Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without synchronization problems.

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National University of Modern Languages
Assignment # 02
Name: Hassaan Ashraf
Roll # 11050
Subject: CCN
Submitted to: Dr. Rizwan

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Table of Contents
Introduction..................................................................................................................................... 4
SONET (Synchronous Optical Network): The standard for fiber optic transport.......................... 5
SONET Heirarchy and Multiplexing.............................................................................................. 5
STS-1 Frame Structure ................................................................................................................... 7
Virtual Tributaries....................................................................................................................... 8
Pointers ....................................................................................................................................... 9
Concatenated Payloads ............................................................................................................. 10
SONET Network Elements........................................................................................................... 11
Why and How Synchronous (vs.Asynchronous or Plesiochronous ) Networks?......................... 11
Synchronous Digital Hierarchy (SDH) -- International super standard........................................ 13
SDH frame ................................................................................................................................ 13
Reference ...................................................................................................................................... 15

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List of Figures
Figure 1: SONET Optical Line Rates ............................................................................................. 5
Figure 2: ISD Channels................................................................................................................... 6
Figure 3: ISDN NW Scenario......................................................................................................... 6
Figure 4: STS-1 Bit Rate................................................................................................................. 7
Figure 5: SONET Transport Overhead........................................................................................... 7
Figure 6: DS-1 Visibility ................................................................................................................ 8
Figure 7: Pointer ............................................................................................................................. 9
Figure 8 : Concatenated STS-1s ................................................................................................... 10
Figure 9: SONET NW Elements................................................................................................... 11
Figure 10 : Digital NW Synchronization Hierarchy..................................................................... 12
Figure 11: Syncronizing a SONET Network................................................................................ 12
Figure 12: SONET/SDH Hierarchies............................................................................................ 13

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SONET/SDH
Introduction
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are
standardized protocols that transfer multiple digital bit streams synchronously over optical fiber
using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission
rates data can also be transferred via an electrical interface. The method was developed to
replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of
telephone calls and data traffic over the same fiber without synchronization problems.
SONET and SDH, which are essentially the same, were originally designed to transport circuit
mode communications (e.g., DS1, DS3) from a variety of different sources, but they were
primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM
format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization
sources of these various circuits were different. This meant that each circuit was actually
operating at a slightly different rate and with different phase. SONET/SDH allowed for the
simultaneous transport of many different circuits of differing origin within a single framing
protocol. SONET/SDH is not a communications protocol in itself, but a transport protocol.
Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH
was the obvious choice for transporting the fixed length Asynchronous Transfer Mode (ATM)
frames also known as cells. It quickly evolved mapping structures and concatenated payload
containers to transport ATM connections. In other words, for ATM (and eventually other
protocols such as Ethernet), the internal complex structure previously used to transport circuit-
oriented connections was removed and replaced with a large and concatenated frame (such as
STS-3c) into which ATM cells, IP packets, or Ethernet frames are placed.
Both SDH and SONET are widely used today: SONET in the United States and Canada, and
SDH in the rest of the world. Although the SONET standards were developed before SDH, it is
considered a variation of SDH because of SDH's greater worldwide market penetration. SONET
is subdivided into four sublayer with some factor such as the path, line, section and physical
layer.
The SDH standard was originally defined by the European Telecommunications Standards
Institute (ETSI), and is formalised as International Telecommunication Union (ITU) standards
G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American
National Standards Institute (ANSI) standard T1.105. which define the set of transmission
formats and transmission rates in the range above 51.840 Mbit/s.

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SONET (Synchronous Optical Network): The standard for fiber optic
transport
In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous
transport signals (STSs) for the fiber-optic based transmission hierarchy. The standard SONET
line rates and STS-equivalent formats are shown
below.
Figure 1: SONET Optical Line Rates
SONET Heirarchy and Multiplexing
The next figure illustrates the basic multiplexing structure of SONET. Any type of service,
ranging from voice to high-speed data and video, can be accepted by various types of service
adapters. A service adapter maps the signal into the payload envelope of the STS-1 or virtual
tributary (VT). New services and signals can be transported by adding new service adapters at
the edge of the SONET network. All inputs are eventually converted to a base format of a
synchronous STS-1 signal (51.84 Mbps or higher). Lower speed inputs such as DS-1s are first
bit- or byte-multiplexed into virtual tributaries. Several synchronous STS-1s are then multiplexed
together in either a single- or two-stage process to form an electrical STS-n signal (n = one or
more).
o mapping—used when tributaries are adapted into VTs by adding justification bits
and POH information.
o aligning—takes place when a pointer is included in the STS path or VT POH, to
allow the first byte of the VT to be located.

