SONET/SDH

SONET/SDH • The high bandwidth of fiber-optic cable is suitable for today's high

data rate technologies (such as video conferencing) and for carrying large numbers of lower-rate technologies at the same time.

• The importance of optical fibers grows in conjunction with the development of technologies requiring high data rates or wide bandwidths for transmission.

• With their prominence, came a need for standardization.

• The ANSI standard is called the Synchronous Optical Network (SONET).

• The ITU-T standard is called the Synchronous Digital are Hierarchy (SDH).

• These two standards are nearly identical.

SONET • SONET is a synchronous network. A single clock is used to.

handle the timing of transmissions and equipment across the entire network.

• Network wide synchronization adds a level of predictability to the system.

• This predictability, coupled with a powerful frame design, enables individual channels to be multiplexed, thereby improving speed and reducing cost.

• SONET contains recommendations for the standardization of fiber optic transmission system (FOTS) equipment sold by different manufacturers.

• The SONET physical specifications and frame design include mechanisms that allow it to carry signals from incompatible tributary systems (such as DS-1 to DS-3). It is this flexibility that gives SONET a reputation for universal connectivity.

SONET

• SONET encodes bit streams into optical signals propagated over optical fiber.

• SONET defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy.

• A bit-way implementation providing end-to-end transport of bit streams.

• All clocks in the network are locked to a common master clock so that simple TDM can be used.

• Multiplexing done by byte interleaving.

SONET Devices

STE: Section Terminating Equipment, e.g. a repeater

LTE: Line Terminating Equipment, e.g. a STS-1 to STS-3 multiplexer

PTE: Path Terminating Equipment, e.g. an STS-1 multiplexer

(a) STS

PT

E LTE STE

STS-1 Path

STS Line

Section Section

Mux Mux reg reg reg

SONET

Terminal

STE STE LTE

STS

PT

E SONET

Terminal

SONET Devices

• STS multiplexer/ demultiplexer:

– An STS multiplexer/demultiplexer either multiplexes signals from multiple sources into an STS or demultiplexes an STS into different destination signals.

• Regenerator:

– An STS regenerator is a repeater that takes a received optical signal and regenerates it. Regenerators in this system, however, add a function to those of physical layer repeaters. A SONET regenerator replaces some of the existing overhead information (header information) with new information. These devices function at the data link layer.

• Add/drop multiplexer:

– An add/drop multiplexer can add signals corning from different sources into a given path or remove a desired signal from a path and redirect it without demultiplexing the entire signal.

SONET Frame

B B B 87B

Information

Payload 9 Rows

125 s Transport

overhead

90 bytes

Section

Overhead 3 rows

6 rows Line

Overhead

SPE Synchronous Payload Envelope

SONET Frame

• A SONET frame can be viewed as a matrix of nine rows of 90 octets each, for a total of 810 octets (6480 bits).

• Some of the octets are used for control; they are not positioned at the beginning or end of the frame (like a header or trailer).

• The first three columns of the frame are used for administration overhead.

• The rest of the frame is called the Synchronous Payload Envelope (SPE). The SPE contains transmission overhead and user data.

Synchronous Transport Signals

• SONET defines a hierarchy of signaling levels called synchronous Transport Signals (STS).

• 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.

• Actual implementation of those specifications is left up to the manufacturers.

• Currently, the most popular implementations are OC-1, OC-3, OC-12, and OC-48.

Synchronous Transport Signals

STS-1 • STS-1 or OC-1 is the lowest rate service provided by SONET.

• STS-1 transmits 8000 frames per second.

• Following figure compares the raw, SPE, and user bit rates.

• The rates reflect the number of columns available.

• For example, the SPE bit rate is less than the raw bit rate due to the three columns for management.

Synchronous Transport Signals Comparison

STS OC Rate (Mbps) SPE (Mbps) User (Mbps)

STS-1 OC-1 51.84 50.12 49.536

STS-3 OC-3 155.52 150.336 148.608

STS-9 OC-9 466.56 451.008 445.824

STS-12 OC-12 622.08 601.344 594.432

STS-18 OC-18 933.12 902.016 891.648

STS-24 OC-24 1244.16 1202.688 1188.864

STS-36 OC-36 1866.23 1804.032 1783.296

STS-48 OC-48 2488.32 2405.376 2377.728

STS-192 OC-192 9953.28 9621.604 9510.912

Synchronous Transport Signals Comparison

Virtual Tributaries

• SONET is designed to carry broadband payloads.

• 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 a frame and combined with other partial payloads to fill out the frame.

• Instead of using all 87 payload columns of an SPE frame for data from one source, we can subdivide the SPE and call each component a VT.

• Four types of VTs have been defined to accommodate existing digital hierarchies.

• Notice that the number of columns allowed for each type of VT can be determined by doubling the type identification number (VT1.5 gets three columns, VT2 gets four columns, etc.).

Virtual Tributaries VT1.5

– The VT1.5 accommodates the U.S. DS-1 service (1.544 Mbps).

VT2

– The VT2 accommodates the European CEPT-1 service (2.048 Mbps).

VT3

– The VT3 accommodates the DS-1C service (fractional DS-1, 3.152 Mbps).

VT6

– The VT6 accommodates the DS-2 service (6.312 Mbps).

• When two or more tributaries are inserted into a single STS-1 frame, they are interleaved column by column.

• SONET provides mechanisms for identifying each VT and separating them without demultiplexing the entire stream.

Virtual Tributaries

Higher-Rate Services

• Lower-rate STSs can be multiplexed to make them compatible with higher-rate systems.

• Figure shows how three STS-1's are multiplexed into a single STS-3.

• To create an STS-12 out of lower-rate services, we could multiplex either 12 STS-l's or 4 STS-3's.

ATM Cells in an STS-3 Frame

Limitation of SONET

• SONET traffic is carried in fixed bandwidth

groups

• SONET has no built-in capability of

dynamically shifting bandwidth usage

Next Generation SONET

• Next-generation SONET (NGS) is the evolution

and enhancement of existing SONET networks

that sets a new economic level for network

efficiency while increasing broadband service

potential.