MAC sublayer.pdf

7/28/2019 MAC sublayer.pdf

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Switched Networkswitched NetworksChapter 3hapter 3

Medium Access Controledium Access Control Sublayerublayer

Dr.-Ing. Torben WeisUniversity Duisburg-Essen

Institut frInformationstechnik

Torben Weis 2

Channel Allocation Problem (1)hannel Allocation Problem (1)

Multiple stations access one channel How to avoid collisions?

Two different approaches Static Channel Allocation Dynamic Channel Allocation

What do we expect? Static Simple Dynamic Efficient


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Torben Weis 3


Example: Cocktail party many people gather in a large room Broadcast medium = air

Human protocols: Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking


Torben Weis 4


If more than 2 users send at the same timecollision

All collided packets are lostwaste of bandwidth

Ideally, the MAC protocol for a broadcastchannel with R bit/sec should satisfy:

If only 1 node is sending than the throughput is R When M nodes have data to send than the throughput

is R / M Decentralized protocol, i.e. no master Simple & inexpensive to implement


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Torben Weis 5

Protocol Layerrotocol Layer

Data Link Layer puts frames on the wire What happens if multiple stations want to put

frames on the medium (wire) at the same time?=> Medium Access Control (MAC) is required

Layer 5Layer 5

Layer 4Layer 4

Layer 3Layer 3

Layer 5Layer 5

Layer 4Layer 4

Layer 3Layer 3Layer 3Layer 3 Layer 3Layer 3

Application Layer

Transport Layer

Network Layer Network Layer

Layer 2Layer 2

Layer 1Layer 1

Layer 2Layer 2

Layer 1Layer 1

Layer 2Layer 2

Layer 1Layer 1

Layer 2Layer 2

Layer 1Layer 1

Data Link Layer

Physical Layer

Data Link Layer

Physical Layer


Torben Weis 6

Layer 5Layer 5

Layer 4Layer 4

Layer 3Layer 3

Layer 5Layer 5

Layer 4Layer 4

Layer 3Layer 3Layer 3Layer 3 Layer 3Layer 3

Application Layer

Transport Layer

Network Layer Network Layer

Layer 2Layer 2 Layer 2Layer 2Layer 2Layer 2 Layer 2Layer 2Data Link Layer Data Link Layer

MACMAC MACMACMACMAC MACMAC

Medium Access Controledium Access Control Sublayerublayer

MAC is not part of the OSI Model MAC is a sublayer of the Data Link Layer

Layer 1Layer 1 Layer 1Layer 1Layer 1Layer 1 Layer 1Layer 1Physical Layer Physical Layer


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Torben Weis 7

Static Channel Allocationtatic Channel Allocation

Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM)


Torben Weis 8

Frequency Division Multiplexing (1)requency Division Multiplexing (1)

Uses static allocation Channel has a bandwidth of H n stations are connected to the cable

Idea Divide the bandwidth in H/ n frequency bands Assign one band to one station

Result No collisions possible Delay increased by factor n


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Torben Weis 9

Frequency Division Multiplexing (2)requency Division Multiplexing (2)

(a) The original bandwidths(b) The bandwidths raised in frequency(b) The multiplexed channel


Torben Weis 10

Time Division Multiplexing (1)ime Division Multiplexing (1)

Uses static allocation Channel transfers B bit/sec n stations are connected to the cable

Idea Allocate n time slots of length t Round robin Each station transfers B*t bits per slot

Result No collisions possible Delay increased by factor n


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Torben Weis 11

Time Division Multiplexing (2)ime Division Multiplexing (2)


Torben Weis 12

Dynamic Channel Allocation (1)ynamic Channel Allocation (1)

Station Model n independent stations Each station wants to send frames is the constant arrival rate Probability of sending a frame in time t is t


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Torben Weis 13


Single Channel Assumption All stations connected to the same channel All stations are equivalent Every station can send/receive Optional

Priorities can be assigned to stations


Torben Weis 14


Collision Assumption Two stations sending at overlapping time intervals

collisionthe transmitted data is garbled

After a collision, data must be sent again Stations can detect collisions No other errors, only collisions


