Synchronous Digital Hierarchy Overview

Synchronous Digital Hierarchy (SDH)

• SDH is a universal standard defined by ITU-T for flexible and common communications architecture.
• SDH overcomes the limitations of 64Kbps digital channels from higher PDH hierarchical signals by removing channel traces and requiring many multiplexers and de-multiplexers.
• SDH was introduced in 1992 and is based on overlaying a synchronized multiplexed signal onto a light stream transmitted with fiber-optic cable.
• SDH also supports radio relay links, satellite links, and electrical interfaces between equipment.
• SDH provides a flexible architecture that accommodates future applications with various transmission rates.
• SDH allows for transmission over multiple media and inter-networking between different manufacturers.
• SDH provides byte-oriented operations for multiplexing, mapping, or alignment.
• SDH requires all nodes to be synchronized with the same master clock, enabling insertion and extraction of tributaries at any point and bit rate.
• SDH offers overhead for operation, administration, and maintenance (OAM) functions, allowing for centralized management.
• SDH provides protection from link or node failures and monitors network performance and manages network events (SDH Resilience).
• SDH is scalable with transmission rates up to 40Gbps.

SDH Network Elements

• Regenerators: They use the received signal to synchronize and replace RSOH bytes before retransmitting. MSOH, POH, and the payload remain unaltered.
• Multiplexers:
  • LTM (Line terminating multiplexer): Combines plesiochronous and synchronous input signals into a higher bit rate STM-N signal.
  • LM (Line multiplexer): Multiplexes/demultiplexes STM-N synchronous signals within STM-M (M>N).
  • ADM (Add/Drop Multiplexer): Allows low bit rate plesiochronous and synchronous signals to be extracted or inserted into the high-speed SDH bit stream.
• Digital Cross-Connects (DXC): Allows mapping of PDH tributary signals into virtual containers and switching multiple containers. Switched traffic can include both SDH streams and selected tributaries.

SDH Topologies

• Ring: Often used to build fault-tolerant architectures. The main advantage is its survivability.
• Hierarchical Master-Slave Synchronization: A master clock directly or indirectly synchronizes slave clocks in a tree topology, organized across two or more levels.

SDH Synchronization Elements

• Primary Reference Clock (PRC): An autonomous clock or one that is synchronized by radio or satellite (GPS or Loran-C) Cesium atomic clock.
• Slave Clocks: SSU (Synchronization Supply Unit) or BITS (Building Integrated Timing Supply). There are transit and local nodes: SSU-T and SSU-L.
• Clocks at Network Element (NE): SEC (SDH Equipment Clock).

SDH Features

• Synchronous Ethernet: Utilizes the same synchronization network model as SDH.
• Higher Bit Rates over STM-1: Achieved through the SDH hierarchy.
• Contiguous Concatenation: Allows bit rates exceeding container capacity. The payload can be distributed across multiple containers, creating large containers that cannot be split during transmission. Examples include VC-4-4c (599.040 Mbps), VC-4-16c (2,396.160 Mbps), VC-4-64c (9,584.640 Mbps), and VC-4-256c (38,338.560 Mbps).

SDH Limitations

• SDH can only transport certain signal rates defined by virtual containers (VC-x) and contiguous concatenation.
• SDH requires a single encapsulation method to accommodate all data packet protocols.
• SDH has bandwidth efficiency concerns.

Next Generation SDH (NGN-SDH)

