CSC 311 Chapter Eight FLOW CONTROL TECHNIQUES

CSC 311 Chapter Eight 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 dramatically different speeds? These are issues that relate to error control and flow control. We have previously examined how to detect errors, how do we handle the errors that we detect? How do we prevent devices from being overwhelmed with data?

CSC 311 Chapter Eight We have learned how to detect errors, what action should we take to correct errors? One common technique is to request a retransmission of lost or damaged frames. There are a number of protocols that operate in this fashion, generally called “automatic repeat request” of ARQ. ERROR CONTROL

CSC 311 Chapter Eight FLOW CONTROL Flow control defines the way in which many frames are sent and 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 a time or no flow control at all, unrestricted..send all frames whenever you wish. More complicated protocols try to strike a balance between unrestricted and overly restricted.

CSC 311 Chapter Eight 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.

CSC 311 Chapter Eight 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 which means there is no distinction between a sender and a receiver. In this instance, A is becoming overloaded with data from B, B is not having a similar problem with data from A. After receiving the X-OFF from A, B will stop sending data to A but A can still send data to B. B will resume only after A sends and X-ON. Because of the time required for the X-OFF message to propagate through the system, A will continue to receive data from B for a short period of time.

CSC 311 Chapter Eight FRAME ORIENTED PROTOCOLS Protocols such as X-ON/X-OFF are byte oriented, typical of asynchronous communication. Frame oriented, synchronous communications require a more sophisticated management technique. We will be looking at transmission of frames of whatever type and size supported by the underlying protocol architecture.

CSC 311 Chapter Eight FRAME ORIENTED FLOW CONTOL PROTOCOLS UNRESTRICTED STOP AND WAIT SLIDING WINDOW GO BACK N SELECTIVE REPEAT

CSC 311 Chapter Eight UNRESTRICTED FLOW CONTROL PROTOCOL Assumes receiver has unlimited capability to receive frames or that 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.

CSC 311 Chapter Eight STOP AND WAIT PROTOCOL Simple to implement. Sender sends a single frame and waits for an acknowledgement before 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 environments and not practicable in many situations. Though only a single frame is ever in transit, the frames are still numbered. 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 will return 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.

CSC 311 Chapter Eight What might go wrong? Note: the author is not entirely correct!! Frame could be damaged. Frame could be lost. Positive acknowledgement could be lost Negative acknowledgement could be lost Protocol must, (AND DOES!!!) handle this. Author does not point out that, upon sending a frame, the sender starts a timer. If this timer times out, sender will resend the frame.

CSC 311 Chapter Eight 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 on the network. Propagation time is time required to traverse the network. Stop and Wait would be terrible for satellite communication. Propagation time is approx seconds each way. Total.5s Consider a network capable of sending 100Mbps and a frame size of bits. It would take 1 ten thousandth of a second to put the frame on the network, it would take one-half second for 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 sent on a network capable of sending 100,000,000 bits per second or a utilization of 0.02%

CSC 311 Chapter Eight