L5CN :MULTIPLE ACCESS AND LOCAL AREA NETWORKS

7/31/2019 L5CN :MULTIPLE ACCESS AND LOCAL AREA NETWORKS

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LECTURE 5MULTIPLE ACCESS AND LOCAL AREA NETWORKS

To ics

Multi le access: CSMA/CD, CSMA/CA, token assin ,

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channelization

LAN: characteristics, basic principles

Protocol architecture

Topologies

Extending LANs: repeater, bridge, router

Virtual LANs

Link La er Services

framing, link access:

3

encapsulate datagram into frame, adding header, trailer

channel access if shared medium

,

different from IP address!

reliable delivery between adjacent nodes

we learned how to do this already !

seldom used on low bit-error link (fiber, some twisted pair)

:

Q: why both link-level and end-end reliability?

Link La er Services more

4

pacing between adjacent sending and receiving nodes

error detection:

errors caused by signal attenuation, noise.

receiver detects presence of errors:

signa s sen er or retransmission or rops rame

error correction:

rece ver en es an correc s error s w ou resor ng oretransmission

half-duplex and full-duplex

with half duplex, nodes at both ends of link can transmit, but not at sametime


2/13

Where is the link la er im lemented?

in each and every host5

link layer implemented inadaptor (aka network host schematic

a lication

Ethernet card, PCMCI card,802.11 card

cpu memorytransportnetwork

link

mp ements n , p ysca ayer

attaches into hosts system

buses

controller

physical

bus(e.g., PCI)

linkphysical

combination of hardware,

software, firmware network adaptercard

Communication on LANs

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Characteristics

small area, limited number of users, all nodes can

commun ca e rec y

short and long sessions

The use of shared medium and broadcast transmission

simple network elements, simple network management Property of LANs

propagation time


3/13

Ideal Multi le Access Protocol

Broadcast channel of rate R bps

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1. when one node wants to transmit, it can send at rate R.

2. when M nodes want to transmit, each can send at average rateR/M

3. fully decentralized:

no speca no e to coor nate transmss ons

no synchronization of clocks, slots

4. simple

MAC Protocols: a taxonom

Three broad classes:

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divide channel into smaller pieces (time slots, frequency, code)

allocate piece to node for exclusive use

Random Access

channel not divided, allow collisions

recover rom co s ons

Taking turns

nodes take turns, but nodes with more to send can take lon er turns

Channel Partitionin MAC rotocols: TDMA

TDMA: time division multiple access11

access to channel in "rounds"

each station gets fixed length slot (length = pkt trans time)n eac roun

unused slots go idle

- , , , , , ,

6-slot

1 3 4 1 3 4

frame

Channel Partitionin MAC rotocols: FDMA

FDMA: frequency division multiple access12

channel spectrum divided into frequency bands

each station assigned fixed frequency band unused transmission time in frequency bands go idle

example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6

bands

ue

ncy

freca e


4/13

Random Access Protocols

When node has packet to send

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transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes collision,

random access MAC protocol specifies:

how to recover from collisions (e.g., via delayed retransmissions)

Examples of random access MAC protocols:

slotted ALOHA

ALOHA

, ,

MAMultiple Access

Aloha

Packet radio protocol

Random-access method based on acknowledgementsand backoffs

Slotted ALOHA

Assumptions: Operation:

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all frames same size

time divided into equal size

when node obtains fresh frame,

transmits in next slots o s me o ransm

frame)

nodes start to transmit only

no co s on: no e cansend new frame in nextslot

slot beginning

nodes are synchronized

if collision: noderetransmits frame in each

win slot, all nodes detect

collision

.p until success

Slotted ALOHA

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Pros

Cons

s ng e ac ve no e can

continuously transmit at fullrate of channel

,

idle slots

nodes may be able to

highly decentralized: only

slots in nodes need to be in

detect collision in less than

time to transmit packet clock s nchronization

simple


5/13


6/13

Carrier Sense Multiple Access (CSMA)

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Listen (sense) before sending

Do not send unless the medium is idle

Reduces the possibility of collisions

Does not eliminate collisions

Propagation delay

Takes time before all other stations can sense a

transmission

Persistence Strate22

Sense carrierSense carrier

Busy?Yes Busy?

Yesa

Send frame

No

Send frame

1-persistent Send as soon as

channel is idle

Non-persistent

Wait a random period of

again

p-Persistent

Sense carrier en c anne s e

Send with probability p

Wait and then sense again Busy?Yeswith probability (1-p)

No

r = random(0, 1)

Wait

r < p ?No

Send frame

Yes

CSMA with Collision Detection

Exponential back-off Wait 2N max ro a at ion t ime after col li sion where N is number of transmission_ _ ,

attempts

Send jam so other stations detect collision as well


7/13

Ethernet: Unreliable connectionless

connectionless: No handshaking between sending and receiving

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NICs unreliable: receiving NIC doesnt send acks or nacks to sending

stream of datagrams passed to network layer can have gaps (missingdatagrams)

gaps will be filled if app is using TCP

otherwise, app will see gaps

:

Ethernet CSMA/CD algorithm

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. rece ves a agram rom

network layer, creates frame

2. If NIC senses channel idle starts

. e ec s ano er

transmission while transmitting,aborts and sends jam signal

frame transmission If NIC senseschannel busy, waits until channel

5. After aborting, NIC enters

exponential backoff: after mth

,

3. If NIC transmits entire frame

without detecting another

,random from

{0,1,2,,2m-1}. NIC waits K512

transmission, NIC is done withframe !

bit times, returns to Step 2

Ethernets CSMA/CD (more)

Jam Si nal: make sure all other Ex onential Backoff:

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transmitters are aware ofcollision; 48 bits

Bit time: .1 microsec for 10 Mb s

Goal: adapt retransmissionattempts to estimated current load

Ethernet ;for K=1023, wait time is about50 msec

be longer

first collision: choose K from {0,1};

times

after second collision: choose Krom , , ,

after ten collisions, choose K from

{0,1,2,3,4,,1023}

See/interact with Java

applet on AWL Web site:

highly recommended !