• Virtual Concatenation (VCAT):
  • Resolves the granularity problem of SDH by adapting transmission speed to user requirements.
  • User data is mapped to groups of virtual containers, using inverse multiplexing (G.805).
  • Optimizes SDH network usage by offering granular bandwidth choices, improving network resource efficiency.
  • Provides transparency in the SDH network as individual VCs behave like traditional virtual containers.
  • Core nodes are transparent to VCAT.
  • End nodes must support VCAT functionalities.
  • The receiver node reassembles the user frame and compensates for delay differences on each path. The maximum delay correction limit is 512 ms, suitable for continental networks (100ms is often sufficient).
• Link Capacity Adjustment Scheme (LCAS):
  • Allows changing the size of a VCAT signal (G.7042).
  • Provides soft protection and load-sharing mechanisms, extending virtual concatenation.
  • Designed to manage the bandwidth allocation of a VCAT path by adding or removing members of a VCG that controls the VCAT channel.
  • Enables dynamic bandwidth changes during service.
  • Bandwidth can be managed by adding or dropping VCs of VCG, allowing for asymmetric configurations.
  • Improves VC availability from failures or changes.
  • Automatically decreases link capacity in case of VC path failure and increases capacity when repaired.
• Generic Framing Procedure (GFP): Maps packet-based signals into constant bit-rate SDH signals.

G.709 Optical Transport Network (OTN)

• OTN technology enables seamless integration of multiple networks and services (legacy SONET/SDH, data, voice, video, and storage) into a common infrastructure.
• OTN (digital wrapper) combines the benefits of SDH/SONET technology with the bandwidth expansion capabilities of DWDM.

Network Evolution

• Large high-speed backbone networks have transitioned from circuit-switched networks based on TDM running SDH/SONET to Ethernet-based networks.
• Primary traffic type: Bursty.
• New models: Peer-to-peer computing and cloud computing.
• Bandwidth demands: Driven by internet-based video applications, with bandwidth demand reaching ZettaByte levels (ExaByte = 109 GB, ZettaByte = 1000 EB).
• TDM limitations: Underutilized or wasted bandwidth, expensive to operate, and limited to 40Gbps.
• Evolution to packet switching: Driven by the growth in packet-based services (L2/L3 VPN, IPTV, VoIP, etc.) and the desire for flexible bandwidth and QoS.
• New packet transport networks retain the same operational model: MPLS (Multi Protocol Label Switching) transport profile defined at IETF (in collaboration with ITU-T).
• MPLS-enabled core networks bring packet-awareness: MPLS enables connection-oriented services for variable-length frames, regardless of type (IP packets, native ATM, SDH, or Ethernet frames).

Packet Network Technology

• Pseudowire (PW): Emulates essential attributes of a native service when transported over a packet-switched network (PSN).
• Challenges for Enterprises: Long-distance connection across the globe, latency issues with TCP application response times, and geographical variations.
• Enterprises aim to keep up with scale and dynamic nature of cloud connectivity: Including mobile users across the globe (dynamic SD-WAN, software and control provisioning).

Software-Defined WAN (SD-WAN)

• SD-WAN is a virtual WAN architecture allowing enterprises to leverage a combination of transport services (including MPLS, LTE, and broadband internet services) to secure user connections to applications.
• MEF (Metro Ethernet Forum) published the SD-WAN Service Attributes and Services (MEF 70) standard in 2019.
• SD-WAN for 5G: Mapping SD-WAN application performance and security to 5G slices.

Key SD-WAN Advantages

• Cost reduction: Transport independence across MPLS, 4G/5G LTE, and other connections. Transmission is adapted to the most suitable service.
• Reliability: Enhanced network reliability.
• Security: Improved network security.
• OPEX Improvement: Replacement of MPLS services with more economical and flexible broadband (including secure VPN connections).
• Application Performance and Agility Improvement: Dynamic route application traffic for efficient delivery and improved user experience.
• User Experience and Efficiency Optimization: Improved user experience and efficiency for SaaS and public cloud applications.
• Simplified Operations: Automation and cloud-based management.

SD-WAN Operation

• SD-WAN combines all data transport types into a single overlay connection managed to route traffic over optimal physical network connections (fiber, DOCSIS, MPLS, 4G, 5G, etc.).
• SD-WAN automates tunnel monitoring, redundancy, and fail-over configurations.
• Hybrid SD-WAN allows for maintaining MPLS tunnels.

This quiz explores the Synchronous Digital Hierarchy (SDH), a standard developed by ITU-T for robust communication infrastructure. It discusses the benefits of SDH, including its ability to overcome limitations of older digital channels, support for various transmission media, and synchronization requirements. Perfect for those interested in telecommunications and networking.