SONET

Introduction SONET SONET - S ynchronous O ptical NET work (North America) It is used as a transport network to carry loads from WANs. SONET was originally designed by ANSI for the public telephone network in the mid-1980s. SDH - Synchronous Digital Hierarchy – Europe. It was developed by ITU-T. SONET is a standard for optical telecommunications transport formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), which sets industry standards in the U.S. for telecommunications and other industries.

SONET is a physical layer network technology designed to carry large volumes of traffic over relatively long distances on fiber optic cabling. SONET defines clear interoperability standards between different vendors' products. SONET performs at high speeds at a cost competitive with alternatives like ATM and Gigabit Ethernet SONET commonly transmits data at speeds between 155 Mbps and 2.5 Gbps.

Architecture 1. Signals – SONET defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy and the hierarchy of electrical signaling levels are called SYNCHRONOUS TRANSPORT SIGNALS (STSs). – Each STSs level (STS-1 to STS-192) supports a certain data rate – Corresponding optical signals are known as Optical Carrier (OCs). – SDH specifies similar system known as Synchronous Transport Module (STM).

Basic SONET signal SONET Hierarchy SignalBit Rate (Mbps)STM STS–1, OC– STS–3, OC– STM-1 STS–12, OC– STM-4 STS–48, OC–482, STM-16 STS–192, OC–1929, STM-64

Architecture 2. SONET Devices – SONET transmission relies on three basic devices: STS multiplexers/demultiplexers, regenerators, add/drop multiplexers and terminals. A simple network using SONET equipment

STS MUX/DEMUX – Marks beginning and end points of SONET links – Provide an interface between an electrical tributary and optical network – STS MUX multiplexes signals from multiple electrical sources and creates the OC signal. – STS DEMUX demultiplexes an optical OC signal into corresponding electrical signals. Regenerator – It is repeater that takes a received OC signal (OC-n), demodulates it into corresponding electric signals (STS- n), regenerates the electric signal, and finally modulates the electric signal into corresponding OC-n signal.

ADD/DROP MUX – Allow insertion and extraction of signals. – Can add STSs coming from different sources into a given path or can remove a desired signal from a path and redirect it without demultiplexing the entire signal. – Use header information to identify individual streams TERMINAL – It is a device that uses the services of SONET network. For example – Router that needs to send packets to another router at the other side of SONET network

3. Connections Devices such as MUX/DEMUX, regenerator, ADD/DROP MUX are connected using : – Sections – It is the optical link connecting two neighboring devices : MUX to MUX, MUX to regenerator – Lines – It is the portion of network between two MUXs: STS MUX to ADD/DROP MUX, two ADD/DROP MUXs, or two STS MUXs. – Paths – It is the end-to-end portion of network between two STS MUX.

SONET Layers The SONET standard includes four functional layers: the photonic, the section, the line, and the path layer. They correspond to both the physical and the data link layers. Photonic, which corresponds to the OSI's physical layer, defines the physical specifications for optical fiber channel, sensitivity of receiver etc. SONET uses NRZ encoding with presence of Light representing 1 and absence of light representing 0. Section Layer is responsible for movement of signal across a physical section. It handles framing, scrambling, error control. Section layer overhead is added to the frame at this layer.

Line Layer is responsible for movement of signal across a physical line. Line layer overhead is added to frame at this layer. STS MUX and ADD/DROP MUX provides line layer functions. Path Layer is responsible for movement of signal from its optical source to its optical destination. Path layer overhead is added at this layer. STS MUX provide path layer functions.

SONET Frames Each STS-n is composed of 8000 frames. Each frame is a two dimensional matrix of bytes with 9 rows and 90*n columns. Each STS-n signal is transmitted at fixed rate of 8000 frames per second. For each frame data is transmitted from left to right, top to bottom. For each byte, bits are transmitted from MSB to LSB. An STS-1 and an STS-n frame STS-1 frames in transmission

EXAMPLE Find the data rate of an STS-3 signal. Solution STS-3, like other STS signals, sends 8000 frames per second. Each STS-3 frame is made of 9 by (3 × 90) bytes. Each byte is made of 8 bits. The data rate is

EXAMPLE What is the duration of an STS-1 frame? STS-3 frame? STS-n frame? Solution In SONET, 8000 frames are sent per second. This means that the duration of an STS-1, STS- 3, or STS-n frame is the same and equal to 1/8000 s, or 125 μs.