A method of transmitting digital information where the data is packed in containers which are synchronized in time enabling relatively simple multiplexing and demultiplexing at the transmitting and receiving ends. The technique is used to carry high capacity information over long distances up to speeds of 10Gbps.
Posted by justfawad
Posted by crouse
Posted by crouse
Synchronous Digital Hierarchy
Posted by sagitraz
every one is absolute right Good Work
Posted by mylife
l Fully cross connection
l Terminal Multiplexer (TM) and Add/Drop Multiplexer (ADM) applications
l Topology: Point to Point, Linear, and Ring
l Remote software downloadable
l Support 1+1 Linear MSP (Multiplex Section Protection) and (SNCP
Sub-Network Connection Protection)
l Friendly GUI (Graphic User Interface) for OAM&P (Operation,
Administration, Maintenance, and Provision)
l Optional point to point EOW (Engineering Order Wire) function
The main unit consists:
Ø Two ports STM-1 (or STM-4) optical interface (for 1+1 MSP or SNCP)
Ø 16 ports E1 interface (120 ohm; 75 ohm for option)
Ø External 2M clock source
Ø RS-232 and LAN port for network management
Ø LED display and alarm output
Sigma provides the following expansion cards:
Ø STM-1o (optical interface)
Ø STM-1e (electrical interface)
Ø E1 (can be configured as T1)
Ø 10/100 BASE-T
Ø DS3 (can be configured as E3)
Ø V.35 (N x 64K)
System Overview of Sigma:
STM-4 Optical Interface:
Ø Interface Type: S4.1, L4.1, or L4.2
Ø Transmission Rate: 622.08M
Ø Meet ITU-T G.957/G.958 requirements
STM-1 Optical Interface:
Ø Interface Type: S1.1, L1.1, or L1.2
Ø Transmission Rate: 155.52M
Ø Meet ITU-T G.957/G.958 requirements
STM-1 Electrical Interface:
Ø Transmission Rate: 155.52M
Ø AC impedance: 75 ohm
T3 (DS3) Interface:
Ø Transmission Rate: 44.736M
Ø AC impedance: 75 ohm
Ø Transmission Rate: 34.368M
Ø AC impedance: 75 ohm
Ø Transmission Rate: 2.048M
Ø AC impedance: Default 120 ohm; 75 ohm for option
Ø Transmission Rate: 1.544M
Ø AC impedance: 100 ohm
Ø Line code: TC-PAM
Ø Transmission Rate: up to 2.3M
Ø AC impedance: 135 ohm
Ø Meet ITU-T G.991.2 requirements
Ø Meet IEEE 802.3 requirements
Ø 10/100 auto negotiation
Ø AC impedance: 100 ohm
Serial Port (Datacom) Interface:
Ø Interface: V.35
Ø Data Rate: N x 64 K, N=1 ~ 32
Ø Interface: RS-232C Asynchronous
Ø Interface: 10/100Base-T
Ø DC: -48VDC
Ø AC: 110/220VAC @ 50/60Hz
Ø Temperature: -10 ~ 60°C (operation)
Ø Humidity: 5 ~ 95% (RH)
Ø EMI: FCC, Part 15, Sub B (Class A)
Ø CISPR22 Class A
Posted by jahangir1983
I think saqlain is absolutely right
Good Work Saqlain
Posted by jahangir1983
What is SDH?
This document is intended as an introductory guide to the Synchronous Digital Hierarchy (SDH) multiplexing standard.
Standards in the telecommunications field are always evolving. Information in this SDH primer is based on the latest
information available from the ITU-T standardisation organization.
Use this primer as an introduction to the technology of SDH. Consult the actual material from ITU-T, paying particular
attention to the latest revision, if more detailed information is required.
For help in understanding the language of SDH telecommunications, a comprehensive Glossary appears at the end
of this document.
Introduction To SDH
SDH (Synchronous Digital Hierarchy) is a standard for telecommunications
transport formulated by the International Telecommunication Union
(ITU), previously called the International Telegraph and Telephone
Consultative Committee (CCITT).
