IBM ATM OC-3c - Manual (Page 27)

Physical Layer 2-15
Physical Layer
The physical layer is responsible for transmitting and
receiving ATM cells over a physical medium. It is also
responsible for checking the integrity of the bits being
transferred over a physical media, and for making
sure that they are error-free.
The ATM Module is compliant with both SONET STS-3c
and SDH STM-1 physical layer standards.
These standards are similar, and most devices allow
you to use either framing standard on each link in the
ATM network.The same framing standard must be
used at each end of the link.
Many users prefer to use the same framing standard
throughout their network (for example SONET STS-3c).
The physical layer is sub-divided into:
■ Path — SONET and SDH are capable of carrying
traffic for a number of uppers layers, and ATM is
only one of those layers. Each upper layer uses its
own Path through the SONET/SDH layer.
■ Line — A line is the whole path between one ATM
device and the adjacent ATM switch or ATM
end-station.
■ Section — When ATM is used for telephone
networks, a line may cover a large distance,
requiring optical repeaters to boost the signal
along its way. The part of a line between an optical
repeater and the adjacent repeater or switch is
known as a section.
SONET STS-3c
Synchronous Optical Network (SONET) is the physical
layer most often associated with ATM. SONET provides,
through a framing structure, the mechanism for the
transport of ATM cells. Data can be transferred at
155.52Mbps.
SDH STM-1
SDH STM-1 is a physical layer similar to the SONET layer,
but with some differences in frame fields. SDH STM-1 is
the physical layer commonly used in Europe.
The physical layer and ATM layers in the network
provide simple performance monitoring functions
between ATM devices; providing basic information
about the health of the link. These functions are
known as “Operation and Maintenance (OAM)”
functions.