Multi Access

7/28/2019 Multi Access

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Multiple Access Protocols

for Link Layer

Multiple Access Protocols

Single shared broadcast channel

nodes: interference

collision if node receives two or more signals atthe same time

Multiple access protoco l

distributed algorithm that determines how nodesshare channel, i.e., determine when node can

communication about channel sharing must usechannel itself!

Typically no out-of-band channel for coordination


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Ideally what we want?

Broadcast channel of rate R bps

1. When one node wants to transmit, it cansend at rate R.

2. When M nodes want to t ransmit, eachcan send at long-run average rate R/M

3. Fully decentralized:

transmissions

no synchronization of clocks, slots

Types of Mult iple Access Channelization

Divide channel into smaller pieces (times o s, requency, co e e c.

Allocate a piece to node for exclusive use

Random Access

channel not divided

revent collisions, or allow and recover

from collisions

Taking turns

Nodes take turns, but nodes with more tosend can take longer turns


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Channelization

Examples: Time-division mult iple access (TDMA)or requency- v s on mu p e acesss .

Take TDMA example:

.

Inefficient when load is unbalanced or unknown.

Random Access Protocols

No a priori pieces (slots, frequency orco e .

Packets are transmitted with li ttle or nocoordination. Collisions possible.

Issue: how to prevent col lisions, orrecover from collisions?

Aloha (slotted and unslotted)

CSMA, CSMA/CD, CSMA/CA


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Aloha Protocols

Initially developed in 1970 for radio

several terminals in Univ. of Hawai.

Mother of all random access protocols,even though very inefficient.

We present an idealized version for easy.

Slotted AlohaAssumptions

Al l f rames same sizeOperation

Time is divided intoequal size slots, time to

transmit 1 frame

Nodes start to transmitframes only at

beginning of slots

frame to transmit, it

transmits in the next slot

If collision, noderetransmits frame in each

subsequent slot with

Slots are synchronized

Collision = more thanone node transmit in a

slot.

.


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Slotted ALOHA

Pros

single active node cancontinuously transmit

Cons

collisions, wastingslots

a u ra e o c anne

highly decentralized:only slots in nodes

need to be in sync

simple

nodes may be able to

detect colli sion in lessthan time to transmitpacket

clock synchronization

Slotted Aloha efficiency

For max efficiencywith N nodes, find p*

that maximizes

Efficiency is the long-runfraction of successful slotswhen there are many nodes,

Suppose N nodes withmany frames to send,

each transmits in slot with

probability p

Np(1-p)N-1

For many nodes, takelimit of Np*(1-p*)N-1as

N goes to infinity,

gives 1/e = .37

each with many frames to send

prob that node 1 hassuccess in a slot = p(1-p)N-1

prob that any node has asuccess = Np(1-p)N-1

At best:channelused for usefultransmissions 37%of time!


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Pure (unslotted) ALOHA

unslotted Aloha: simpler, no synchronization

when frame first arrives

transmit immediately

collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Pure Aloha efficiency

P(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1,p0].

P(no other node transmits in [p0-1,p0]

= p . (1-p)N-1 . (1-p)N-1

=p . (1-p)2(N-1)

choosing opti mum p and then letting n -> infty ...

= 1/(2e) = .18Even worse !


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Carrier Sense Multiple Access

(CSMA)

In Aloha, nodes transmit immediatelyw en a new pac e arr ves.

What if we are able to sense carrier andtell busy and idle periods apart?

Busy carrier indicates ongoingtransmission. Wait until carrier id le.T icall needs a carrier sense intervalfor detecting the state of the carrier.

Can be used in both s lotted and unslottedsystems.

CSMA : Persistence

New arrival senses busy carrier

Continue sensing until idle (persistentCSMA).

Retry after a random time interval (non-persistent CSMA). Retry again if still busy.

Two types of persistent CSMA

Transmit new arrival as soon as carrier

- . Transmit new arrival after a random time

interval after carrier becomes idle (p-persistent CSMA).


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Note on Backoff

Random backoff interval, b slots

Tr in to transmit in subse uent slots with some

Some event causes transmission to defer

(e.g., medium busy, collision etc.)

probabili ty p is equivalent to sending after arandom interval.

Length of random interval in slots = b.

Prob [b=k] = p(1-p)(k-1)

Coll ision Detection

Need to know whether t ransmission is.

First idea: Detect col lision. Retransmitafter backoff (as in Aloha) if collision.

CSMA/CD 1-persistent CSMA withcoll ision detection. Used in Ethernet. IEEE

. .

Listen while transmitting. If noticetransmission being garbled, flag collision.


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Ethernet CSMA/CD

Important parametera = max. signalpropagation delay in network / smallest

packet transmission time.

Packet transmission time = packet size /channel bandwidth.

Parametera must be less than 0.5 so thatthe packet lasts long enough at the

transmitter for it be able to detect collisionif it happens anywhere in the network.


Another nodePropagation time

TimeTransmitter


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TimeTransmitter

Sender detects collision



To detect collis ion, min

TimeTransmitter

Some other node

pac e ransm me >max propagation delay innetwork.

Thus Ethernet has a max cable length and aminimum packet size.