SDH was first introduced into the telecommunications network in 1992
and has been deployed at rapid rates since then. It’s deployed at all
levels of the network infrastructure, including the access network and
the long-distance trunk network. It’s based on overlaying a synchronous
multiplexed signal onto a light stream transmitted over fibre-optic cable.
SDH is also defined for use on radio relay links, satellite links, and at
electrical interfaces between equipment.
The comprehensive SDH standard is expected to provide the transport
infrastructure for worldwide telecommunications for at least the next two
or three decades.
The increased configuration flexibility and bandwidth availability of SDH
provides significant advantages over the older telecommunications system.
These advantages include:
A reduction in the amount of equipment and an increase in network reliability.
The provision of overhead and payload bytes – the overhead bytes permitting
management of the payload bytes on an individual basis and facilitating centralised
The definition of a synchronous multiplexing format for carrying lower-level
digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies
the interface to digital switches, digital cross-connects, and add-drop
The availability of a set of generic standards, which enable multi-vendor
The definition of a flexible architecture capable of accommodating future
applications, with a variety of transmission rates.
In brief, SDH defines synchronous transport modules (STMs) for the
fibre-optic based transmission hierarchy.
Before SDH, the first generations of fibre-optic systems in the public
telephone network used proprietary architectures, equipment line codes,
multiplexing formats, and maintenance procedures. The users of this
equipment wanted standards so they could mix and match equipment
from different suppliers.
The task of creating such a standard was taken up in 1984 by the
Exchange Carriers Standards Association (ECSA) in the U.S. to establish
a standard for connecting one fibre system to another. In the late stages
of the development, the CCITT became involved so that a single international
standard might be developed for fibre interconnect between telephone
networks of different countries. The resulting international standard
is known as Synchronous Digital Hierarchy (SDH).
Synchronisation of Digital Signals
To correctly understand the concepts and details of SDH, it’s important
to be clear about the meaning of Synchronous, Plesiochronous, and
In a set of Synchronous signals, the digital transitions in the signals
occur at exactly the same rate. There may however be a phase difference
between the transitions of the two signals, and this would lie within
specified limits. These phase differences may be due to propagation
time delays, or low-frequency wander introduced in the transmission
network. In a synchronous network, all the clocks are traceable to one
Stratum 1 Primary Reference Clock (PRC). The accuracy of the PRC is
better than ±1 in 1011 and is derived from a cesium atomic standard.
If two digital signals are Plesiochronous, their transitions occur at
“almost” the same rate, with any variation being constrained within tight
limits. These limits are set down in ITU-T recommendation G.811. For
example, if two networks need to interwork, their clocks may be derived
from two different PRCs. Although these clocks are extremely accurate,
there’s a small frequency difference between one clock and the other.
This is known as a plesiochronous difference.
In the case of Asynchronous signals, the transitions of the signals don’t necessarily
occur at the same nominal rate. Asynchronous, in this case, means
that the difference between two clocks is much greater than a plesiochronous
difference. For example, if two clocks are derived from free-running quartz
oscillators, they could be described as asynchronous.
The primary reason for the creation of SDH was to provide a long-term
solution for an optical mid-span meet between operators; that is, to
allow equipment from different vendors to communicate with each other.
This ability is referred to as multi-vendor interworking and allows one
SDH-compatible network element to communicate with another, and to
replace several network elements, which may have previously existed
solely for interface purposes.
The second major advantage of SDH is the fact that it’s synchronous.
Currently, most fibre and multiplex systems are plesiochronous. This
means that the timing may vary from equipment to equipment because
they are synchronised from different network clocks. In order to multiplex
this type of signal, a process known as bit-stuffing is used. Bit-stuffing
adds extra bits to bring all input signals up to some common bit-rate,
thereby requiring multi-stage multiplexing and demultiplexing. Because
SDH is synchronous, it allows single-stage multiplexing and demultiplexing.
This single-stage multiplexing eliminates hardware complexity, thus
decreasing the cost of equipment while improving signal quality.
In plesiochronous networks, an entire signal had to be demultiplexed in
order to access a particular channel; then the non-accessed channels had
to be re-multiplexed back together in order to be sent further along the
network to their proper destination. In SDH format, only those channels
that are required at a particular point are demultiplexed, thereby eliminating
the need for back-to-back multiplexing. In other words, SDH makes
individual channels “visible” and they can easily be added and dropped.
