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The Evolution of SDH/SONET in Digital Transmission Systems

Learn about SDH and SONET, their advantages, basic transmission rates, and why they replaced PDH systems in flexible high-capacity networks. Understand the history, multiplexing techniques, and standardization efforts that shaped these vital technologies.

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The Evolution of SDH/SONET in Digital Transmission Systems

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  1. Chapter 8 Digital Transmission Systems Part 2 Bahman R. Alyaei

  2. 10 SDH and SONET • SDH is an acronym for Synchronous Digital Hierarchy. It is an European development. • SDH: is a hierarchical set of digital transport structures (Overhead), standardized for the transport of suitably adapted Payloads over physical transmission networks. • SONET is an acronym for Synchronous Optical Network . It is a North American development. • The two (SDH and SONET) are very similar. Bahman R. Alyaei

  3. Continue… Bahman R. Alyaei

  4. Continue… • Either one can accommodate the standard E1 family (i.e., 2.048 Mbps, etc.) and DS1family (i.e., 1.544 Mbps, etc.) of line rates. • SDH/SONET is replacing PDH systems in the Transport Network. • By Transport Network we mean the flexible high-capacity transmission network that is used to carry all types of information. • By Flexible we mean that telecommunications operators are able to easily modify the structure of the transport network from the centralized management system. Bahman R. Alyaei

  5. 10.1 Advantages of SDH/SONET • SDH/SONET is based on the principal of direct synchronous multiplexing, where separate, slower signals can be multiplexed directly onto higher speed SDH/SONET signals without intermediate stages of multiplexing. • SDH/SONET is more flexible and reliable than PDH and provides advanced network management and maintenance features. Bahman R. Alyaei

  6. Continue… • Can be used in Long-Haul Networks, Local Networks and Loop Carriers, and it can also be used to carry CATVvideo traffic, ATM, and ISDN. • In SDH/SONET 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/SONET makes individual channels “visible” and they can easily be added and dropped. Bahman R. Alyaei

  7. Continue… • The data rates for optical transmission are standardized (i.e., vendor independent). • Different systems are included in standards, for example, Terminal, Add/Drop, and Cross-Connection Systems. • These systems make SDH/SONET networks more flexible than PDH systems, which include only terminal multiplexer functionality. Bahman R. Alyaei

  8. 10.2 Why SDH/SONET • Originally, all communications in the telephone network was analog. • Analog lines or analog microwave links were used to connect to switching offices. Bahman R. Alyaei

  9. Continue… • In about 1962, the network providers began using digital communications between switching centers. • This was PDH system (DS-Carrierin US and E-Carrierin Europe). Bahman R. Alyaei

  10. E1 or DS1 E1 or DS1 Continue… • As communications needs grew, many DS-Carrier or E-Carrier lines were needed between switching centers. • In the late 1970’s optical communications began to be used to interconnect switching offices. Bahman R. Alyaei

  11. Continue… • Prior to standardization, every manufacturer of optical communications used their own framing. • The ANSI and the ITU began work in 1986 to define standards for optical communications. • Both bodies finalized their first set of standards in 1988. Bahman R. Alyaei

  12. Continue… • They defined the following: • Optical and Cupper interfaces (wavelength, frequency, power, etc.). • Rates, frame formats, and network elements (Layers). • Operations, Administration, and Maintenance (OAM) functions including monitoring for valid signal, defect reporting, and alarms due to abundant overhead bits. Bahman R. Alyaei

  13. 10.3 Basic SDH/SONET Transmission Rates (Hierarchy) • SONET and SDH converge at SDH’s 155 Mbps base level, defined as STM-1 (Synchronous Transport Module-1). • The base level for SONET is STS-1 (Synchronous Transport Signal-1) or OC-1 (Optical Carrier-1) and is equivalent to 51.84 Mbps. • Thus, SDH’s STM-1 is equivalent to SONET’sSTS-3 (3 x 51.84 Mbps = 155.52 Mbps). • Higher SDH rates of STM-4 (622 Mbps), STM-16 (2.4 Gbps), and STM-64 (10 Gbps) have also been defined. Bahman R. Alyaei