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Torben Weis 17


Station Model Single Channel Assumption Collision Assumption (a) Continuous Time

(b) Slotted Time (a) Carrier Sense

(b) No Carrier Sense


Torben Weis 18

Multiple Access Protocolsultiple Access Protocols

ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols

Limited-Contention Protocols Next week

Wavelength Division Multiple Access Protocols Wireless LAN Protocols


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Torben Weis 19

ALOHA (1)LOHA (1) Norm Abramson at the Univ. of Hawaii mountainous islands

land network difficult to install

ACK

ACK ACK

ACK


Torben Weis 20

ALOHA (2)LOHA (2)

M nodes can create new frames at any time The node immediately transmits its frame

completely on a shared frequency (Different in slotted ALOHA)

Base station acknowledges packages on adedicated frequency

If the frame collidesNo ACK is received

Retransmission with probability pavoids efficiency to become 0


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Torben Weis 21

Pure ALOHAure ALOHA

In pure ALOHA, frames are transmitted at completelyarbitrary times.


Torben Weis 22

Pure ALOHA (2)ure ALOHA (2)

Vulnerable period for the shaded frame = 2 t.


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Torben Weis 23

Slotted ALOHAlotted ALOHA

Idea Create time slots of length t Transmission only at beginning of each time slot

Result Vulnerability reduced from 2 t to t We doubled the throughput

Problem How does everybody agree on time slots?


Torben Weis 24

Pure and Slotted ALOHAure and Slotted ALOHA

Throughput versus offered traffic for ALOHAsystems


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Torben Weis 27

Carrier Sense Multiple Access (2)arrier Sense Multiple Access (2)

If everyone is sensing the medium,how can collisions still occur?

channelpropagationdelay


Torben Weis 28

1-persistent CSMAersistent CSMA

The protocol Listen before transmitting If channel busy, wait until is channel idle If channel idle, transmit If collision occurs, wait a random amount of time and

start all over again It is called 1-persistent because the station

transmits with a probability of 1 whenever itfinds the channel idle.


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Torben Weis 29

Nonpersistentonpersistent CSMASMA

The protocol Listen before transmitting If busy, wait a random amount of time

and sense the channel again If idle, transmit a packet immediately If collision occurs, wait a random amount of time and

start all over again What is special?

The station does not sense all the timeless greedy than 1-persistent CSMA


Torben Weis 30

pp -persistent CSMAersistent CSMA

Applies to slotted channels The protocol

Listen before transmitting If channel busy, wait until is channel idle If channel idle

Transmit with probability p Wait for next slot with probability q=1- p

If collision occurs, repeat the above procedure


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Torben Weis 31

Persistent andersistent and Nonpersistentonpersistent CSMASMA

Comparison of the channel utilization versus

load for various random access protocols


Torben Weis 32

Collision Detection (1)ollision Detection (1)

1. Waiting for an acknowledge (ACK) ACK is sent after the frame has been transmitted

Frames with collisions are transmitted completely Waiting for ACK wastes time and bandwidth

2. Sending and receiving Check, whether you receive what you have sent

Collisions can be detected early This is called Collision Detection or just CD


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Torben Weis 33

Collision Detection (2)ollision Detection (2)

When is a collision detected ? Station A sends at time t

0 Signal needs to the most distant station X At t0+ - station X sends, too

X sensed that the channel is idle The signal of X needs to station A Result: A must wait 2 - before it can detect a

collision Problem: Maximum value of depends on the

maximum distance between stations


Torben Weis 34

CSMA with Collision DetectionSMA with Collision Detection

Senders can detect collisions while sendingCollision Detection


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Torben Weis 35

Collisionollision -Free Protocols (1)ree Protocols (1)

The basic bit-map protocol


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The bitmap protocol is a reservation protocol Overhead calculation for the reservation

Every station ( N in total) wants to send a frameOverhead = 1 contention slot per N frames = 1 bit

Only one station wants to send a frameOverhead = 1 contention slot for 1 frame = N bits


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Torben Weis 37


The binary countdown protocol A dash indicates silence


Torben Weis 38


Binary countdown is a reservation protocol, too Overhead calculation for the reservation

N stations in total At least one wants to send Contention slot has length log 2 N

Optimization Many frames carry the sender address

Efficiency can become 100%


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Torben Weis 39


Binary countdown uses priorities High addresses always win Low addresses can starve to death Possible fix

Address numbers circulate After sending a frame, address numbers are shifted The sending station gets a low address