Plesiochronous Digital Hierarchy (PDH)
Traditionally, digital transmission systems and hierarchies have been based
on multiplexing signals which are plesiochronous (running at almost the same
speed). Also, various parts of the world use different hierarchies which lead to
problems of international interworking; for example, between those countries
using 1.544 Mbit/s systems (U.S.A. and Japan) and those using the
2.048 Mbit/s system.
To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary
to demultiplex the signal all the way down to the 2 Mbit/s level
before the location of the 64 kbit/s channel can be identified. PDH
requires “steps” (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140
multiplex) to drop out or add an individual speech or data channel
(see Figure 1). This is due to the bit-stuffing used at each level.
Limitations of PDH Network
The main limitations of PDH are:
Inability to identify individual channels in a higher-order bit stream.
Insufficient capacity for network management;
Most PDH network management is proprietary.
There’s no standardised definition of PDH bit rates greater than 140 Mbit/s.
There are different hierarchies in use around the world. Specialized interface
equipment is required to interwork the two hierarchies.
Posted by saqlain231
The introduction of any new technology is usually preceded by much hyperbole and rhetoric. In many cases, the revolution predicted never gets beyond this. In many more, it never achieves the wildly over optimistic growth forecasted by market specialists - home computing and the paperless office to name but two. It is fair to say, however, by whatever method you use to evaluate a new technology, that synchronous digital transmission does not fall into this category. The fundamental benefits to be gained from its deployment by PTOs seem to be so overwhelming that, bar a catastrophe, the bulk of today's plesiochronous transmission systems used for high speed backbone links will be pushed aside in the next few years. To quote Dataquest:, "It has been claimed by many industry experts that the impact of synchronous technology will equal that of the transition from analogue to digital technology or from copper to fibre optic based transmission."
For the first time in telecommunications history there will be a world-wide, uniform and seamless transmission standard for service delivery. Synchronous digital hierarchy (SDH) provides the capability to send data at multi-gigabit rates over today's single-mode fibre-optics links. This first issue of Technology Watch looks at synchronous digital transmission and evaluates its potential impact. Following issues of TW will look at customer oriented broad-band services that will ride on the back of SDH deployment by PTOs. These will include:
* Frame relay
* SMDS (Switched Multi-Megabit Data Service)
* ATM (asynchronous transfer mode)
* High speed LAN services such as FDDI
The use of synchronous digital transmission by PTOs in their backbone fibre-optic and radio network will put in place the enabling technology that will support many new broad-band data services demanded by the new breed of computer user. However, the deployment of synchronous digital transmission is not only concerned with the provision of high-speed gigabit networks. It has as much to do with simplifying access to links and with bringing the full benefits of software control in the form of flexibility and introduction of network management.
In many respects, the benefits to the PTO will be the same as those brought to the electronics industry when hard wired logic was replaced by the microprocessor. As with that revolution, synchronous digital transmission will not take hold overnight, but deployment will be spread over a decade, with the technology first appearing on new backbone links. The first to feel the benefits will be the PTOs themselves, as demonstrated by the technology's early uptake by many operators including BT. Only later will customers directly benefit with the introduction of new services such as connectionless LAN-to-LAN transmission capability.
According to one market research company it will take until the mid or late 1990s before 70% of revenue for network equipment manufacturers will be derived from synchronous systems. Remembering that this is a multi-billion $ market, this constitutes a radical change by any standard (Figure 2).
Users who extensively use PCs and workstations with LANs, graphic layout, CAD and remote database applications are now looking to the telecommunication service suppliers to provide the means of interlinking these now powerful machines at data rates commensurable with those achieved by their own in-house LANs. They also want to be able to transfer information to other metropolitan and international sites as easily and as quickly as they can to a colleague sitting at the next desk.
