ch 12. multiple access

Ch 12. Multiple Access

Multiple Access for Shared Link

• Dedicated link– Point-to-point connection is sufficient

• Shared link– Link is not dedicated – Wireless medium, Ethernet– Controlling the access to the medium• Who will use the medium?

Taxonomy of Multiple Access

12.1 Random Access

• Random access (or contention)– No station is superior to another station (no

centralized authority)– No scheduled time for transmission – transmission

is random– No rules specify which station would send next –

stations compete with each other (contention)

Pure (Original) ALOHA• Earliest random access, developed at the University

of Hawaii in early 1970s• Basic procedure– If a station has a frame to send, then it sends a frame

immediately (hoping that none transmits simultaneously)– Then, check whether the transmission is successful (by

receiving an ACK)• Collision: two or more users’ transmissions overlap

resulting in packet loss

t

Collision occurs

Station 1 decides to Xmit

Station 2 Xmits

Example of Four Stations

• Only two frames (1.1 and 3.2) are successful

Reliable Transmission• Retransmission

– The sender checks whether a transmission is successful by receiving an ACK from the receiver

– If unsuccessful, the sender retransmits the frame• Binary Exponential Back-off

– Before retransmission, wait for random amount of time– If it is the first retransmission, random within [0,T], where T is a time

unit (often, max round-trip propagation delay)– If it is the second retransmission, random within [0, 2T]– If it is the third retransmission, random within [0, 4T]– …– If it is the k-th retransmission, random within [0, 2kT]

Detailed Algorithm of Pure ALOHA

Example 12.1

• The stations on a wireless ALOHA network are a maximum 600km apart. If we assume that signals propagate at 3x108 m/s, we find T = (600x103)/(3x108) = 2ms. Now we can find the value of backoff time TB for different Ks. – For K = 1, TB = 0 or 2ms

– For K = 2, TB = 0, 2, 4, or 6ms

– For K = 3, TB = 0, 2, 4, 6, 8, 10, 12, or 14ms– In many cases, there are maximum K. For example,

if K > 10, the range is set to [0, …, 210-1] TB

Vulnerable Time• Vulnerable time is a time interval, in which

there is a possibility of collisionAssume that all stations send fixed-length frames

Throughput Analysis : Pure Aloha

mm

N S

sec / packets 1sec / packets N

rateion transmisschannel Max.

packets new ing transmittof Rate (S)t throughpuNormalized

N stations Each station transmits λ (new)

packets/ sec Arrival from each station is Poisson Fixed Packet lengths Time to transmit a packet m=Tfr


• Due to collisions : Total rate of packets attempting transmission = λ′

λ > ′ λ• G = Actual traffic intensity or utilization

G = N λ m′


• Any transmissions by any station between times t-m and t+m will cause a collisionPr {Successful transmission}= Pr {No arrivals occur in time [t-m, t+m] | An arrival occurs at time t}= Pr {No arrivals occurring in time (t-m,t+m)}= Pr {No arrivals occur in time 2m}

= e-N λ (2m)′

• Hence, Ps = Pr {Successful transmission} = e-2G (Because G = N λ m)′

vulnerable time

Throughput Analysis : Pure Aloha (Throughput with new packets) (Throughput with all packets)

# of successfully transmitted packetssec

# of totally transmitted packetssec

# of successfully transmitted packets# of totally tran

sSPG

S

G

smitted packets

2

2

/

/S

G

G

P S G

S G e

S Ge


• S = G e-2G : Pure Aloha throughput equationdS/dG = e-2G - 2G e-2G

At the peak of S, e-2G (1 - 2G) = 0G* = 1 / 2 = 0.5Smax = 0.5e-1 = 1 / (2e)Smax ≈ 0.18

• For Pure ALOHA the max. throughput is 18% of channel capacity.

Suitable for highly bursty traffic (computer traffic)


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.015…

0.03125

0.0625

0.125

0.25 0.5 1 2 4 8

Ge-G

Ge-2G

G

S0.184

G*

Smax

Slotted ALOHA• Time-slotted system• Slotted ALOHA: stations must send a frame

only at the beginning of the time slot

Vulnerable Time of Slotted ALOHA• Improves the efficiency– Vulnerable time is Tfr

(Recall that pure ALOHA has vulnerable time of 2Tfr)

