Chap 4 Multiaccess Communication(Part 2)

Ling-Jyh Chen

2

Classification of Multiple Access Protocols

Multiple access protocols

Contention-based Conflict-free

Random access Collision resolution

FDMA,

TDMA,

CDMA,

Token Bus,

etc

ALOHA,

CSMA,

BTMA,

etc

TREE,

WINDOW, etc

BTMA: Busy Tone Multiple Access

3

Contention Protocols

ALOHA Developed in the 1970s for a packet radio network

by Hawaii University. Whenever a station has a data, it transmits.

Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision.

Slotted ALOHA Improvement: Time is slotted and a packet can

only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.

4

Slotted ALOHA

1 2&3 2Time

Collision

Retransmission Retransmission

3

Slot

Node 1 Packet

Nodes 2 & 3 Packets

5

Slotted ALOHA (cont.)

1),,1(),0(0)],,1(1)[,0(),0(),1(1)],,0(1)[,1(

)(2),,(

,

inQnQinQnQnQnQinQnQ

nminiQ

P

ra

rara

ra

a

inn

),1(),0(),0(),1( nQnQnQnQP rarasucc

6

Throughput of Slotted ALOHA

GeP 0

• The probability of no collision is given by

GeGPGS 0

• The throughput S is

368.01max

eS

• The Maximum throughput of slotted ALOHA is

7

ALOHA

1 2 3 3 2Time

Collision

Retransmission Retransmission

Node 1 Packet Waiting a random time

Node 2 Packet

Node 3 Packet

8

Throughput of ALOHA

n

!n

(2G)nP

e 2G

• The probability that n packets arrive in two packets time is given by

where G is traffic load.

GeP 20

• The probability P(0) that a packet is successfully received without collision is calculated by letting n=0 in the above equation. We get

GeGPGS 20

• We can calculate throughput S with a traffic load G as follows:

184.021

max e

S

• The Maximum throughput of ALOHA is

9

Comparison of Aloha and S-Aloha

G 86420

0.5

0.4

0.3

0.2

0.1

0

Slotted Aloha

Aloha

0.368

0.184

G

S

10

CSMA: Carrier Sense Multiple Access

11

Contention Protocols

CSMA (Carrier Sense Multiple Access) Improvement: Start transmission only if no

transmission is ongoing

CSMA/CD (CSMA with Collision Detection) Improvement: Stop ongoing transmission if a

collision is detected

CSMA/CA (CSMA with Collision Avoidance) Improvement: Wait a random time and try again

when carrier is quiet. If still quiet, then transmit

12

Carrier Sense Multiple Access In many multiaccess systems--e.g., LANs--ready

station can determine if medium is idle before transmitting if medium is sensed as busy, ready station defers until

it becomes idle collisions are still possible if two (or more) ready

stations sense idle at same time

1 2 3TimeCollision

4

Node 4 sense

Delay

5

Node 5 sense

Delay

Node 1 PacketNode 2 Packet

Node 3 Packet

13

CSMA

14

CSMA Slotted Aloha The major difference between CSMA Slotted Aloha

and ordinary slotted Aloha is that idle slots in CSMA have a duration β.

If a packet arrives at a node while a transmission is in progress, the packet is regarded as backlogged and begins transmission with probability qr after each subsequent idle slot.

Packets arriving during an idle slot are transmitted in the next slot as usual.

a.k.a. nonpersistent CSMA

15

nonpersistent CSMA

Idle Period

Busy Period

Collision!!

Time

16

CSMA Slotted Aloha Variations persistent CSMA

frames arriving during an idle slot β are transmitted at end of the minislot

arrivals during busy period are transmitted as soon as medium is sensed as idle (after β)

backlogged stations (holding collided frames) retransmit at end of each idle minislot with probability qr

P-Persistent CSMA frames arriving during an idle minislot are transmitted at end

of the minislot arrivals during busy period are transmitted at end of each

idle minislot with probability p backlogged stations retransmit at end of each idle minislot

with probability qr < p

17

Mathematical analysis of nonpersistent

Markov chain model (discrete time) state is number n of backlogged stations

each busy (success or collision) slot has unit length

each busy slot is followed by one (idle) minislot

each time step in the MC corresponds to a real time interval of either if no station transmits) or 1+ if at least one station transmits)

18

CSMA Slotted Aloha Analysis

At a transition into state n (i.e., at the end of an idle slot), the prob. of no transmissions in the following slot is e-λβ(1-qr)n. The first term is the prob of no arrivals in the

previous idle slot The second term is the prob of no

transmissions by the backlogged nodes

19

CSMA Slotted Aloha Analysis (cont.)

The expected time (T) between state transitions in the state n is β+(1-e-λβ(1-qr)n).

