Chapter 9

Synchronous Optical Network/Synchronous Digital

Hierarchy

Part II: Framing and Architecture

SONET/SDH

• SONET is an acronym standing for synchronous optical network. SDH stands for synchronous digital hierarchy.

• SONET has a North American flavor. SDH has a European flavor.

• Both SONET and SDH are similar digital transport formats that were developed for the specific purpose of providing a reliable and versatile digital structure to take advantage of the higher bit-rate capacity of optical fiber.

• Both SONET and SDH use basic building block techniques.

How SONET and SDH Came into Being

1960s --- T1/E1, electrical transmission1970s --- The concept of using optical fiber to transmit signals was conceived

Research on using fiber to transmit T1/E1 started.1984 --- Bell Breakup, a group of vendors started to develop optical transmission

standard. 1985 --- ANSI T1X1 Committee submitted a proposal of 50.668Mbit/s transfer

rate.1987 --- ANSI T1X1 Committee published draft document on SONET (STS-1)

About the same time ITU-T established SDH – 155.52 Mbit/s

Then: STS-1 made 51.84 and STS-1 X 3 = 155.52 Mbit/s

SONET and SDH transmission rate

Network elements• Synchronous Transport Signal (STS) multiplexer/demultiplexer: It either

multiplexes signal from multiple sources into a STS signal or demultiplexesan STS signal into different destination signals.

• Regenerator: It is a repeater that takes a received optical signal and regenerates it.

• Add/drop Multiplexer: Can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it.

Add-Drop multiplexers

• An important SONET/SDH network element is the add/drop multiplexer (ADM), that is used to add and drop sub-channels within an OC-N signal. Upon entering an ADM, the sub-channels can be individually droped by the ADM and others can be added.

Section, line and path• A section is an optical link, connecting two neighboring devices• A line is a portion of network between two multiplexers • A Path is the end-to-end portion of the network between two STS

multiplexers

SONET signal structure• Synchronous transport signal (STS)

STS-N bit rate = N * 51.84 Mbps(STS-1)• Optical Carrier (OC-N)

Resultant optical signal after electrical-to-optical conversion• N=1,3,12,24,48,192

SONET-Framing

The basic building block• The STS-1 frame: the smallest SONET frame is visualized as a two-

dimensional matrix of 9-row by 90-column bytes. The first 3 columns of STS-1 frame contain the transport overhead. The remaining 87 columnscontain path overhead and end user data, known as the synchronous payload envelope (SPE). Path overhead information is in column 4 of the STS-1 frame. Two SPE columns (30 and 59) do not contain any information and are known as “fixed stuff”.

STS-1 frame transmission• STS-1 frames are transmitted consecutively (i.e. when one ends the next

one starts). Bytes are transmitted serially, starting with the most significant bit of the byte in column 1 and row 1 and ending with the last byte in row 9 and column 90. At the end of the frame, the process continues with the next frame and so on.

STS-1=(90 bytes/row)(9 rows/frame)(8 bits/byte) / (125 µs/frame) = 51.84 Mbps

STS-1 Overhead

Section overhead

Responsible for movement of a signal across a physical section.

Line overheadResponsible for the movement of a signal across a physical line.

Path overhead

Responsible for movement of a signal across a physical path.

STS-N frame

SPE• The STS-1 SPE may begin anywhere in the STS-1 envelope capacity.

Typically, the SPE begins in one STS-1 frame and ends in the next.

• Payload pointer resides in the line overhead and designates the location of the byte where the STS SPE begins. It provides a method for allowing flexible and dynamic alignment of the STS SPE within the STS envelope capacity, independent of the actual contents of the SPE.

• Recalculating or updating the payload pointer to achieve dynamicalignment.

Virtual tributary• Virtual tributary (VT) is a structure designed for the transport of sub-STS-1

payloads.• Four sizes: VT1.5, VT2, VT3, VT6

VTs mapped in groups and groups in SPE

• A number of VTs may fit in a SPE. However, VTs are mapped in larger containers known as groups; a SPE fits seven groups each of 12 columns. Thus, in a group either four VT1.5s, or three VT2s, or two VT3s, or one VT6 fit.

SONET multiplex hierarchy• VT group (VTG): VTs of similar types are grouped together• VTG has 12 columns by 9 rows• 7 VTGs can be accommodated in the STS-1 SPE

Network architecture• A key characteristic of SONET/SDH is that they usually configured as a

ring configuration. This is done to create loop diversity for uninterrupted service protection purposes in case of link or equipment failures. The SONET/SDH rings are commonly called self-healing rings, since the traffic flowing along a certain path can automatically be switched to an alternate or standby path following failure or degradation of the link segment.

Two popular SONET/SDH rings

• Two-fiber, unidirectional, path-switched ring (two-fiber UPSR)• Two-fiber or four-fiber, bidirectional, line-switched ring (two-fiber or four-

fiber BLSR), having a capacity advantage

Network architecture

Include point-to-point links, linear chains, UPSR, and BLSR

DWDM employment

• As another step toward realizing the full potential of optical fiber transmission capacity, researchers have looked at all-optical WDM networks to extend the versatility of communication networks beyond architectures such as those provided by SONET.

• These networks can be classified as either broadcast-and-select or wavelength-routing networks.

• In general, broadcast-and-select techniques employing passive optical stars, buses, or wavelength routers are used for local network applications, whereas active optical components form the basis for constructing wide-area wavelength-routing networks.

Broadcast-and-select WDM Networks

• Broadcast-and-select single-hop networks• Broadcast-and-select multihop networks• The shuffleNet multihop networks

Wavelength-routed networks