Multiple Access Control+aloha.pptx

8/10/2019 Multiple Access Control+aloha.pptx

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Presented by:

Manisha KauraM.Tech (ECE)

Multiple Access Control&

ALOHA Protocol


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Content

Introduction-MAC

Functions performed in MAC layer

MAC Address

Channel allocations Static channel allocation

Queen theory

Dynamic channel allocation

ALOHA protocol Advantages and disadvantages of ALOHA.


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Introduction

The MAC sub layer provides addressingand channel access control mechanisms that

make it possible for several terminals or

network nodes to communicate within

a multiple access network that incorporates ashared medium, e.g. Ethernet.

The hardware that implements the MAC is

referred to as a medium access controller.


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Link Layer Control (LLC)

Responsible for error

and flow control

Control

Responsible framing

and MAC address and

Multiple Access Control

Figure a Data l ink layer divided into two functional ity-oriented sublayers


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Functions performed in MAC

layer

Frame delimiting and recognition.

Addressing of destination stations (both as

individual stations and as groups of stations).

Conveyance of source-station addressinginformation.

Protection against errors, generally by means

of generating and checking frame check

sequences. Control of access to the physical

transmission medium.


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MAC Address

It is a unique identifier assigned to networkinterfaces for communications on the physical

network segment. MAC addresses are used as

a network address for most IEEE 802 network

technologies.

MAC addresses are most often assigned by

the manufacturer of a network interface

controller (NIC) and are stored in its

hardware.If assigned by the manufacturer, a

MAC address usually encodes the

manufacturer's registered identification

number.


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Channel allocations:

Static channel allocation

Dynamic channel allocation


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Static Channel Allocation Bandwidth divided into equal sized portions;

so no interference of users.

For fewer, very large and varying number ofusers, it is a poor fit.

Poor fit can be proved by queen theory.T=1/(C-)

Where T: Mean time delay

C: Channel Capacity

: average arrival rate of frame

: average length of frame


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Dynamic Channel allocation

In DCA schemes, all channels are kept in acentral pool and are assigned dynamically to

new calls as they arrive in the system.

After each call is completed, the channel isreturned to the central pool. It is fairly

straightforward to select the most appropriate

channel for any call based simply on current

allocation and current traffic, with the aim ofminimizing the interference.


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ALOHA Protocol A station that has data can transmit at any time

After transmitting a frame, the sender waits for an acknowledgment

for an amount of time (time out) equal to the maximum round-trip

propagation delay

If no ACK was received, sender assumes that the frame or ACK has

been destroyed and resends that frame after it waits for a random

amount of time

If station fails to receive an ACK after repeated transmissions, it gives

up

Channel utilization or efficiency or Throughput is the percentage of

the transmitted frames that arrive successfully (without collisions) or

the percentage of the channel bandwidth that will be used for

transmitting frames without collisions

ALOHA Maximum channel utilization is 18% (i.e, if the system

produces F frames/s, then 0.18 * F frames will arrive successfully on

average without the need of retransmission).


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Figure b Procedure for ALOHA protocol


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Advantages and Disadvantages of

ALOHA protocols

Advantages

A node that has frames to be transmitted can transmit continuously at the

full rate of channel (R bps) if it is the only node with frames.

Simple to be implemented.

No master station is needed to control the medium.

Disadvantage

If (M) nodes want to transmit, many collisions can occur

This causes low channel utilization.


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Thank you

For

Listening !!

Multiple Access Control+aloha.pptx

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