Digital data and voice transmission is based on a 2.048Mbit/s bearer consisting of 30 time division multiplexed (TDM) voice channels, each running at 64Kbps (known as E1 and described by the CCITT G.703 specification). At the E1 level, timing is controlled to an accuracy of 1 in 1011 by synchronising to a master Caesium clock. Increasing traffic over the past decade has demanded that more and more of these basic E1 bearers be multiplexed together to provide increased capacity. During this time rates have increased through 8, 34, and 140Mbit/s. The highest capacity commonly encountered today for inter-city fibre optic links is 565Mbit/s, with each link carrying 7,680 base channels, and now even this is insufficient.
Unlike E1 2.048Mbit/s bearers, higher rate bearers in the hierarchy are operated plesiochronously, with tolerances on an absolute bit-rate ranging from 30ppm (parts per million) at 8Mbit/s to 15ppm at 140Mbit/s. Multiplexing such bearers (known as tributaries in SDH speak) to a higher aggregate rate (e.g. 4 x 8Mbit/s to 1 x 34Mbit/s) requires the padding of each tributary by adding bits such that their combined rate together with the addition of control bits matches the final aggregate rate. Plesiochronous transmission is now often referred to as plesiochronous digital hierarchy (PDH).
Because of the large investment in earlier generations of plesiochronous transmission equipment, each step increase in capacity has necessitated maintaining compatibility with what was already installed by adding yet another layer of multiplexing. This has created the situation where each data link has a rigid physical and electrical multiplexing hierarchy at either end. Once multiplexed, there is no simple way an individual E1 bearer can be identified in a PDH hierarchy, let alone extracted, without fully demultiplexing down to the E1 level again as shown in Figure 3.
The limitations of PDS multiplexing are:
* A hierarchy of multiplexers at either end of the link can lead to reduced reliability and resilience, minimum flexibility, long reconfiguration turn-around times, large equipment volume, and high capital-equipment and maintenance costs.
* PDH links are generally limited to point-to-point configurations with full demultiplexing at each switching or cross connect node.
* Incompatibilities at the optical interfaces of two different suppliers can cause major system integration problems.
* To add or drop an individual channel or add a lower rate branch to a backbone link a complete hierarchy of MUXs is required as shown in figure 3.
* Because of these limitations of PDH, the introduction of an acceptable world-wide synchronous transmission standard called SDH is welcomed by all.
In the USA in the early 1980s, it was clear that a new standard was required to overcome the limitations presented by PDH networks, so the ANSI (American National Standards Institute) SONET (synchronous optical network) standard was born in 1984. By 1988, collaboration between ANSI and CCITT produced an international standard, a superset of SONET, called synchronous digital hierarchy (SDH).
US SONET standards are based on STS-1 (synchronous transport signal) equivalent to 51.84Mbit/s. When encoded and modulated onto a fibre optic carrier STS-1 is known as OC-1. This particular rate was chosen to accommodate a US T-3 plesiochronous payload to maintain backwards compatibility with PDH. Higher data rates are multiples of this up to STS-48, which is 2,488Gbit/s.
SDH is based on an STM-1 (155.52Mbit/s) rate, which is identical to the SONET STS-3 rate. Some higher bearer rates coincide with SONET rates such as: STS-12 and STM-4 = 622Mbit/s, and STS-48 and STM-16 = 2.488Gbit/s. Mercury is currently trialing STM-1 and STM-16 rate equipment.
SDH supports the transmission of all PDH payloads, other than 8Mbit/s, and ATM, SMDS and MAN data. Most importantly, because each type of payload is transmitted in containers synchronous with the STM-1 frame, selected payloads may be inserted or extracted from the STM-1 or STM-N aggregate without the need to fully hierarchically de-multiplex as with PDH systems.
Benefits of SDH Transmission
SDH transmission systems have many benefits over PDH:
* Software Control allows extensive use of intelligent network management software for high flexibility, fast and easy re-configurability, and efficient network management.
* Survivability. With SDH, ring networks become practicable and their use enables automatic reconfiguration and traffic rerouting when a link is damaged. End-to-end monitoring will allow full management and maintenance of the whole network.
* Efficient drop and insert. SDH allows simple and efficient cross-connect without full hierarchical multiplexing or de-multiplexing. A single E1 2.048Mbit/s tail can be dropped or inserted with relative ease even on Gbit/s links.
