"Normal response mode allows the secondary-to-primary link to be shared without contention, because it has the primary give the secondaries permission to transmit one at a time. It also allows operation over half-duplex communication links, as long as the primary is aware that it may not transmit when it has given permission to a secondary."
Ah! That explains what the GREIS manual means by "When enabled, synchronization packets will be transmitted to avoid possible data damage."
I wonder why that's no longer recommended. Seems to make sense that you'd want the base radio to say "OK, repeaters, when you get this, you may transmit one at a time and I'll shut up for a moment so we're not all blasting the airwaves at once."
Thank you Shawn Billings, you have correctly described the SNRM packet meaning. SNRM packet used by base and rover to establish connection. We can say, that this packet is management packet, in previous firmware versions we have supported also connection oriented protocols (open connection, then exchange data), but now we have simplified this part, and modems can exchange data without any management packet. SNRM packet have also "secondary useful role": when long time modems don't exchange data, SNRM as first packet helps receiver tune to base station, correct frequency offset, and synchronize with base. For repeater case, base and repeater divide the transmission time. For 1 second we have about 16 packet time which is divided for base (8) and repeater(8). Base wait when repeater finish and ready to receive packets from transmitter for retranslation. With new type corrections, data count is large and when we use SNRM packet, for 1 second interval we "lost" 2 packet time.