Synchronous Digital Hierarchy

The Synchronous Digital Hierarchy (SDH ) is one of the multiplexing techniques in the field of telecommunication that allows the merging of lower rate data streams to a high-rate data stream. The entire network is synchronous.

General

1985 was started in the USA, under the name of SONET ( Synchronous Optical Network) to specify a new generation of optical digital transmission systems ( Plesiochronous Digital Hierarchy ) should have significant advantages over the widespread PDH technology. For compatibility reasons, they should be able to also carry signals of the PDH technique, but otherwise form a new hierarchy of bit rates. As a basic bit rate in the U.S. was therefore 51 Mbit / s chosen and called STS-1 ( Synchronous Transport System, Step 1). With this bit rate, the bit rate of 45 Mbit plesiochronic / s could be transmitted. The next stage uses the multiplex factor of 3 and supplies 155 Mbit / s (STS -3). It transports three individual STS -1, so it has three structured information fields that carry the payload. But this is often inconvenient, which is why a variant is defined, instead of three fields has a coherent field with triple size. This process is called STS- 3c, the c for " concatenated " ( concatenated, connected) is. Another method is called concatenation Virtual Concatenation and serves the same goal: increasing the associated information field. It is marked on the label with a trailing -vc ( virtual concatenation ). By introduction of virtual concatenation, it became possible over the SDH network bit rate in increments of n times of 2 Mbit / s (for example, 2M, 4M, 6M, 10M, 20M, 40M, 50M, 100M ) as well as high bit rate data signals ( such as to multiplex Gigabit Ethernet) without data loss rates and transfer.

From the International Organization for Standardization (ITU-T Recommendations G.707 ) the concept of a new hierarchy for digital transmission systems has been addressed and standardized under the name SDH. However, in contrast to the North American SONET chosen as the basis, the 155 Mbit / s level, called STM -1 ( Synchronous Transport Module, Step 1).

The data is transferred transparently in containers using " Link Connections" and " trails " through the SDH network. In case of failure of an SDH network node or a fiber optic SDH network elements, the data streams within a few milliseconds to a backup path automatically switch ( Protection).

Compared to the previous PDH SDH networks is equipped with significantly enhanced OAM functions, ie faults (defects and anomalies) can be clearly identified and reported differentiated. The interfaces used must have a maximum bit error rate of 10-10 and even single bit errors in an SDH signal any rate are detectable. Total SDH networks are designed for the highest quality of service and availability.

Introduction

SDH is standardized by the ITU -T ( G.707, G.783, G.803 ). It is derived from the SONET ( Synchronous Optical Network ), has been developed by AT & T and Bell Core 1985. The standardization of SONET was carried out by ANSI. Today the differences between SONET and SDH are small, the two concepts are interoperable. Since PDH for broadband ISDN with bit rates above 100 Mbit / s is of limited use, SDH was primarily designed as a transmission system for B -ISDN. However, it is also suitable for the transparent transport of all interest bearing capacities ( ATM cells multiplexed signals of the PDH hierarchy, SAN signals, Ethernet aggregation, etc.).

• Physical interface: In general, fiber, microwave or satellite link
• Regenerator: refreshing the muffled and distorted signals with respect to timing and amplitude
• Multiplexer: plesiochronic and / or merge or insert / decouple signals synchronous signals to high-bandwidth SDH bit streams
• VC ( Virtual Container ): transport container with user data. VC-4 layer governs Ein-/Ausgliedern (mapping) of 140 Mbit / s signals, (E4), of the VC-3 mapping of Layer 34/45 Mbit / s signals ( E3/T3 ) and VC layer 12, the mapping of 2 Mbit / s signals ( E1)

Basic properties

SDH constitutes a synchronous time-division multiplexing method, the similar PDH multiplex hierarchy includes. The aim is the best possible utilization of the available transmission capacity of optical fibers. In contrast to PDH, the clocks of the individual transmission sections are in synchronism with a very small deviation. The PDH technology works with a maximum tolerance of 50 ppm, the SDH technology more than 10 times more accurate. The principle of SDH is simple: the byte streams from n sources at the rate R n · R are summarized by synchronous multiplex to a byte stream in the rate.

In contrast to PDH, it is possible by the synchronous operation of the SDH, multiplex signal of order n 1 directly from the signals of all lower hierarchy levels 1, ..., n to form. Similarly, a multiplexed signal of low order are extracted directly from the higher levels of the hierarchy framework. These functions are referred to as add / drop. Said synchronous multiplex process also enables the transport of bit streams such as ATM cells and PDH multiplex signals. This function is called " cross-connect ".

