoid collision in shared medium  More efficient channel utilization  Less variability in delays  Can provide fairness to stations  Increased computational or procedural complexity • Two main approaches – Reservation – Polling
  • 24. 24 Reservation Systems Time Cycle n Reservation interval Frame transmissions r d d d r d d d Cycle (n + 1) r = 1 2 3 M • Transmissions organized into cycles • Cycle: reservation interval + frame transmissions • The reservation intervals has a minislot for each station to request reservations for frame transmissions
  • 25. 25 Reservation System Options • Centralized or distributed system – Centralized systems: A central controller listens to reservation information, decides order of transmission, issues grants – Distributed systems: Each station determines its slot for transmission from the reservation information • Single or Multiple Frames – Single frame reservation: Only one frame transmission can be reserved within a reservation cycle – Multiple frame reservation: More than one frame transmission can be reserved within a frame • Channelized or Random Access Reservations – Channelized (typically TDMA) reservation: Reservation messages from different stations are multiplexed without any risk of collision – Random access reservation: Each station transmits its reservation message randomly until the message goes through
  • 26. 26 t r 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3 (a) t r 3 5 r 3 5 r 3 5 8 r 3 5 8 r 3 8 (b) Example • Initially stations 3 & 5 have reservations to transmit frames • Station 8 becomes active and makes reservation • Cycle now also includes frame transmissions from station 8
  • 27. 27 Efficiency of Reservation Systems • Assume minislot duration = vX • TDM single frame reservation scheme – If propagation delay is negligible, a single frame transmission requires (1+v)X seconds – Link is fully loaded when all stations transmit, maximum efficiency is: • TDM k frame reservation scheme – If k frame transmissions can be reserved with a reservation message and if there are M stations, as many as Mk frames can be transmitted in XM(k+v) seconds – Maximum efficiency is: max 1 1 MX M MX r     vX v max 1 1 MkX M MkX k r     v vX
  • 28. 28 Random Access Reservation Systems • Large number of light traffic stations – Dedicating a minislot to each station is inefficient • Slotted ALOHA reservation scheme – Stations use slotted Aloha on reservation minislots – On average, each reservation takes at least e minislot attempts – Effective time required for the reservation is 2.71vX max 1 (1 ) 1 2.71 X X ev r     v
  • 29. 29 Example: GPRS • General Packet Radio Service – Packet data service in GSM cellular radio – GPRS devices, e.g. cellphones or laptops, send packet data over radio and then to Internet – Slotted Aloha MAC used for reservations – Single & multi-slot reservations supported
  • 30. 30 Polling Systems • Centralized polling systems: A central controller transmits polling messages to stations according to a certain order • Distributed polling systems: A permit for frame transmission is passed from station to station according to a certain order • A signaling procedure exists for setting up order Central Controller
  • 31. 31 Polling System Options • Service Limits: How much is a station allowed to transmit per poll? – Exhaustive: until station’s data buffer is empty (including new frame arrivals) – Gated: all data in buffer when poll arrives – Frame-Limited: one frame per poll – Time-Limited: up to some maximum time • Priority mechanisms – More bandwidth & lower delay for stations that appear multiple times in the polling list – Issue polls for stations with message of priority k or higher
  • 32. 33 Average Cycle Time • Assume walk times all equal to t’ • Exhaustive Service: stations empty their buffers • Cycle time = Mt’ + time to empty M station buffers • /M be frame arrival rate at a station • NC average number of frames transmitted from a station • Time to empty one station buffer: t 1 3 2 4 5 1 … M t’ t’ t’ t’ t’ t’ Tc Tstation = Nc X = ( l M Tc )X = rTc M • Average Cycle Time: r r           1 t M T T t M MT t M T c c station c X  r 
  • 33. 34 Efficiency of Polling Systems • Exhaustive Service – Cycle time increases as traffic increases, so delays become very large – Walk time per cycle becomes negligible compared to cycle time: c c T Mt Efficiency T r     Can approach 100% • Limited Service – Many applications cannot tolerate extremely long delays – Time or transmissions per station are limited – This limits the cycle time and hence delay – Efficiency of 100% is not possible X t t M MX MX Efficiency / 1 1       Single frame per poll
  • 34. 35 Application: Token-Passing Rings Free Token = Poll Listen mode Delay Input from ring Output to ring Ready station looks for free token Flips bit to change free token to busy Transmit mode Delay To device From device Ready station inserts its frames Reinserts free token when done token Frame Delimiter is Token Free = 01111110 Busy = 01111111
