Synchronous Optical Network (SONET) and PoS
by Frank Zhiyi Lin
As a typical representation of first-generation optical networks, SONET are widely
and used today in voice traffic and IP based network. This paper is intended to
provide an introduction
and overview of technologies used in SONET, as long as the recent development of
(Packet over SONET).
SONET is current transmission and multiplexing standard for high-sped signals
within the carrier
infrastructure in North American specified in ANSI T1.105, ANSI T1.106 and ANSI
T1.117. A similar
standard SDH (Synchronous Digital Hierarchy) is adopted in Europe and Japan.
SONET and SDH
are technically consistent standards. SONET standard defines the rates and
formats, the physical layer,
network element architectural features, and network operational criteria.
SONET, ESCON (Enterprise Serial Connection, developed by IBM), Fiber Channel,
(high-performance parallel interface) are typical first-generation optical network,
which use fiber as a
transmission medium, but do all switching, processing, and routing electronically. All
of them are single-
wavelength networks as well. SONET now is considered as “Old World” standard
because it is a time
division-multiplexing (TDM) technology optimized for voice traffic. The recent
development in optical
network, such as DWDM, has been concentrated on the optical routing rather than
the traditional electronic
Prior to SONET/SDH, the existing infrastructure was based on the plesiochronous
digital hierarch (PDH).
At the time of PDH, the primary focus of optical network was on multiplexing digital
voice circuits. PDH
suffered from several problems, such as significantly complicated
multiplexer/demultiplexer, difficulty in
management and monitoring the traffic, incompatibility between different vendor’s
equipment due to lack
of standard format on the transmission link, which move the carriers and vendors in
seeking a new transmission
and multiplexing standard in the late 1980. The task of creating such a standard was
taken up in 1984 by the
Exchange Carriers Standards Association (ECSA) to establish a standard for
connecting one fiber system to
SONET network layer and frame structure
Traditionally, transmission systems have been asynchronous, with each terminal in
the network running on its
own clock. In digital transmission, clocking is one of the most important
considerations. Clocking means using
a series of repetitive pulses to keep the bit rate of data constant and to indicate
where the ones and zeroes are
located in a data stream.
Because these clocks are totally free-running and not synchronized, large variations
occur in the clock rate and
thus the signal bit rate. For example, a DS–3 signal specified at 44.736 Mbps + 20
parts per million (ppm) can
produce a variation of up to 1,789 bps between one incoming DS–3 and another.
Asynchronous multiplexing uses multiple stages. Signals such as asynchronous
DS–1s are multiplexed, and extra
bits are added (bit-stuffing) to account for the variations of each individual stream
and combined with other bits
(framing bits) to form a DS–2 stream. Bit-stuffing is used again to multiplex up to
DS–3. DS–3s are multiplexed
up to higher rates in the same manner. At the higher asynchronous rate, they cannot
be accessed without
In a synchronous system such as SONET, the average frequency of all clocks in the
system will be the same
(synchronous) or nearly the same (plesiochronous). Every clock can be traced back
to a highly stable reference
supply. Thus, the STS–1 rate remains at a nominal 51.84 Mbps, allowing many
synchronous STS–1 signals to
be stacked together when multiplexed without any bit-stuffing. Thus, the STS–1s are
easily accessed at a higher
Low-speed synchronous virtual tributary (VT) signals are also simple to interleave
and transport at higher rates.
At low speeds, DS–1s are transported by synchronous VT–1.5 signals at a constant
rate of 1.728 Mbps. Single-
step multiplexing up to STS–1 requires no bit stuffing, and VTs are easily accessed.
Pointers accommodate differences in the reference source frequencies and phase
wander and prevent frequency
differences during synchronization failures.
How SONET works
Digital switches and digital cross-connect systems are commonly employed in the
digital network synchronization
hierarchy. The network is organized with a master-slave relationship with clocks of
the higher-level nodes feeding
timing signals to clocks of the lower-level nodes. All nodes can be traced up to a
primary reference source, a
Stratum 1 atomic clock with extremely high stability and accuracy. Less stable clocks
are adequate to support the
The internal clock of a SONET terminal may derive its timing signal from a building
integrated timing supply (BITS)
used by switching systems and other equipment. Thus, this terminal will serve as a
master for other SONET nodes,
providing timing on its outgoing OC–N signal. Other SONET nodes will operate in a
slave mode called loop timing
with their internal clocks timed by the incoming OC–N signal. Current standards
specify that a SONET network must
be able to derive its timing from a Stratum 3 or higher clock.
Before we discuss the functionality of the SONET, it is necessary to have an
overview of the SONET system layer,
corresponding to OSI network architecture.