SDH hierarchy knows the stages according to the table. The level frames are denoted by n STM- n ( Synchronous Transport Module -n). The steps of the STM-1, STM -4, STM -16 and STM-64 are used frequently. SDH reserved for OAM functions ( Operations, Administration and Maintenance) about 5 % of the gross data rate.

SDH hierarchy levels

The fields marked with * levels are included in the standard. Items marked with ** in SONET hierarchy levels have the greatest spread. STM-1 can be electrical or optical interfaces are executed STM- 4 and above only with optical interfaces.

SDH Network Elements

Basic types of network elements in SDH multiplex technology are defined as follows:

• REG ( regenerators ) amplify optical signals. Distinction from the purely optical amplifier, as used in the OTN SDH based, is the conversion of the received optical signal into an electrical one. The first electrical signal is amplified, synchronized in time and corrected in its shape. Thereafter, it is converted back into an optical signal and transmitted. The purely optical amplifier comes without the conversion into an electrical signal.
• TM ( Terminal Multiplexer) usually have several plesiochronic subscriber interfaces and one or more interfaces for SDH network. Grasp tributary signal ( engl. adj. Tributary, on forward ) derived from its sub- network elements or from terminals to an aggregate signal of SDH hierarchy, for example, STM -1, together, which is passed into the SDH network.
• ADM ( Add- Drop Multiplexer ), are an extension of the terminal multiplexer. They have two aggregate- side interfaces for SDH signals of the same hierarchical level. An ADM can split the received signals of the two aggregates interface in the partial signals contained in it and one of them the corresponding Tributaryschnittstellen forward (drop), but otherwise by passing signals unchanged between the two aggregates interfaces. In the reverse direction adds to the ADM signals arriving at the Tributaryschnittstellen, instead of the removed part signals back into the aggregate side signals a (add). SDH networks in a ring require ADMs TMs shall not be used in rings.
• Cross-connect multiplexer and DCS ( Digital Cross -Connect System) ( in Europe also called DXC ) in turn are extensions of ADM. You have at least 4 sets side interfaces and can both taken from these partial signals or the arrived at the Tributaryschnittstellen signals on the VC- level interconnect arbitrarily.

Functional model of the SDH

SDH contains features that are assigned to the OSI layer 1. The functional blocks and their stratification are characterized by the following expressions:

• Optical sections ( photonic ) refer optical to optical signals on optical fibers and conversions - electric and vice versa.
• Regenerator section ( regenerator section ) indicates a fiber portion which is disposed between regenerator (REG ), or between a regenerator and another network element. The regenerator section is assigned to the RSOH.
• Multiplexer section (Multiplex Section) connects two multiplexers (also for several regenerators away ). The multiplex section terminating connecting two ports of the same STM-N rate. The multiplex section is assigned to the MSOH.
• HO path ( High Order path or trail ) can be transmitted through multiple network elements (eg ADM, DCS and regenerators ) away (without re-synchronization ). AU4 mapped signal as it contains a VC4 (or a concatenation of VC4 containers, for example, for the ATM data signals ) having a user data signal rate or E4 serves as a transport layer for the LO paths. The HO- path associated with the VC4 POH. Furthermore, there is also the rate VC3 HO paths when they are in a eingemappt AU3.
• LO path ( low order path or trail ) the rates VC11, VC12, VC3, VC4 and are packed in a transport the actual data signals with bit rates equivalent to DS1 to E3. The LO path associated with the VC11/12/3-POH.

These layers are characterized by independent OAM functions (eg transmission error monitoring, alerting, Protection), which operate independently of the parent transfer layer. For example, HO- level, the bit error rate can be measured without having to rely on data from the multiplex sections. In the reverse direction, but the subordinate layer is occupied by an error signal, ie the failure of a multiplex section all contained HO- LO paths and paths are discarded upon failure of the higher layer.

Topology of SDH networks

In most countries, the transport networks in SDH technology have now been removed and the old PDH technology is largely replaced. Therefore topologies various forms, are realized, they are based on the geographical requirements. An important feature of SDH technology is the automatic switchover to alternative paths in case of failure ( Protection). As an example to explain the operation of the Protection of the double ring is often chosen, during normal operation, a ring is used, the so-called commuting. The second ring is used as a cold reserve, as a substitute. Byte streams are introduced by the ADM ( add-drop multiplexer ) in the working path and taken from him. When a failure in the working path, the APS (Automatic Protection System) switches from the working path to the backup path. This topology is MS -SPRING (Multi- Section Shared Protection Ring ) from the STM -16 standardized under the name 4- fiber upwards.

