1. Digital Transmission Systems Part 2
Chapter 8
Digital Transmission Systems
Part 2
07/03/2013
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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.
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3. Continue…
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4. Continue…
Either one can accommodate the standard E1 family (i.e., Mbps, etc.) and DS1 family (i.e., 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.
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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.
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6. Continue…
Can be used in Long-Haul Networks, Local Networks and Loop Carriers, and it can also be used to carry CATV video 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.
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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.
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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.
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9. Continue…
In about 1962, the network providers began using digital communications between switching centers.
This was PDH system (DS-Carrier in US and E-Carrier in Europe).
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10. 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.
E1 or DS1
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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.
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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.
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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 Mbps.
Thus, SDH’s STM-1 is equivalent to SONET’s STS-3 (3 x Mbps = Mbps).
Higher SDH rates of STM-4 (622 Mbps), STM-16 (2.4 Gbps), and STM-64 (10 Gbps) have also been defined.
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14. SDH Bit Rate SDH Capacity SONET Bit Rate SONET Capacity
STM-0
51.84 Mbps
21 E1
STM-1
Mbps
63 E1 or 1 E4
STM-4
Mbps
252 E1 or 4 E4
STM-16
Mbps
1008 E1 or 16 E4
STM-64
Mbps
4032 E1 or 64 E4
STM-256
Mbit/s
16128 E1 or 256 E4
SONET
Bit Rate
SONET Capacity
STS-1, OC-1
51.84 Mbps
28 DS1 or 1 DS3
STS-3, OC-3
Mbps
84 DS1 or 3 DS3
STS-12, OC-12
Mbps
336 DS1 or 12 DS3
STS-48, OC-48
Mbps
1344 DS1 or 48 DS3
STS-192, OC-192
Mbps
5376 DS1 or 192 DS3
STS-768, OC-768
Mbps
21504 DS1s or 768 DS3
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15. Data Rates of SONET and Corresponding SDH Data Streams
Continue…
Data Rates of SONET and Corresponding SDH Data Streams
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16. Continue…
Multiplexing is accomplished by combining or interleaving multiple lower-order signals (1.5 Mbps DS1 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-1 signal 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.
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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.
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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.
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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.
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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.
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21. Segment no. 7 270 Bytes in (125/9) μS The STM-1 frame 07/03/2013
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22. STM-1 frame visualized as a block, and the direction of transmission
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23. Continue…
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24. STM-1 frame visualized as a block
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25. Continue… Hence, the STM-1 frame rate Rf is
Rf = 8000 frames per second
The bit rate Rb of STM-1 frame is calculated as follow:
Rb = Rf x Cf , where
Rf is 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.
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26. Continue… We know that the frame rate is Rf = 8000 frames/second.
Cf is 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
= Mbps
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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 Interleaving N STM-1 signal.
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28. Multiplexing of STM-1 to generate STM-N
Continue…
Multiplexing of STM-1 to generate STM-N
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29. STM-N signal frame structure
Continue…
STM-N signal frame structure
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30. Continue…
Example:
An STM-4 signal will be created by Byte Interleaving four STM-1 signals.
The basic frame rate remains 8,000 frames per second, but the capacity is quadrupled, resulting in a bit rate of x Mbps or Mbps.
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31. Multiplexing of STM-1 to generate STM-4
Continue…
Multiplexing of STM-1 to generate STM-4
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32. 11.1 SDH STM-1 Frame Structure
As we know that, the SDH frame STM-1 consists of two parts:
The First Nine Columns comprise the Overhead (OH), occurs at a rate x 9 x 8 x 8000 = Mbps.
While the remainder is called the Payload, which is also called Virtual Container (VC), occurs at a rate x 261 x 8 x 8000 = Mbps.
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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 x 260 x 8 x 8000 = Mbps.
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34. Virtual Container (VC)
SOH
AU-PTR
Container
POH 1 3 4 9
9 columns
Overhead (OH)
271 columns
Virtual Container (VC)
125 μS
9 rows
SDH frame STM-1 structure
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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 = Mbps.
MSOH = 6 x 9 x 8 x 8000 = Mbps.
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36. Virtual Container (VC)
RSOH
MSOH
AU-PTR
Container
POH 1 3 4 9
9 columns
Overhead (OH)
261 columns
Virtual Container (VC)
125 μS
9 rows
SDH frame STM-1 structure
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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.
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38. Continue… The contents carrier in the container are real goods.
SDH frame
The contents carrier in the container are real goods.
