The document provides an overview of SONET (Synchronous Optical Network), an ANSI standard for synchronous network technology that utilizes a single clock for timing and allows for improved speed and cost efficiency through multiplexing. It details the SONET frame structure, the devices involved such as STS multiplexers and regenerators, and the use of virtual tributaries for compatibility with existing digital hierarchies. Additionally, it discusses the hierarchical levels of SONET signaling, data rates, and how lower-rate signals can be multiplexed into higher-rate services.

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SONET

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TopicsTopics
SONET Devices
SONET Frame
Frame Transmission
Synchronous Transport Signals
STS-1
Virtual Tributaries
Higher-Rate Service

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SONET
• SONET – Synchronous Optical Network
• SONET is ANSI standard
• The IUT-T (International Telecommunication Union - Telecommunication)
standard is SDH (Synchronous Digital Hierarchy)
• These two standards are nearly identical.
• Addresses three concerns are:
• First, SONET is a synchronous network. A single clock is used to handle the
timing of transmissions and equipment across the network. Network-wide
synchronization adds a level of predictability to the system. This
predictability, coupled with a powerful frame design, enables individual
channels to be multiplexed, thereby improving speed and reducing costs.
• Second, SONET contains recommendations for the standardization of fiber-
optic transmission system (FOTS) equipment sold by different
manufacturers.
• Third, The SONET physical specifications and frame design include
mechanisms that allow it to carry signals from incompatible tributary systems.
It is this flexibility that gives SONET a reputation for universal connectivity.
• SONET is a good example of a TDM system.

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Figure 9.10 A SONET
• SONET transmission relies on three basic devices:
• Synchronous Transport Signal (STS) multiplexers
• Regenerators, and
• Add/Drop Multiplexers
SONET is a synchronous TDM system
controlled by a master clock.

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SONET Devices
• STS Multiplexers/Demultiplexers
• It either multiplexes signals from multiple sources to an STS or
demultiplexes an STS into different destination signals.
• Regenerators:
• It is a repeater.
• It takes a received optical signal and regenerates it.
• It adds a function to those physical layer repeaters.
• It replaces some of the existing overhead information (header
information) with new information.
• These devices function at the data link layer.
• Add/Drop Multiplexers:
• It can add signals coming from different sources into a given path or
• remove a desired signal from a path and
• redirect it without demultiplexing the entire signal.

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Figure 9.11 Frame Format
• A SONET frame is a matrix of 90 octets each, for a total of 810 octets.
• Some of them are used for control: they are not positioned at the beginning
or end of the frame (like a header or trailer).
• The first 3 columns are used for administration overhead.
• The rest is called SPE (Synchronous Payload Envelope)
• The SPE contains transmission overhead and user data.
• The payload does not have to start at row 1, column 4.
• It can start anywhere in the frame and can even span two frames.

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Frame Transmission and STS
• Frame Transmission
• SONET frames are transmitted one after another without any gap in
between, even if there are no real data.
• Empty frames carry dummy data.
• A sequence of frames looks like a sequence of bits.
• The first 2 bytes of each frame is called the alignment bytes define the
beginning of each frame.
• The 3rd
byte is the frame identification
• STS
• STS is Synchronous Transport Signal
• It defines a hierarchy of signaling levels.
• There are 9 levels and they are STS-1, STS-3, STS-9, STS-12, STS-18,
STS-24, STS-36, STS-48 and STS-192 .
• Each STS level supports a certain data rate, specified in Mbps.
• The physical links defined to carry each level of STS are called OCs
(Optical Carriers).
• OC levels describe the conceptual and physical specifications of the links
to support each level of signaling.

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Figure 9.12 Data rate

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Table 9.1 SONET ratesTable 9.1 SONET rates
STS OC Rate (Mbps) SPE (Mbps) User (Mbps)
STS-1STS-1 OC-1 51.84 50.12 49.536
STS-3STS-3 OC-3 155.52 150.336 148.608
STS-9STS-9 OC-9 466.56 451.008 445.824
STS-12STS-12 OC-12 622.08 601.344 594.432
STS-18STS-18 OC-18 933.12 902.016 891.648
STS-24STS-24 OC-24 1244.16 1202.688 1188.864
STS-36STS-36 OC-36 1866.23 1804.032 1783.296
STS-48STS-48 OC-48 2488.32 2405.376 2377.728
STS-192STS-192 OC-192 9953.28 9621.604 9510.912

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Figure 9.13 VT (Virtual Tributaries) Types
Virtual Tributaries:
• SONET is designed to carry broadband payloads.
• Current digital hierarchy data rates (DS-1 to DS-3) are lower than STS-
1.
• To make SONET backward-compatible with the current hierarchy, its
frame design includes a system of virtual tributaries (VTs).
• VT is a partial payload that can be inserted into a frame and combined
with other partial payloads to fill out the frame.
• Instead of using all 87 payload columns of an SPE frame for data from
one source, we can subdivide the SPE and call each component a VT.

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VT (Virtual Tributaries)
Virtual Tributaries
• SONET is designed to carry broadband payloads.
• Current digital hierarchy data rates (DS-1 to DS-3) are lower than STS-1.
• To make SONET backward-compatible with the current hierarchy, its frame
design includes a system of virtual tributaries (VTs).
• VT is a partial payload that can be inserted into a frame and combined with
other partial payloads to fill out the frame.
• Instead of using all 87 payload columns of an SPE frame for data from one
source, we can subdivide the SPE and call each component a VT.
VT Types
• Four types of VTs have been defined.
• VT1.5: The VT1.5 accommodates the US. DS-1 service (1.544 Mbps)
• VT2: The VT2 accommodates the European. CEPT-1 service (2.048 Mbps)
• VT3: The VT3 accommodates the DS-1C service (3.152 Mbps)
• VT6: The VT6 accommodates the DS-2 service (6.312 Mbps)

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Figure 9.14 STS multiplexing
Higher-Rate Services:
• Lower-rate STSs can be multiplexed to make them compatible with
higher-rate systems.
• For example, STS-1 can be combined into one STS-3, four STS-3’s can
multiplexed into one STS-12, and so on.
• Figure shows that three STS-1’s are multiplexed into a single STS-3.
• To create an STS-12 out of lower -rate services, we could multiplex
either 12 STS-1’s or 4 STS-3’s.

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Quiz Contents
•SONET
•Addresses of the SONET Standard
•SONET Devices
•SONET Frame Format
•Frame Transmission and STS
•Data Rate
•SONET Rates
•VT and VT Types
•STS multiplexing