Torben Weis 40

Limitedimited -Contention Protocolsontention Protocols

Acquisition probability for asymmetric contention channel


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Torben Weis 41

Adaptive Tree Walk Protocol (1)daptive Tree Walk Protocol (1)

The tree for eight stations


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Adaptive Tree Walk Protocol (2)daptive Tree Walk Protocol (2)

Idea Limit the number of participants in the contention First, everybody may try to send Collision halve the participants

Best of both worlds Low delay when there are few collisions

As good as slotted ALOHA Low delay when many station bid for the channel

Almost as good as reservation protocols


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Torben Weis 43

Wavelength Division Multiple Access Protocolsavelength Division Multiple Access Protocols


Torben Weis 44

Manchester Code (1)anchester Code (1)

How to represent 0 and 1 on the wire? Idea 1

1 = +5 Volt, 0 = 0 Volt Problem: How to differentiate 00000000 from an idle

sender ? Idea 2

1 = +5 Volt, 0 = -5 Volt Problem: We cannot be sure that all receivers sample

the signal at the same rate (clock drift) We need a self-clocking encoding


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Torben Weis 45


Encode 1 bit as 2 bits 1 = 10 0 = 01 Good news: the signal oscillates in the middle of

every bit Bad news: We waste half of the bandwidth

This is called Manchester Code It is used by Ethernet

(we will come to that later)


Torben Weis 46



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Torben Weis 47

Differential Manchester Codeifferential Manchester Code

Encode 1 bit as 2 bits The encoding of bit n depends on the encoding

of bit n -1 If n = 1

n-1 encoded as 10 ? -> n = 01 n-1 encoded as 01 ? -> n = 10

If n = 0 n-1 encoded as 10 ? -> n = 10 n-1 encoded as 01 ? -> n = 01

Advantage: Better noise immunity Disadvantage: Requires more complex

equipment


Torben Weis 48

IEEE Standard 802 for LANsEEE Standard 802 for LANs

Ethernet: IEEE 802.3 Token Bus: IEEE 802.4 Token Ring: IEEE 802.5

Wireless: IEEE 802.11


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Torben Weis 49

IEEE 802.5 Token RingEEE 802.5 Token Ring


Torben Weis 50

Token Ringoken Ring Physical Length (1)hysical Length (1)

A 3 byte token circulates on the ring Sender must not receive its own token data

before it sends the last token bit

How many bits fit on the ring?


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Torben Weis 51

Token Ringoken Ring Physical Length (2)hysical Length (2)

Each station buffers 1 bit Problem: station can be taken off the net

The cable carries some bits Data rate of R Mbit/sec

A bit is emitted every 1/ R sec Signal propagation speed 200 m/ sec

Each bit occupies 200/ R m Example: R=1 Mbit/sec, length = 1000m

Each bit occupies 200m 5 bits on the ring cable Additional buffers are required


Torben Weis 52

Token Passing (1)oken Passing (1)

The Tokenrotates aroundthe ring

Station Acaptures theToken andconverts it to aPacket

Station Atransmits thepacket

Staton A waitsfor the Packet

Station Areleases theToken

Release After Reception (RAR)

TK

P K T

P K T

P K T

T K


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Torben Weis 53


Station A transmitsthe Packet andimmediately releasesthe Token

Release After Transmission (RAT)

The Tokenrotates aroundthe ring

TK

Station Acaptures theToken andconverts it to aPacket

P K T

P K T

T K


Torben Weis 54


The ring can contain: A token A data frame (or both in release after transmission)

The token must fit on the ring The size of a data frame is not limited

A data frame is not entirely on the ring The sender can receive its own frame while still

sending Can be used for error detection


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Torben Weis 55


Is starvation possible? i.e. a station wants to send but never gets the token ? Answer: No

Stations s 1 , s 2 , s n Station s i fetches the token and transmits data Then, s i issues a new token

s i+1 can get the token Full load round robin Token ring is fair

as long as only one priority is used


Torben Weis 56

bypassrelays

Wire center

Token Ring Wiringoken Ring Wiring


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Torben Weis 57

Token Ringoken Ring Multistationultistation Access Unit (MSAU)ccess Unit (MSAU)


Torben Weis 58

Token Ringoken Ring - Active Monitorctive Monitor

Some station must inject the token detect circulating frames send a continuous signal

(e.g. Differential Manchester encoded 0)to synchronize all stations

This station is the active monitor (AM) All other stations are standby monitors (SM) The active monitor is elected

The station with the largest MAC wins


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Torben Weis 59

Joining a Token Ringoining a Token Ring

1. (Lobe Check) - Performs a lobe media check to seewhether frames are received without error.

