Time Sequence diagrams

Solution Preview

check attachment.

Please reference this figure for the question:

55. Draw 3 time sequence diagrams that illustrate the flow of frames between points B and G in Figure 1 using the following information:

a. Stop and Wait ARQ, SENDER sends four frames (F1, F2, F3, and F4). F1, F2 and F4 are received error free, but F3 is lost in the network during its initial transmission. Continue the diagram until all frames are received are error free and properly acknowledged.

b. Go-Back-N ARQ, SENDER sends five frames (F1, F2, F3, F4, and F5). F1 through F2 are received error free but F3 is lost in the network after F4 and F5 are sent. Assume that F4 and F5 are received error free. Continue the diagram until all frames are received error free and properly acknowledged.

c. Selective Reject ARQ, SENDER sends six frames (F1, F2, F3, F4, F5 and F6) but only has a four frame send window. F1,F2 and F4 are received error free but an error is detected in F3 after F4 is received. When sent F5 is received error free while F6 is lost in the network. Continue the diagram until all frames are received error free and properly acknowledged.

ANSWER:
a. Stop and Wait ARQ:
Stop and Wait transmission is the simplest reliability technique and is adequate for a very simple communications protocol. A stop and wait protocol transmits a Protocol Data Unit (PDU) of information and then waits for a response. The receiver receives each PDU and sends an Acknowledgement (ACK) PDU if a data PDU is received correctly, and a Negative Acknowledgement (NACK) PDU if the data was not received. In practice, the receiver may not be able to reliably identify whether a PDU has been received, and the transmitter will usually also need to implement a timer to recover from the condition where the receiver does not respond.
Under normal transmission the sender will receive an ACK for the data and then commence transmission of the next data block. For a long delay link, the sender may have to wait an appreciable time for this response. While it is waiting the sender is said to be in the "idle" state and is unable to send further data.

Stop and Wait ARQ - Waiting for Acknowledgment (ACK) from the remote node.
The blue arrows show the sequence of data PDUs being sent across the link from the sender (top to the receiver (bottom). A Stop and Wait protocol relies on two way transmission (full duplex or half duplex) to allow the receiver at the remote node to return PDUs acknowledging the successful transmission. The acknowledgements are shown in green in the diagram, and flow back to the original sender. A small processing delay may be introduced between reception of the last byte of a Data PDU and generation of the corresponding ACK.
When PDUs are lost, the receiver will not normally be able to identify the loss (most receivers will not receive anything, not even an indication that something has been corrupted). The transmitter must then rely upon a timer to detect the lack of a response.

Stop and Wait ARQ - Retransmission due to timer expiry
In the diagram, the second PDU of Data is corrupted during transmission. The ...