These are analogous to customer traffic, being carried in the Payload area of SDH frame.
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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.
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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.
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41. Continue… The POH is responsible for the Path layer OAM functions.
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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.
SDH Pointers allow this limited asynchronous operation within the synchronous network.
It points the location of the VC in the STM frame.
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43. Typical SDH Communication Network
12 SDH Signal Hierarchy
Typical SDH Communication Network
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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.
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45. Continue…
The SDH Layer Model
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46. 12.1 SDH Network Elements
The SDH signal is layered to divide responsibility for transporting the Payload through the network.
Each SDH Network 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.
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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).
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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
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49. Continue… PTE is a terminating multiplexer.
It is responsible for adding first order POH, RSOH, and MSOH 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).
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50. Regenerator
A regenerator is needed when, due to the long distance between multiplexers, the signal level in the fiber becomes too low.
Regenerator.
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51. Continue…
The regenerator recovers timing from the received signal and replaces the existing Regenerator Section overhead (RSOH) bytes of the received STM signal before retransmitting the signal; the Multiplex Section Overhead (MSOH), Path Overhead (POH), and Container (C) are not altered.
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52. ADM
It is responsible for adding higher order RSOH, MSOH, and POH to the received STM signal. It is also responsible for evaluating RSOH, MSOH, and POH.
A single-stage Multiplexer/Demultiplexer can multiplex various inputs into an STM-N signal.
At an Add/Drop site, only those signals that need to be accessed are dropped or inserted.
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53. Continue…
The remaining traffic continues through the network element without requiring special pass through units or other signal processing.
In rural applications, an Add/Drop Multiplexer (ADM) can be deployed at a terminal site or any intermediate location for consolidating traffic from widely separated locations.
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54. Add/Drop multiplexer example.
Continue…
STM-N
Add/Drop multiplexer example.
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55. A Synchronous Add–Drop Multiplexer (ADM)
Continue…
A Synchronous Add–Drop Multiplexer (ADM)
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56. DXC
An SDH Digital Cross-Connect (DXC) accepts various E-carrier and STM rates, accesses the STM-1 signals, and switches at this level.
It is responsible for adding RGSO, and MSOH without altering the POH.
One major difference between a DXC and an ADM is that a DXC may be used to interconnect a much larger number of STM-1s.
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57. Digital Cross-Connect (DXC)
Continue…
Digital Cross-Connect (DXC)
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58. Continue… It is ideally used at an SDH Hub Network.
The DXC can be used for grooming (consolidating or segregating) of STM-1s or for broadband traffic management.
For example, it may be used to segregate high-bandwidth from low-bandwidth traffic and send them separately to the high-bandwidth (for example video) switch and a low-bandwidth (voice) switch.
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59. Example: Assume that the traffic is traveling from west to east.
If regenerator section errors are detected at Site 4.
A problem will be somewhere between Site 3 and Site 4.
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60. Continue…
The observed problem can not be west of Site 3, since all regenerator section results are recalculated at every point in the network.
If multiplexer section and regenerator errors are found at Site 4.
A problem exists between Site 2 and Site 4, since multiplexer section results are recalculated only at major network nodes, such as the SDXS at Site 2.
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61. Continue…
Furthermore, if multiplexer section errors, but not regenerator errors, are found at Site 4.
Then a problem exists between Site 2 and Site 3.
Finally, if path errors are detected at Site 4.
Then a problem exists anywhere between Site 1 and Site 4.
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62. 13 SDH Network
SDH Core Transmission Stations (SDH-CTSs) are usually located at each of the trunk and international exchanges and many of the larger local exchanges.
Figure in the next slide illustrates the concept with an example of five CTSs (A to E), which are supporting the core transmission between a set of trunk telephone switching units, data nodes and private circuit nodes.
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63. Core Transmission Network Configuration
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64. Continue…
An SDH-CTS uses a combination of ADMs and DXC equipment to provide the necessary transmission flexibility.
The configuration of the ADM and the DXC are managed through a computer-based controller, which may be co-sited or remotely located.
The SDH configuration controller allows the network operator to manage the configuration of the CTS flexibility points, through planning and assignment processes, as well as reconfigurations in real time to compensate for transmission link breakdowns.
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65. Continue…
An SDH-CTS comprises a DXC on which the high speed transmission links terminate.
For the example of a 2 Mbps block extraction from the incoming 155 Mbps link, the DXC needs to be able to identify and manipulate the appropriate 2 Mbps tributary from the incoming SDH link and pass it to the outgoing link.
