Protocols specify the rules for communicating over a channel, much as one person politely waiting for another to finish before they speak. Protocols coupled with channel characteristics determine the net efficiency of communications over the channel.
Protocols can improve the effective channel quality. An example is an ARQ (automatic repeat request) protocol in which a source automatically retransmits a message if it fails to receive an acknowledgment from the destination within some predefined time period following the original transmission of the message. The destination knows whether to acknowledge the message based on some error detection capability, which is typically based on redundant information added to the message, such as a parity code or cyclic redundancy check (CRC).
Figure 0.3 depicts a message ("Pick-up at 1:30 PM.") being transmitted from a source to a destination via "packets" which contain four characters at a time. Additional redundant information in each packet allows the destination to know whether or not it has received that packet correctly. Once the destination is satisfied that it has correctly received a packet, it sends an acknowledgment ("ack") message to the original packet source. When the source receives the acknowledgment, it may transmit the next packet in sequence. In an ARQ arrangement, failure to receive an acknowledgment within a specific time period causes the source to retransmit the packet which was not acknowledged. Only a single transmitted packet remains outstanding (i.e., unacknowledged) at a time.
Error detection and recovery mechanisms can be much more sophisticated than this simple ARQ scheme. One way to enhance the effective channel performance is to allow multiple packets to be outstanding at a time. Individual packets are assigned a sequence number reflecting their order in a sequence of packets flowing from a specific source to a specific destination, which allows them to be separately acknowledged or retransmitted in the event of a failure.
This type of windowing scheme is commonly used when significant delays are involved in the end-to-end data transmission or when the channel has a relatively high quality. When an individual packet is not received correctly, the destination could request retransmission of either the individual bad packet, called selective packet rejection, or that packet plus all succeeding packets. Which of these modes is employed depends on the nature of the communication and the medium used.
Figure 0.4 depicts such a windowing scheme applied to the packet-based transmission example from above, but with each packet individually numbered in sequence. In this example, up to three packets may be outstanding at a time and the destination must notify the host of the next expected packet number, implicitly acknowledging all preceding packets. Unless a time-out occurs at the source, it will continue to transmit packets until the window-size of three outstanding unacknowledged packets is reached. The destination periodically acknowledges all received packets.
If sufficient redundant information is added to a message, it could enable the receiver of the message to not only detect an error but also correct it. Although this requires some additional processing by the destination, it could obviate the need for retransmission. This error correction capability is generally desirable in channels which are expensive, prone to distortion, or suffer from long latency in the dialogue cycle.