A Short Tutorial on CSMA - IEEE 802 Modes and their Throughput-Delay ... • Sense the channel...

May, 1991 Doc: IEEE P802.11191-44-A

A Short Tutorial on CSMA

• Related work and history • Analytical results • Ethernet & measured results • Pros and Cons

A Short Tutorial on CSMA 3 Paulette Altmaier

May, 1991 Doc: IEEE P802.11/91-44-A

What this Is

• Presentation of established work in the field

• Highlights of well-known work

What this is Not

• A pitch for CSMA

• Attempt to apply these or other results to 802.11 work (merits a separate submission)


'l

'; . . , May, 1991 Doc: IEEE P802.11191-44-A

Context for CSMA Discussion1

Definitions -Autonomous Networks

networks operated by independent entities not actively coordinated with each other

Collocated Networks networks operating on the same physical channel in the same vicinity

Basic System and Market Assumptions • A few cells will exist • The system architecture should exploit the existence of cells to the maximum extent possible but • There will not be enough cells to guarantee every autonomous collocated network exclusive use of a cell • Users will not accept a solution that requires coordinated network engineering by unrelated collocated users it follows that • Spectrum sharing among user communities or independent access points using a common ceillchannel is an essential requirement for the system architecture.

Consequently, 2 channel access issues exist

• To regulate channel sharing by autonomous collocated networks

• To regulate channel sharing by nodes within an autonomous system

Any proposed system architecture must address both issues to be viable

1 This introductory section does not fonn part of the Tutorial since it represents the opinions of the author. .


May, 1991 Doc: IEEE P802.11191-44-A

Context for CSMA discussion, contd

Why propose CSMA? • A serious contender • Well suited to small cells and work-groups • Supports cooperative spectrum use between independent systems • Supports portable computer environments well (dynamic topologies)

A Short Tutorial on CSMA 2 Paulette Altrnaier

May, 1991 Doc: IEEE P802.11/91-44-A

History and Related Work

History of CSMA

• First proposed in 1971 for use in packet radio channels

• Research funded by DoD for use in Packet Radio

• The Darpa Packet Radio Network (PRNET) is CSMA based

• Use in packet radio preceded use in wired networks

CSMA and related protocols

• Aloha

• CSMA

• Ethernetl802.3

Conceptual basis

• The channel carries its own control information

• Control is distributed among all nodes


May, 1991 Doc: IEEE P802.11191-44-A

References

The research of L. Kleinrock, F. Tobagi, S. Lam et al

Primary source materials for this presentation:

"Packet Switching in Radio Channels: Part I - Carrier Sense Multiple-Access Modes and their Throughput-Delay Characteristics" by Klenirock and Tobagi

"Packet Switching in Radio Channels: Part 11- The hidden terminal problem in Carrier Sense Multiple Access and the busy tone solution" by Tobagi and Kleinrock

Unless otherwise stated, all graphs are reproduced from these sources.


May, 1991 Doc: IEEE P802.11191·44·A

Analytical Results

• System assumptions • non-persistent CSMA • p-persistent CSMA • 1-persistent CSMA


May, 1991 Doc: IEEE P802.11/91-44-A

System Assumptions (basis for analysis)

• Wideband channel

• Propagation delay a small fraction of packet transmission time

• Fixed length packets

• Infinite population

• Independent Poisson source of packets

• Retransmissions increase the offered load on the channel

• Half-duplex nodes

• Channel noiseless

• Propagation delay is identical for all source-destination pairs

• No capture (overlap causes destruction of all transmitted pkts)

Simulation results • in agreement with analytic results


May, 1991 Doc: IEEE P802.11/91-44-A

Aloha protocols

• Aloha Node transmits whenever it has a packet ready (total chaos model!)

• Slotted Aloha Node transmits when it has a packet ready, but only on time slot boundaries

Performance

,-,.' Q,

:e 0.40 -G

'" Slotted ALOHA: S=G~

E <V .:: ~ 0.30 4> Ql

~ .0::1 C7'

~I 0.20

enl Pure ALOHA: S=GE -20

0.10

o O.s 1.0 loS 2.0

G (attempts per packet time)

Fig. 3-3. throughput versus off~r.d traffic for ALOHA syst~ms .

3.0


May, 1991 Doc: IEEE P802.11191-44-A

CSMA protocols

• Sense the channel before transmitting

• Execute a channel-state-dependent algorithm

• Different algorithms yield different "CSMA-class" protocols

• Main variants • non-persistent • p-persistent • 1-persistent • Ethernet (1-persistent with collision detect)

• Important parameter: The collision window caused by propagation delay Normalized collision window is defined as

Propagation time2

a= Packet transmission time

2In practice, Effective propagation time = actual propagation time + carrier sense time + rcv-xmt turn-around time and is dominated by the carrier sense and rcv-xmt turn-around times.

If carrier sense time < 1 0 ~ec rcv-xmt turnaround time < 10 ~ec (these are good representative numbers)

then effective propagation time i\ 20 ~ec

If data rate = 1 Mb/s, range = SOm, packet length = 1000 bytes (8000 bits)a = 0.0025 If data rate = I Mb/s, range = SOm, packet length = 100 bytes (800 bits)-a=On~ ,


May, 1991 Doc: IEEE P802.11191-44-A

Non-persistent CSMA

Algorithm: Sense the channel before transmitting. If channel idle, transmit If channel busy, backoff, then repeat entire algorithm (sense +

transmit/wait) Backoff algorithm is usually exponential

Performance curves

... 0 .......... .._ 0

.B NON - PERS ISTENT CSMA

. 6

.4

.2

Q (OFFERED CHANNEL ~"'FFIC)

Throughput In nonpersistent CSMA.

