3Com CoreBuilder 7000, Operation Manual

Page: 225 / 264

Physical Layer 225
Cell Scrambling: Provides a solution to bit patterns in the ATM cell
payload (much like in the SONET). Cell scrambling can be enabled or
disabled.
PLCP Framing and Cell Delineation:
Mapping of ATM cells into the
DS-3 is accomplished by inserting the 53 bytes of ATM cells into the
DS-3 PLCP. The PLCP consists of a 125µs frame within a standard DS-3
payload. Once the ATM cells are inserted into the PLCP, it is mapped
into the DS-3 payload information. Extraction of ATM cells from the
DS-3 is accomplished by framing on the PLCP and then extracting the
ATM cells directly.
Path Overhead Utilization: The following PLCP overhead bytes/nibbles
are required to support the coding and functions across the UNI:
Frame alignment, bit interleaved parity, cycle/stuff counter, PLCP path
status, path overhead identifier, growth octets and trailer nibbles.
PLCP Timing: The PLCP frame from the network equipment to the
customer has a timing function available. The customer can use the
clock recovered from the PLCP.
Nibble (4 bits) Stuffing: Although the DS-3 PLCP is not aligned to the
DS-3 framing bits, the ATM cell octets in the PLCP frame are nibble
aligned to the DS-3 payload envelope. Nibble stuffing is required after
the twelfth cell to align the PLCP frame.
Other physical layer interfaces supported by ATM include 100 Mbps
4B/5B and 155 Mbps 8B/10B.