Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized protocols
that transfer multiple digital bit streams synchronously over optical fiber using lasers
or highly coherent light from light-emitting diodes. At low transmission rates data can
also be transferred via an electrical interface. The method was developed to replace the Plesiochronous
Digital Hierarchy system for transporting large amounts of telephone calls and data
traffic over the same fiber without synchronization problems. SONET generic criteria are detailed
in Telcordia Technologies Generic Requirements document GR-253-CORE. Generic criteria applicable
to SONET and other transmission systems are found in Telcordia GR-499-CORE.
SONET and SDH, which are essentially the same, were originally designed to transport circuit
mode communications 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 itself a communications protocol per se, 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 frames
also known as cells. It quickly evolved mapping structures and concatenated payload containers
to transport ATM connections. In other words, for ATM, the internal complex structure previously
used to transport circuit-oriented connections was removed and replaced with a large and
concatenated frame 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.
The SDH standard was originally defined by the European Telecommunications Standards
Institute, and is formalized as International Telecommunication Union standards G.707, G.783,
G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards
Institute standard T1.105.
Difference from PDH SDH differs from Plesiochronous Digital Hierarchy
in that the exact rates that are used to transport the data on SONET/SDH are tightly synchronized
across the entire network, using atomic clocks. This synchronization system allows entire
inter-country networks to operate synchronously, greatly reducing the amount of buffering required
between elements in the network. Both SONET and SDH can be used to encapsulate earlier
digital transmission standards, such as the PDH standard, or they can be used to directly
support either Asynchronous Transfer Mode or so-called packet over SONET/SDH networking.
Therefore, it is inaccurate to think of SDH or SONET as communications protocols in and
of themselves; they are generic, all-purpose transport containers for moving both voice
and data. The basic format of a SONET/SDH signal allows it to carry many different services
in its virtual container, because it is bandwidth-flexible. Protocol overview
SONET and SDH often use different terms to describe identical features or functions.
This can cause confusion and exaggerate their differences. With a few exceptions, SDH can
be thought of as a superset of SONET. SONET is a set of transport containers that
allow for delivery of a variety of protocols, including traditional telephony, ATM, Ethernet,
and TCP/IP traffic. SONET therefore is not in itself a native communications protocol
per se and should not be confused as being necessarily connection-oriented in the way
that term is usually used. The protocol is a heavily multiplexed structure,
with the header interleaved between the data in a complex way. This permits the encapsulated
data to have its own frame rate and be able to "float around" relative to the SDH/SONET
frame structure and rate. This interleaving permits a very low latency for the encapsulated
data. Data passing through equipment can be delayed by at most 32 microseconds (µs),
compared to a frame rate of 125 µs; many competing protocols buffer the data during
such transits for at least one frame or packet before sending it on. Extra padding is allowed
for the multiplexed data to move within the overall framing, as the data is clocked at
a different rate than the frame rate. The protocol is made more complex by the decision
to permit this padding at most levels of the multiplexing structure, but it improves all-around
performance. The basic unit of transmission
The basic unit of framing in SDH is a STM-1, which operates at 155.520 megabits per second.
SONET refers to this basic unit as an STS-3c. When the STS-3c is carried over OC-3, it is
often colloquially referred to as OC-3c, but this is not an official designation within
the SONET standard as there is no physical layer difference between an STS-3c and 3 STS-1s
carried within an OC-3. SONET offers an additional basic unit of transmission,
the STS-1 or OC-1, operating at 51.84 Mbit/s—exactly one third of an STM-1OC-3c carrier. This speed
is dictated by the bandwidth requirements for PCM-encoded telephonic voice signals:
at this rate, an STS-1/OC-1 circuit can carry the bandwidth equivalent of a standard DS-3
channel, which can carry 672 64-kbit/s voice channels. In SONET, the STS-3c signal is composed
of three multiplexed STS-1 signals; the STS-3c may be carried on an OC-3 signal. Some manufacturers
also support the SDH equivalent of the STS-1/OC-1, known as STM-0.
Framing In packet-oriented data transmission, such
as Ethernet, a packet frame usually consists of a header and a payload. The header is transmitted
first, followed by the payload. In synchronous optical networking, this is modified slightly.
