Basics of SONET(Synchronous Optical Network)

Level: - Intermediate
Platform: - Optical networks
Keywords: - : what is SONET- Synchronous Optical Network, single-mode and Multi mode optical fiber cables
Author: - Surender Singh

What is SONET:-

SONET (Synchronous Optical Network) is a standard ratified by the American National Standards Institute (ANSI) for high-capacity optical communications. It is currently the leading backbone transmission technology implemented by large carriers.

SONET is divided into an electrical connection and an optical connection with a multiplexer in between. SONET signals are called Synchronous Transport Signal (STS) on the electrical connection side and Optical Carrier (OC) on the optical connection side. The lowest SONET signal rate is the STS-1 (or OC-1) that has a rate of 51.84 Mbps. Higher rates are achieved by multiplexing STS-1s. A STS-1 is the equivalent of a DS3 (or T3) connection. The different connection rates are as follows:


OC-1

STS-1

51.84 Mbps

OC-3

STS-3

155.52 Mbps

OC-12

STS-12

622.08 Mbps

OC-48

STS-48

2488.32 Mbps

OC-96

STS-96

4976.64 Mbps

 

Generally SONET uses fiber optic cable as a transmission medium. There are two main types of optical cable, single-mode and multi-mode. The difference between the two cable types is the diameter of the silicon dioxide core. Multi-mode cable has a core that is 50 to 100 microns in diameter, while single-mode cable has a diameter of 7 to 9 microns. SONET interfaces use single-mode fiber optic cable that is defined in three different ranges: Short Reach (SR) with a range of up to 2km, Intermediate Reach (IR) with a range of up to 15km, and Long Reach (LR) with a range of up to 40km. For short-run or interoffice applications, SONET can run over coaxial cable with maximum distances of 900ft for STS-1 rates and 450ft for STS-3 rates.

The fiber optic cable used for SONET is immune to electrical interference, but can suffer from attenuation and dispersion. In SONET networking, a laser converts the ones and zeros of a traditional electrical signal into pulses of light (using a laser or diode). An optical receiver receives these pulses at the far end of the cable. As the signal travels down the cable, the pulses get smaller with distance—this is attenuation (same concept as for copper cable).
There are three types of dispersion. Modal dispersion is caused by light waves bouncing around in the core of the cable. Some of the light rays travel a longer distance than others causing the pulses to spread out and become harder to differentiate. The smaller the core diameter of the cable, the less modal distortion. Different colors of light travel at different speeds, causing the pulses to spread out as they travel down the cable. This is called chromatic dispersion. Polarization mode dispersion (PMD) occurs when the core of the cable is not perfectly round. PMD is a crucial problem in long haul fiber optic networks.

The frame structure of a SONET STS-1 frame is depicted as a two dimensional grid consisting of 9 rows by 90 columns of 8-bit bytes, or 6480 bits per frame. The frame rate is 8000 frames per second for a total of 51,840,000 bps (6480x8000). The first 3 columns are used for transport overhead with the rest of the frame used for data.

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