C-Bit Parity Framing

In M13 framing, every C-bit in a DS3 frame is used for bit stuffing. However, because multiplexers first use bit stuffing when multiplexing  DS1 signals into DS2 signals, the incoming DS2 signals are already synchronized. Therefore, the bit stuffing that occurs when DS2 signals are multiplexed is redundant.

C-bit parity framing format redefines the function of C-bits and X-bits, using them to monitor end-to-end path performance and provide in-band data links. The C-bit parity framing structure is shown in Figure 4.

Figure 4: DS3 C-Bit Parity Framing

In C-bit parity framing, the X-bits transmit error conditions from the far end of the link to the near end. If no error conditions exist, both X-bits are set to 1. If an out-of-frame (OOF) or alarm indication signal (AIS) error is detected, both X-bits are set to 0 in the upstream direction for 1 second to notify the other end of the link about the condition.

The C-bits that control bit stuffing in M13 frames are typically used in the following ways by C-bit parity framing:

• Application identification channel (AIC)—The first C-bit in the first subframe identifies the type of DS3 framing used. A value of 1 indicates that C-bit parity framing is in use.
• N_a—A reserved network application bit.
• Far-end alarm and control (FEAC) channel—The third C-bit in the first subframe is used for the FEAC channel. In normal transmissions, the FEAC C-bit transmits all 1s. When an alarm condition is present, the FEAC C-bit transmits a code word in the format 0xxxxxxx 11111111, in which x can be either 1 or 0. Bits are transmitted from right to left.

  Table 18 lists some C-bit code words and the alarm or status condition indicated.

  Table 18: FEAC C-Bit Condition Indicators

  Alarm or Status Condition	C-Bit Code Word
  DS3 equipment failure requires immediate attention.	00110010 11111111
  DS3 equipment failure occurred—such as suspended, not activated, or unavailable service—that is non-service-affecting.	00011110 11111111
  DS3 loss of signal.	00011100 11111111
  DS3 out of frame.	00000000 11111111
  DS3 alarm indication signal (AIS) received.	00101100 11111111
  DS3 idle received.	00110100 11111111
  Common equipment failure occurred that is non-service-affecting.	00011101 11111111
  Multiple DS1 loss of signal.	00101010 11111111
  DS1 equipment failure occurred that requires immediate attention.	00001010 11111111
  DS1 equipment failure occurred that is non-service-affecting.	00000110 11111111
  Single DS1 loss of signal.	00111100 11111111

• Data links—The 12 C-bits in subframes 2, 5, 6, and 7 are data link (DL) bits for applications and terminal-to-terminal path maintenance.
• DS3 parity—The 3 C-bits in the third subframe are DS3 parity C-bits (also called CP-bits). When a DS3 frame is transmitted, the sending device sets the CP-bits to the same value as the P-bits. When the receiving device processes the frame, it calculates the parity of the M-frame and compares this value to the parity in the CP-bits of the following M-frame. If no bit errors have occurred, the two values are typically the same.
• Far–end block errors (FEBEs)—The 3 C-bits in the fourth subframe make up the far-end block error (FEBE) bits. If a framing or parity error is detected in an incoming M-frame (via the CP-bits), the receiving device generates a C-bit parity error and sends an error notification to the transmitting (far-end) device. If an error is generated, the FEBE bits are set to 000. If no error occurred, the bits are set to 111.

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