CSMA CD

MAC header contains a MAC destination address which

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ensure only the specified destination actually forwards

the received frame (Others discard the frames).


8/13

CSMA CD

When a collision occurs i.e. excess current above what it is eneratin

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> 24 mA for coaxial Ethernet), it stops transmission immediately.

Transmits a 32-bit jam sequence such that receivers discard the framedue to a CRC error.

To ensure collision detection, Ethernet defines a minimum frame size.

The minimum frame size is related to distance, media, and number ofnodes (Ethernet Slot Time).

Collision Detection

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,

A.

A short time later, B also transmits. (In this case, the

medium, as observed by the NIC at B happens to be

idle too).

After Tr , the NIC at B detects the other

transmission from A, and is aware of a collision,

but NIC A has not yet observed that NIC B was also

transmitting. B continues to transmit, sending the

Ethernet Jam sequence (32 bits).

After one complete round trip propagation time

(twice Tr ), both NICs are aware of the collision. A

transmits a complete Jam Sequence. Finally the

cable becomes idle.

Collision Detection

Requires that stations still transmit when the colliding packet arrives

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Collisions detected by all stations

Minimum packet size and maximum bus length

(Thick Ethernet)

TT prtr 2P: packet size [bit]

v

L

B

P

2

L: bus length [m]v: signal propagation speed [m/s]

Packet Size Versus Bus Length

S: frame size [bit]

v

L

C

STT prtr

22 >>

C: link capacity [bit/s]

L: bus length [m]

v: signal propagation speed

Collisions must be detected byTtr

TprTpr

[m/s]

all stations

Minimum packet size and

maximum bus len th

72 bytes (64 bytes at data

link layer) and 500 meters

Station 1 Station 2

Ethernet)


9/13

Wireless LAN CSMA CA

Problem with CSMA/CD in combination with radio33

Collision detection is not reliable

E.g., asymmetry: computer 3 might not receive the signals from computer1 to com uter 2

Hidden station problem, signal fading Requires ability to send data and detect collisions at the same timeMore costly hardware, higher bandwidth

CSMA CD Collision Detection

CSMA/CD: carrier sensing, deferral as in CSMA34

collisions detectedwithin short time colliding transmissions aborted, reducing channel

wastage

collision detection:

easy in wired LANs: measure signal strengths, comparetransmitted, received signals

cu n w re ess s: rece ve s gna s rengoverwhelmed by local transmission strength

Wireless NetworksHidden

erm na35

Stations may not detect each others signals

CSMA CA contd

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Carrier Sense Multiple Access with Collision

Avoidance

Control signals before transmission

Carrier sense:

do not transmit immediately when medium getsidle (p-persistence)

ait ran om time

lower collision probability


10/13

Figure 12.16 Timing in CSMA/CA

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Note

n , e can a so e use odefine the priority of a station or a frame.

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Note

n , t e stat on n s t e c annebusy, it does not restart the timer of the

con en on w n ow;it stops the timer and restarts it when the

c anne ecomes e.

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CSMA/CA Wait Procedure

When medium is busy =

Wait a random amount of

time

,

Sense carrier But only decrement timer

when medium is idle

IEEE 802.11 Wireless LANBusy?

Yes

NoDecrement t

t = 0 ?No

Yes

Send frame


11/13

Figure 12.17 Flow diagram for CSMA/CA

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CSMA/CA With RTS/CTS

Request-to-send/clear-to-send (RTS/CTS) handshake

RTS/CTS frames contain Duration field

Period of time the medium is reserved for transfer

Other stations remain quite during this period Need not be used for all frames

Overhead too high for small frames

CONTROLLED ACCESSCONTROLLED ACCESS

nn con ro econ ro e accessaccess,, ee s a onss a ons consuconsu oneone ano erano er oo

findfind whichwhich stationstation hashas thethe rightright toto sendsend.. AA stationstation cannotcannot

..

discussdiscuss threethree popularpopular controlledcontrolled--accessaccess methodsmethods..

Controlled

TokenPassing

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Token Passing

Token (a control frame) circulates among the nodes

e no e a o s e o en as e r g o ransm

Current station, predecessor, and successor

Used in Token Ring LAN


12/13

Token Passin Procedure45

Takin Turns MAC rotocols

Token passing:46

r contro to en passe rom

one node to next

sequentially.

r token message

r concerns:(nothing

to sendm token overhead

m latency

m single point of failure

(token)

data

Logical ring and physical topology in

token- assin access method Reservation48


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Pollin49

Select Poll

Takin Turns MAC rotocols

Polling:

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master node invitesslave nodes to transmitpoll

data

typically used withdumb slave devices

masterdata

concerns:

polling overhead

atency

single point of failure(master)

s aves

Channelization51

A station is allocated a frequency band on an FDM link

TDMA Entire bandwidth is one channel

A station is allocated time slots on a TDM link

Entire bandwidth is one channel

Data from all inputs are transmitted at the same time

Based on coding theory, and uses sequences of numbers calledchips generated using Walsh Tables.

Summar of DLL52

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L5CN :MULTIPLE ACCESS AND LOCAL AREA NETWORKS