The header is termed the overhead, and instead of being transmitted before the payload, is
interleaved with it during transmission. Part of the overhead is transmitted, then part
of the payload, then the next part of the overhead, then the next part of the payload,
until the entire frame has been transmitted. In the case of an STS-1, the frame is 810
octets in size, while the STM-1/STS-3c frame is 2,430 octets in size. For STS-1, the frame
is transmitted as three octets of overhead, followed by 87 octets of payload. This is
repeated nine times, until 810 octets have been transmitted, taking 125 µs. In the
case of an STS-3c/STM-1, which operates three times faster than an STS-1, nine octets of
overhead are transmitted, followed by 261 octets of payload. This is also repeated nine
times until 2,430 octets have been transmitted, also taking 125 µs. For both SONET and SDH,
this is often represented by displaying the frame graphically: as a block of 90 columns
and nine rows for STS-1, and 270 columns and nine rows for STM1/STS-3c. This representation
aligns all the overhead columns, so the overhead appears as a contiguous block, as does the
payload. The internal structure of the overhead and
payload within the frame differs slightly between SONET and SDH, and different terms
are used in the standards to describe these structures. Their standards are extremely
similar in implementation, making it easy to interoperate between SDH and SONET at any
given bandwidth. In practice, the terms STS-1 and OC-1 are
sometimes used interchangeably, though the OC designation refers to the signal in its
optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3
can be said to contain 3 STS-1s. SDH frame
The 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 identify administrative units 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 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 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 of a STM-1 signal is divided into two parts: the regenerator section
overhead and the multiplex section overhead. 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. Transport overhead
The transport overhead is used for signaling and measuring transmission error rates, and
is composed as follows: Section overhead
Called RSOH in SDH terminology: 27 octets containing information about the frame structure
required by the terminal equipment. Line overhead
Called MSOH in SDH: 45 octets containing information about error correction and Automatic Protection
Switching messages as may be required within the network.
AU Pointer Points to the location of the J1 byte in the
payload. Path virtual envelope
Data transmitted from end to end is referred to as path data. It is composed of two components:
Payload overhead Nine octets used for end-to-end signaling
and error measurement. Payload
User data For STS-1, the payload is referred to as the
synchronous payload envelope, which in turn has 18 stuffing bytes, leading to the STS-1
payload capacity of 756 bytes. The STS-1 payload is designed to carry a full
PDH DS3 frame. When the DS3 enters a SONET network, path overhead is added, and that
SONET network element is said to be a path generator and terminator. The SONET NE is
line terminating if it processes the line overhead. Note that wherever the line or path
is terminated, the section is terminated also. SONET regenerators terminate the section,
but not the paths or line. An STS-1 payload can also be subdivided into
seven virtual tributary groups. Each VTG can then be subdivided into four VT1.5 signals,
each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into three
VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is
a TUG-2; VT1.5 is equivalent to VC-11, and VT2 is equivalent to VC-12.
Three STS-1 signals may be multiplexed by time-division multiplexing to form the next
level of the SONET hierarchy, the OC-3, running at 155.52 Mbit/s. The signal is multiplexed
by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing
2,430 bytes and transmitted in 125 µs. Higher-speed circuits are formed by successively
aggregating multiples of slower circuits, their speed always being immediately apparent
from their designation. For example, four STS-3 or AU4 signals can be aggregated to
form a 622.08 Mbit/s signal designated OC-12 or STM-4.
The highest rate commonly deployed is the OC-768 or STM-256 circuit, which operates
at rate of just under 38.5 Gbit/s. Where fiber exhaustion is a concern, multiple SONET signals
can be transported over multiple wavelengths on a single fiber pair by means of wavelength-division
multiplexing, including dense wavelength-division multiplexing and coarse wavelength-division
multiplexing. DWDM circuits are the basis for all modern submarine communications cable
systems and other long-haul circuits. SONET/SDH and relationship to 10 Gigabit Ethernet
Another type of high-speed data networking circuit is 10 Gigabit Ethernet. The Gigabit
Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant with a line
rate of 10.3125 Gbit/s, and a wide area variant with the same line rate as OC-192/STM-64.
