Question 19B1

A DSC (Digital Selective Calling) distress alert begins with a specific preamble to ensure reliable reception and synchronization by receiving stations. This preamble consists of an alternating dot pattern, followed by a phasing signal, and then the actual message data. The alternating dot pattern is a sequence of 200 bits that switch between mark and space frequencies. Its purpose is to allow the receiving equipment's modem to synchronize accurately with the incoming signal's bit timing, even under varying signal conditions or when the receiver has just started monitoring the channel. After this robust synchronization, the phasing signal (a specific 10-bit pattern) provides frame synchronization, indicating the start of the data characters. Therefore, an alternating dot pattern of 200 bits directly precedes the phasing signal, as mandated by the ITU-R M.493 standard for DSC. Options A, B, and D specify incorrect lengths for this essential synchronization pattern.