Page 101 - Optical Switching And Networking Handbook

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86 Chapter 4

Using the same fiber, a synchronous network is able to increase
the available bandwidth while reducing the amount of equipment in
the network. Moreover, the provisioning of SDH for sophisticated
network management introduces much more flexibility into the
overall networking strategies for the carriers.

Synchronous Digital Hierarchy

As synchronous equipment was rolled into the network, the full ben-
efits became more apparent. The carriers experienced significant
cost reductions and avoidances, less hardware, and increased effi-
ciencies in multiplexing of the various rates established by the ITU.
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However, other benefits required less spares to be maintained in the
network. Additional benefits were had by the use of the SDH stan-
dard multiplexing formats, which could encapsulate the PDH multi-
plexed signals inside the SDH transport. This protects the carriers’
investments and prevents the use of forklift technology. In fact, SDH
offered the network operators the ability to future-proof their net-
works, allowing them to offer metropolitan area network (MAN),
wide-area network (WAN), and B-ISDN services on a single platform.
The SDH forms a multiplexing rate based on the STM-n frame
format. STM stands for synchronous transmission module. The
STM-n general frame format works as follows: Similar to the
SONET OC-1 (albeit larger), the basic STM-1 frame consists of

270 columns 9 rows 2430 octets

9 columns 9 rows 81 octets section overhead

The remaining 2349 octets create the payload. Higher-rate frames
are derived from multiples of STM-1 according to the value of n. The
standard STM-1 frame is shown in Figure 4-1.This is similar but dif-
ferent from the frame in an OC-3 stream.

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