The WAN PHY variant encapsulates Ethernet data using a lightweight SDH/SONET frame,
so as to be compatible at a low level with equipment designed to carry SDH/SONET signals,
whereas the LAN PHY variant encapsulates Ethernet data using 64B/66B line coding.
However, 10 Gigabit Ethernet does not explicitly provide any interoperability at the bitstream
level with other SDH/SONET systems. This differs from WDM system transponders, including both
coarse and dense wavelength-division multiplexing systems that currently support OC-192 SONET
signals, which can normally support thin-SONET–framed 10 Gigabit Ethernet.
SONET/SDH data rates User throughput must not deduct path overhead
from the payload bandwidth, but path-overhead bandwidth is variable based on the types of
cross-connects built across the optical system. Note that the data-rate progression starts
at 155 Mbit/s and increases by multiples of four. The only exception is OC-24, which is
standardized in ANSI T1.105, but not a SDH standard rate in ITU-T G.707. Other rates,
such as OC-9, OC-18, OC-36, OC-96, and OC-1536, are defined but not commonly deployed; most
are considered orphaned rates. Physical layer
The physical layer refers to the first layer in the OSI networking model. The ATM and SDH
layers are the regenerator section level, digital line level, transmission path level,
virtual path level, and virtual channel level. The physical layer is modeled on three major
entities: transmission path, digital line and the regenerator section. The regenerator
section refers to the section and photonic layers. The photonic layer is the lowest SONET
layer and it is responsible for transmitting the bits to the physical medium. The section
layer is responsible for generating the proper STS-N frames which are to be transmitted across
the physical medium. It deals with issues such as proper framing, error monitoring,
section maintenance, and orderwire. The line layer ensures reliable transport of the payload
and overhead generated by the path layer. It provides synchronization and multiplexing
for multiple paths. It modifies overhead bits relating to quality control. The path layer
is SONET's highest level layer. It takes data to be transmitted and transforms them into
signals required by the line layer, and adds or modifies the path overhead bits for performance
monitoring and protection switching. SONET/SDH network management protocols
Overall functionality Network management systems are used to configure
and monitor SDH and SONET equipment either locally or remotely.
The systems consist of three essential parts, covered later in more detail:
Software running on a 'network management system terminal' e.g. workstation, dumb terminal
or laptop housed in an exchange/ central office. Transport of network management data between
the 'network management system terminal' and the SONET/ SDH equipment e.g. using TL1/ Q3
protocols. Transport of network management data between
SDH/ SONET equipment using 'dedicated embedded data communication channels' within the section
and line overhead. The main functions of network management thereby
include: Network and network-element provisioning
In order to allocate bandwidth throughout a network, each network element must be configured.
Although this can be done locally, through a craft interface, it is normally done through
a network management system that in turn operates through the SONET/SDH network management network.
Software upgrade Network-element software upgrades are done
mostly through the SONET/SDH management network in modern equipment.
Performance management Network elements have a very large set of
standards for performance management. The performance-management criteria allow not
only monitoring the health of individual network elements, but isolating and identifying most
network defects or outages. Higher-layer network monitoring and management software allows
the proper filtering and troubleshooting of network-wide performance management, so that
defects and outages can be quickly identified and resolved.
Consider the three parts defined above: Network management system terminal
Local Craft interface Local "craftspersons" can access a SDH/ SONET
network element on a "craft port" and issue commands through a dumb terminal or terminal
emulation program running on a laptop. This interface can also be attached to a console
server, allowing for remote out-of-band management and logging.
Network management system This will often consist of software running
on a Workstation covering a number of SDH/SONET network elements
TL1/ Q3 Protocols TL1
SONET equipment is often managed with the TL1 protocol. TL1 is a telecom language for
managing and reconfiguring SONET network elements. The command language used by a SONET network
element, such as TL1, must be carried by other management protocols, such as SNMP, CORBA,
or XML. Q3
SDH has been mainly managed using the Q3 interface protocol suite defined in ITU recommendations
Q.811 and Q.812. With the convergence of SONET and SDH on switching matrix and network elements
architecture, newer implementations have also offered TL1.
