csc 311 chapter eight flow control techniques. csc 311 chapter eight how do we manage the large...

CSC 311 Chapter Eight

FLOW CONTROL TECHNIQUES


How do we manage the large amount of data on the network?

How do we react to a damaged transmission?

How do we ensure that messages are correctly delivered?

How can we transmit multiple frames efficiently?

How can we reconcile computers that operate at dramaticallydifferent speeds?

These are issues that relate to error control and flow control.

We have previously examined how to detect errors, how do wehandle the errors that we detect?

How do we prevent devices from being overwhelmed with data?


We have learned how to detect errors, what action should we taketo correct errors?

One common technique is to request a retransmission of lost ordamaged frames.

There are a number of protocols that operate in this fashion,generally called “automatic repeat request” of ARQ.

ERROR CONTROL


FLOW CONTROL

Flow control defines the way in which many frames are sentand tracked and how the stations do error control.

There are a number of different flow control protocols ranging

from simple to complex.

Simple techniques might involve sending a single frame at atime or no flow control at all, unrestricted..send all frameswhenever you wish.

More complicated protocols try to strike a balance betweenunrestricted and overly restricted.


Basic Flow Control …. Signaling

Sender sends data as long as receiver is able to receive it.

Sender will send until receiver says stop and will not start

again, until notified by receiver that it is OK to do so.


X-ON/ X-OFF

Rather than use separate lines to signal, the signals can be embedded in the data.The figure shown below assumes full duplex communication whichmeans there is no distinction between a sender and a receiver.

In this instance, A is becoming overloaded with data from B, B is nothaving a similar problem with data from A. After receiving the X-OFF fromA, B will stop sending data to A but A can still send data to B. B will resumeonly after A sends and X-ON. Because of the time required for the X-OFFmessage to propagate through the system, A will continue to receive datafrom B for a short period of time.


FRAME ORIENTED PROTOCOLS

Protocols such as X-ON/X-OFF are byte oriented, typical of asynchronouscommunication.

Frame oriented, synchronous communications require a more sophisticatedmanagement technique.

We will be looking at transmission of frames of whatever type and sizesupported by the underlying protocol architecture.


FRAME ORIENTED FLOW CONTOL PROTOCOLS

• UNRESTRICTED

• STOP AND WAIT

• SLIDING WINDOW

• GO BACK N• SELECTIVE REPEAT


UNRESTRICTED FLOW CONTROL PROTOCOL

Assumes receiver has unlimited capability to receive frames orthat unlimited buffering capacity is available.

It assumes that every frame will arrive without damage and

in the order sent.

No error checking is done and no checking is done for

lost or delayed frames.


STOP AND WAIT PROTOCOL

Simple to implement.

Sender sends a single frame and waits for an acknowledgementbefore sending another.

Essentially, it sends a single frame and “stops and waits” for a response from the receiver before sending another.

It is simple, but slow. Not effective in high traffic environmentsand not practicable in many situations.

Though only a single frame is ever in transit, the frames are stillnumbered. Frames are numbered alternately, 0 or 1. This is necessary to avoid confusion and ensure correctness of the protocol.

After receiving a frame number 0, for example, the receiver willreturn an ACK1, which indicates to the sender that frame 0 was received correctly, without errors, and that the receiver now expects a frame numbered 1.


What might go wrong? Note: the author is not entirely correct!!

Frame could be damaged.Frame could be lost.Positive acknowledgement could be lostNegative acknowledgement could be lost

Protocol must, (AND DOES!!!) handle this.Author does not point out that, upon sending a frame, thesender starts a timer. If this timer times out, sender will resendthe frame.


When is STOP AND WAIT not suitable.

When traffic is heavy because, it does not make good utilization of network capacity

When propagation time is much larger than transmission time.

Transmission time is the time required to put the message onthe network.

Propagation time is time required to traverse the network.Stop and Wait would be terrible for satellite communication.

Propagation time is approx. 0.25 seconds each way. Total .5sConsider a network capable of sending 100Mbps and a framesize of 10000 bits. It would take 1 ten thousandth of a secondto put the frame on the network, it would take one-half secondfor the frame to arrive and the ACK to get back to sender.So, in this instance, in one second only 20,000 bits would be senton a network capable of sending 100,000,000 bits per second or a utilization of 0.02%

csc 311 chapter eight flow control techniques. csc 311 chapter eight how do we manage the large...

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