Tektronix CTS 850, User Manual

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Functional Overview
2–30
CTS850 SDH/PDH Test Set User Manual
The figure on the next page illustrates the ITU–T SDH multiplexing
structure. The notations in the boxes, such as C–1, VC–3, and AU–4,
are explained in the table after the figure. Not all containers and rates
shown in this figure are specifically handled by the CTS 850 test
unit.
At the lowest level, containers (C) are input to virtual containers
(VC). The purpose of this function is to create a uniform VC payload
by using bit–stuffing to bring all inputs to a common bit–rate ready
for synchronous multiplexing. Various containers (ranging from
VC–11 at 1.728 Mb/s to VC–4 at 139.264 Mb/s) are covered by the
SDH hierarchy. Next, VCs are aligned into tributary units (TUs),
where pointer processing operations are implemented.
These initial functions allow the payload to be multiplexed into TU
groups (TUGs). As the figure illustrates, the xN label indicates the
multiplexing integer used to multiplex the TUs to the TUGs. The
next step is the multiplexing of the TUGs to higher level VCs, and
TUG–2 and TUG–3 are multiplexed into VC–3 (ANSI mappings)
and VC–4. These VCs are multiplexed with fixed byte–stuffing to
form administration units (AUs) which are finally multiplexed into
the AU group (AUG). This payload then is multiplexed into the
Synchronous Transport Module (STM).