  • 35. 36 Methods of Token Reinsertion • Multi-token operation – Free token transmitted immediately after last bit of data frame • Single-token operation – Free token inserted after last bit of the busy token is received back – Transmission time at least ring latency – If frame transmission time is longer than ring latency, equivalent to multi-token operation • Single-Frame operation – Free token inserted after transmitting station has received last bit of its frame – Equivalent to attaching trailer equal to ring latency Busy token Free token Frame Idle Fill
  • 36. 37 Token Ring Throughput • Definition – t’: ring latency (time required for bit to circulate ring) – X: maximum frame transmission time allowed per station • Multi-token operation – Assume network is fully loaded, and all M stations transmit for X seconds upon the reception of a free token – This is a polling system with limited service time: M a MX MX MX / 1 1 / 1 1 max          t t r latency ring normalized the is X a t  
  • 37. 38 • Single-token operation – Effective frame transmission time is maximum of X and t’ , therefore Token Ring Throughput ρmax = = MX t΄+ M(X+ t΄) 1 1+a΄(1 + 1/M) ρmax = = MX t΄+ M max{(X,t΄} 1 max{1, a΄} + a΄/M • Single-frame operation – Effective frame transmission time is X+ t’ ,therefore
  • 38. 39 Token Reinsertion Efficiency Comparison Maximum throughput a  Multiple token operation 0 0.2 0.4 0.6 0.8 1 1.2 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 Single frame operation M = 50 M = 10 Single token operation M = 50 M = 10 • a <<1, any token reinsertion strategy acceptable • a ≈1, single token reinsertion strategy acceptable • a >1, multitoken reinsertion strategy necessary
  • 39. 40 Application Examples • Single-frame reinsertion – IEEE 802.5 Token Ring LAN @ 4 Mbps • Single token reinsertion – IBM Token Ring @ 4 Mbps • Multitoken reinsertion – IEEE 802.5 and IBM Ring LANs @ 16 Mbps – FDDI Ring @ 50 Mbps • All of these LANs incorporate token priority mechanisms
  • 40. 41 Comparison of MAC approaches • Aloha & Slotted Aloha – Simple & quick transfer at very low load – Accommodates a large number of low-traffic bursty users – Highly variable delay at moderate loads – Efficiency does not depend on a • CSMA-CD – Quick transfer and high efficiency for low delay-bandwidth product – Can accommodate a large number of bursty users – Variable and unpredictable delay
  • 41. 42 Comparison of MAC approaches • Reservation – On-demand transmission of bursty or steady streams – Accommodates large number of low-traffic users with slotted Aloha reservations – Can incorporate QoS – Handles large delay-bandwidth product via delayed grants • Polling – Generalization of time-division multiplexing – Provides fairness through regular access opportunities – Can provide bounds on access delay – Performance deteriorates with large delay-bandwidth product
  • 42. 43 Note 6: Medium Access Control Protocols Channelization
  • 43. 44 Why Channelization? • Channelization – Semi-static bandwidth allocation of portion of shared medium to a given user • Highly efficient for constant-bit rate traffic • Preferred approach in – Cellular telephone networks – Terrestrial & satellite broadcast radio & TV
  • 44. 45 Why not Channelization? • Inflexible in allocation of bandwidth to users with different requirements • Inefficient for bursty traffic • Does not scale well to large numbers of users – Average transfer delay increases with number of users M • Dynamic MAC much better at handling bursty traffic 0 5 10 15 20 25 30 0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 r E[T]/X M=16 M=8 M=4 M=2 M=1
  • 45. 46 Channelization Approaches • Frequency Division Multiple Access (FDMA) – Frequency band allocated to users – Broadcast radio & TV, analog cellular phone • Time Division Multiple Access (TDMA) – Periodic time slots allocated to users – Telephone backbone, GSM digital cellular phone • Code Division Multiple Access (CDMA) – Code allocated to users – Cellular phones, 3G cellular
  • 46. 47 Channelization: FDMA • Divide channel into M frequency bands • Each station transmits and listens on assigned bands • Each station transmits at most R/M bps • Good for stream traffic; Used in connection-oriented systems • Inefficient for bursty traffic Frequency Guard bands Time W 1 2 M M–1 …
  • 47. 48 Channelization: TDMA • Dedicate 1 slot per station in transmission cycles • Stations transmit data burst at full channel bandwidth • Each station transmits at R bps 1/M of the time • Excellent for stream traffic; Used in connection-oriented systems • Inefficient for bursty traffic due to unused dedicated slots 1 Time Guard time One cycle 1 2 3 M W Frequency ...
