3Com CoreBuilder 7000, Operation Manual

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B
P
ROTOCOLS AND
I
NTERFACES
This appendix provides you with basic concepts and information about
interfaces and management protocols used in the ATM system including
the following topics:
Physical Layer
ATM Layer
Operation & Maintenance (OAM)
ATM Adaptation Layer (AAL)
Physical Layer
Traditionally, the physical layer deals with bits and its function is to move
a bit from place to place. The physical layer in ATM works with cells,
much in the same way that the traditional physical layer deals with bits,
defining physical media characteristics, line coding and framing. The
physical layer transmits and receives ATM cell streams over a physical
medium. It also checks the integrity of the bits being transferred over a
physical medium, and makes sure that they are error-free.
SONET STS-3c
Physical Layer
Interface
Synchronous Optical Network (SONET) is the physical layer most often
associated with ATM. Its rate is 155.52 Mbps which enables a payload
rate of 149.760 Mbps. SONET provides, through a framing structure, the
payload envelope necessary for the transport of ATM cells. The functions
of the SONET physical layer include:
HEC Generation: The entire header, including the HEC byte, is
protected by the Header Error Control (HEC) sequence. The HEC
sequence is capable of single-bit error correction and multiple-bit error
detection. When the receiver side is set to correction mode, only
single-bit errors can be corrected, while in detection mode,
multiple-bit errors can be detected. All cells with detected errors in the
header are discarded.