43952335-SDH | Electrical Engineering | Data Transmission

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SDH/SONET : SDH/SONET: Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are two closely related multiplexing protocols for transferring 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 fiber wire without synchronization problems. SONET and SDH wer
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  SDH/SONET : SDH/SONET: Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are two closelyrelatedmultiplexingprotocols for transferring multipledigitalbit streams usinglasersor light-emitting diodes    (LEDs) over the sameoptical fiber . The method was developed to replace thePlesiochronous Digital Hierarchy    (PDH) system for transporting larger amounts of telephonecalls anddatatraffic over the same fiber wire   without synchronization problems.SONET and SDH were srcinally designed to transportcircuit modecommunications (eg, T1, T3) from avariety of different sources. The primary difficulty in doing this prior to SONET was that the synchronizationsource of these different circuits were different, meaning each circuit was actually operating at a slightlydifferent rate and with different phase. SONET allowed for the simultaneous transport of many differnet circuitsof differing srcin 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 neturality and transport-oriented features, SONET was the obvious choicefor transporting ATM (Asynchronous Transfer Mode) frames, and so quickly evolved mapping structures andconcatenated payload containers so as to transport ATM connections. In other words, for ATM (and eventuallyother 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 whichATM frames, IP packets, or ethernet is placed.Both SDH and SONET are widely used today: SONET in theU.S.andCanadaand SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide marketdictate that SONET now is considered the variation.The two protocols are standardized according to the following: y   SDH or  Synchronous Digital Hierarchy standard developed by theInternational TelecommunicationUnion(ITU), documented in standard G.707 and its extensionG.708  y   SONET or  Synchronous Optical Networking standard as defined byGR-253-COREfromTelcordia  andT1.105fromAmerican National Standards Institute  Structure of SONET/SDH signals SONET and SDH often use different terms to describe identical features or functions, sometimes leading toconfusion that exaggerates their differences. With a few exceptions, SDH can be thought of as a superset of SONET. The two main differences between the two: y   SONET can use either of two basic units for framing while SDH has one y   SDH has additional mapping options which are not available in SONET. The basic unit of transmission The basic unit of framing in SDH is aSTM-1(Synchronous Transport Module level - 1), which operates at155.52 Mbit/s. SONET refers to this basic unit as anSTS-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, theSTS-1(Synchronous Transport Signal - 1), operatingat 51.84 Mbit/s - exactly one third of an STM-1/STS-3c. Some manufacturers also support the SDH equivalentSTM-0, but this is not part of the standard. Framing In packet oriented data transmission such asEthernet, a packet frame usually consists of a header and a payload,with the header of the frame being transmitted first, followed by the payload (and possibly a trailer, such as aCRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists, but instead of the overhead being transmitted before the payload, it is interleaved, with partof the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next partof the payload, until the entire frame has been transmitted. In the case of an STS-1, the frame is 810 octets insize 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 beentransmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster thanSTS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload. This is also repeated nine timesover until 2,430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH, this isnormally 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 theoverhead 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. However, the standards are extremelysimilar in implementation, such that it is 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. What it means is that thesame octet position in each frame comes past every 125 microseconds. If one octet is extracted from the bitstream every 125 microseconds, this gives a data rate of 8 bits per 125 microseconds - or 64 kbit/s, the basicDS0telecommunications rate. This relation allows an extremely useful behaviour of synchronous opticalnetworking, which is that 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. Furthermore, it shows that a relatively simple device is all that isneeded to extract a datastream from an SDH framed connection and insert it into a SONET framed connectionand vice versa.In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers tothe signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be saidto contain 3 STS-1s. SDH Frame ASTM-1Frame. The first 9 columns contain the overhead and the pointers. For the sake of simplicity, theframe is shown as a rectangular structure of 270 columns and 9 rows, but the protocol does not transmit the bytes in this order in practiceFor the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and 9 rows. The first 3rows and 9 columns contain Regenerator Section Overhead (RSOH) and the last 5 rows and 9 columns containMultiplex Section Overhead (MSOH). The 4th row from the top contains pointersTheSTM-1(Synchronous Transport Module level - 1) frame is the basic transmission format for SDH or thefundamental frame or the first level of the synchronous digital hierarchy. The STS-1 frame is transmitted inexactly 125 microseconds, therefore there are 8000 frames per second on a fiber-optic circuit designated OC-1(optical carrier one). The STM-1 frame consists of overhead plus a virtual container capacity. The first 9columns of each frame make up the Section Overhead, and the last 261 columns make up the Virtual Container (VC) capacity. The VC plus the pointers (H1, H2, H3 bytes) is called the AU (Administrative Unit).Carried within the VC capacity, which has its own frame structure of 9 rows and 261 columns, is the PathOverhead and the Container. The first column is for Path Overhead; it¶s followed by the payload container,which can itself carry other containers. Virtual Containers can have any phase alignment within theAdministrative Unit, 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 systemitself, 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 y   Total content : 9 x 270 bytes = 2430 bytes y   overhead : 9 rows x 9 bytes y    payload : 9 rows x 261 bytes  y   Period : 125 sec y   Bitrate : 155.520 Mbit/s (2430 x 8 bits x 8000 frame/s ) y    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 top left corner.
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