BTM509 Digital Data Communication Systems

KOCAELI UNIVERSITYGraduate School of

Natural and Applied Sciences

Prepared By: Mohammed ABUIBAIDEmail: m.a.abuibaid@gmail.com

Submitted to: Dr. Halil YİĞİT

Electronic and Communication Engineering

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Presentation Outline

1. Introduction to CSMA

2. CSMA access modes

3. CSMA Protocol Modifications

4. MATLAB code of CSMA/CD

Introduction videoCSMA Access Modes- 1-Persistent

- Non-Persistent

- P-Persistent

- O-Persistent

CSMA Protocol Modifications- CSMA with collision detection

- CSMA with collision avoidance

- Virtual time CSMA CSMA/CD & CSMA/CA made simple – Youtube

CSMA access modes 1-Persistent : an aggressive transmission algorithm

Non-Persistent : a non aggressive transmission algorithm

P-Persistent : an approach between 1-persistent and non-persistent

O-Persistent: Each node is assigned a transmission order by a supervisory node

1-Persistent CSMA access mode

In case of a collision, the sender waits for a random period of time and attempts to transmit again unconditionally (i.e. with probability=1).

1-persistent CSMA is used in CSMA/CD systems including Ethernet.

Non-Persistent CSMA access mode

This approach reduces collision, results in overall higher medium throughput

but with a penalty of longer initial delay compared to 1–persistent.

P-Persistent CSMA access mode

P-Persistent CSMA is used in CSMA/CA

systems including Wi-Fi and other

packet radio systems.

Each node is assigned a transmission order by a supervisory node.

When the transmission medium goes idle, nodes wait for their time slot in accordance with their assigned transmission order.

The node assigned to transmit first transmits immediately.

The node assigned to transmit second waits one time slot (but by that time the first node has already started transmitting).

Nodes monitor the medium for transmissions from other nodes and update their assigned order with each detected transmission (i.e. they move one position closer to the front of the queue).

O-Persistent CSMA access mode

O-Persistent CSMA access mode

O-Persistent CSMA is used by CobraNet, LonWorks and the controller area network.

It is used for the automation of various functions within buildings such as lighting and HVAC, …etc.

It is used in large audio installations such as convention centers, stadiums, airports, … etc.

CSMA Protocol Modifications:

CSMA/CD: improves CSMA performance by terminating transmission as soon

as a collision is detected.

CSMA/CA: improves CSMA performance by reducing the incidence of collision.

VTCSMA : improves CSMA performance by avoiding collision generated by

nodes transmitting signals simultaneously.

CSMA/CD : CSMA with Collision Detection

CSMA Procedure Collision Detected Procedure

CSMA/CD Applications CSMA/CD was used in now obsolete shared media Ethernet variants

(10BASE5, 10BASE2) and in the early versions of twisted-pair Ethernet which used repeater hubs.

10BASE2

CSMA/CD Applications Modern Ethernet networks, built with switches and full-duplex

connections, no longer need to utilize CSMA/CD because each collision

domain is now isolated.

CSMA/CD is still supported for

backwards compatibility and for

half-duplex connections.

CSMA with Collision AvoidanceCollisions are avoided by three strategies: Inter-frame space The contention window Acknowledgments

CSMA/CA Applications GNET - Apple's LocalTalk 802.11 RTS/CTS IEEE 802.15.4 (Wireless PAN) NCR WaveLAN HomePNA Bus networks ITU-T G.hn standards ( G.9960)

VTCSMA : Virtual time CSMA Designed to avoid collision generated by nodes transmitting signals simultaneously.

It used mostly in hard real-time systems (such as mission-critical applications)

It uses two clocks at every node, a virtual clock VC and a real clock RC which tells "real time".

When the transmission medium is sensed to be busy, the VC freezes.

When the transmission medium is free, VC is reset.

Calculating VC runs faster than RC when channel is free

VC is not initiated when the transmission medium is busy.

MATLAB Code

Mohammed AbuibaidLive & Breathe Wireless