Most SONET NEs have a limited number of management interfaces defined:
TL1 Electrical interface The electrical interface, often a 50-ohm coaxial
cable, sends SONET TL1 commands from a local management network physically housed in the
central office where the SONET network element is located. This is for local management of
that network element and, possibly, remote management of other SONET network elements.
Dedicated embedded data communication channels SONET and SDH have dedicated data communication
channels within the section and line overhead for management traffic. Generally, section
overhead is used. According to ITU-T G.7712, there are three modes used for management:IP-only
stack, using PPP as data-link OSI-only stack, using LAP-D as data-link
Dual stack using PPP or LAP-D with tunneling functions to communicate between stacks.
To handle all of the possible management channels and signals, most modern network elements
contain a router for the network commands and underlying protocols.
Equipment With advances in SONET and SDH chipsets, the
traditional categories of network elements are no longer distinct. Nevertheless, as network
architectures have remained relatively constant, even newer equipment can be examined in light
of the architectures they will support. Thus, there is value in viewing new, as well as
traditional, equipment in terms of the older categories.
Regenerator Traditional regenerators terminate the section
overhead, but not the line or path. Regenerators extend long-haul routes in a way similar to
most regenerators, by converting an optical signal that has already traveled a long distance
into electrical format and then retransmitting a regenerated high-power signal.
Since the late 1990s, regenerators have been largely replaced by optical amplifiers. Also,
some of the functionality of regenerators has been absorbed by the transponders of wavelength-division
multiplexing systems. Add-drop multiplexer
Add-drop multiplexers are the most common type of network elements. Traditional ADMs
were designed to support one of the network architectures, though new generation systems
can often support several architectures, sometimes simultaneously. ADMs traditionally have a
high-speed side, and a low-speed side, which can consist of electrical as well as optical
interfaces. The low-speed side takes in low-speed signals, which are multiplexed by the network
element and sent out from the high-speed side, or vice-versa.
Digital cross connect system Recent digital cross connect systems support
numerous high-speed signals, and allow for cross-connection of DS1s, DS3s and even STS-3s/12c
and so on, from any input to any output. Advanced DCSs can support numerous subtending rings
simultaneously. Network architectures
SONET and SDH have a limited number of architectures defined. These architectures allow for efficient
bandwidth usage as well as protection, and are fundamental to the worldwide deployment
of SONET and SDH for moving digital traffic. Every SDH/SONET connection on the optical
physical layer uses two optical fibers, regardless of the transmission speed.
Linear Automatic Protection Switching Linear Automatic Protection Switching, also
known as 1+1, involves four fibers: two working fibers, and two protection fibers. Switching
is based on the line state, and may be unidirectional, or bidirectional.
Unidirectional path-switched ring In unidirectional path-switched rings, two
redundant copies of protected traffic are sent in either direction around a ring. A
selector at the egress node determines which copy has the highest quality, and uses that
copy, thus coping if one copy deteriorates due to a broken fiber or other failure. UPSRs
tend to sit nearer to the edge of a network, and as such are sometimes called collector
rings. Because the same data is sent around the ring in both directions, the total capacity
of a UPSR is equal to the line rate N of the OC-N ring. For example, in an OC-3 ring with
3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, 100 percent of
the ring bandwidth would be consumed by nodes A and D. Any other nodes on the ring could
only act as pass-through nodes. The SDH equivalent of UPSR is subnetwork connection protection;
SNCP does not impose a ring topology, but may also be used in mesh topologies.
Bidirectional line-switched ring Bidirectional line-switched ring comes in
two varieties: two-fiber BLSR and four-fiber BLSR. BLSRs switch at the line layer. Unlike
UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes
adjacent to the failure reroute the traffic "the long way" around the ring on the protection
fibers. BLSRs trade cost and complexity for bandwidth efficiency, as well as the ability
to support "extra traffic" that can be pre-empted when a protection switching event occurs.
In four-fiber ring, either single node failures, or multiple line failures can be supported,
since a failure or maintenance action on one line causes the protection fiber connecting
two nodes to be used rather than looping it around the ring.
BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities.
