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Physical Layer Interfaces 437
DS-3 Physical Layer
Interface
DS-3 is an important interface because it is widely deployed on public
UNIs. Although the DS-3 rate is 44.76 Mbps, the nominal bit rate that is
available for the transport of ATM cells is 40.704 Mbps. This is due to the
overhead induced by the Physical Layer Convergence Protocol (PLCP). The
PLCP for DS-3 defines the mapping of ATM cells onto existing DS-3
facilities. 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 is then mapped
into the DS-3 information payload. Extraction of ATM cells from the DS-3
operates by framing on the PLCP and then extracting the ATM cells
directly.
The functions of the DS-3 physical layer include:
HEC Generation and Verification: The Header Error Control (HEC)
byte protects the entire cell header. It is capable of one bit error
detection based on the HEC field.
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. After 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.