What are SDH Standards?

Relevant Standards:
• ITU-T G.703 Physical/electrical Characteristics of Digital Interfaces
Plesiochronous Digital Hierarchy (PDH)
• ITU-T G.732/G.744/G.751
Synchronous Digital Hierarchy (SDH)
• ITU-T G.707 Network Node Interface for the Synchronous Digital Hierarchy
• ITU-T G.783 Characteristics of SDH Equipment Functional Blocks
• ITU-T G.784 SDH Management
• ITU-T G.957 Optical Interfaces for Equipment ans Systems relating to SDH
• ETS 300 147 Synchronous Digital Hierarchy Multiplexing Structure
• ETS 300 417 Generic functional requirements for SDH equipment

What are SDH virtual Containers?

• Once a container has been created, path overhead byte are added to create a virtual container.
• Path overheads contain alarm, performance and other management information.
• A path through an SDH network exists from the point where a PDH signal is put into a container to where the signal is recovered from the container.
• The path overheads travel with the container over the path.

What are Possbile SDH Rates?

SDH is a transport hierarchy based on multiples of 155.52 Mbit/s

The basic unit of SDH is STM-1:
STM-1 = 155.52 Mbit/s
STM-4 = 622.08 Mbit/s
STM-16 = 2588.32 Mbit/s
STM-64 = 9953.28 Mbit/s

Each rate is an exact multiple of the lower rate therefore
the hierarchy is synchronous.

PDH vs. SDH Hierarchy.

PDH transmission rates:
 SDH is designed to unify all transmission rates into a single Mapping hierarchy

What is PDH: Plesiochronous Digital Hierarchy?

Plesiochronous Multiplexing

• Before SDH transmission networks were based on the PDH hierarchy.Plesiochronous means nearly synchronous.
• 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber or radio.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4 DME.
• Alarm and performance management requires separate equipment in PDH.

What is Add/Drop Multiplexer?

• used in rings
• two STM-N line interfaces (East/West)
• large number of tributary interfaces STM-1, 140 / 34 / 2 Mbit/s(equipped up to full ring capacity)

What is Synchronous Cross-Connect?

Synchronous Cross Connect in SDH Network Elements Perform following action.

• used in meshed networks
• mainly STM-N interfaces
• large switching matrix (1024 STM-1)
• Wideband SXC: VC-12 granularity
• Broadband SXC: AU-4 granularity

AU-4 Pointer

The SDH pointer mechanism is a very elegant way of multiplexing multiple data
containers without the need to align the containers to a common frame start. Thus the framebuffers in SDH multiplexing equipment can be kept small and the transmission delay due to buffering is minimized.

The VC-4 container is allowed to float freely within the AU-4. The H1 and H2 pointer
bytes form a word with a range of 0 to 782 which indicates the offset, in three byte
increments, between the pointer and the first byte of the VC-4.

Administrative Unit (AU-4) and Virtual Container (VC-4)

The actual payload carried in an STM-1 frame is encapsulated in an administrative unit
(AU-4). The AU-4 consists of a VC-4 virtual container comprising 261 columns plus a 9
octet wide AU-4 pointer that points to the first octet of the VC-4 payload container.

SDH Section Overhead (SOH)

A relatively large number of 72 octets in an STM-1 frame have been reserved for various management and monitoring purposes. This so-called section overhead (SOH) is further divided into a regenerator section overhead (RSOH) and multiplex section overhead(MSOH).