Because a BLSR does not send redundant copies from ingress to egress, the total bandwidth
that a BLSR can support is not limited to the line rate N of the OC-N ring, and can
actually be larger than N depending upon the traffic pattern on the ring. In the best case,
all traffic is between adjacent nodes. The worst case is when all traffic on the ring
egresses from a single node, i.e., the BLSR is serving as a collector ring. In this case,
the bandwidth that the ring can support is equal to the line rate N of the OC-N ring.
This is why BLSRs are seldom, if ever, deployed in collector rings, but often deployed in
inter-office rings. The SDH equivalent of BLSR is called Multiplex Section-Shared Protection
Ring. Synchronization
Clock sources used for synchronization in telecommunications networks are rated by quality,
commonly called a stratum. Typically, a network element uses the highest quality stratum available
to it, which can be determined by monitoring the synchronization status messages of selected
clock sources. Synchronization sources available to a network
element are: Local external timing
This is generated by an atomic cesium clock or a satellite-derived clock by a device in
the same central office as the network element. The interface is often a DS1, with sync-status
messages supplied by the clock and placed into the DS1 overhead.
Line-derived timing A network element can choose to derive its
timing from the line-level, by monitoring the S1 sync-status bytes to ensure quality.
Holdover As a last resort, in the absence of higher
quality timing, a network element can go into a holdover mode until higher-quality external
timing becomes available again. In this mode, the network element uses its own timing circuits
as a reference. Timing loops
A timing loop occurs when network elements in a network are each deriving their timing
from other network elements, without any of them being a "master" timing source. This
network loop will eventually see its own timing "float away" from any external networks, causing
mysterious bit errors—and ultimately, in the worst cases, massive loss of traffic.
The source of these kinds of errors can be hard to diagnose. In general, a network that
has been properly configured should never find itself in a timing loop, but some classes
of silent failures could nevertheless cause this issue.
Next-generation SONET/SDH SONET/SDH development was originally driven
by the need to transport multiple PDH signals—like DS1, E1, DS3, and E3—along with other groups
of multiplexed 64 kbit/s pulse-code modulated voice traffic. The ability to transport ATM
traffic was another early application. In order to support large ATM bandwidths, concatenation
was developed, whereby smaller multiplexing containers are inversely multiplexed to build
up a larger container to support large data-oriented pipes.
One problem with traditional concatenation, however, is inflexibility. Depending on the
data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth
left over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s
Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to considerable waste.
More important is the need for all intermediate network elements to support newly introduced
concatenation sizes. This problem was overcome with the introduction of Virtual Concatenation.
Virtual concatenation allows for a more arbitrary assembly of lower-order multiplexing containers,
building larger containers of fairly arbitrary size without the need for intermediate network
elements to support this particular form of concatenation. Virtual concatenation leverages
the X.86 or Generic Framing Procedure protocols in order to map payloads of arbitrary bandwidth
into the virtually concatenated container. The Link Capacity Adjustment Scheme allows
for dynamically changing the bandwidth via dynamic virtual concatenation, multiplexing
containers based on the short-term bandwidth needs in the network.
The set of next-generation SONET/SDH protocols that enable Ethernet transport is referred
to as Ethernet over SONET/SDH. See also
List of device bandwidths Routing and wavelength assignment
Multiwavelength optical networking Optical mesh network
Optical Transport Network G.709
Notes
References
External links Understanding SONET/SDH
The Queen's University of Belfast SDH/SONET Primer
SDH Pocket Handbook from Acterna/JDSU SONET Pocket Handbook from Acterna/JDSU
The Sonet Homepage SONET Interoperability Form
Network Connection Speeds Reference Next-generation SDH: the future looks bright
The Future of SONET/SDH Standards
Telcordia GR-253-CORE, SONET Transport Systems: Common Generic Criteria
Telcordia GR-499-CORE, Transport Systems Generic Requirements: Common Requirements
ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats
ANSI T1.119/ATIS PP 0900119.01.2006: SONET - Operations, Administration, Maintenance,
and Provisioning - Communications ITU-T recommendation G.707: Network Node Interface
for the Synchronous Digital Hierarchy ITU-T recommendation G.783: Characteristics
of synchronous digital hierarchy equipment functional blocks
ITU-T recommendation G.803: Architecture of Transport Networks Based on the Synchronous
Digital Hierarchy
