Introduction about congestion control. various reason for congestion control

1. Topic: Congestion control
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Traffic Jam
Data Jam
Congestion Control -

2. Data Traffic
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The main focus of Congestion Control and
quality of service is data traffic.
Congestion control: To avoid Traffic Congestion
Quality of Service: To create an appropriate environment for the traffic
Traffic Description
1. Average data rate
2. Peak data rate
3. Maximum Burst size
4. Effective Bandwidth

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Data Traffic
ADR=Amt of Data/Time
Average Data Rate: Indicates the average bandwidth needed by the traffic
Peak Data Rate: Maximum data rate of the traffic. Peak bandwidth needs for traffic to pass
through without changing its data flow.
Max Burst Size: Maximum length of time the traffic is generated at the peak rate.
Effective bandwidth: Network needs to allocate for the flow of traffic:

4. Traffic Profiles
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Data flow have one of the traffic profiles:
Constant bit Rate: Fixed rate does not change. Both avg and peak rate are same. Very easy
since it is predictable
Variable bit Rate: May change in time smooth, average data rate and peak data rate are
different
Bursty data: Data rate changes suddenly in very short time. One of the main causes of
congestion in a network.

5. Traffic Profiles
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6. Congestion
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Congestion: The number of packets sent to the network is greater than the
capacity of the networks
Congestion Control: Mechanism and techniques to control
the congestion and keep the load below the capacity

7. Congestion
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Router: Have input and output queue for each interface.
Three steps before departing: The number of packets sent to the network is greater than
the capacity of the networks
1. Packets are put at the end of the input queue.
2. Router removes the packet from the input queue once it reaches the front of
the queue
3. Packet is put in the appropriate output queue and waits its turn to be sent

8. Network Performance
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Two factor for measuring performance of networks are:
1. Delay Vs Load: When the load is much less than the capacity of the network the delay is
at a minimum. When a packet is delayed the source not receiving the
acknowledgement, retransmits the packet, which makes delay and congestion.
2. Throughput Vs Load: Number of packets passing through the network in a unit of time.
When the load exceeds the capacity the queues become full and the routers have to
discard the packets.

9. Congestion control
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Mechanisms that can either
1.Prevent congestion, before it happens (Open loop congestion) or
2.Remove congestion, after it has happened(Closed loop congestion)

10. Open Loop congestion control
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1. Retransmission policy: If any lose or corrupted then Packet can be retransmit.
2. Window Policy: Use Selective reject window method
3. Ack Policy: Sending fewer acknowledgements means imposing less load on the network.
4. Discarding Policy: Router discard less sensitive packets when congestion is likely
happened.
5. Admission Policy: Quality of service mechanism(Router may deny establishing a virtual
circuit if there is congestion in network or possibility of future congestion.
Closed Loop congestion control
Backpressure method for alleviating congestion ( Warning is from one node to its upstream node
It may reach the source station.

11. Closed Loop congestion control
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Choke packet: Sent by a node to the source to inform it of congestion. (warning from router to
source station directly)
Implicit Signaling : When a source sends several packets and there is no acknowledgement
then the assumption is that the network is congested. The source should slow down.
Explicit Signaling : Send the direct signal is included in the packets that carry data to source
or destination in backward or forward direction.
Backward Signaling : A bit can be set in packet moving opposite direction to the congestion.
Source slow down the process
Forward Signaling : A bit can be set in packet moving in the direction of congestion. Warn
the destination to slow down the ack.

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