Clearly, β T ≦ ≦ β+1

Using Little’s Theorem, the expected number of arrivals between state transitions is:

E{arrival} = λ (β+1-e-λβ(1-qr)n)

20

CSMA Slotted Aloha Analysis (cont.)

The expected number of departure between state transitions in state n is simply the probability of a successful transmission, that is given by:

The drift in state n is defined as the expected number of arrivals less the expected number of departures between state transitions.

nr

r

rnrn qe

qnqqeD )1(

1)1(1

nr

r

r

rn

rn

r

succ

qeqnq

qqneqe

PPP

)1(1

)1(!0)()1(

!1)(

]retx one arrival, no[]retx no arrival, one[

101

21

CSMA Slotted Aloha Analysis (cont.)

For small qr, (1- qr)n-1 (1- q≒ r)n e≒ -qrn

Therefore,

where g(n) = λβ+ qrn is the expected number of attempted transmissions following a transition to state n

The drift is negative if

The numerator is the expected number of departures per state transition, and the denominator is the expected duration of a state transition period; thus the ratio can be interpreted as departure rate.

)()( )()1( ngngn engeD

)(

)(

1)(

ng

ng

eeng

22

Departure Rate (i.e., throughput)

g

g

ege

121

1

2g

λ Arrival rate

Departure rate:

Equilibrium

large backlog

23

Throughput vs β

Using GNUPlot

4.4.2: skip

24

CSMA unslotted Aloha When a packet arrives, the transmission starts

immediately if the channel is sensed to be idle.

If the channel is sensed to be busy, or if the transmission results in a collision, the packet is regarded as backlogged.

Each backlogged packet repeatedly attempts to retransmit at randomly selected times separated by independent, exponentially distributed random delays τ, with prob density xe-xτ

25

CSMA unslotted Aloha (cont.) We assume a propagation and detection delay of β,

so that if one transmission starts at time t, another node will not detect that the channel is busy until t+β, thus causing the possibility of collisions.

Consider an idle period that starts with a backlog of n. The time until the first transmission starts is an exponentially distributed R.V. with rate G(n)=λ+nx

G(n) is the attempt rate in packets per unit time.

26

CSMA unslotted Aloha (cont.) A collision occurs if the next sensing is done

within time β. Thus, the prob that this busy period is a collision is 1-e-βG(n)

The prob of a transmission following an idle period is e-βG(n)

The expected time from the beginning of one idle period until the next is 1/G(n) + (1+ β) The first term is the expected time until the first

transmission starts The second term is the time until the first

transmission ends and the channel is detected as being idle again.

27

CSMA unslotted Aloha (cont.) The departure rate when the backlog is n is given by:

For small β, the maximum value occurs when G(n)≒β-1/2, and the value is

The MAX value is slightly smaller than the MAX value of CSMA slotted Aloha. The reason is when CSMA is not being used, collisions are somewhat more likely fit a given attempt rate in an unslotted system than a slotted system.

)1()(/1

)(

nGe nG

211

28

CSMA unslotted Aloha (cont.)

However, in a slotted system, β would have to be larger than in an unslotted system to compensate for synchronization inaccuracies and worst-case propagation delay.

Thus, unslotted Aloha appears to be the natural choice for CSMA.

4.4.4: skip

29

CSMA/CD: CSMA + Collision detection

30

CSMA/CD In CSMA protocols

If two stations begin transmitting at the same time, each will transmit its complete packet, thus wasting the channel for an entire packet time

In CSMA/CD protocols The transmission is terminated immediately upon the

detection of a collision CD = Collision Detect

31

CSMA/CD

32

CSMA/CD (cont’d) Sense the channel

If idle, transmit immediately If busy, wait until the channel becomes idle

Collision detection Abort a transmission immediately if a collision is

detected Try again later after waiting a random amount of time

33

CSMA/CD (cont’d) Carrier sense

reduces the number of collisions

Collision detection reduces the effect of collisions, making the

channel ready to use sooner

34

Slotted CSMA/CD We visualize S-CSMA/CD in terms of slots and

minislots.

The minislots are of duration β, which denotes the time required for a signal to propagate from one end of the cable to the other and to be detected.

If the nodes are all synchronized into minislots of this duration, and if one node transmits in a minislot, all the other nodes will detect the transmission and not use subsequent minislots until the entire packet is completed.

35

Slotted CSMA/CD (cont.)

If more than one node transmits in a minislot, each transmitting node will detect the condition by the end of the minislot and cease transmitting.

Thus, the minislots are used in a contention mode, and when a successful transmission occurs in a minislot, it effectively reserves the channel for the completion of the packet.

