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Effect of Communication Failures

The emulation was run with a given probability of failure on each packet type of the Network Control Protocol. The following results are based on the output of the finite state machine (FSM) transition output of the emulation and an explanation is given for each case.

A dropped MYCALL packet has no effect as long as at least one of the MYCALL packets from each ES is received at the master ES. This is the only use of the AX.25 broadcast mode in the ES configuration. The broadcast AX.25 mode is a one time, best effort delivery; therefore, MYCALL packets are repeatedly broadcast at the NCP layer.

The Maisie emulation demonstrated that a dropped NEWSWITCH packet caused the protocol to fail. This is because the master ES will wait until it receives all SWITCHPOS packets from all ES nodes for which it had received MYCALL packets. The NEWSWITCH packet is sent over the AX.25 in connection-oriented mode, e.g. a mode in which corrupted frames are retransmitted; the probability of loosing a packet in this mode is very low. A dropped SWITCHPOS packet has the same effect as a dropped NEWSWITCH packet. In order to avoid this situation, the NCP will resend the NEWSWITCH if no response is received.

Finally, the Maisie emulation showed that a lost TOPOLOGY packet results in a partitioned network. The ES which fails to receive the TOPOLOGY packet is not joined with the remaining ES nodes; however, this ES node continued to receive and process USER_POS packets from all RNs. It therefore attempts to form an initial connection with all RNs. The solution for this condition is not to allow RN associations with an ES node until the TOPOLOGY packet is received. Because MYCALL packets are transmitted via broadcast AX.25, each ES node can simply count the number of MYCALL packets and estimate the time for the master ES node to calculate the topology using the number of MYCALL packets as an estimate for the size of the network. If no TOPOLOGY packet is received within this time period, the ES node retransmits its SWITCHPOS packet to the master ES node in order to get a TOPOLOGY packet as a reply.


next up previous
Next: Summary Up: Emulation Results Previous: ES Mobility

Steve Bush
Tue Nov 12 11:48:31 CST 1996