SONET is a four-layer system hierarchy as shown in Figure 1.
PHOTONIC: SONET physical layer, describes optical network specifications
SECTION: Creates SONET frames, converts electronic signals to optical ones
LINE: Synchronizes, multiplexes and switches data into SONET frames
PATH: End-to-end transport of user data at the appropriate signaling speed
Figure Error! Bookmark not defined.1: SONET Network Layer
The functions of each layer is described in the following Table 1:
Photonic Conversion between STS signal and OC signals
Section Framing, Scrambling, Error Monitoring, Section Maintenance
Line Synchronization, Multiplexing, Error monitoring, Line Maintenance,
Path Map the signals into a format required by the line layer. It also reads,
interprets, and modifies the path overhead for performance monitoring
and automatic protection switching.
Before we discuss the SONET frame and its overheads, we listed the factors, which
determines the usage of the
overhead bytes and the ways the input signals are mapped into the Synchronous
Payload Envelope (SPE).
All SONET network elements are integrated into a synchronization hierarchy.
There is no need to send
preamble for clock synchronization.
Similar to digital signals, framing bits are required to indicate the beginning of
STS-N frame is sent every 125 sec whether there is data to be sent or not.
Since data arrives asynchronously, data may start any where in the SPE.
Pointer is used to indicate the
starting address of data.
The input data and the output data may have a different clock rate.
Positive/negative stuffing is used for
SONET functions map closely into the physical layer of the OSI seven-layer
stack. Error checking is not
required in this layer. However, error checking is done in SONET for
equipment monitoring and automatic
SONET integrates OAM&P in the network; overhead channels are
established for administrative functions
SONET has a fix size SPE. In order to accommodate different signal rates, bit
stuffing is needed to map
various signal into the SPE.
Same as the common digital carriers, SONET adopts the frame length of 125 sec
or frame rate of 8000 frames
per second. The STS-1 (Synchronous Transport Signal level 1) is the basic signal of
SONET. Each STS-1 frame
can be viewed as a 9-row by 90-column structure, a total of 810 bytes. The following
Figure 2 shows the STS-1
The first 3 columns are the transport overhead. Nine of the 27 (3 * 9) bytes of the
transport overhead is used for
section overhead. 18 of the 27 (3 * 9) bytes of the transport overhead is used for line
overhead. From column 4
to column 90 are the Synchronous Payload Envelope (SPE). The byte transmission
order is row-by-row, left to
right. At a rate of 8000 frames per second, that works out to a rate of 51.840 Mb/s,
as the following equation
9 × 90 bytes/frame × 8 bits/byte × 8000 frames/s = 51,840,000 bits/s = 51.840
Figure 3 depicts the STS-1 SPE, which occupies the STS-1 Envelope Capacity. The
STS-1 SPE consists of 783
bytes, and can be depicted as an 87 column by 9-row structure. Column 1 contains
nine bytes, designated as the
STS Path Overhead (POH). Two columns (columns 30 and 59) are not used for
payload, but are designated as the
"fixed stuff" columns. The 756 bytes in the remaining 84 columns are designated as
the STS-1 Payload Capacity.
Figure Error! Bookmark not defined.3: STS1 Payload Capacity
An STS-N is a specific sequence of N × 810 bytes. The STS-N is formed by byte-
interleaving STS-1 modules
(see Figure 4). The Transport Overhead of the individual STS-1 modules are frame
aligned before interleaving, but
the associated STS SPEs are not required to be aligned because each STS-1 has a
Payload Pointer to indicate
the location of the SPE (or to indicate concatenation).
The STS frame consists of two parts, the STS payload and the STS overhead. The
STS payload carries the
information portion of the signal. The STS overhead carries the signaling and
protocol information. This allows
communication between intelligent nodes on the network, permitting administration,
and control of a network from a central location.
The overhead information has several layers, which are shown in Figure 5. Path-
level overhead is carried from
end-to-end; it's added to DS1 signals when they are mapped into virtual tributaries
and for STS-1 payloads that
travel end-to-end. Line overhead is for the STS-N signal between STS-N
multiplexers. Section overhead is used
for communications between adjacent network elements, such as regenerators.
Figure Error! Bookmark not defined.5: Overhead of SONET frame
The following Table 2 details the different SONET overhead information:
Section Overhead Performance monitoring (STS-N signal), Local orderwire,
Data communication channels to carry information for
Line Overhead Locating the SPE in the frame, Multiplexing or
concatenating signals, Performance monitoring,
Automatic protection switching, and Line maintenance.
