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102 Chapter 4

The Model as It Pertains to SDH

Everything we do usually ties back to a reference to the interna-
tional standards. For this we use the Open Systems Interconnect
(OSI) as a reference model. In general, SDH operates at the bottom
layer (layer 1, physical) of the OSI. This physical layer is subdivided
into three separate components.When we deal with this mapping on
the OSI, we usually refer to four sublayer components.
Similar to the SONET architecture, SDH networks are divided
into various layers that are directly related to the network topology.
The lowest layer is the physical transmission section, the medium in
the form of glass (fiber), but it also can represent radio systems such
as microwave or satellite. Actually, the photonic layer is below the
physical layer, where we turn the pulses into light (photons) and
propagate them on the fibers. In many cases, the photonic and the
physical layers are combined. The regenerator section is a path
between repeaters. This is also at the physical layer because it
involves that length of fiber between two repeaters. The overhead is
called the repeater (or regenerator) section overhead (RSOH). Sig-
naling across the medium also occurs at this level. Following the
repeater portion, we have the multiplexer overhead (MSOH) used by
the multiplexers for the necessary overhead to track operation of the
circuits and OAM&P. The multiplex section deals with the part of
the SDH link between two multiplexers where we map and multi-
plex our services (DS-1) into the SDH transport. Sitting on top of the
MSOH are two VC layers.These two layers are a part of the mapping
process whereby the individual tributary signals are placed inside
the SDH payload. VC4 is the mapping for the 139-Mbps payload,
whereas VC12 contains the individual E-1 signals mapped into the
SDH.
An overall picture of this (the SDH layer model) is shown in Fig-
ure 4-13. The lowest layer is the physical transmission section, the
medium in the form of glass (fiber), but it also can represent radio
systems such as microwave or satellite. In the figure, the photonic
layer is shown as a subset to the physical interface. Note that the
ATM, POTS, and Internet Protocol (IP) networks are also shown in
this figure in as much as they fit into the overall scheme of the
model.