A simplified version of Ring Protection is called a 2- fiber MS- SPRing, where half of the available bandwidth is kept free to substitute routes circuit, or filled with low priority traffic. This bandwidth will be applied in the event of a fault with the traffic of the failed ring distance and the low priority traffic is discarded.

MS- SPRing mechanisms are only suitable for ring structures and therefore particularly applicable in backbone structures. For linear structures, the protocol MSP ( Multiplex Section Protection) was developed, there usually protects the back up exactly one fiber link (1 1). Developments demonstrate the Ersatzschaltungsweg with low priority traffic (1:1) or protect multiplex multiple sections with a substitute (1: N). These protocols work on the multiplex section level, that is, the equivalent circuit is used for the entire optical fiber.

For heavily meshed structures offers the path-based Sub - Network Connection Protection, which offers on VC level, a 1 1 protection.

All such protective mechanisms have in common that, according to the standard equivalent circuit measures after detecting a fault must be completed automatically within 50 milliseconds. In modern SDH devices actually achieved switching times are remarkably lower (depending on the cable length / propagation delay is about 1 millisecond per 200 km ).

In general, all compounds are 1 1 protected in today's SDH transmission networks.

Frame assembly and multiplexing structure

SDH transmit useful data and control data in a sequence of frames (frame) that are sent serially. Each frame consists of overhead ( control data ) and payload (user data and other data). The STM-1 frame consists of the areas of payload, RSOH ( regenerator section overhead ) and MSOH (Multiplex Section Overhead ) and AU pointer. The frame is transmitted row-wise from left to right and from top to bottom. The AU pointer ( Administrative Unit ) point to the location of the payload in the payload area.

The terms of this frame structure are defined as follows:

• Container (C ) areas in the frame, corresponding to a particular payload. The size of the container was fitted to the data rates in the PDH - defined. Inserting plesiochronic data streams requires justification operations (bit - or Bytesynchron ). Each container is added to the POH ( path overhead ) to describe the user data.
• Virtual container (VC -i) are divided into VC low-order ( VC11 VC12 up, VC2 and VC3 ) and higher order VC (VC -4). Some VC low-order can be combined into such a higher order, but need not.
• Tributary Unit (TU -i) are required because of the difference out of the SDH VC next phase positions may have on the multiplexed frame in relation. Therefore, the VC are embedded in the larger University. The beginning of a VC within a TU is indicated by pointer.
• Tributary Unit Group ( TUG ) summarize TU -i together according to the diagram.
• Administrative Unit ( AU i) have the advantage over higher order VC the same function as the tributary unit group compared with VC low-order.
• Administrative Unit Group ( AUG) are formed analogously to the tributary unit group of AU -3 and AU-4. The associated pointers are the AU pointer in row 4 bytes 1-9.
• Synchronous Transport Module (STM -n): the context of higher order () are formed of many frames of the next lower level in the hierarchy accordingly by multiplexing.

The introduction of the pointer allowed ( in contrast to PDH), the direct addressing of a Nutzdatensignales in a high bit-rate signal, to have the complete signal without demultiplexing. Furthermore, small clock differences between the network elements can be compensated via pointers.

Note: ATM signals can be mapped directly into a C4 with a transfer rate of about 150 Mbit / s ( the DSLAM).

Adaptation of the AU pointer

An adaptation of the Administrative Unit pointer at any time. For this purpose, the following situations are responsible:

• Virtual containers are not bonded to frame boundaries.
• Under some circumstances "wandering " virtual container (VC )
• In every fourth frame pointer can be customized according to notice.
• If desired, the pointer structure is linked (transport container groups in turn contain, etc.).

Construction of an STM -N signal

Standards in accordance with ITU- T

• G.707/Y.1322 " Network Node Interface for the Synchronous Digital Hierarchy (SDH) ", defines the bit rates, the principles of the multiplex structure and the signal for SDH structure at the network node interface ( NNI)
• G.780 " Vocabulary of terms for synchronous digital hierarchy (SDH ) networks and equipment ," a glossary
• G.783 " Characteristics of synchronous digital hierarchy (SDH ) equipment functional blocks", defines the belonging to SDH function blocks in the form of information models
• G.784, " Synchronous digital hierarchy (SDH ) management ," describes the operation associated with SDH technology
• G.803, " Architecture of transport networks based on the synchronous digital hierarchy (SDH ) "

LAPS

In order packet-oriented IP data to be mapped directly into an SDH container transport protocol LAPS (Link Access Procedure SDH) has been developed. ITU-T X -85 defined IP over SDH and ITU-T - X 86 defines Ethernet over SDH using LAPS.

