C-bit parity mode – Allied Telesis 86241-06 User Manual

Page 23

Patch 86241-06 For Rapier Switches

Patch 86241-05 for Software Release 2.4.1
C613-10340-00 REV E

into 8 blocks of 85 bits with 84 bits available for payload, and one bit for
framing overhead. The frame structure is shown in Figure 1 on page 23.

Figure 1: DS3 Framing Structure.

The switch with the DS3 interface is called the near end. The entity the switch
connects to is called the far end. X1 and X2 are set to 1 if the near end is
receiving an Alarm Indication Signal (AIS), a Loss Of Frame (LOF), or a Loss Of
Signal (LOS). This allows the near end to indicate to the far end that it is
experiencing a problem and is known as Far End Receive Failure (FERF).

P1 and P2 form the P-bit channel. They provide parity information for the
preceding M-frame.

M1, M2, and M3 form a frame alignment channel used by the hardware to locate
all seven M-subframes.

F1, F2, F3, and F4 form an M-subframe alignment channel which is used by the
hardware to identify all frame overhead bit positions.

C1, C2, and C3 form the C-bit channel.

C-bit Parity Mode

In C-bit parity mode the C-bits are described as follows:

■

The first C-bit in M-subframe 1 is set to 1 to identify the format as C-bit
parity. If this is zero the format is assumed to be M23.

■

The second C-bit in M-subframe 1 is designated Nr and is set to 1.

■

The third C-bit in M-subframe 1 provides the Far End Alarm and Control
signal (FEAC) which is used to:

•

Send alarm or status information from the far end back to the near end.

•

Initiate DS3 loopbacks.

The three C-bits in M-subframe 3 are designated as CP-bits and are used to
implement CP-bit parity. At the near end the CP-bits are set to the same value
as the P-bits. The parity of the CP-bits of frame N are compared with the parity
of the CP-bits of frame N+1. A difference in parity between N and N+1 is
deemed a CP-bit parity error.

679

Bits

679

Bits

679

Bits

679

Bits

679

Bits

679

Bits

679

Bits

INFO

M-Frame 4760 bits

First M-Subframe 680 bits

DS31