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o multiplexing—used when multiple lower order path-layer signals are adapted into
a higher-order path signal, or when the higher-order path signals are adapted into
the line overhead.
o stuffing—SONET has the ability to handle various input tributary rates from
asynchronous signals; as the tributary signals are multiplexed and aligned, some
spare capacity has been designed into the SONET frame to provide enough space
for all these various tributary rates; therefore, at certain points in the multiplexing
hierarchy, this space capacity is filled with fixed stuffing bits that carry no
information but are required to fill up the particular frame
Figure 2: ISD Channels
o
Figure 3: ISDN NW Scenario

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STS-1 Frame Structure
STS-1 is a specific sequence of 810 bytes (6,480 bits), which includes various overhead bytes
and an envelope capacity for transporting payloads. It can be depicted as a 90-column by 9-row
structure. With a total frame length of 125 µs (8,000 frames per second), STS-1 has a bit rate of
51.840 Mbps -- viz.
(9) x (90 bytes/frame) x (8 bits/byte) x (8,000 frames/s) = 51,840,000 bps = 51.840 Mbps
The order of transmission of bytes is row-by-row from top to bottom and from left to right (most
significant bit first). As shown in the figure below, the first three columns of the STS-1 frame
are for the transport overhead. The three columns contain 9 bytes. Of these, 9 bytes are overhead
for the section layer (for example, each section overhead), and 18 bytes are overhead for the line
layer (for example, line overhead). The remaining 87 columns constitute the STS-1 envelope
capacity (payload and POH).
Figure 4: STS-1 Bit Rate
Figure 5: SONET Transport Overhead

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Virtual Tributaries
In addition to the STS-1 base format, SONET also defines synchronous formats at sub-STS-1
levels. The STS-1 payload may be subdivided into virtual tributaries, which are synchronous
signals used to transport lower-speed transmissions. There are three sizes of VTs:
Figure 6: DS-1 Visibility

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Pointers
SONET uses a concept called “pointers” to compensate for frequency and phase variations.
Pointers allow the transparent transport of synchronous payload envelopes (either STS or VT)
across plesiochronous boundaries, i.e., between nodes with separate network clocks having
almost the same timing. The use of pointers avoids the delays and loss of data associated with the
use of large (125-microsecond frame) slip buffers for synchronization.
Figure 7: Pointer

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Concatenated Payloads
For future services, the STS-1 may not have enough capacity to carry some services. SONET
offers the flexibility of concatenating STS-1s to provide the necessary bandwidth. The next
illustrates SONET flexibility by concatenating three STS-1s to 155.52 Mbps to provide the
capacity to transport an H4 digital-video channel. STS-1s can be concatenated up to STS-3c.
Beyond STS-3, concatenation is done in multiples of STS-3c. For ATM services, concatenation
is done in multiples of STS-12c.
Figure 8 : Concatenated STS-1s

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SONET Network Elements
Figure 9: SONET NW Elements
Why and How Synchronous (vs.Asynchronous or Plesiochronous ) Networks?
A Simple Asynchronous Signal -- terminal serial communication.
A Basic Synchronous (or Plesiochronous) Signal -- High-Level Data Link Control (HDLC)
frame structure.
 Asynchronous -- simple, cheap and inefficient (slow)

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o Synchronous -- complex, expensive and efficient (fast)
Figure 10 : Digital NW Synchronization Hierarchy
Figure 11: Synchronizing a SONET Network

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Synchronous Digital Hierarchy (SDH) -- International super standard
Figure 12: SONET/SDH Hierarchies
SDH frame
An STM-1 frame. The first nine columns contain the overhead and the pointers. For the sake of
simplicity, the frame is shown as a rectangular structure of 270 columns and nine rows but the
protocol does not transmit the bytes in this order.
For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and nine
rows. The first three rows and nine columns contain regenerator section overhead (RSOH) and
the last five rows and nine columns contain multiplex section overhead (MSOH). The fourth row
from the top contains pointers.
The Synchronous Transport Module, level 1 (STM-1) frame is the basic transmission format for
SDH—the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in
exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic
circuit. The STM-1 frame consists of overhead and pointers plus information payload. The first
nine columns of each frame make up the section overhead and administrative unit pointers, and
the last 261 columns make up the information payload. The pointers (H1, H2, H3 bytes) identify
administrative units (AU) within the information payload. Thus, an OC-3 circuit can carry
150.336 Mbit/s of payload, after accounting for the overhead.
Carried within the information payload, which has its own frame structure of nine rows and 261
columns, are administrative units identified by pointers. Also within the administrative unit are
one or more virtual containers (VCs). VCs contain path overhead and VC payload. The first
column is for path overhead; it is followed by the payload container, which can itself carry other

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containers. Administrative units can have any phase alignment within the STM frame, and this
alignment is indicated by the pointer in row four.
The section overhead (SOH) of a STM-1 signal is divided into two parts: the regenerator section
overhead (RSOH) and the multiplex section overhead (MSOH). The overheads contain
information from the transmission system itself, which is used for a wide range of management
functions, such as monitoring transmission quality, detecting failures, managing alarms, data
communication channels, service channels, etc.
The STM frame is continuous and is transmitted in a serial fashion: byte-by-byte, row-by-row.

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Reference
https://en.wikipedia.org/wiki/Synchronous_optical_networking
http://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture12/sonet/sonet.html