Next Generation SDH

The SONET / SDH was created to transmit voice and data traffic optically with higher transmission rates. The payload of the container is therefore defined backwards compatible with the data transfer rates of PDH hierarchy. The original idea was that the data traffic of IT facilities is first transmitted electrically through a standard PDH bit rate such as 2 - Mbit / s ( E1), and then in an SDH multiplexer with other PDH signals together to form a optical SDH aggregate signal is multiplexed. This process is still running, but at higher data rates of the unused part of the transmission capacity is high: for example, is required for the data transmission rate of Ethernet traffic of 100 Mbit / s STM -1 signal at 155 Mbit / s.

In order to efficiently transmit voice and data over a common platform, the ITU was the GFP protocol, the virtual concatenation ( VCAT ) and defines the granular addition or disconnection of capacity ( LCAS ). These extensions of the conventional SDH is called Next Generation SDH.

General Framing Procedure

When GFP protocol (ITU- T G.7041 ) Ethernet frame and under other common network technologies ( Fibre Channel, ESCON, FICON, Gigabit Ethernet, Digital Video ) will be imaged by GFP mapping into SDH containers. Two modes are defined: Transparent GFP ( GFP -T) and frame -mapped GFP ( GFP -F).

Virtual Concatenation ( VCAT )

But because the SDH defined container sizes for transmitting data packets were not optimal, in addition, the (English virtual concatenation, ITU-T G.707 ), several containers ( VC12, VC3 and VC4 ) " virtual concatenation " was introduced. This results in a correspondingly larger payload. Only two VC3 are now supported for Fast Ethernet instead of a VC4 needed. The advantage of the virtual concatenation: The individual containers are transported separately through the network, the hardware needs - as opposed to " contiguous concatenation " - only at the network edges to the new functionality to be customized.

Link Capacity Adjustment Scheme ( LCAS )

By means of the LCAS protocol ( ITU- T G.7042 ) can arise during operation single virtual container to or disconnected, so that a quasi- dynamic change of the transport capacity in the network with relatively short reaction time and without operator intervention ( for example, disturbances in network) is possible. This allows, for example, compounds ( Ethernet over SDH, ...) on two paths ( 50/50) is divided, whereby, when a failure of a path, the connection can continue to operate, albeit with reduced / half bandwidth. A protective function using LCAS has over other methods such as the SNCP advantage that no additional transmission capacity is needed (for SNCP is twice the bandwidth used - main and substitute each with full target bit rate ).

Future of NG- SDH and NG- SONET

GFP and LCAS allow the SDH packet data without loss of bandwidth cost to transmit. 50 % of the transmission capacity are, however, needed to protect the SDH trails, which is seen unfavorably priced. Restoration using GMPLS allows the SDH high-speed lines ( STM16 STM64 or ) exploit more effectively. At the Restoration (Shared mesh) is a substitute dynamically calculated in advance; multiple routes share a standby link. The NG - SDH is competitive in the wide area networks with the IP/MPLS- and Ethernet networks.

In the optical testbed VIOLA in Germany the latest optical network techniques such as Ason GMPLS and the Next Generation SDH testing.

Multiservice Platforms

IP-enabled NG- SDH network nodes use the SDH or WDM as a transport network is called a MSPP (Multi Service Provisioning Platform) or as MSTP ( Multiservice Transport Platform).

Examples:

• SDH and MPLS - capable Cisco ONS 15454 SDH Multiservice Provisioning Platform ( MSPP ), Corrigent CM -100 Packet ADM
• DWDM, SDH, MPLS, ATM, and Ethernet -enabled: Alcatel 1660SM ( Rel.5 )
• OTN, GMPLS, SDH, MPLS and ATM - capable Alcatel 1678MCC, ADVA FSP 3000

In October 2005, the first multifunktonsfähige platform was at the Broadband World Forum in Madrid, presented which combines a 100 % mix of Ethernet / MPLS, SONET / SDH, and WDM / OTN in a single device. The Alcatel- Lucent 1850 Transport Service Switch makes no difference between packet-oriented (IP) and circuit-switched services. It transports data services independently.