2. (Physical Insertion) - A station then sends a 5 volt signalto the MSAU to open the relay.

3. (Address Verification) - A station then sends a messageto is own MACIf the Copied flag is set -> The MAC is already used

4. (Participation in Ring Poll) - The station must participatein the periodic (every 7 seconds) ring poll process.

5. (Request Initialization) - Finally a station sends out aspecial request to a parameter server to obtainconfiguration information.


Torben Weis 60

Token Structure (1)oken Structure (1)

SD AC EDSD = Starting DelimiterAC = Access ControlED = Ending Delimiter

1 1 1

JK0JK000 J - Phase Violation JK - Phase Violation K

0 - Binary Zero

PPPTMrrr

PPP - Priority BitsT - Token BitM - Monitor Bitrrr - Reservation Bit

SD

AC

J - Phase Violation JK - Phase Violation K1 - Binary OneI - Intermediate BitE - Error Bit

JK1JK1IEED


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Torben Weis 61

Token Structure (2)oken Structure (2)

The manchester codeviolation enables areceiver to detect the startof a token

Violation is achieved bynot switching in themiddle of a bit

J K assures that the

signal is still oscillating

SD AC EDSD = Starting DelimiterAC = Access ControlED = Ending Delimiter

1 1 1

JK0JK000 J - Phase Violation JK - Phase Violation K0 - Binary Zero

SD


Torben Weis 62

Data Frame Structureata Frame Structure

SD AC FC DA SA DATA ED FS

SD = Starting Delimiter

AC = Access ControlFC = Frame ControlDA = Destination AddressSA = Source AddressED = Ending DelimiterFS = Frame Status

1 1 1 6 6 n 1 1

FFZZZZZZ FF - Frame TypeZZZZZZ - Control Bits

FC

AC00AC00A = Address

RecognizedC = Frame Copied

FS


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Torben Weis 63

Token Ring Priorityoken Ring Priority


Torben Weis 64

IEEE Standard 802 for LANsEEE Standard 802 for LANs

Ethernet: IEEE 802.3 Token Bus: IEEE 802.4 Token Ring: IEEE 802.5

Wireless: IEEE 802.11


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Torben Weis 65

IEEE 802.4 Token BusEEE 802.4 Token Bus

Physical: Bus (solid line)

Logical: Ring (dashed line)


Torben Weis 66

Why Token Bus ?hy Token Bus ?

Combines: Cabling of Ethernet (single wire) Determinism of Token Ring

Different application domains Token Ring / Ethernet: Office Token Bus: Industrial automation

Pushed by General Motors Real-time important


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Torben Weis 67

Token Busoken Bus Frame Formatrame Format

Comparable to IEEE 802.5 format,but unfortunately different


Torben Weis 68

Token Busoken Bus - Messagesessages


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Torben Weis 69

Token Busoken Bus MAC Protocol (1)AC Protocol (1)

Initializing a ring

Station 90 wants to join Station 90 detects that there is no traffic Station 90 sends claim_token (90) The station with the highest MAC wins

90


Torben Weis 70


Joining a ring

Station 80 wants to join 90 periodically sends solicit_successor (90,70) Station between 90 and 70 may respond 80 sends set_successor (90,80)

90 80 70 60


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Torben Weis 73


Leaving a ring

Stations 80 wants to leave 80 waits for the token 80 sends set_successor (90,70)

90 has new successor70 has new predecessor

90 80 70 60


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Station failure

Station 80 fails 90 sends token . If 80 does not reply, 90 sends again Still no response 90 sends who_follows (90,80) 70 sends set_successor (90,70)

90 80 70 60


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Torben Weis 75


Station failure, part 2

Stations 80 and 70 fail 90 sends token . If 80 does not reply, 90 sends again No response 90 sends who_follows (90,80) No response 90 sends solicit_successor_2 (90) Any alive station sends set_successor (90,xx)

90 80 70 60

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MAC sublayer.pdf