– When a data arrives, it waits until the next slot

Throughput Analysis: Slotted Aloha Pr {Success} = e-G

Similar to pure Aloha: S = G e-G , dS/dG = 0G* = 1 Smax = 1/e ≈ 0.36 Slotted Aloha improves throughput by a factor of 2!!! However, maintaining slots consistently in a distributed

system is difficult

Throughput Analysis: Slotted Aloha

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.015…

0.03125

0.0625

0.125

0.25 0.5 1 2 4 8

Ge-G

Ge-2G

G

S0.184

0.368

G*

Smax

Simulating Slotted-ALOHA Network• nonAttempt = 0; collision = 0• for i=0 to 9 do

– throughput(i) = 0– variable p(i) is given within [0, 1]

• for t=1 to 1000 do /* simulate for 1000 time slots */– for i=0 to 9 do /* simulate with 10 stations */

• X(i) = 0– for i=0 to 9 do

• X(i) = 1 with probability p(i) /* station i transmits at this time slot */– Y = X(0) + X(1) + … + X(9)– case Y

• == 0: nonAttempt = nonAttempt + 1• == 1: throughput(i) = throughput(i) + 1• >= 2: collision = collision + 1

• endfor• print nonAttempt, collision, throughput(0), throughput(1),…, throughput(9)

Carrier Sense Multiple Access (CSMA)

• Intended to reduce collision– A station senses the medium before trying to use

it (i.e., listen to the medium before sending)– If a station finds that there is an ongoing

transmission in the medium, it should not send

Collision under CSMA

• CSMA can reduce collision, but cannot eliminate

B

C

Vulnerable Time of CSMA

• Vulnerable time = propagation time Tp

Persistence: CSMA• 1-persistent

– If the medium is busy, sense continuously

– If the medium is idle, send a frame

• non-persistent – If busy, wait for a random

amount of time– If idle, send a frame

• p-persistent– If busy, sense continuously– If idle, send a frame with

prob. p, and wait for a time slot with prob. 1-p

High chance of collision

Low efficiency

Procedure of Persistence Methods

CSMA/CD• CSMA with Collision Detection • Station monitors the medium after it sends a frame

– If it detects a collision, it stops transmitting immediately– Before sending the last bit of the frame, the sending station

must detect the collision restriction on the frame size

CSMA/CA• CSMA with Collision Avoidance– Signal attenuation makes it hard to detect a collision (usual in

wireless environments)– Inter-Frame Space (IFS): a station defers transmission for an IFS

time, even if the channel is found idle– Contention windows: a station randomly picks a time slot of

contention window as its wait time– Acknowledgement: data still can be corrupted

• Cautions in CSMA/CA– IFS can be used to prioritize stations: stations with

a higher priority will have a shorter IFS– Fairness: when a station finds that the channel is

busy while it waits in the contention window, it freezes the timer, which will restart when the channel becomes idle

12.2 Controlled Access

• Reservation access– A station needs to make a reservation before

sending data

time

Controlled Access (2)• Polling

– Primary station (central entity) manages other secondary stations– Select: check whether the primary station can send a frame to a

secondary station– Poll: check whether a secondary station has a frame to send to

the primary station

Controlled Access (3)

• Token passing– Stations in a network are organized in a logical ring– A special packet called token (there is only one token in

the network) is circulated in the network– A station should wait for the token before sending data– The station who holds the token can send data – Once the station finishes its transmission, it passes the

token to the next station– Token management: the token has to be monitored to

ensure that it has not been lost or destroyed

12.3 Channelization

• Frequency Division Multiple Access (FDMA)

Channelization (2)

• Time Division Multiple Access (TDMA)

Channelization (3)

• Code Division Multiple Access (CDMA)

• Chip sequence of CDMA– Orthogonal sequences: ci * cj = 0 if i ≠ j

– Each station j has its sequence cj, and sends cj for to send 1 or –cj for to send 0

CDMA (Cont)

• Decoding for Station 2’s receiver in CDMA

CDMA (Cont)

• At sender– di = 0 or 1

• At receiver of Station 3:– Result = c3 * (d1* c1 + d2 * c2 + d3 * c3 + d4 * c4)

= 0 + 0 + d3 * (c3 * c3) + 0

Homework

• Exercise in Chap. 12– 11, 12, 13, 14, 15, 23

• Additional problem– Simulate Slotted-ALOHA networks (see Slide 22)– 1) Draw nonAttempt when p(i) = p for all i, changing

p = 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2– 2) Repeat 1) with collision– 3) Repeat 1) with i throughput(i)– 4) Obtain throughput(i)’s when p(i) = 0.02 x i, AND

compare with the results when p(i) = 0.1 for all i

ch 12. multiple access

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