STS Path Overhead Performance monitoring of the STS SPE, Signal label (the
content of the STS SPE, including status of mapped
payloads), Path status, Path trace
VT Path Overhead Provides communication between the point of creation of
a VT SPE and its point of disassembly, error checking,
signal label, and path status.
Due to likely differences in the clocks from different system, or due to faults, the
frames from different systems will
not aligned perfectly. 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 (that is, 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. Pointers provide a simple means of dynamically and flexibly phase-
aligning STS and VT payloads,
thereby permitting ease of dropping, inserting, and cross-connecting these payloads
in the network. Transmission
signal wander and jitter can also be readily minimized with pointers.
The secret to making SONET work is the payload pointer. The tributaries coming
into a multiplexer may have been
created with a clock running at a different speed. They are not necessarily aligned
with each other or with the clock
in the multiplexer. To resolve this problem, remember that this is a
SYNCHRONOUS network, the SONET multiplexer
finds the beginning of a frame for each tributary. It then calculates a pointer
designating where in the STS-1 frame it
has placed the beginning of the tributary frame. This way, it is not necessary for the
multiplexer to either get the signals
in sync (impossible) or stuff the frame with bits.
If a tributary signal's clock slips over time with respect to the
multiplexer's clock, the SONET multiplexer simply recalculates
The STS-1 pointer (H1 and H2 bytes) performs two functions:
1. Locate the STS-1 SPE
2. Indicate the need of frequency justification
The STS-1 pointer is divided into three parts:
1. four bits of New Data Flag (NDF)
2. two bits of unassigned bits
3. 10 bits for pointer value, which are then separated into increment (I) bits and
decrement (D) bits. The 10 bit pointer
is required to represent the maximum SPE offset of 782 (9 rows * 87 columns - 1).
The VT pointers provide a method of allowing flexible and dynamic alignment of the
VT SPE within the VT superframe.
The VT pointer (V1 and V2 bytes) can be divided into 3 parts:
1. four bits of New Data Flag (NDF)
2. two bits of VT size indication -- '00' indicates VT-6, '01 indicates VT-3, '10'
indicates VT-2 and '11' indicates VT-1.
3. 10 bits for pointer value, which are then separated into increment (I) bits and
decrement (D) bits. The 10 bit pointer is
a binary number that indicates the offset from V2 to the first byte of the VT SPE.
The multiplexing principles of SONET are:
Mapping - A process used when tributaries are adapted into Virtual Tributaries
(VTs) by adding justification bits and Path Overhead (POH) information.
Aligning - This process takes place when a pointer is included in the STS Path or
VT Path Overhead, to allow the first byte of the Virtual Tributary to be located.
Multiplexing - This process is 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.
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.
One of the benefits of SONET is that it can carry large payloads (above 50 Mb/s).
However, the existing digital hierarchy signals can be accommodated as well, thus
protecting investments in current equipment.
To achieve this capability, the STS Synchronous Payload Envelope can be sub-
divided into smaller components or
structures, known as Virtual Tributaries (VTs), for the purpose of transporting and
switching payloads smaller than the
STS-1 rate. All services below DS3 rate are transported in the VT structure.
Figure 6 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.
Figure Error! Bookmark not defined.6: SONET multiplexing hierarchy
It is worth mention that each byte of the tributary signal, and thus the tributary signal
itself is visible in the frame. This
makes it relatively easy to pull a tributary signal out of the main signal as compared
to de-multiplexing a PDH signal.
For example, an STS-3 signal is formed by byte-interleaving 3 STS-1 frames as
shown in Figure 6. Each byte is
separately visible. The various STS-1's could be carrying different types of traffic
(voice, data...) and can be heading
to different destinations.
STS multiplexing is performed at the Byte Interleave Synchronous Multiplexer.
Basically, the bytes are interleaved together in a format such that the low-speed
signals are visible. No additional signal processing occurs except a direct conversion
from electrical to optical to form an OC-N signal
If two transitions are said to be plesiochronous, their transitions occur at almost the
same rate, with any variation being constrained within tight limits. This is slightly
different with SONET where two transitions will happen at the same rate.
SONET defines different layer in the network level, it use different SONET Network
The following Table 3 shows the typical equipment elements will found in a SONET.
Table 3: SONET elements
Terminal multiplexer Acts as a concentrator of DS1s as well as other tributary
Regenerator Acts as amplifier to enhance the signal level.
Add/Drop Can multiplex various inputs into an OC-N signal. At an
Multiplexer (ADM) add/drop site, only those signals that need to be accessed
are dropped or inserted.
Wideband Digital Accepts various optical carrier rates, accesses the STS-1
Cross-Connects signals, and switches at this level.