Collision at some

node. Sender unable

to detect.


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Exponential Backoff In Ethernet

Transmitter transmits a jam signal aftercoll ision is detected to make sure every

transmitter aborts transmission.

Then it enters an exponential backoffphase. Retransmits the coll iding packet

after 512*K bit time.

K is random from {0,1,2,3,,2m-1}, m =

min(n,10). n = no. of collisions for this packet.

Performance Metrics

Throughput: Bits/sec or packets/secreceived correctl at the receiver.

Offered load: Bits/sec or packets/secactually transmitted on the network.

Normalized throughput or load: The abovenormalized with respect to bandwidth.

Delay: Delay between when the first bit of

t e pac et starts transm tt ng or t e rsttime and the when the last bit of thepacket correctly received.


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Less Common Metrics

Stability: Whether throughput decreases withincrease in load.

Could happen in a random access scheme, ifcollision increases with increase in load.

Ideally, the protocol should try to admit less load ifoverload (frequent collisions) is detected.

Fairness: Whether all nodes with packets totransmit have e ual shots at transmittin . i.e. all

contending nodes get equitable share of the

bandwidth; nobody hogs the channel.

CSMA/CD Performance Much harder to analyze analytically than Aloha.

Max. throughput is very dependent on parameter a.

With some conservative assumptions, max.normalized throughput = 1/(1+constant*a).

Very small a close to zero -> almost idealthroughput. Large a means more waste beforecollision detect. Bad for CSMA/CD.

constant 5 for Ethernet LAN. Find a from Ethernetspecs.


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CSMA on Wireless

Cannot depend on collision detectionabilit .

Full duplex radios are complex andexpensive.

Even if sender is able to detect collision, itmay so happen that sender senses nocollision but collision ha ens at thereceiver. This is due to path loss of wireless

signals. Doesnt happen in Ethernet.

Busy Tone Multiple Access(BTMA) Divide the available frequency band into two channels:

message (data) channel and control channel.

,control channel. Any hidden terminal upon hearing this

busy tone will refrain from transmitting .

Carrier sense on the busy tone only. Solves both h iddenand exposed terminal problems.

Problems:

Use of two channels makes radios complex. Also, channels

may nee o we -separa e . Propagation characteristics on two channels may be

different.


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CSMA/CA, IEEE 802.11 WLAN

CSMA with coll ision avoidance. A form of p-persistent CSMA.

se n s an ar . or re ess .

Transmit after the channel is continuously sensedidle for an amount of time equal to or greater thanthe Distributed Inter Frame Space (DIFS).

If channel is busy, defer until the channel issensed idle for DIFS duration. Then go through an

.Transmit after the backoff timer expires.

The timer counts down as long as the channel isidle, but stops counting during the time thechannel is busy.

CSMA/CA Contd.

Avoids collision by randomization, but . ,

from receiver unlike CSMA/CD.

Receiver sends a short ACK packet after aSIFS (Short Inter Frame Spacing) period

after correctly receiving a packet.

- ,value of backoff timer is now doubled

much l ike Ethernet (exponential backoff).


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CSMA/CA in 802.11

DIFSDIFScontention window(randomized back-offmechanism)

t

medium busy Transmitted frame

slot time

direct access ifmedium is idle DIFS

Ready for transmission

ACK

SIFS

Backoff clock stops if medium is sensed busy. DIFS > SIFS

Hidden Terminal Problem

A and C cannot hear each other.

A B

,transmission using carrier sense.

If C transmits as well, packets collide at B.

A and C are hidden from each other.


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Hidden Terminal Problem

Cannot happen in wired Ethernet. Peculiar

Need to sense carrierat receiver, notsender!

Solution: Do virtual carrier sensing . Askreceiver whether it can hear anything. If it

, .

Solving Hidden Terminal Problem:RTS-CTS Handshake

When A wants to send a packet to B, node A fi rstsends a short Re uest-to-Send RTS acket to B.

On receiving RTS, node B responds by sendingClear-to-Send (CTS).

When a hidden node (e.g., C) overhears a CTS,it keeps quiet for the duration of the wholetransfer.

Transfer duration is included in both RTS and

CTS.

A B C

RTS

CTS CTS

DATA

D

RTS

ACK


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RTS-CTS Handshake Contd

Note: Any node that hears RTS or CTSmust keep quiet for the duration of the

.

Note the RTS packets may collide.Probabili ty small as they are short.

Retransmit RTS after backoff if no CTS.

A B C

RTS

CTS CTS

DATA

D

RTS

ACK

Timeline

SIFS

DIFS

ACK

senderdataRTS

CTSSIFS SIFS

t

data

defer access

otherstations

recever

DIFS

Contentionperiod

NAV (RTS)NAV (CTS)

Note Data > RTS/CTS/ACK > DIFS > SIFS. Timesnot to scale for clarity.

NAV = network allocation vector. During the periodNAV is set the node must act as if medium busy.Thus a packet from them ready during th is timemust go through a contention period.


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Exposed Terminal Problem

In princ iple, A->B and C->D transmissions can go

AC

D

.

But A and C can hear each other. C will wait for A-

>B to end before starting C->D. A and C are exposed terminals.

Solutions??

Multi Access

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