3.2.4.2 Send Preamble
The initiator MUST send the Preamble Message as described in section 2.2.6.
In the case of Simplex mode, the initiator MUST send the Preamble End record as described in section 2.2.3.7.
In the case of Singleton Unsized, and Duplex modes, the initiator MUST perform the following additional steps:
If an upgrade is required, send the Upgrade Request Record as described in section 2.2.3.5.
If an upgrade is sent, read the Upgrade Response Record as described in section 3.1.4.8.
Send the Preamble End Record as described in section 2.2.3.7.
Read the Preamble Ack Record as described in section 3.1.4.10.
The initiator MUST set Send Allowed to TRUE.
If the mode is Duplex, the initiator MUST set Receive Allowed to TRUE.