  14. Bahman R. Alyaei

  15. Continue… Data Rates of SONET and Corresponding SDH Data Streams Bahman R. Alyaei

  16. Continue… • Multiplexing is accomplished by combining or interleaving multiple lower-order signals (1.5 MbpsDS1 carrier, 2 Mbps E1 carrier, etc.) into higher-speed circuits (51 Mbps STS-1, 155 Mbps STM-1, etc.). • By changing the SONET standard from Bit- Interleaving to Byte-Interleaving, it became possible for SDH to accommodate both transmission hierarchies. • This modification allows an STM-1signal to carry multiple 1.5 Mbps or 2 Mbps signals and multiple STM signals to be aggregated to carry higher orders of SONET or SDH tributaries. Bahman R. Alyaei

  17. 11 SDH STM Signal • SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbps signals with required Overhead to form a frame called STM-1. • SDH is a Byte-Interleaving multiplexing system. • An STM is the information structure used to support Section Layer Connections in the SDH. Bahman R. Alyaei

  18. Continue… • It consists of information Payload and Overhead (OH) information fields organized in a block frame structure which repeats every 125 μS. • The information is suitably conditioned for serial transmission on the selected media at a rate which is synchronized to the network. • STM-1 is the base level of SDH. Bahman R. Alyaei

  19. Continue… • The STM-1 frame, is created by 9 segments of 270 bytes each (1-byte = 8-bits) • The first 9 bytes of each segment carry Overhead (OH) information. • The remaining 261 bytes carry Payload. • When visualized as a block, the STM-1 frame appears as 9 rows by 270 columns of bytes. • The STM-1 frame is transmitted row-by-row. • Row #1 first, with the most significant bit (MSB) of each byte transmitted first, then the Row #2 and so on, up to Row #9. Bahman R. Alyaei

  20. Continue… • The STM-1 frame lasts for 125 μS, in other words, the 9 row segments will be transmitted in a total time equal to 125 μS. • This will permit SDH to easily integrate existing digital services into its hierarchy. • Therefore, there are 8000 frames per second. Bahman R. Alyaei

  21. Segment no. 7 270 Bytes in (125/9) μS The STM-1 frame Bahman R. Alyaei

  22. STM-1 frame visualized as a block, and the direction of transmission Bahman R. Alyaei

  23. Continue… Bahman R. Alyaei

  24. STM-1 frame visualized as a block Bahman R. Alyaei

  25. Continue… • Hence, the STM-1 frame rateRf is Rf = 8000 frames per second • The bit rate Rb of STM-1 frame is calculated as follow: Rb = Rf x Cf , where Rfis the frame rate (frames/second). Cf is the frame capacity (bits/frame). • The frame capacity of a signal is the number of bits contained within a single frame. Bahman R. Alyaei

  26. Continue… • We know that the frame rate is Rf = 8000 frames/second. • Cfis calculated as follow Cf = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame • Then, the bit rate Rb of the STM-1 signal is calculated as follows: Rb = 8000 frames/second x 19,440 bits/frame = 155.52 Mbps Bahman R. Alyaei

  27. Continue… • The multiplexing of multiple data stream, plays an important role in SDH. • Byte Interleaving scheme is used to multiplex multiple data stream. • The higher transmission levels (Multiplex) such as STM-4 and STM-16 of the SDH Hierarchy are generated from integer multiples of STM-1 signal. • In general, STM-N signal is generated by Byte InterleavingN STM-1signal. Bahman R. Alyaei

  28. Continue… Multiplexing of STM-1 to generate STM-N Bahman R. Alyaei

  29. Continue… STM-N signal frame structure Bahman R. Alyaei

  30. Continue… • Example: • An STM-4 signal will be created by Byte Interleavingfour STM-1 signals. • The basic frame rate remains 8,000 frames per second, but the capacity is quadrupled, resulting in a bit rate of 4 x 155.52 Mbps or 622.08 Mbps. Bahman R. Alyaei

  31. Continue… Multiplexing of STM-1 to generate STM-4 Bahman R. Alyaei

  32. 11.1 SDH STM-1 Frame Structure • As we know that, the SDH frameSTM-1 consists of two parts: • The FirstNine Columns comprise the Overhead (OH), occurs at a rate 9 x 9 x 8 x 8000 = 5.184 Mbps. • While the remainder is called the Payload, which is also called Virtual Container (VC), occurs at a rate 9 x 261 x 8 x 8000 = 150.336 Mbps. Bahman R. Alyaei

