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*Keypoints:Encapsulation is the process of adding header information to data. Be very familiar with theabove 5 steps of data encapsulation and the order in which they occur.Tunneling
The process in which frames from one network system are placed inside the frames of another network
system.
*Keypoints:Know the definition for tunneling.Flow Control
Flow control is a function that prevents network congestion by ensuring that transmitting devices do not
overwhelm receiving devices with data.
There are a number of possible causes of network congestion. Usually it is because a high-speed com-
puter generates data faster than the network can transfer it, or faster than the destination device can re-
ceive and process it.
There are three commonly used methods for handling network congestion:
·
Buffering
·
Source Quench Messages
·
Windowing
Buffering
Buffering is used by network devices to temporarily store bursts of excess data in memory until they
can be processed. Occasional data bursts are easily handled by buffering. However, buffers can over-
flow if data continues at high speeds.
Source Quench Messages
Source quench messages are used by receiving devices to help prevent their buffers from overflowing.
The receiving device sends a source quench message to request that the source reduce its current rate of
data transmission.
Windowing
Windowing is a flow-control method in which the source device requires an acknowledgement from the
destination after a certain number of packets have been transmitted.
1.
The source device sends a few packets to the destination device.
2.
After receiving the packets, the destination device sends an acknowledgment to the source.
3.
The source receives the acknowledgment and sends the same amount of packets.
4.
If the destination does not receive one or more of the packets for some reason (such as over-
flowing buffers), it does not send an acknowledgment. The source will then retransmits the
packets at a reduced transmission rate.
Windowing is very reliable because it uses positive acknowledgement. Positive acknowledgement
requires the recipient device to communicate with the sending device, sending back an acknow-
ledgement when it receives data. If the sending device does not receive an acknowledgement it
knows to retransmit the packets at a reduced transmission rate. It the receiving device sends a
packet with a zero window size, it means it's buffers are full and it cannot receive any more data.