The overall goal of this lab is to design, implement, and test a peer-to-peer communication system. Peer-to-peer means people on two computers communicate without the people on the other computers seeing the information. The system must use a ring-connected RS232 serial channel (Figure 14.4), must use interrupt-driven I/O, and must have a layered software configuration. The lowest-level software performs serial I/O. The middle-level software sends message packets from one computer to another. The highest-level software interfaces with the human operator (keypad/LCD) and provides a mechanism to create a peer-to-peer connection. In a layered system, software in one layer can call routines only within that layer or the layer immediately below it. You need a way to see who is on the network and a way to request/accept or terminate connection between two operators. Local operator input/output will occur via a keypad and LCD. You may assume all nodes on the system are willing to cooperate and will not perform malicious activity. On the other hand, it is possible that another computer on the network may not be plugged in, or the network connection may be broken
The communication system between two or more microcomputers will be designed in three layers. The first layer, the physical layer is implemented by the SCI hardware and the interrupt-driven device driver. The second layer may consist of a simplified binary synchronous communication protocol (BSC). At this level, message packets will be transmitted between the two machines. Possible formats of the control-code packet and the data packet are shown in Figure 14.25. This control-code packet contains exactly six bytes.
Figure 14.4
Figure 14.25
In the data packet, the data c1, c2, . . ., cn are ASCII characters that constitute the information being sent from source to destination. It is OK to limit message sizes to a maximum of 20 bytes. The destination computer will respond with an ACK control packet if the message was received properly and will respond with a NAK if there are any framing, overrun, noise, or LRC errors. The transmitter will send a message and "stop and wait" for either an ACK or a NAK. If an ACK is received, then it can continue. If a NAK is received or if no response is received after a reasonable delay, then the message is retransmitted Because there is a ring physical channel, there is no possibility of a collision. You must handle the situations when the destination computer does not exist or when the ring is broken. To solve this fault you will need some time-out mechanism to retransmit the packet if an ACK is not received in some reasonable time. You should choose an upper limit (e.g., three) on the number of times a packet is retransmitted. After three tries an error is reported to the operator. The highest level will be a keypad interpreter and an LCD display. The LCD should show interactive feedback to the operator creates messages to be sent and should display messages received.