Synchronous Digital Hierarchy (SDH)

Synchronous Digital Hierarchy (SDH)

• ITU established the Synchronous Digital Hierarchy (SDH) as an international standard for synchronous data transmission on optical media
• SDH specifications set optical interfaces that allow lower-rate signals, like PDH, to transmit at a common synchronous rate
• SDH enables compatibility of optical transmission equipment from multiple vendors within the same span
• SDH enables dynamic drop-and-insert capabilities on the payload

Advantages of SDH Transmission

• SDH achieves transmission rates up to 10 Gbit/s, suitable for backbones in telecommunications networks
• SDH is much easier to extract and insert low-bit rate channels from high-speed bit streams, compared to older PDH systems
• Network providers can react quickly and easily to customer requirements using SDH
• SDH networks have automatic back-up and repair mechanisms to cope with system faults
• SDH is an ideal platform for services ranging from POTS, ISDN, and mobile radio to data communications
• SDH makes it easier to set up gateways between different network providers and SONET systems due to globally standardized interfaces, reducing equipment costs

Synchronous Transport Module (STM)

• STM is an information structure of information payload and overhead bits in a block frame structure, repeating every 125 microseconds
• STM information is suitably conditioned for serial transmission on the selected media at the rate synchronized to the network
• STM followed by an integer indicates the level of SDH, with STM 1 as the first level of SDH bit rates
• SDH bit rates increase as integer multiples, with higher rate levels denoted by the corresponding multiplication fraction of the first level

SDH Bit Rates

• STM-1 has a basic rate of 155.520 Mbps
• STM-4 indicates it includes 4 STM-1s, each STM-1 is independent
• STM-16 means 16 STM-1s, and STM-64 means 64 STM-1s

SDH Multiplexing Structure

• Inputs to SDH system are PDH bit streams including standardized E1, E3, or E4 of E hierarchy and/or T1, T2 and T3 of T hierarchy
• Synchronous multiplexing involves assembling PDH data flows/flows from other sources in appropriate containers
• Virtual containers are generated by attaching the POH (Path Overhead)
• Tributary units are assembled by attaching pointers and inserting containers at the appropriate positions
• Generation of administrative units is similar to tributary units
• Basic transport frames are generated and multiple basic transport frames are multiplexed into a superior transport frame

Mapping, Aligning and Multiplexing

• Mapping is a process for adapting PDH tributaries into VCs by adding POH information
• Aligning is the process when a pointer is included into Tributary Unit or Administrative Unit to locate the 1st byte of VC
• Multiplexing is the process when multiple low-order path signals are adapted into a higher-order path signal, or high-order path signals are adapted into a Multiplexing Section

Mapping Elements

• Mapping elements are packaging units with fixed sizes, depending on the traffic path they follow in the multiplexing hierarchy
• Container: The entry point of the PDH signal and is the basic packing unit for tributary channels filled with information from a plesiochronous signal
• Justification facilities adapt plesiochronous tributaries to synchronous network clock
• A container suits the rate of the inputted signal and the synchronous frame structure, with fixed stuffing bits inserted for synchronous tributaries
• Signal is prepared so as to enter into the next stage, the virtual container and containers can be Basic or Higher Order
• Virtual Container (VC = C + POH): Each container is added with control information known as Path Over Head (POH) for the service provider to achieve end-to-end path monitoring and the POH fields are organized in a block frame structure
• Tributary unit (TU): An information structure which provides adaptation between the lower order path layer and the higher order path layer that consists of information payload of the virtual container and the tributary unit pointer
• Tributary unit group (TUG): One or more tributary units grouped/multiplexed by byte interleaving to form higher bit stream rate as part of multiplexing structure
• Pointer: An indicator whose value defines frame offset of a virtual container with reference to the frame reference of transport entity on which it is supported and indicates phase alignment of the virtual containers
• Administrative Unit (AU): An information structure, that provides adaptation between higher order path layer and the multiplex section layer Consists of information payload and AU pointer which indicates offset of the payload frame start relating to the multiplex section frame start.
• Administrative Group Unit (AUG): A homogenous assembly of AU-3s or AU-4

STM-1 Frame

• The STM frame standards with line rate are as shown in Table1
• The first level of the synchronous digital hierarchy shall be 155.52 Mbps
• The overhead and transport functions are divided into layers using a client/server layer approach, and they are: Regenerator Section, Multiplex Section & Path