Source: ''Packet Switching in Radio Channels: Part I - Carrier Sense Multiple-Access Modes and their Throughput-Delay Characteristics" by Klenirock and Tobagi, IEEE Trans on Comm Dec 75


May, 1991 Doc: IEEE P802.11/91-44-A

p-persistent CSMA

• Assumes a very fine-grained slotted channel • If P = 1 protocol similar to I-persistent CSMA • p=O is not the same as non-persistent CSMA

Algorithm:

Sense the channel before transmitting on mini-slot boundaries If busy, wait till channel goes idle If idle, transmit with probability p Else wait for the next mini-slot, and repeat.

Performance curves

p - PERS IS TENT CSMA (a - 0.01) .1

5' .6

I en .4

.2

0~c=~~ ____ L-__________ L-~~ __ ~~L-____ ~ __ ~ 0 .01 0.1 10 100

G (OFFERED CHANNEL 'TRAFFIC)

Channel ttV"oughput In p - persistent CSMA.


,


May, 1991 Doc: IEEE P802.11/91-44-A

1-persistent CSMA

Algorithm:

Sense the channel before transmitting on mini-slot boundaries If idle, transmit If busy, wait till channel goes idle and then transmit

• Collision will occur if > 1 node ready to transmit during a packet transmission time

• Ethernetl802.3 is I-persistent with Collision Detect.

Performance curves

.8

5' .6 ":I: g o a:: E (J) .'1

.2

0.1

1 - PERS ISTENT CSMA

1 10 100

G ( OFFERED CHANNEL lRAFF IC)

Throughput in 1-persistent CSMA.



May, 1991 Doc: IEEE P802.11191-44-A

CSMA throughput determinants and characteristics

• Access mode • Propagation Delay • p in p-persistent CSMA


May, 1991 Doc: IEEE P802.11/91-44-A

Access mode

.8 a= 0.01

.6

.4

1 - PERS ISlENT CSMA.

.2

PURE ALOHA

oL-__ ~~=-__ ~ __________ ~ __ ~~~~.L ________ ~ 0.01 0.1 10

G (OFFERED CHANNEL TRAFF IC)

Throughput for the various access modes (a = 0.01)



100

May, 1991 Doc: IEEE P802.11/91-44-A

>-!::: u < 0.. < U ...J I.&J Z Z < :I: u

8

.6

.2

Propagation Delay

~ SLOTTED NON- PERS ISTENT CSM A , .r-0PTIMUM p - PERSISTENT CSMA

./ / NON - PERS ISTENT CSM A

SLOTTED 1-PERS ISTENT CSM A~. '"

1 - PERSISTENT CSMA SLOTTED ALOH A

PURE ALOHA

O~ ____ ~ ________ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ ____ ~ .001 .002 .003 .005 .01 .02 .03 .06 .1 .2 .3 .5

FIG. 10. EFFECT OF PROPAGATION DELAY ON CHANNEL CAPACITY.

Source: "Packet Switching in Radio Channels: Part I - Carrier Sense Multiple-Access Modes and their Throughput-Delay Characteristics" by Klenirock and Tobagi, IEEE Trans on Comm Dec 75


May, 1991 Doc: IEEE P802.11191-44-A

p in p-persistent CSMA

&=0

~ ______________ ~~-!&~=0~.0~1 __ ~ __ ___

.8t-

&= 0.06

~ .6b-----------------=~ u -< "-< u -' w z z ~ .4 u

.2

&=0 .1

p - PERSISTENT CSMA

o ~-----------+.~----~--------~----------------~------------~--------~--------~------------~ .01 .1 .3

p (p - PERS ISlENT P ARA~lERJ

p - persistent CSMA: en'ect of p on channel capacity.



May, 1991 Doc: IEEE P802.11191-44-A

Ethernet

Profile • I-persistent CSMA with Collision Detect • Collision detected within 64 byte (512 bit) times • Collision Detect reduces wasted bandwidth when collision

occurs - transmission aborted • I-persistent protocols not practical for heavily loaded

channels without collision detect

Algorithm: Sense the channel before transmitting. If idle, transmit If busy, wait till transmission is complete, then transmit with

probability 1 If collision detected, do exponential backoff and repeat entire

algorithm


May, 1991 Doc: IEEE P802.11/91-44-A

Measured Performance of Ethernet

Channel Utilization

100~

50~

0:1

O~ 50S

Bytes/packet

.....-:::::========== 612 (~1t)~) ___ -------1~ (~BS,.,)

~:---------- s-1Hj~

100S 150S

Total Offered Load

Measured Performance of Ethernet

Source: Measured Performance of an Ethernet Local Network, John Shoch and Jon A Hupp


May, 1991 Doc: IEEE P802.1V91-44-A

Benefits

Pros and Cons (non-persistent CSMA)

• Distributed control • Algorithm run independently at each node • Simple • Extensive analytical and simulation work available • Stable over loads well in excess of channel capacity

Cons • Degrades in the presence of hidden nodes, Aloha in limiting case • Throughput rises slowly at high load

If offered load = channel capacity, throughput = -50% • Delay increases with load

Areas for study • Generalized Busy Tone algorithm to reduce hidden node degradation4

Suitability to our application and consequent overall system characteristics deferred to a separate submission.

4Busy Tone is a contentious algorithm. While this author does not personally favour it, other IEEE 802.11 members may have supporting arguments.


A Short Tutorial on CSMA - IEEE 802 Modes and their Throughput-Delay ... • Sense the channel...

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