 this problem, 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 C-Bits M13 framing is a bit outdated because it assumes that DS2 links may be terminated from remote locations, just as DS1s are.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 were 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 of the benefits 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 21-2 shows how FEBEs are sent on a DS3 with SONET in the middle of the link. C-bit framing also allows for the introduction of far-end out-of-frame (FEOOF) sig- nals. When a break is detected on the receiving interface of the remote end of the link, it sends a FEOOF signal back to the source. Figure 21-3 shows an example of FEOOFs in action. Chapter 21: DS3 DS3 with SONET in the middle C-bit Fiber optic terminal Fiber optic terminal C-bit multiplexer multiplexer Error FEBES are Detects error or framing detected and errors and sends a FEBE incremented here back to the source Figure 21-2. Far-end block errors DS3 with SONET in the middle C-bit Fiber optic terminal Fiber optic terminal C-bit multiplexer multiplexer alarm FEOOF FEOOF FEOOFs are Detects blue alarm detected and and sends a FEOOF incremented here back to the source Figure 21-3. Far-end out-of-frame errors Additional codes, called far-end alarm and control (FEAC) codes, are also available, and include: DS3 Equipment Failure-Servi