dual T1s. Remember that each T1 will have its own clock master and slave. The DS3 needs to support its own clocking without interfering with that of the indi- vidual T1s (modern networks that use SONET in the core do not really have this prob- lem, but this technology was designed many years ago, before SONET existed). C-Bits M13 framing is a bit outdated, because it assumes that DS2 links may be terminated from remote locations, just as DS1s are. In practice, DS2s are not deployed and, as a result, exist only within the multiplexer. This means that the timing issues that require bit stuffing occur only at the M12 stage and never at the M23 stage. Still, the M13 framing process provides positions for bit stuffing at the M23 stage. Another framing technique was developed to take advantage of the unused bits. The original purpose of these bits (called C-bits) was to signal the presence of bits stuffed at the M23 stage of the multiplexing process. C-bit framing uses the C-bits in the DS3 frame differently than originally planned. One benefit of C-bit framing is the inclusion of far-end block errors (FEBEs) reporting. FEBEs (pronounced FEE-bees) are DS3-specific alarms that indicate the far end of the link has received a C-parity or framing error. Figure 22-2 shows how FEBEs are sent on a DS3 with SONET in the middle of the link. DS3 with SONET in the middle C-bit Fiber optic terminal Fiber optic terminal C-bit multiplexer multiplexer Error FEBÉs are Detects error or framing detected and errors and sends a FEBE incremented here back to the source Figure 22-2. Far-end block errors C-bit framing also allows for the introduction of far-end out-of-frame (FEOOF) signals. When a break is detected on the receiving interface of the remote end of the link, it Framing sends a FEOOF signal back to the source. Figure 22-3 shows an example of FEOOFs in action. DS3 with SONET in the middle C-bit Fiber optic terminal Fiber opticterminal C-bit multiplexer multiplexer alarm FEOOF FEOOF FEOOFs are Detects blue alarm detected and and s