Terminal, SDH and Add/Drop Multiplexers

• Terminal multiplexers combine plesiochronous and synchronous input signals into higher bit rate STM-N signals
• SDH regenerators regenerate the clock and amplitude relationships of incoming data signals
• Add/drop multiplexers(ADMs) can extract or insert plesiochronous /lower bit rate synchronous signals into high-speed SDH bit streams, which makes it possible to set up ring structures for automatic back-up path switching

STM-1 Frame Structure

• An STM-1 frame structure is made up of payload blocks, overhead blocks and pointers
• Frame lasts for 125µSec arranged in matrix format of 9 rows X 270 columns forming a line rate of 155.52 Mbps
• Divided into three main areas: Section Over Head (SOH), Path Over Head (POH) and Pay Load
• The Section Over Head (SOH): Contains maintenance, monitoring, operational functions, split into Regenerator Section and Multiplex Section Overhead
• Regenerator Section Overhead (RSOH): Contains info. required for elements of a section found in the first three rows of Columns 1 through 9 of the STM-1 frame

Description of RSOH Bytes

• A1, A2: Framing bytes that indicate beginning of the STM-N frame, providing a frame alignment pattern
• JO: Regenerator section trace used to transmit a Section Access Point Identifier so a that section receiver can verify its continued connection to the intended transmitter and supports continuity testing between transmitting/receiving device on each regenerator section
• B1: A parity code used for transmission error checks over a regenerator section, which provides end-to-end error performance monitoring, over all bits of the previous STM-N frame after scrambling and the computed value is placed in B1 byte before scrambling
• E1: Provides local order wire channel for voice communications between regenerators, hubs and remote terminal locations
• F1: Allocated for user's purposes like temporary data/voice channel connection special maintenance applications
• D1-D3: Bytes are used for message-based data communications channel providing administration, monitoring, alarm and maintenance functions between regenerator section termination equipment

Multiplex Section Overhead (MSOH)

• Contains the info. required between the multiplex section termination equipment at each end of the Multiplex section
• B2: Provides multiplex section error monitoring of an STM-N frame and is a computed value across all bits of the previous STM-N frame, except for the first three rows of SOH
• K1, K2: Two bytes allocated for APS signaling for multiplex section protection that is used for MSP between multiplex level entities for bi-directional automatic protection switching and for communicating Alarm Indication Signal and Remote Defect Indication conditions
• D4 - D12: MS Data Communications Channel (DCC) bytes. These nine bytes are used for OAM information, such as control, maintenance, remote provisioning, monitoring, administration and other communication needs
• S1: Synchronisation status message byte which carries synchronisation messages
• M1: MS remote error indication (MS-REI)

STM-1 Payload

• Payload: The bytes containing data from the tributaries are transferred here without buffering and are in relation with the STM-N frame
• Adapts the flow rates of the plesiochronous signals to the required rates of them in VCs, some additional bits are added as pointers as per the process of justification

Mapping PDH Tributaries into SDH Containers

• The SDH system has appropriate mapping to transport the PDH flows in the SDH systems
• The mapping has to solve the rate matching problem between the local clock of multiplexer and the received flow.
• There is a separate mapping algorithm exists for each PDH flow that uses positive justification for rate matching between the multiplexer and the received plesiochronous flow
• Two categories of mapping algorithms are synchronous and asynchronous mapping

Synchronous and Asynchronous Mapping

• Synchronous mapping: inserts bits from plesiochronous flows in containers using the clock, extracted from the received flow and rate matching between the formed containers and the synchronous transport frames is achieved with the help of the transport units pointers
• Asynchronous mapping: inserts bits from plesiochronous flows in containers using the local clock of the multiplexer and rate matching is realized with the help of positive justification, which there are not necessary continuous pointer operations only for mapping of the plesiochronous tributaries

Mapping of E-4 into STM-1

• The path is from Plesiochronous signal -> Container -> Path Overhead -> Virtual Container -> Pointer -> Administrative Unit -> Section Overhead -> Synchronous Transport Module

