initial time from NMEA frames
With the new releases of the WR Swtich Software (starting from v3.4) there will be no longer support for getting initial time from NMEA frames through the serial port. It was used in Grand Master mode to set the local date and time (up to full seconds). Instead, there will be an NTP client that can be configured to query any NTP server in the network through the Management Ethernet port. This solution is more generic. It uses well defined and standardized NTP protocol so the Switch can get time from any NTP server regardless its type and manufacturer. It is more common for time labs to have NTP servers with GNSS antenna and Rubidium/OCXO oscillators than serial port with raw NMEA frames coming from a single receiver.
Setting the fraction of a second in Grand Master mode stays unchanged and is based on 1-PPS and 10MHz signal from external source (e.g. Cesium).