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Understanding SONET and SDH: A Comprehensive Tutorial

sonet sdh optical fiber hdlc communication

This tutorial provides an overview of SDH/SONET, covering basics, HDLC framing, terminologies, rates, and the SONET STS-1 SDH Frame. SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) serve the same purpose: communication over optical fiber links. They are physical layer standards designed to ensure reliable transmission of information between a source and destination via optical fiber cables. ANSI developed SONET, while ITU developed SDH.

SONET/SDH addresses the following communication objectives:

  • Carrying multiple T or E carrier lines between switching centers to accommodate increased communication lines.
  • Data framing, achieved using the HDLC frame format.
  • Managing information transfer between optical fiber nodes.
  • Multiplexing and de-multiplexing.
  • Error checking.
  • Handling clock rate variations.
  • Mapping Plesiochronous voice and data traffic.
  • Signaling for automatic switching during fiber cable or optical node failures.

SDH/SONET Terminologies

SDH network terminologies

As shown in Figure 1, an end-to-end connection between SDH/SONET networks is called a “path.” The connection between major ADM (Add/Drop Multiplexer) nodes is the “line.” The “section” is the connection between an ADM and a regenerator, or between regenerators.

Based on these definitions, the following abbreviations are commonly used in SDH:

  • PTE - Path Terminating Equipment
  • LTE - Line Terminating Equipment
  • STE - Section Terminating Equipment
  • ADM - Add/Drop Multiplexer

HDLC Frame

HDLC frame format

All communication over SONET/SDH optical links uses the HDLC frame format, as illustrated in the figure. The HDLC frame consists of flags at the beginning and end, an address field, a control field, information fields, and a CRC.

The character 0x7E serves as the flag, also called the framing character. This is crucial for identifying the beginning and end of the frame because HDLC frames have variable lengths due to the information field.

CRC (Cyclic Redundancy Check) is used for error detection. This field is typically 2 or 4 bytes (octets).

  • Address: Defines the transmission mode, either unicast (0x0F) or multicast (0x8F).
  • Control: Defines the frame transmission type: Information data frame (I), Supervisory frame (S), or Unnumbered frame (U).