The DXC is divided into a higher-order DXC switch-block handling the SDH transmission rates and a lower-order DXC switch-block handling the 2 Mbps and other PDH rates.
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66. Continue… SDH Core Transmission Network Station – DXCs 07/03/2013
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67. Continue…
At smaller transmission nodes, ADMs only are used to extract digital transmission blocks (at the PDH rates of 2, 8 and 34 Mbps) for the co-sited telephone switching unit, private circuit, and data units within the exchange building, associated with the CTS.
In order to maximize their (DXCs and ADMs) management capability, SDH networks are usually structured in a set of hierarchical levels.
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68. SDH Core Transmission Network Station – ADMs
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69. SDH Transmission Network Structure
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70. Continue…
Figure in S#69 illustrates a typical SDH network structure using DXCs and ADMs.
At the top level (Tier 1) of the national network is a mesh of high-capacity SDH transmission links between flexibility nodes (CTS) of DXCs.
This forms the inner portion, or backbone of the Core Transmission Network and links the major trunk exchanges, as well as private circuit and data nodes.
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71. Continue…
Hanging off this level at Tier 2 is a set of SDH rings linking ADMs within a region of the country, serving smaller trunk exchanges, local exchanges and other nodes.
Above the Tier 1 of the national network is the international portion of the Core Transmission Network, the Tier 0, which links to the network’s transmission gateways to the transmission networks of other countries – via submarine cable landing stations, microwave radio stations or satellite Earth stations.
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72. Continue…
A typical suite consisting of several racks of SDH transmission equipment. 
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73. Continue…
The yellow leads are optical patch cords that connect from Optical Distribution Frames (ODFs), where Backhaul Network based Optical Fiber Cables are terminated (see next slide).
The white cables running across the upper parts of these racks feed to a Digital Distribution Frame (DDF) that then connects to Main Telephony switches, Internet and Routers/Switches, Television and Radio Program distribution networks. 
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74. Optical Distribution Frames (ODFs)
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75. 13.1 SDH Network Configurations
There are four major configurations:
Point-to-Point.
Point-to-Multipoint.
Hub Architecture .
Ring Architecture .
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76. 13.1.1 Point-to-Point Is the simplest network configuration.
It involves two terminal multiplexers linked by fiber with or without a regenerator in the link.
In this configuration, the SDH path and the Service path (for example, E1 or E4 links end-to-end) are identical.
This synchronous island can exist within an asynchronous network world.
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77. Point-to-Point Network Configuration
Continue…
STM-1 E1 E3
STM-1 E1 E3
Point-to-Point Network Configuration
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78. 13.1.2 Point-to-Multipoint Also called Linear Add/Drop architecture.
It includes adding and dropping circuits along the way (link) to facilitate adding and dropping tributary channels at intermediate points in the network.
The SDH ADM is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross-connecting, adding and dropping channels, and then remultiplexing.
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79. Point-to-Multipoint Network Configuration
Continue…
Point-to-Multipoint Network Configuration
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80. Hub Architecture
It accommodates unexpected growth and change more easily than simple point-to-point networks.
It concentrates traffic at a central site using two or more ADMs, and a DXC switch, and allows easy re-provisioning of the circuits.
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81. Continue…
There are two possible implementations of this type of network function:
Cross-connection at higher-order path levels, for example, using three E3 and E4 tributary in the switching matrix.
Cross-connection at lower-order path levels, for example, using 63 E1 tributary in the switching matrix.
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82. Hub Network Architecture
Continue…
Hub Network Architecture
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83. Ring Architecture
The SDH building block for a ring architecture is the ADM.
Multiple ADMs can be put into a ring configuration for either Bidirectional or Unidirectional traffic.
The main advantage of the ring topology is its survivability; if a fiber cable is cut, for example, the multiplexers have the local intelligence to send the services affected via an alternate path through the ring without a lengthy interruption.
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84. Continue…
The demand for survivable services, diverse routing of fiber facilities, flexibility to rearrange services to alternate serving nodes, as well as automatic restoration within seconds, have made rings a popular SDH topology.
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85. Continue…
Ring Architecture
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86. 13.2 Hybrid Network
The mixture of different applications is typical of the data transported by SDH.
Synchronous networks must be able to transmit Plesiochronous signals and at the same time be capable of handling future services such as ATM.
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87. Schematic Diagram of Hybrid Communication Network
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