3Com CoreBuilder 9000, Руководство пользователя

Страница: 435 / 468

C
P
ROTOCOLS AND
I
NTERFACES
This appendix describes basic concepts and information about interfaces
and management protocols that are used in the ATM system including
the following topics:
Physical Layer Interfaces
ATM Layer
Operation and Maintenance (OAM)
ATM Adaptation Layer (AAL)
Physical Layer
Interfaces
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 as the traditional physical layer deals with bits, defining
physical media characteristics, line coding, and framing. The physical layer
is responsible for transmitting and receiving ATM cell streams over a
physical medium. It is also responsible for checking the integrity of the
bits that are being transferred over a physical medium, and making 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.