  • 48. 49 Guardbands • FDMA – Frequency bands must be non-overlapping to prevent interference – Guardbands ensure separation; form of overhead • TDMA – Stations must be synchronized to common clock – Time gaps between transmission bursts from different stations to prevent collisions; form of overhead – Must take into account propagation delays
  • 49. 50 Channelization: CDMA • Code Division Multiple Access – Channels determined by a code used in modulation and demodulation • Stations transmit over entire frequency band all of the time! Time W Frequency 1 2 3
  • 50. 51  Binary information R1 bps W1 Hz Unique user binary random sequence Digital modulation Radio antenna Transmitter from one user R >> R1bps W >> W1 Hz  CDMA Spread Spectrum Signal • User information mapped into: +1 or -1 for T sec. • Multiply user information by pseudo- random binary pattern of G “chips” of +1’s and -1’s • Resulting spread spectrum signal occupies G times more bandwidth: W = GW1 • Modulate the spread signal by sinusoid at appropriate fc
  • 51. 52 Signal and residual interference Correlate to user binary random sequence Signals from all transmitters Digital demodulation Binary information   CDMA Demodulation • Recover spread spectrum signal • Synchronize to and multiply spread signal by same pseudo-random binary pattern used at the transmitter • In absence of other transmitters & noise, we should recover the original +1 or -1 of user information • Other transmitters using different codes appear as residual noise
  • 52. 53 R0 R1 R2 g(x) = x3 + x2 + 1 g0 g2 g3 The coefficients of a primitive generator polynomial determine the feedback taps Time R0 R1 R2 0 1 0 0 1 0 1 0 2 1 0 1 3 1 1 0 4 1 1 1 5 0 1 1 6 0 0 1 7 1 0 0 Sequence repeats from here onwards output Pseudorandom pattern generator
  • 53. 54 Channelization in Code Space • Each channel uses a different pseudorandom code • Codes should have low cross-correlation – If they differ in approximately half the bits the correlation between codes is close to zero and the effect at the output of each other’s receiver is small • As number of users increases, effect of other users on a given receiver increases as additive noise • CDMA has gradual increase in BER due to noise as number of users is increased • Interference between channels can be eliminated if codes are selected so they are orthogonal and if receivers and transmitters are synchronized – Shown in next example
  • 54. 55 Example: CDMA with 3 users • Assume three users share same medium • Users are synchronized & use different 4-bit orthogonal codes: {-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1} +1 -1 +1 User 1 x -1 -1 +1 User 2 x User 3 x +1 +1 -1 Shared Medium + Receiver
  • 55. 56 Channel 1: 110 -> +1+1-1 -> (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1) Channel 2: 010 -> -1+1-1 -> (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1) Channel 3: 001 -> -1-1+1 -> (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1) Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 1 Channel 2 Channel 3 Sum Signal Sum signal is input to receiver
  • 56. 57 Example: Receiver for Station 2 • Each receiver takes sum signal and integrates by code sequence of desired transmitter • Integrate over T seconds to smooth out noise x Shared Medium + Decoding signal from station 2 Integrate over T sec
  • 57. 58 Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1) Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1) Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1) Integrated Output: -4, +4, -4 Binary Output: 0, 1, 0 Sum Signal Channel 2 Sequence Correlator Output Integrator Output -4 +4 -4 Decoding at Receiver 2 X =
  • 58. 59 W1= 0 W2= 0 0 0 1 W4= 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 W8= 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 0 0 0 1 Walsh Functions • Walsh functions provide orthogonal code sequences by mapping 0 to -1 and 1 to +1 • Walsh matrices constructed recursively as follows: W2n= Wn Wn Wn Wn c
  • 59. 60 Channelization in Cellular Telephone Networks • Cellular networks use frequency reuse – Band of frequencies reused in other cells that are sufficiently far that interference is not a problem – Cellular networks provide voice connections that are steady streams • FDMA used in AMPS • TDMA used in IS-54 and GSM • CDMA used in IS-95
  • 60. 61 Further Reading • Textbook: 6.1, 6.2, 6.3, 6.4