Container C4 and Total Rate VC4

• Input to container C4 is E4 -139.264 mbps where the 260 columns and 9 rows matrix structure for payload is processed
• Each row is split into 20 blocks of 13 bytes each containing information bits from the 140Mbit/s signal and the 13th byte is used as W, X, Y and Z bytes for different purposes
• Pay load area is divided as: 20 Blocks X 9 Rows = 180 Blocks that contain information bytes and Overhead bytes Total rate VC4 (without POH) = 149760 kbps where t he bit rate is 149.760 mbps, which is higher than input to C4, (139.264 mbps)
• Each row is stuffed with different types of bytes with W being the normal information byte, Y being stuffing byte with undefined structure, X being a byte having defined structure (a justification control bit), and Z bit having is defined structure
• E4 equals bit rate of 139.264 mbps + 15 ppm that is 139264+2.088 kbps where j ustification bit is added in row in Z byte

Justification

• The operation which fit a variable rate signal into a fixed rate frame
• It is necessary to allocate highest bit rate to transmit tributary within S frame

Path Overhead (POH)

• Oversees task of monitoring quality and the container type, where format and size of the POH, depends on container type and it is assigned, and transported with the payload until the payload is demultiplexed

POH Bytes

• J1: Higher-Order VC-N path trace byte transmits a 15-byte, E.64 format string/A 64-byte free-format that allows the receiving terminal in a path to verify its continued connection to the intended transmitting terminal

• B3: Path bit interleaved parity code is a parity code (even), used to determine if a transmission error has occurred over a path. Its value is calculated over all the bits of the previous virtual container before scrambling and placed in B3 byte.

• C2 Path signal label byte specifies the mapping type in the VC-N

• G1: Path status byte to convey path terminating status and performance back to the originating path terminating equipment and therefore allows the bi-directional path in its entirety to be monitored, from either end of the path

• F2: Path user channel byte to faciltate user communication between path elements

• H4: Position and Sequence indicator byte to provide a multi frame and sequence indicator for virtual VC-3/4 concatenation or payload

• F3: Path user channel byte to enable purposes between path elements and whose payload is dependent

Functions of a Pointer

• Minimizes multiplexing Delay due to signals from different originating points differing in their phases , because of different transmission length and different clock generation with signals are written into memories and read out using a new phase of the frame to be multiplexed to align a new phase
• Assigned to each VC to be multiplexed and indicates relative phase shift between the VC and new frame by using the address number in the new frame such that every VC has different pointer value which is renewed at every multiplexing process, so it is not necessary to introduce undesirable additional delays
• Payload Pointers indicates where in the container capacity a VC starts, and the byte stuffing process allows dynamic alignment of the VC in case it slips in time by identifying the location of first byte of the VC in the payload
• VC is allowed to "float" within the STM-1 frame capacity

AU-4 Pointer bytes

• H1 and H2 Pointer bytes: The two bytes of VC payload pointer designate location of the VC frame and are used to align the VC and STM-1 Section Overheads in an STM-N signal, to perform frequency justification, and to indicate STM-1 concatenation.
• H3 Pointer action byte for frequency justification where,depending on pointer value, the byte adjusts the fill input buffers,and only carries a valid information in the event of negative justification
• Details are given by New Data Flag indicating arbitrary change of the value of a pointer that may increment/decrement bits and are determined by set values under direct filling by 140 Mbps

Frequency Justification

• Absorbs frequency differences between payload and frames if VCs is transported over different networks and the network element is in an abnormal condition
• Types in SDH includes Positive which the frame speed of the STM is higher than the payload arrival speed that has Frame (n+3) Pointer value incremented by 1, if the frame rate of the VC-n is too slow with respect to that of the AUG-N, then the alignment of the VC-n must periodically slip back in time
• Frame speed is considered to be the Negative type, If the frame speed of the STM is lower than the payload arrival speed if the frame rate of the VC-n is too fast with respect to that of the AUG-N such that frame (n+3)Pointer value decremented by 1 and that are applied in 4 consecutive frames with the 3rd varying H bits

Mapping of E-3 into STM-1 and E1 traffic

• Multiplex C3 container in STM-N frame
• Traffic to form STM-1 frame through AU-4 mapping

Ethernet over SDH (EoS)

• A simple, flexible and cost-effective solution by developing a set of new technologies to offer Ethernet based services for customers, which addresses following key issues and is by Generic Framing Procedure (GFP)
• The framing protocol to encapsulate Ethernet frames to generate an SDH payload in SDH networks enables several functions to adapt Ethernet traffic to transportion

Advantages GFP

• Traffic management and QoS control are significantly easier
• GFP is more robust than HDLC and less susceptible to bit errors
• GFP is supported by OTN /WDM interfaces in addition to SDH
• GFP permits multiple protocols to share the same transport path from different ports or links, resulting greater bandwidth efficiency

