2 cissp ® Official Study Guide Eighth Edition

Synchronous Digital Hierarchy and Synchronous Optical Network
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) are 
fiber-optic high-speed networking standards. SDH was standardized by the International 
Telecommunications Union (ITU) and SONET by the American National Standards 

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Chapter 12 
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Secure Communications and Network Attacks
Institute (ANSI). SDH and SONET are mostly hardware or physical layer standards defin-
ing infrastructure and line speed requirements. SDH and SONET use synchronous time-
division multiplexing (TDM) to high-speed duplex communications with minimal need for 
control and management overhead.
These two standards have only slight variations and use the same hierarchy of band-
width levels. The transmission service supports a foundational level of speed of 51.48 Mbps, 
which supports the Synchronous Transport Signals (STS) of SDH and/or the Synchronous 
Transport Modules (STM) of SONET. The term Optical Carrier (OC) can also be substi-
tuted for STS. The main bandwidth levels of SDH and SONET are shown in Table 12.4.
TA b l e 12 . 4
Bandwidth levels of SDH and SONET
SONET
SDH Data
Rate
STS-1 / OC-1
STM-0
51.84 Mbps
STS-3 / OC-3
STM-1
155.52 Mbps
STS-12 / OC-12
STM-4
622.08 Mbps
STS-48 / OC-48
STM-16
2.488 Gbps
STS-96 / OC-96
STM-32
4.876 Gbps
STS-192 / OC-192
STM-64
9.953 Gbps
STS-768 / OC-768
STM-256
39.813 Gbps
SDH and SONET both support mesh and ring topologies. These fiber solutions are often 
implemented as the backbone of a telco service and divisions or fractions of the capacity are 
subscribed out to customers. The interconnection points or nodes of SDH and SONET
are often Add-Drop Multiplexers (ADMs), which allow for the addition or removal of low-
rate bit stream connections or products into the main trunk line.