* Standardisation enables the interconnection of equipment from different suppliers through support of common digital and optical standards and interfaces.
* Robustness and resilience of installed networks is increased.
* Equipment size and operating costs are reduced by removing the need for banks of multiplexers and de-multiplexers. Follow-on maintenance costs are also reduced.
* Backwards compatibly will enable SDH links to support PDH traffic.
* Future proof. SDH forms the basis, in partnership with ATM (asynchronous transfer mode), of broad-band transmission, otherwise known as B-ISDN or the precursor of this service in the form of Switched Multimegabit Data Service, (SMDS).
The introduction of synchronous digital transmission in the form of SDH will eventually revolutionise all aspects of public data communication from individual leased lines through to trunk networks. Because of the state-of-the-art nature of SDH and SONET technology, there are extensive field trials taking place in 1992 throughout the world prior to introduction in the 1993 - 1995 time scale.
There is still a lack of understanding of the ramifications of the introduction of SDH within telecommunications operations. In practice, the use of extensive software control will impact positively all parts of the business. It is not so much a question of whether the technology will be taken up, but when.
Introduction of SDH will lead to the availability of many new broad-band data services providing users with increased flexibility. It is in this area where confusion reigns with potential technologies vying for supremacy. These will be discussed in future issues of Technology Watch.
Importantly for PTOs, SDH will bring about more competition between equipment suppliers designing essentially to a common standard. One practical effect could be to force equipment prices down, brought about by the larger volumes engendered by access to world rather than local markets. At least one manufacturer is currently stating that they will be spending up to 80% of their SDH development budgets on management software rather than hardware. Such was the situation in the computer industry in the early 1980s. Not least, it will have a great impact on such issues as staffing levels and required personal skills of personnel within PTOs.
SDH deployment will take a great deal of investment and effort since it replaces the very infrastructure of the world's core communications networks. But it must not be forgotten that there are still many issues to be resolved.
The benefits to be gained in terms of improving operator profitability, and helping them to compete in the new markets of the 1990s, are so high that deployment of SDH is just a question of time.
Posted by saqlain231
Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are multiplexing protocols that transfer multiple digital bit streams using lasers or light-emitting diodes (LEDs) over the same optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fibre wire without synchronization problems.
SONET and SDH were originally designed to transport circuit mode communications (eg, T1, T3) from a variety of different sources. The primary difficulty in doing this prior to SONET was that the synchronization sources of these different circuits were different. This meant each circuit was actually operating at a slightly different rate and with different phase. SONET allowed for the simultaneous transport of many different circuits of differing origin within one single framing protocol. In a sense, then, SONET is not itself a communications protocol per se, but a transport protocol.
Due to SONET's essential protocol neutrality and transport-oriented features, SONET was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as TCP/IP and Ethernet), the internal complex structure previously used to transport circuit-oriented connections is removed and replaced with a large and concatenated frame (such as STS-3c) into which ATM frames, IP packets, or Ethernet are placed.
A rack of Alcatel STM-16 SDH Add Drop Multiplexors
Both SDH and SONET are widely used today. SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET is considered the variation.
The two protocols are standardized according to the following:
* SDH or Synchronous Digital Hierarchy standard was originally defined by the ETSI or European Telecommunications Standards Institute
* SONET or Synchronous Optical Networking standard as defined by GR-253-CORE from Telcordia and T1.105 from American National Standards Institute
Difference from PDH
Synchronous networking differs from PDH in that the exact rates that are used to transport the data 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 used directly to support either Asynchronous Transfer Mode (ATM) or so-called Packet over SONET/SDH (POS) networking. As such, it is inaccurate to think of SDH or SONET as communications protocols in and of themselves, but rather as generic and all-purpose transport containers for moving both voice and data. The basic format of an SDH signal allows it to carry many different services in its Virtual Container (VC) because it is bandwidth-flexible.
 Structure of SONET/SDH signals
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.
