media access control

8/3/2019 Media Access Control

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Medium Access Control


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The Media Access Control (MAC) data

communication protocol sub-layer, also known

as the Medium Access Control, is a sublayer of

the Data link layer specified in the seven-layerOSI model(layer 2). It provides addressing and

channel access control mechanisms that make it

possible for several terminals or network nodes

to communicate within a multi-point network,typically a local area network(LAN) or

metropolitan area network(MAN).


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Some Multiple Access Protocols

CSMA/CD

Token passing

Wireless LAN Protocols


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Random Access MAC Protocols

Non-Carrier-Sense protocols: doesnt listen to

the channel before transmitting

ALOHA

Carrier-Sense protocols: senses the channel

before transmitting

CSMA (Carrier Sense Multiple Access): does not

detect collision.

CSMA/CD (Ethernet): A node listens before/while

transmitting to determine whether a collision happens.


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ALOHA(Additive Link Off Hawaiian Access)

Radio-based communication network

Developed in 1970s at the Univ of Hawaii

Basic idea: transmit when a node has data to be

sent.

Receiver sends ACK for data

Detect collisions by timing out for ACK

Recover from collision by trying afterrandom delay

Too short: large number of collisions

Too long: underutilization


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Ethernet MAC

If line is idle (no carrier sensed) send packet

immediately

If line is busy (carrier sensed) wait until idle and

transmit packet immediately

If collision detected

Stop sending and jam signal

Jam signal: make sure all other transmitters areaware of collision

Wait a random time (Exponential backoff), and try

again


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Ethernet Performance

Ethernets work best under light loads

Utilization over 30% is considered heavy

Peak throughput worse with

More hosts

More collisions needed to identify single sender

Smaller packet sizes

More frequent arbitration

Longer links

Collisions take longer to observe, more wasted

bandwidth


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Ethernet MAC Protocol

Collision detection can take as long as 2 .X


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CarrierSense Multiple Access (CSMA)

Listen to medium and wait until it is free

(no one else is talking)

Wait a random backoff time

Advantage: Simple to implement

Disadvantage: Cannot recover from a collision


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Wireless Interference

Two transmitting stations interfere with each

other at the receiver

Receiver gets garbage

A B

C


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CarrierSense Multiple Access

with Collision Detection (CSMA-CD)

Procedure

Listen to medium and wait until it is free

Start talking, but listen to see if someone else starts talking too

If collision, stop; start talking after a random backoff time

Used for hub-based Ethernet

Advantage: More efficient than basic CSMA

Disadvantage: Requires ability to detect collisions

More difficult in wireless scenario


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Collision Detection in Wireless

No fate sharing of the link High loss rates

Variable channel conditions

Radios are not full duplex Cannot simultaneously transmit and receive

Transmit signal is stronger than received signal


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with Collision Avoidance (CSMA-CA)

Similar to CSMA but controlframes are

exchanged instead of data packets

RTS: request to send CTS: clear to send

DATA: actual packet

ACK: acknowledgement


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with Collision Avoidance (CSMA-CA)

Small control frames lessen the cost of collisions

(when data is large)

RTS + CTS provide virtual carrier sense

protects against hidden terminal

A B


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MAC Address The IEEE 802.3 Ethernet and 802.5 Token Ring protocols

specify that the MAC sub-layer must supply a 48-bit (6 byte)

address. The MAC address is most frequently represented as

12 hexadecimal digits. The MAC address uniquely identifies a

specific network device and MAC addresses must be unique

on a given LAN (a network of computing devices in a singlesubnet of IP addresses). The first 12-bit portion of the MAC

address identifies the framework of the network device, the

last 12-bit portion identifies the unique id of the device itself.

When looking at a hexadecimal representation of the MAC

address, the first six hexadecimal digits identify the vendorand the last six hexadecimal digits identify the specific

network interface card.


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MAC Address As Displayed by

Vendor/Manufacturer

Command Used

to display MAC

00:00:0C:12:B1:CF Cisco, Unix/SUN, Linux ifconfig -a

00000C-12B1CF ProCurve Switches show bridge

00-00-0C-12-B1-CF Microsoft ipconfig /all


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Thank YOU

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