SONET Overview


  • SONET is a transport protocol based on Time Division Multiplexing, originally designed to transport voice channels and high speed bandwidth over fiber.

SONET Overview

  • SONET stands for Synchronous Optical NETworking (SONET).
  • Originally developed to transport circuit-switched (DS1, DS3..etc) voice encoded in PCM (Pulse Code Modulation).
  • SONET and SDH are very similar protocols. SONET is used in North America and SDH thoughtout the rest of the world. SDH stands for Synchronous Digital Hierarchy.
  • SONET allows for encapsulation of different circuit types of different rates within its Virtual Container (VC). Each encapsulated data can have it's own frame rate with very low processing delay of at most 32 microseconds. How is padding used for that?
  • Designed to be provide bandwidth of 50Mbps +.

Unit of Transmission

  • SONET uses Synchronous Transport Signal (STS-n) as a unit of transmission in electrical state.
  • SDH uses Synchronous Transport Module (STM-n) as a unit of transmission in electrical state.
  • Optical Channel (OC) is the signal transmitted over fiber.
  • Note that the data-rate progression starts at 155 Mbit/s and increases by multiples of four. With exception of STS-24.
SONET SDH Optical Line Rate (Mbps) Payload Bandwidth
STS-1 STM-0 OC-1 51.84 50.112
STS-3 STM-1 OC-3 155.52 150.336
STS-12 STM-4 OC-12 622.08 601.344
STS-24 - OC-24 1,244.160 1,202.688
STS-48 STM-16 OC-48 2,488.32 2,405.376
STS-192 STM-64 OC-192 9,953.28 9,621.504
STS-768 STM-256 OC-768 39,813.120 38,486.016
  • Line rate:
90 columns * 9 rows * 8bit/byte * 8000 frame/sec = 51.84 Mbps
  • Payload rate:
87 columns * 9 rows * 8bits/byte * 8000 frames/sec = 50.112 Mbps.
  • Columns 1-3 are used for Transport Overhead (1-3) which leaves 4 to 90 for payload.

SONET's Physical Layer

  • SONET's physical layer consists of the following 4 layers:
    • 1. Photonic Layer: responsible for electronic to optical conversion from STS to OC signal. Deals with pulse shape, wavelengths and power levels.
    • 2. Section Layer: Interaction between any Section Terminating Equipment (STE) which include regenerators and multiplexers. STS-n blocks are encapsulated and final framing and scrambling of traffic occurs. Add its own overhead used for frame detection, section error monitoring, order wire communications and section maintenance.
    • 3. Line Layer : Supports traffic flow between adjacent SONET multiplexers (this excludes any regenerators that might exist. Provides the capability of multiplexing/demultiplexing. Interaction between Line Terminating Equipment (LTE) on this layer. Adds it's own overhead. Lines are made up of several Sections. Responsible for reliable transport of the path layer.
    • 4. Path Layer : Used between Path Terminating Equipment (PTE). Path layer is made up of other Lines and Segments.

SONET Framing

  • Framing is the structure of frame formats.
  • SONE transmits 8000 frame per second. The size of the frame varies based on the STS or STM rate. In other words 1 frame every 125 microseconds.
  • SONET Frame consists of two part:
    • Transport Overhead (TOH)
      • Section Overhead (SOH)
      • Line Overhead (LOH)
    • Synchronized Payload Envelope (SPE) - payload use to carry actual a traffic.

SONET Network Elements

Path Terminating Equipment

  • Any equipment used for the full path passage to it destination PTE.
  • One such device is a Terminal Multiplexer.
  • Terminal Multiplexer (TM) concentrates traffic from T1, T3 or E1 line into appropriate STS frame. For example T1s incoming and outgoing OC-n.
  • Multiplexing Gateways are used to concentrate ATM or Ethernet.
  • Usually edge devices on the customer premise.

Add/Drop Multiplexers

  • ADMs main function is to add and/or drop individual STS signals without fully demultiplexing.
  • Usually have at least 3 sets of fiber: two to the ring and one to a Terminating Multiplexer.


  • Regenerators take the incoming signal, convert optical to electrical and regenerate back to optical without any distortion.
  • Section Overhead (SOH) has to be processed and new SOH generated. Processing include the following task:
    • Framing and synchronization
    • Error monitoring
    • Alarms Monitoring
  • Amplifiers just amplify the signal without changing any information within the SONET layers. Signal distortion is possible.

Digital Cross-Connects

  • Automatically switch signal from one wire to anther.
  • Within SONET, DCC can be used to automatically cross-connect OC-n signals to other fiber.


Point to Point

  • Simplest topology with two PTE devices.
  • PTEs can be router or switches or any device that support OC-n interface.

Point to Multipoint/Linear

  • Similar to a bus topology, intermediate points are used to adding and dropping traffic along the path.
  • Usually network consists of PTEs and ADMs.


  • Most common topology within service providers.
  • Provides most resilience.
  • Topology consists of ADM.
  • Rings have two or four fibers. Half of the fiber is used for transmission in one direction and the other half in reverse direction. If failure occurs, transmission direction is reversed.
  • Rings can be cross-connected to other rings.
  • Two types of rings:
    • unidirectional path-switched rings (UPSR)
    • bidirectional line-switched rings (BLSR)
  • Rings have to operate at the same speed.
  • Rings can be path switched or line switched
  • Rings can be unidirectional or bidirectional.
  • With all combinations, there should be 8 different types of rings that can be build.

Full Mesh

  • Topology consists of ADMs and digital cross-connects.


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