Concatenation

• Process of summing the bandwidth of X containers of same type into larger container, where methods include:
• Contiguous: Creates big containers w/o ability to split into smaller pieces transmission
• Virtual: Transports the individual VCs and aggregates, and for its concatenation, functionality is only needed at path termination

Virtual Concatenation (VCAT)

• It allows the separation of GFP-adapted traffic into different paths in the SDH network having two bandwidth provisioning schemes

• allows bandwidth to be tuned in small increments on demand to match desired data rate and avoid wastage as the Traditional contiguous concatenation comes in coarse increments -Is easily routed through a network and aids to eliminate stranded bandwidth. Allows for more efficient usage of an existing network's available bandwidth and allows only transparency to core network elements and only requires end nodes of the network to be aware of the containers -Resilient Individual members of a virtually concatenated group can be routed as diversely as possible across a network

Link Capacity Adjustment Scheme (LCAS)

• Dynamically adjusts capacities of SDH paths according to source and/or destination needs with help to manage bandwidth allocation of a VCAT path

• LCAS can add and remove members of a VCG that control a VCAT channel however it cannot adapt with traffic pattern

LCAS Applications

• VCAT bandwidth allocation enables the resizing of the VCAT pipe in use when it receives an order from the NMS to increase or decrease size.
• Network Resilience In the event of partial failure of one path, LCAS reconfigures connection using the members still up and able to continue carrying traffic
• Asymmetric Configurations LCAS is a unidirectional protocol the provision of asymmetric bandwidth between two MSSP nodes to configure asymmetric links however its cross-domain operation requires multiple edge nodes
• Auto Negotiation enables detection using ability detection feature that enables link partner abilities to advertise, acknowledge receipt as segment is detected
• Link Integrity allows detecting faults along the end-to-end Ethernet transport connection such as Near-end Ethernet Link Failure SDH Link Failures CSF/Far-end Ethernet Link Failure for and any traffic affecting a VCG that will prevent ETH port from functioning

Network Topology and Automatic Protection Switching (APS) in SDH

• A network element which must be live traffic equipped, has topologies capable faults management of their fibre cables and Linear Mesh Ring or Star types
• Linear Topology involves high-speed network access and does not require protection from faults
• Mesh topology involves both a wide variety for channels between nodes with great protection that offer circuit solution so Multiplexers use is avoided
• Ring topology offer all linear capabilities and terminals in a protected pair by the following features Automatic protection switching APS occurs to facilitate that that both high capacity can be met but also provide detections, switching and selections
• automatic protection switching (APS) is both a functionality and requirement of all high quality transport networks

ITU-T Recommendation G.841

• Provides equipment of various layers, rings, and physical nodes and has a classified sublayers which contain trails of switches, and transmissions

Protection Switch

• Protection requires traffic to have three main components

APS Classifications (1 + 1, 1:1, etc) Multiplex, and ITU Reccomendations

Ring Protection

• Can use either unidirectional or bidirectional path or just protection and may requires two fiber

• Can use either unidirectional or bidirectional path or protection.

Fiber Protection

• Done in the two forms of switching path with other elements in a line that's controlled

Ring Protection

• For the purpose of connecting each fibre, in some cases requires the additional fibers or ports

• BSLR = Bi-Directional Line-Switched Rings

• USPR = Directional Rings

Subnetwork Switching

• For layered networks, they take portion of segment or path, which switches various servers etc..
• Is best by the N to provide multi functions

SDH

• 841

Frequency

• To not have various changes , has various clock signals, can use, PRC SSU and SEC

Free-Running Mode

• No signals are supported that can oscillate at the element to find it
• ClockLostControl

LockMode

• Signal has limited bounds and average with the reference.

  SSM

  • Used with settings of protected channels

Setting/Clocks

• has configurations to control and see quality etc in list all elements

Jitter

• Is the variation’s of digital signal to ideal positioning of timing source It accumulates at any connection where it cause issues with signal The Temporal Resolution Is Necessary and Measures Transient Phase

• Parameters are for Synchronization and Specifications which Time Interval Error, Max Time Dev and and require sufficient duration get good statistics average

• The causes that effects this are below, cabling various frequences, and transmissions

Functional Tests

• Check corrected connectors and their signal routes