 Protocol overview
The protocol is an extremely heavily multiplexed structure, with the header interleaved between the data in a complex way. This is intended to permit the encapsulated data to have its own frame rate and to 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, compared to a frame rate of 125 microseconds and many competing protocols buffer the data 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 due to it being on a different clock than the frame rate. The decision to allow this at most of the levels of the multiplexing structure makes the protocol complex, but gives high all-round performance. SONET is the standard defined by the ANSI T1 for synchronous operation used in North America
 The basic unit of transmission
The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module level - 1), which operates at 155.52 Mbit/s. SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal - 3, concatenated), but its high-level functionality, frame size, and bit-rate are the same as STM-1.
SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal - 1), operating at 51.84 Mbit/s - exactly one third of an STM-1/STS-3c. That is, in SONET the associated OC-3 signal will be composed of three STS-1s (or, more recently in packet transport, the OC-3 signal will carry a single concatenated STS-3c.) Some manufacturers also support the SDH equivalent: STM-0.
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 (and possibly a trailer, such as a CRC). 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 2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload. This is repeated nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster than STS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine times over until 2,430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH, this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for STS-1; and 270 columns and 9 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 particular bandwidths.
It is worth noting that the choice of a 125-microsecond interval is not an arbitrary one. If one octet is extracted from the bitstream every 125 microseconds, this produces a data rate of 8 bits per 125 microseconds - or 64 kbit/s, the basic digital signaling rate for telecommunication systems world wide. This allows an extremely useful technique to be used in synchronous optical networking. The low data-rate channels or streams of data can be extracted from high data-rate streams by simply extracting octets at regular time intervals—there is no need to understand or decode the entire frame. This is not possible in PDH networking. It shows that a relatively simple device is all that is needed to extract a datastream from an SDH-framed connection and insert it into a SONET-framed connection and vice versa.
In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format 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
A STM-1 Frame. The first 9 columns contain the overhead and the pointers. For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and 9 rows but, in practice, 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 9 rows. The first 3 rows and 9 columns contain Regenerator Section Overhead (RSOH) and the last 5 rows and 9 columns contain Multiplex Section Overhead (MSOH). The 4th row from the top contains pointers
The STM-1 (Synchronous Transport Module level - 1) frame is the basic transmission format for SDH or the fundamental frame or the first level of the synchronous digital hierarchy. The STM-1 frame is transmitted in exactly 125 microseconds, therefore there are 8000 frames per second on a fiber-optic circuit designated OC-3 (optical carrier three). The STM-1 frame consists of overhead and pointers plus information payload. The first 9 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.
Carried within the Information Payload, which has its own frame structure of 9 rows and 261 columns, are Administrative Units identified within the information payload by pointers. Within the Administrative Unit is one or more Virtual Containers (VC). VC contain Path Overhead and VC payload. The first column is for Path Overhead; it’s 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 an STM-1 signal (SOH) is divided into two parts: the Regenerator Section Overhead (RSOH) and the Multiplex Section Overhead (MSOH). The overheads contain information from the 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.
STM–1 frame contains
* 1 octet = 8 bit
* Total content : 9 x 270 octets = 2430 octets
* overhead : 8 rows x 9 octets
* pointers : 1 row x 9 octets
* payload : 9 rows x 261 octets
* Period : 125 μsec
* Bitrate : 155.520 Mbit/s (2430 octets x 8 bits x 8000 frame/s )
* payload capacity : 150.336 Mbit/s (2349 x 8 bits x 8000 frame/s)
The transmission of the frame is done row by row, from the left to right and top to bottom.
 Framing structure
The frame consists of two parts, the transport overhead and the path virtual envelope.
 Transport overhead
The transport overhead is used for signaling and measuring transmission error rates, and is composed as follows:
* Section overhead - called RSOH (Regenerator Section Overhead) in SDH terminology: 27 octets containing information about the frame structure required by the terminal equipment.
* Line overhead - called MSOH (Multiplex Section Overhead) in SDH: 45 octets containing information about alarms, maintenance and error correction as may be required within the network.
* Pointer – It 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 (POH): 9 octets used for end to end signaling and error measurement.
