SONET (Synchronous Optical Network)
Definition : SONET (Synchronous Optical Network) is an ANSI standard. It encodes bit streams into optical signals that are propagated over optical fiber. SONET’s high speed and its frame structure permit it to support a very flexible set of bearer services.
Synchronous Transport Signals
SONET defines a hierarchy of signaling levels called synchronous transport signals (STSs). Each STS level (STS -1 to STS 192) supports a certain data rate, specified in megabits per second.
The physical links defined to carry each level of STS are called optical carriers (OCs). OC levels describe the conceptual and physical specifications of the links required to support each level of signaling.
The basic SONET signal is STS -1. It has a bit rate of 51.84Mbps. The STS -n has a rate equal to n x 51. 84Mbps.
Section Section Section Section
A SONET system
STS Multiplexer / Demultiplexer:
* An STS multiplexer/demultiplexer either multiplexes signals from multiple sources into an STS signal or demultiplex an STS signal into different destination signals.
A regenerator (REG) is a device that boosts the power of the optical signal. The device has three components .
* The first component converts the optical signal into an electrical signal.
* The second amplifies the electrical signal, and the
* Third coverts the amplified electrical signal to more powerful optical signal.
Add / drop multiplexer:
An add/drop multiplexer can add signals coming from different sources into a given path.
Sections, Lines and Paths
Various levels of SONET connections are called sections, lines and paths.
Sections: A section is the optical link connecting two neighbor devices: multiplexer to multiplexer, multiplexer to regenerator, or regenerator to regenerator.
Line: Line is the transmission medium (optical fiber). It is the portion of the network between two multiplexers. A line has a certain bit rate.
Path: A path is the end-to-end portion of the network between two STS multiplexers.
SONET defines four layers. The photonic layer is the lowest and performs physical layer activities. The section, line and path layers correspond to the OSI model’s data link layer.
The photonic layer corresponds to the physical layer of the OSI model. It includes physical specifications for the optical fiber channel, the sensitivity of the receiver, multiplexing functions and so on.
The photonic layer converts the electrical bit stream from the section layer into an optical signal.
SONET uses NRZ encoding with the presence of light representing 1 and the absence of light representing 0
The section layer is responsible for the movement of a signal across a physical section.
The section layer provides the framing, error control and scrambling for the bits being transmitted.
Section layer overhead is added to the frame at this layer.
The line layer is responsible for the movement of a signal across a physical section.
The line layer provides line maintenance and protection and multiplexing of STS–1 signal.
STS multiplixers and add/drop multiplexers provide line layer functions. I
The path layer is responsible for the movement of a signal from its optical source to its optical destination. STS multiplexers provide path layer functions.
End- to -end error detection
Multiplexing, frame and frequency alignment
Framing, scrambling and error control
Electrical to optical and optical to electrical signal
The layers of SONET and some of their functions:
Data received from an electronic interface, such as a T-1 line, is encapsulated in a frame at the path layer and over head is added.
Additional overhead is added, first at the line layer and then at the section layer. Finally, the frame is passed to the photonic layer, where it is transformed into an optical signal.
O verhead 90 Octets per Row
SPE 87 * 9 = 783 Octets
U SER Data = 86 * 9 = 774 Octets 9 Rows
The four layers of the SONET over/lead
Consider an example where two path layer processes are exchanging DS -3 (digital signal) frames (The 45 -Mbps DS -3 signal belongs to the current telephone signal hierarchy).
The DS -3 frames, plus POH (Path Over Head), are mapped into an STS -1 SPE (Synchronous Payload Envelope) which is then given to the line layer.
The line layer may multiplex several different payloads from the path layer. In addition to multiplexing the line overhead provide other functions such as protection switching.
Finally, the SOH (Section Over Head) provides framing and scrambling prior transmission by the photonic layer.
The synchronous payload envelope (SPE) contains user data and details about charges and payment required for transmission.
Path overhead includes end-to-end tracking information.
The SONET frame
The fig. represents overall features of the SONET frame. The 810-byte STS -1 frame is organized into nine rows of 90 bytes (transmitted left to right, top to bottom).
A frame is 125 s in duration, corresponding to one 8-KHz voice sample. (This gives the STS -1 rate of 51.840Mbps)
(810 bytes / frame x 8,000 frames / s ~ 51.840Mbps)
STS -1 frame
One frame = 810 bytes ( or) 810 x 8 bits = 6,480bits
Within one second 8000 frames will be transmitted. 1
Total data rate of STS -1 frame = 6,480 x 8000= 51,840,000 bits
The first three columns (27 bytes) are for SOH and LOH. Thus an SPE is 9 rows x 87 columns, of which the first column (9bytes) is devoted to POH.
SONET is designed to carry broadband payloads. Current digital hierarchy data rates (DS -1 to DS -3 ), however lower than STS -1. To make SONET backward compatible with the current hierarchy, its frame design includes a system of Virtual Tributaries (VTs).
A virtual tributary is a partial payload that can be inserted into an STS -1 and combined with other partial payloads to fill out the frame.
Types of VTs:
Four types of VTs have been defined to accommodate existing digital hierarchies.
VT 1.5 accommodates the US DS -1 service ( 1.544 Mbps )
VT2 accommodates the European CEPT- 1 service
VT3 accomodates the DS -1 C service (fractional DS -1, 3.152Mbps)
VT6 accomodates the DS -2 service (6.312Mbps)
When two or more tributaries are inserted into a single STS -1 frame, they are interleaved column by column.
SONET is designed to provide a backbone network for WANs.
SONET can replace existing T-1 or T-3 lines.
SONET links and interfaces will also be used as the physical layer in high –speed LANs.
SONET can be the carrier for ISDN and B-ISDN .
SONET can replace the fiber -optic cables used in cable TV network.
SONET links are used in Gigabit Ethernets and A TM
The synchronous clock in the nodes of a SONET network facilitates byte interleaving, reducing the cost of time-division multiplexing equipment.