Cell rate decoupling is used when a continuous data stream is required at the physical layer, as in SONET and channelized facilities such as DS1. Cell rate decoupling sends special "idle" cells over the framed facility and discards any idle cells it receives. Idle cells are necessary to maintain a connection in channelized facilities because the channel bank equipment must always see a constant bit rate transmission, or it disconnects the channel. When nothing is being sent over a channelized facility, idle flags are transmitted (this is also used to maintain clock synchronization between two endpoints). Idle cells are not recognized by the ATM layer.

The functions of the transmission Convergence Sublayer (CS) differ depending on the medium being used. For instance, if SONET is the medium, the physical layer requires a different set of functions than a DS-3 medium would require. This sublayer provides whatever services are needed by each type of medium.

There are some specific functions required for DS3 and 100-Mbps interfaces. The ATM physical layer provides a convergence protocol (Physical Layer Convergence Protocol, PLCP), which maps ATM cells onto a DS3. The interface supports 44.736 Mbps. ATM cells are mapped into a DS3 PLCP data unit, which is then mapped into the DS3 payload. The DS3 PLCP is not aligned to the DS3 framing bits.

The 100-Mbps access was intended for private UNIs. Private UNIs are not as complex as public UNIs, which must provide higher reliability and complex monitoring. The specification is based on the FDDI physical layer.

I mentioned earlier that two 155-Mbps interfaces were defined. One is for the public UNI, while the other is for the private UNI. The difference lies in the distances supported by each interface. The 155-Mbps private UNI interface can be used over fiber optics or twisted-pair copper. The public UNI requires fiber optics using single-mode fiber.

NEXT PAGE: ATM Layers (con't)

Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.