* Payload: user data (774 bytes for STM-0/STS-1, or 2340 octets for STM-1/STS-3c)
For STS-1, the payload is referred to as the synchronous payload envelope (SPE), 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 (NE) is said to be a path generator and terminator. The SONET NE is said to be 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 7 VTGs, or Virtual Tributary Groups. Each VTG can then be subdivided into 4 VT1.5 signals, each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into 3 VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG2; VT1.5 is equivalent to VC11, and VT2 is equivalent to VC12.
Three STS-1 signals may be multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125 microseconds.
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 as OC-12 or STM-4.
The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under evaluation. [Few vendors are offering STM-256 rates now, with speeds of nearly 40Gbit/s]. Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of Wavelength division multiplexing, including dense wave division multiplexing (DWDM) and Coarse Wave Division Multiplexing (CWDM). DWDM circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.
 SONET/SDH and relationship to 10 Gigabit Ethernet
Another circuit type amongst data networking equipment is 10 Gigabit Ethernet (10GbE). This is similar to the line rate of OC-192/STM-64 (9.953 Gbit/s). The Gigabit Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant (LAN PHY), with a line rate of exactly 10,000,000 kbit/s and a wide area variant (WAN PHY), with the same line rate as OC-192/STM-64 (9,953,280 kbit/s). The Ethernet wide area variant encapsulates its data using a light-weight SDH/SONET frame so as to be compatible at low level with equipment designed to carry those signals.
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-WDM systems (CWDM, DWDM) that currently support OC-192 SONET Signals, which can normally support thin-SONET framed 10 Gigabit Ethernet.
 SONET/SDH data rates
SONET/SDH Designations and bandwidths SONET Optical Carrier Level SONET Frame Format SDH level and Frame Format Payload bandwidth (kbit/s) Line Rate (kbit/s)
OC-1 STS-1 STM-0 50,112 51,840
OC-3 STS-3 STM-1 150,336 155,520
OC-12 STS-12 STM-4 601,344 622,080
OC-24 STS-24 – 1,202,688 1,244,160
OC-48 STS-48 STM-16 2,405,376 2,488,320
OC-192 STS-192 STM-64 9,621,504 9,953,280
OC-768 STS-768 STM-256 38,486,016 39,813,120
OC-3072 STS-3072 STM-1024 153,944,064 159,252,240
In the above table, Payload bandwidth is the line rate less the bandwidth of the line and section overheads. User throughput must also deduct path overhead from this, 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 155Mb/s and increases by multiples of 4. The only exception is OC-24 which is standardised 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, and OC-96, and OC-1536 are sometimes described, but it is not clear if they were ever deployed, and are certainly not common, and are not standards compliant.
The next logical rate of 160 Gb/s OC-3072/STM-1024 has not yet been standardised, due to the cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40 Gb/s.
 Physical layer
The physical layer actually comprises a large number of layers within it, only one of which is the optical/transmission layer (which includes bitrates, jitter specifications, optical signal specifications and so on). The SONET and SDH standards come with a host of features for isolating and identifying signal defects and their origins.
 SONET/SDH network management protocols
SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom language for managing and reconfiguring SONET network elements. TL1 (or whatever command language a SONET Network Element utilizes) must be carried by other management protocols, including SNMP, CORBA and XML.
There are some features that are fairly universal in SONET Network Management. First of all, most SONET NEs have a limited number of management interfaces defined. These are:
* Electrical Interface. The electrical interface (often 50 Ω) sends SONET TL1 commands from a local management network physically housed in the Central Office where the SONET NE is located. This is for "local management" of that NE and, possibly, remote management of other SONET NEs.
* Craft Interface. Local "craftspersons" can access a SONET NE on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be hooked-up to a console server, allowing for remote out-of-band management and logging.
* SONET and SDH have dedicated Data Communication Channels (DCC)s within the section and line overhead for management traffic. Generally, section overhead (regenerator section in SDH) 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 (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks.
An interesting fact about modern NEs is that, to handle all of the possible management channels and signals, most NEs actually contain a router for routing the network commands and underlying (data) protocols.
The main functions of Network Management include:
* Network and NE Provisioning. In order to allocate bandwidth throughout a network, each NE must be configured. Although this can be done locally, through a craft interface, it is normally done through a Network Management System (sitting at a higher layer) that in turn operates through the SONET/SDH Network Management Network.
