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Thursday, July 25, 2013

PPT On Computer Network Topology

Presentation On Computer Network Topology

Computer Network Topology Presentation Transcript:
1.Computer network Topology & Architecture

2.Access Methods Contention
An Access method – set of rules – how network nodes share the transmission medium.
A pair of fundamental philosophies are
First come First served
Take turn
Media Access Methods of three types
    Token Passing
The way devices on the network – physically connected is known as the topology .
Topology is a map of the network – plan for how the cabling interconnect the nodes.

3.Contention / Polling / Token Passing
Contention – computers are contending for use of the transmission medium.
Any computer – network transmit data @ any time (first come first served)
Polling – one device is responsible for polling– other devices to see –ready for transmission / receiving data.
Token Ring – Computers take turns – to use transmission medium.
New Access Method – Demand Priority – resolves contention and doing accounts – data type priorities.

In pure contention based control – any computer can transmit – any time.
Collision occurs -- two computers – transmit – at the same time –disrupt each other’s signaling
Modern Contention Based networks – devices listens for other signals on the media before transmitting – Collisions are not totally eliminated  – kept down to manageable levels – technique known as Carrier Sense Multiple Access or CSMA
Two types of CSMA are ………
CSMA/CD Carrier Sense Multiple Access / Collision Detection
CSMA/CA Carrier Sense Multiple Access / Collision Avoidance

Device “listens” to media – any other transmission
Media quiet -- device proceeds to start transmitting data
After transmits its data – listens to the media to detect any collision
If collision detects – device send a signal for all other device not to send data for a small period of time to clear all signals from the media
Trasmitting device – wait a random amount of time b’fore sending its data
If second collision occurs with same device – double the random time out b’fore transmit again – once transmission successfully, other devices – allowed to transmit again


Device wanting to send -- checks the media for any active transmission
If media clear - device sends Request to Send message
If it is okay to transmit – network server responds with Clear to Send signal
When device receives Clear to Send signal – it transmits its data
After the successful transmission – device send Abort sequence signal – it is finished
CSMA/CA most often used by Apple’s LocalTalk network
Ethernet uses CSMA/CD method

Polling based systems – device – controller / master device – poll other – on network – whether – ready or transmit or receive data.
Polling system – not used – on networks. – fair amount of network traffic.
Computer – polls – printer to receive a print job.

9.Polling based access

10.Token Passing
Special packet is passed from computer to computer -- Token
A computer that wants to transmit must wait for a free token.
Computer takes control of the token and transmits data. Only this computer is allowed to transmit; others must wait for control of the token.
Receiving computer strips the data from the token and sends an acknowledgment.
Original sending computer receives the acknowledgment and sends the token on.

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