  33. Continue… • The OH is further divided into: • Section Overhead (SOH). • Administrative Unit Pointer (AU-PTR). • The Payload or Virtual Container (VC) is further divided into: • Path Overhead (POH): One column. • Container (C): 260 columns and data rate given by 9 x 260 x 8 x 8000 =149.76 Mbps. Bahman R. Alyaei

  34. 125 μS 1 9 10 11 270 1 3 SOH Container 4 9 rows AU-PTR SOH POH 9 9 columns Overhead (OH) 261 columns Virtual Container (VC) SDH frame STM-1 structure Bahman R. Alyaei

  35. Continue… • The SOH dedicates • Three Rows for the Regenerator Section Overhead (RSOH) and • Six Rows for the Multiplexer Section Overhead (MSOH). • Rate of RSOH and MSOH is given by RSOH = 3 x 9 x 8 x 8000 = 1.728 Mbps. MSOH = 6 x 9 x 8 x 8000 = 3.456 Mbps. Bahman R. Alyaei

  36. 125 μS 1 9 10 11 270 1 3 RSOH Container 4 9 rows AU-PTR MSOH POH 9 9 columns Overhead (OH) 271 columns Virtual Container (VC) SDH frame STM-1 structure Bahman R. Alyaei

  37. 11.2 The Truck Analogy • SDH frame functions as a transport truck which has a tractor and a container type trailer. • It packs the signals of different hierarchies into packages of different sizes like packing cargoes and then loads them into the truck. Bahman R. Alyaei

  38. Continue… • The contents carrier in the container are real goods. • These are analogous to customer traffic, being carried in the Payload area of SDH frame. SDH frame Bahman R. Alyaei

  39. 11.3 The Function of OHs • The OH within the SDH signal supports network management at both the Path and Section levels. • To realize layered monitoring, the OH is classified into SOH and POH. • SOH and which includes RSOH and MSOH, is responsible for the section layer OAM. Bahman R. Alyaei

  40. Continue… • SOH functions are: • Frame alignment pattern. • Parity check. • STM-1 identification. • Alarm information. • Automatic protection switching. • Data communication channel. • Voice communication channel. • User channel. Bahman R. Alyaei

  41. Continue… • The POH is responsible for the Path layer OAM functions. Bahman R. Alyaei

  42. 11.4 Function of The Pointer • SDH network is intended to be synchronous network. • However, there will always be slight timing differences because different clocks are being used or the same clock is being distributed over long distances. • SDHPointers allow this limited asynchronous operation within the synchronous network. • It points the location of the VC in the STM frame. Bahman R. Alyaei

  43. 12 SDH Signal Hierarchy Typical SDH Communication Network Bahman R. Alyaei

  44. Continue… • There are three Sections in the SDH signal hierarchy: • Path. • Multiplex Section. • Regenerator Section. • The Overheads (OHs) are always generated at the beginning of a section and only evaluated at the end of a section. Bahman R. Alyaei

  45. Continue… The SDH Layer Model Bahman R. Alyaei

  46. 12.1 SDH Network Elements • The SDHsignal is layered to divide responsibility for transporting the Payload through the network. • Each SDHNetwork Element (NE) is responsible for • Interpreting and generating its overhead layer, • Communicating control and status information to the same layer in other equipment, • Terminating its overhead layer. Bahman R. Alyaei

  47. Continue… • As the Payload travels through the SDH network, each layer is terminated by one of a general class of NEs named • Regenerator Section Terminating Equipment (RSTE), • Multiplexer Section Terminating Equipment (MSTE), • Path Terminating Equipment (PTE). Bahman R. Alyaei

  48. 12.1.1 Path Terminating Equipment (PTE) • PTE is an entry-level path-terminating terminal multiplexer, acts as a concentrator of E1s as well as other tributary signals. Terminal multiplexer example Bahman R. Alyaei

  49. Continue… • PTE is a terminating multiplexer. • It is responsible for adding first order POH, MSOH,and RSOH to the data Container (C). • Its simplest deployment would involve two terminal multiplexers linked by fiber with or without a regenerator in the link. • This implementation represents the simplest SDH link (Regenerator Section, Multiplex Section, and Path, all in one link). Bahman R. Alyaei

  50. 11.1.2 Regenerator • A regenerator is needed when, due to the long distance between multiplexers, the signal level in the fiber becomes too low. Regenerator. Bahman R. Alyaei

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