* Software Upgrade. NE Software Upgrade is in modern NEs done mostly through the SONET/SDH Management network.
* Performance Management. NEs have a very large set of standards for Performance Management. The PM criteria allow for monitoring not only the health of individual NEs, but for the isolation and identification of most network defects or outages. Higher-layer Network monitoring and management software allows for the proper filtering and troubleshooting of network-wide PM so that defects and outages can be quickly identified and responded to.
With recent advances in SONET and SDH chipsets, the traditional categories of NEs are breaking down. Nevertheless, as Network architectures have remained relatively constant, even newer equipment (including "Multiservice Provisioning Platforms") 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.
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 (ADMs) are the most common type of NEs. 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" (where the full line rate signal is supported), 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 NE and sent out from the high speed side, or vice versa.
 Digital cross connect system
Recent digital cross connect systems (DCSs or DXCs) 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
Currently, SONET (and SDH) have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection (i.e. the ability to transmit traffic even when part of the network has failed), and are key in understanding the almost worldwide usage of SONET and SDH for moving digital traffic. The three main architectures are:
* Linear APS (Automatic Protection Switching), also known as 1+1: This involves 4 fibers: 2 working fibers (1 in each direction), and two protection fibers. Switching is based on the line state, and may be unidirectional, with each direction switching independently, or bidirectional, where the NEs at each end negotiate so that both directions are generally carried on the same pair of fibers.
* UPSR (Unidirectional Path Switched Ring): In a UPSR, two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines the higher-quality copy and decides to use the best copy, thus coping if deterioration in one copy occurs 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 if we had an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, then 100% of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring, say B and C could only act as pass through nodes. The SDH analog of UPSR is Subnetwork Connection Protection (SNCP); however, SNCP does not impose a ring topology, but may also be used in mesh topologies.
* BLSR (Bidirectional Line Switched Ring): BLSR comes in two varieties, 2-fiber BLSR and 4-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. BLSRs trade cost and complexity for bandwdith efficiency as well as the ability to support "extra traffic", which can be pre-empted when a protection switching event occurs. 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. The best case of this is that 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 (MS-SPRING).
Clock sources used by Synchronization in telecommunications networks are rated by quality, commonly called a 'stratum' level. Typically, a network element uses the highest quality stratum available to it, which can be determined by monitoring the Synchronization Status Messages(SSM) of selected clock sources.
As for Synchronization sources available to an NE, these are:
* Local External Timing. This is generated by an atomic Caesium clock or a satellite-derived clock by a device in the same central office as the NE. The interface is often a DS1, with Sync Status Messages supplied by the clock and placed into the DS1 overhead.
* Line-derived timing. An NE can choose (or be configured) 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, an NE can go into "holdover" until higher quality external timing becomes available again. In this mode, an NE uses its own timing circuits as a reference.
 Timing loops
A timing loop occurs when NEs in a network are each deriving their timing from other NEs, 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, the technique of concatenation was developed, whereby smaller multiplexing containers (eg, STS-1) are inversely multiplexed to build up a larger container (eg, STS-3c) 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.
Virtual Concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly arbitrary size (e.g. 100 Mbit/s) without the need for intermediate NEs to support this particular form of concatenation. Virtual Concatenation increasingly leverages X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.
Link Capacity Adjustment Scheme (LCAS) 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 to enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).
 See also
* List of device bandwidths
* Routing Wavelength Assignment (RWA)
1. ^ International Engineering Consortium SONET Tutorial, undated, URL retrieved on 21 April 2007
 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 (SIF)
* Network Connection Speeds Reference
* Next-generation SDH and MSPP
* The Future of SONET/SDH (pdf)
* Telcordia GR 253 CORE: SONET Transport Systems: Common Generic Criteria
* 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 (OAM&P) - Communications
* ITU-T recommendation G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)
* ITU-T recommendation G.783: Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks
* ITU-T recommendation G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)
Retrieved from "http://en.wikipedia.org/wiki/Synchronous_optical_networking"
Categories: SONET | Fiber-optic communications | Network protocols
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