DS3 byte counter counts {HEAD ~ the DS3 AIS/Idle code bytes} and generating an AIS/Idle bytecount enable control signal if the number of DS3 AIS/Idle code bytes is less than a predetermined number of {TAIL ~ DS3 bytes} to be mapped in a given row of the STS 1 frame 
DS3 byte counter counts { HEAD ~ the DS3 AIS / Idle code bytes } and generating an AIS / Idle bytecount enable control signal if the number of DS3 AIS / Idle code bytes is less than a predetermined number of { TAIL ~ DS3 bytes } to be mapped in a given row of the STS 1 frame
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH