Error Control

Error control
is responsible for checking each transmission and verifying that all of the PDUs
are contiguous and not erroneous. If there are missing or damaged PDUs, the destination will
not send an ACK packet for the previous transmission. (Refer to Figure 36.6.)
Once all of the data is transferred without errors, the originating system sends a termination
request, which tells the destination system that no more data needs to be transmitted. The destination
system then responds with a termination acknowledgment.
As you can see, both systems do a lot of communicating, aside from the exchange of data.
From the connection request to the termination acknowledgment, every exchange is accompanied
with control information that keeps the data transfer reliable and error free. Table 36.1
gives examples of several connection-oriented protocols. 1102

Flow Control

Although flow control was briefly described earlier, this section contains more detail.
Flow control
is responsible for ensuring that the transmitting station does not send data faster than the
receiving station can process it. This is done by establishing a window size for the transmission.
Look at Figure 36.6 to see how windowing works. Notice that the originating system sends
out a specified number of PDUs. Once that number is reached, the originating system waits for
a response from the destination system. After the response is received, the system continues to
transmit data.