36

Slotted CSMA/CD (cont.) We assume each backlogged node transmits

after each idle slot with prob qr

The node transmitting rate after an idle slot is Poisson with parameter g(n)=λβ+ qrn

Consider state transitions at the ends of idle slots: if no transmissions occur, the next idle slot ends after time β; if one transmission occurs, the next idle slot ends after 1+ β

37

Slotted CSMA/CD (cont.) Variable-length packets are allowed here, but

the packet durations should be multiples of the idle slot durations.

For simplicity, we assume the expected packet duration is 1.

Finally, if a collision occurs, the next idle slot ends after 2β, i.e. nodes must hear an idle slot after the collision to know that it is safe to transmit.

38

Slotted CSMA/CD (cont.)

The expected length of the interval between state transitions is: E{interval}=β+g(n)e-g(n)+β[1-(1+g(n)) e-g(n)] The second term is 1 times the success prob The third term is the additional β times the

collision prob

The prob of success is g(n)e-g(n)

The drift in state n is λE{interval} - Psucc

39

Slotted CSMA/CD (cont.) The departure rate in state n is

This quantity is maximized over g(n) at g(n)=0.77, and the resulting value is 1/(1+3.31β)

The constant (i.e. 3.31) is dependent on the detailed assumptions of the system. However, if β is very small, this constant is not very important.

Unslotted CSMA/CS makes more sense due to the difficulty of perfect synchronizing on short minislots.

)()(

)(

))(1(1)()(

ngng

ng

engengeng

40

Unslotted CSMA/CD Suppose a node at one end starts to transmit, and

then, almost β time units later, a node at the other end starts. The 2nd node ceases its transmission almost immediately upon hearing the 1st node, but nonetheless causes errors in the first packet and forces the 1st node to stop transmission another β time units later.

Node 1 starts

Node 2 starts Node 1 heardNode 2 stops

Node 2 heardNode 1 stops

TimePropagation

delay

41

Unslotted CSMA/CD (cont.) Nodes closer together on the cable detect collisions

faster than those more spread apart.

As a result, the MAX throughput achievable with Ethernet depends on the arrangement of nodes on the cable and is very complex to calculate exactly.

Goal: to find bounds on all the relevant parameters from the end of one transmission to the end of the next in order to get a conservative bound on max throughput!

42

Unslotted CSMA/CD (cont.) Assume that each node initiates transmissions according to an

independent Poisson process whenever it senses the channel idle, and the overall rate is G.

All nodes sense the beginning of an idle period at most β after the end of a transmission.

The expected time to the beginning of the next transmission is at most 1/G.

The next packet will collide with some later starting packet with prob at most 1-e-βg

The colliding packet will cease transmission after at most 2β

The packet will be successful with prob at least e-βg and will occupy 1 time unit.

43

Unslotted CSMA/CD (cont.) The departure rate S for a given G is the success

prob divided by the expected time of a success or collision; so

The MAX occurs at

The corresponding MAX value is

This analysis is very conservative, but if β is very small, throughputs very close to 1 can be achieved.

GG

G

eeGeS

)1(2/1

43.06

113

G

2.611

S

44

Unslotted CSMA/CD (cont.) The MAX stable throughput approaches 1 with decreasing

β; whereas the approach is as a constant times β1/2 for CSMA.

The reason for the difference is that collisions are not very costly with CSMA/CD, and thus much higher attempt rates can be used.

CSMA/CD (and CSMA) becomes increasingly inefficient with increasing bus length (i.e. β), with increasing data rate (i.e. C), and with decreasing data packet size (i.e. L). ps:

LC

45

IEEE 802 LANs LAN: Local Area Network What is a local area network?

A LAN is a network that resides in a geographically restricted area

LANs usually span a building or a campus

46

Characteristics of LANs Short propagation delays

Small number of users

Single shared medium (usually)

Inexpensive

47

Common LANs Bus-based LANs

Ethernet (*) Token Bus (*)

Ring-based LANs Token Ring (*)

Switch-based LANs Switched Ethernet ATM LANs

(*) IEEE 802 LANs

48

OSI Layers and IEEE 802

802.2 Logical Link Control

802.3 802.4 802.5Medium Access Control

Data Link Layer

Physical Layer

Higher Layers

OSI layers IEEE 802 LAN standards

Higher Layers

CSMA/CD Token-passing Token-passing bus bus ring

49

IEEE 802 Standards802.1: Introduction802.2: Logical Link Control (LLC)802.3: CSMA/CD (Ethernet)802.4: Token Bus802.5: Token Ring802.6: DQDB 802.11: CSMA/CA (Wireless LAN)

50

Summary

4.5.1, 4.5.3, 4.5.4, 4.5.5, 4.5.6, 4.6: skip