Broadband Digital Accesses the STS-1 signals, and switches at this level
Digital Loop Carrier Considered as a concentrator of low-speed services
before they are brought into the local central office for
The following Figure 7 shows a typical SONET:
Packet over SONET
SONET/SDH is a high-speed TDM physical-layer transport technology, inherently
optimized for voice. PoS provides a means for using the speed and excellent
management capabilities of SONET/SDH to optimize data transport.
The current Internet Engineering Task Force (IEFT) PoS specification is RFC 2615
(PPP over SONET), which obsoletes RFC 1619. PoS provides a method for
efficiently carrying data packets in SONET/SDH frames. RFC 1661 (Point-to-Point
Protocol) and RFC 1662 (PPP in HDLC-like framing) are related. High-bandwidth
capacity coupled with efficient link utilization makes PoS largely preferred for
building the core of data networks. PoS overhead, which averages about 3 percent,
is significantly lower than the 15 percent average for the asynchronous transfer
mode (ATM) cell tax. Figure 7 illustrates the PoS hierarchy.
Figure Error! Bookmark not defined.7: PPP over SONET
PoS uses PPP in High-Level Data Link Control (HDLC)-like framing (as specified in
RFC 1662) for data encapsulation at Layer 2 (data link) of the Open System
Interconnection (OSI) stack. This method provides efficient packet delineation and
error control. The frame format for PPP in HDLC-like framing is shown in Figure 3.
Figure 8: PPP in HDLC-Like Framing
RFC 2615 specifies the use of PPP encapsulation over SONET/SDH links. PPP was
designed for use on point-to-point links and is suitable for SONET/SDH links, which
are provisioned as point-to-point circuits even in ring topologies. PoS specifies STS-
3c/STM-1 (155 Mbps) as the basic data rate, and it has a usable data bandwidth of
149.760 Mbps. PoS frames are mapped into SONET/SDH frames and they sit in the
payload envelop as octet streams aligned on octet boundaries. Figure 9 shows the
framing process. RFC 2615 recommends payload scrambling and a safeguard
against bit sequences, which may disrupt timing.
Figure 9: PoS Framing Sequence
PoS interfaces are usually connected over carrier SONET/SDH networks, where
timing is synchronized to a reference Layer 1 clock running at 0.000001 parts per
minute (ppm). The PoS interfaces derive timing information from the incoming data
stream. This information may be distributed and synchronized throughout the rest of
the whole network. A PoS interface retrieving timing in this fashion is said to be loop
(or line) timed.
Figure 10: PoS Synchronization
In addition to high-bandwidth efficiency, PoS offers secure and reliable transmission
for data. Reliable data transfer depends on timing integrity. SONET/SDH timing
information is obtained by filtering state transitions through a phased-locked loop
(PLL). Since effective synchronization of the PLL depends on the density of "one" bit
received, a long continuous sequence (about 80 or more) of "zero" bits could distort
the expected density of "one" bits, impacting synchronization and resulting in loss of
timing. A severe drift of the receiving clock will cause signal or data loss. Although
the probability that this scenario will occur is very low under normal circumstances, it
is prudent to guard against accidental or possibly malicious payload bit sequences
that could cause it to happen.
PoS has adopted the ATM-style, self-synchronous payload scrambler which is
based on the polynomial, x^43+1. This scrambler is specified in RFC 2615 and
supported on most PoS interfaces. When in use, the signal label in the path
overhead (C2) is set to 0x16 to indicate scrambled payload. ATM-style payload
scrambling is also recommended in updates to the Telecordia GRE-253 for SONET
and also the ITU SDH specification. A graphical representation of the self-
synchronous scrambler is shown in Figure 11.
Figure 11: ATM-Style Self-Synchronous Scrambler
SONET/SDH has been most successfully used for high-speed IP transport in wide
area networking (WAN) applications. In most WAN applications today, routers with
PoS interfaces are connected to carrier SONET rings via ADMs. With the
proliferation of PoS in the wide area, available bandwidth in the local area network
(LAN) severely lagged as the traditional solutions for point-of-presence (PoP)
interconnectivity, such as Fiber Distributed Data Interface (FDDI) and Fast Ethernet
were unable to keep up. Therefore, service providers adopted PoS for PoP LAN
In recent times the emergence of DWDM, as a viable technology for bandwidth
multiplication, has further promoted the significance of PoS in the quest for more
bandwidth to meet insatiable demand by emerging data applications such as voice
over IP and video streaming.
Computer Networks: A System Approach. By Larry L Peterson & Bruce S. Davie
Optical Networks: A Practical Perspective. By Rajiv Ramaswami & Kumar N.
BellCore: SONET transport systems: Common generic criteria, 1995.