Often retransmissions at the same time are to phones connected to the same switch, but sometimes retransmissions occur together to phones at opposite ends of the network. Those in each cluster are mainly between the PBX and some set of the VoIP phones, but not always the same set. The Wireshark log shows about 2 clusters of retransmissions a day ranging from 5 packets to hundreds. Wireshark picks up a clump of retransmitted TCP packets at the times when we record phone restarts. I have been doing some Wireshark monitoring on the connection between the VoIP PBX and the rest of the network.

Simultaneously there are often signs of temporary loss of connection on computers: freezes in explorer while accessing network shares, errors in our administration software due to loss of connection to the database server. Since their installation around a year ago, every week or so, we notice a VoIP phone resetting itself - occasionally in the middle of a call. I have an irritating problem with a LAN of about 100 computers, 2 Windows domain servers, and 12 VoIP phones.

Wireshark tcp retransmission causes