- A 24 bit interleaved even parity code used to determine if transmission errors have occurred over the Section Layer
- Its calculated over all the bits of the STS-3 frame except those in the Section overhead.
- The computed checksum is placed in the TOH of the following STS-N frame
K1 Byte (APS-Linear)
- Used to activate and deactivate the switching between the main and protection paths on a multiplexer section
- APS message type conforms to ITU-T G.783 and Bellcore GR.253 for Linear network architectures
o Bits 1-4
§ 1111 Lockout of protection
§ 1110 Forced switch
§ 1101 Signal fail, high priority (1:n only)
§ 1100 Signal fail, low priority
§ 1011 Signal degrade, high priority (1:n only)
§ 1010 Signal degrade, low priority
§ 1000 Manual switch
§ 0110 Wait to restore
§ 0100 Exercise
§ 0010 Reverse request (bi-directional systems only)
§ 0001 Do not revert
§ 0000 No request
§ Other codes are unused
o Bits 5-8 selects channel used by APS messages
§ 0000 Null channel
§ 0001 to 1110 Channels 1 thru 14
TX150 e-Manual D07-00-023 Rev A02
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