A Wide Area Network(WAN), SONET, that is used as a transport network to carry loads from other WANs.

The high bandwidths of fibre-optic cable are suitable for today’s high data rate technologies e.g. Video conferencing and carrying large numbers of lower-rate technologies at the same time. For this reason, the importance of fibre optics grows in conjunction with the development of technologies requiring high data rates or wide bandwidths for transmission.

The United States(ANSI) and Europe(ITU-T) have responded by defining standards that, through independent, are fundamentally similar and ultimately compatible. The ANSI standard is called Synchronous Optical Network(SONET) and the ITU-T standard is called the Synchronous Digital Hierarchy(SDH).

SONET/SDH is a synchronous network using synchronous TDM(Time Division Multiplexing) multiplexing. All clocks in the system are locked to a master clock.

Architecture:

The architecture of a SONET system consists of Signals, Devices and Connections.

SONET Signals

SONET defines a hierarchy of electrical signalling 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 corresponding optical signals are called Optical Carriers (OCs). SDH specifies a similar system called a Synchronous Transport Module (STM). STM is intended to be compatible with existing European hierarchies, such as E lines, and with STS levels. To this end, the lowest STM level, STM-1, is defined as 155.520 Mbps, which is Exactly equal to STS-3.

SONET Devices

SONET transmission relies on three basic devices: STS multiplexers/ Demultiplexers, regenerators, add/drop multiplexers and terminals.

1. STS Multiplexers/ Demultiplexers:

STS Multiplexers/ Demultiplexers are the beginning and endpoints of a SONET link. They provide the interface between an electrical tributary network and the optical network. An STS multiplexers multiplexes signal from multiple electrical sources and creates the corresponding OC signal. An STS demultiplexer demultiplexes an optical OC signal into corresponding electric signals.

2. Regenerator:

A regenerator is a repeater that takes a received optical signal(OC-n), demodulates it into the corresponding electrical signals (STS-n), regenerates the electrical signal, and finally modulates the electrical signal into its correspondent OC-n signal. A SONET regenerator replaces some of the existing overhead information (header information) with new information.

3. Add\ Drop Multiplexer:

Add/Drop multiplexers allow insertion and extraction of signals. An add/ drop multiplexer (ADM) can add STSs coming from different sources into a given path or can remove the desired signal from a path and redirect it without demultiplexing the entire signal.

A number of incoming electronic signals are fed into the STS multiplexers, where they are combined into a single optical signal. The optical signal is transmitted to a regenerator, where it is recreated without the noise it has picked up in transit. The regenerated signals from a number of sources are then fed into an add/drop multiplexer. The add/drop multiplexer reorganizes these signals, if necessary, and sends them out as directed by information in the data frames. These remultiplexed signals are sent to another regenerator and from there to the receiving STS demultiplexer, where they are returned to a format usable by the receiving links.

4. Terminals:

A terminal is a device that uses the services of a SONET network. For example, in the Internet, a terminal can be a router that needs to send packets to another router at the other side of a SONET network.

5. Connections:

The devices in a network are connected using sections, lines and paths.

• Sections:

A section is an optical link connecting two neighbouring devices: multiplexers to multiplexers, a multiplexer to regenerator or regenerator to a regenerator.

• Lines:

A line is the portion of the network between two multiplexers: STS multiplexer to add/drop multiplexer, two add/drop multiplexers, or two STS multiplexers.

• Paths:

A path is an end to end portion of the network between two STS multiplexes. In a simple SONET of two STS multiplexers linked directly to each other, the section, line and path are the same.

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.

Path Layers

The Path layer is responsible for the movement of a signal from its optical source to its optical destination. At the optical source, the signal is changed from an electronic form into an optical form, multiplexed with other signals, and encapsulated in a frame. At the optical destination, the received frame is demultiplexed, and the individual optical signals are changed back into their electronic forms. Path layer overhead is added at this layer. STS multiplexers provide path layer functions.

Line layer

The line layer is responsible for the movement of a signal across a physical line. Line layer overhead is added to the frame at this layer. STS multiplexers and add/drop multiplexers provide line layer functions.

Section Layer

The Section layer is responsible for the movement of a signal across a physical section. It handles framing, scrambling and error control. Section layer overhead is added to the frame at this layer.

Photonic 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. SONET uses NRZ encoding, with the presence of light representing 1 and the absence of light representing 0.

Device layer Relationship

Figure 1.5 shows the relationship between the devices used in SONET transmission and the four layers of the standard. The STS multiplexer is a four layer device. An add/drop multiplexer is a three layer device. A regenerator is a two layer device.

Figure 1.5

SONET Frames

Each synchronous transfer signal STS- n is composed of 8000 frames. Each frame is a two-dimensional matrix of bytes with 9 rows by 90 X n columns. For example, an STS-1 frame is 9 rows by 90 columns (810 bytes), and an STS-3 is 9 rows by 270 columns (2430 bytes). Figure 1.6 shows the general format of an STS-1 and STS-n.

Figure 1.6

Frame, Byte and Bit transmission

One of the interesting points about SONET is that each STS-n Signal is transmitted at a fixed rate of 8000 frames per second. This is the rate at which voice is digitized. For each frame the bytes are transmitted from the left to the right, top to the bottom. For each byte, the bits are transmitted from the most significant to the least significant. Figure 1.7 shows the order of frame and byte transmission.

Figure 1.7

If we sample a voice signal and use 8 bits(1 byte) for each sample, we can say that each byte in a SONET frame can carry information from a digitized voice channel. In other words, an STS-1 signal can carry 774 voice channels simultaneously (810- required bytes for overhead).

STS-1 Frame Format

The basic format of an STS-1 frame is shown in Figure 1.8 SONET frame is a matrix of 9 rows of 90 bytes (octets) each, for a total of 810 bytes.

Figure 1.8

The first three columns of the frame are used for section and line overhead. The upper three rows of the first three columns are used for section overhead (SOH). The lower six are line overhead (LOH). The rest of the frame is called the synchronous payload envelope (SPE). It contains user data and path overhead (POH) needed at the user data level.

Section Overhead

The Section overhead consists of nine octets. The labels, Functions and organization of these octets are shown in Figure 1.9

Figure 1.9

Alignment Bytes (A1 and A2): Bytes A1 and A2 are used for framing and synchronization and are called alignment bytes. These bytes alert a receiver that a frame is arriving and give the receiver a predetermined bit pattern on which to synchronize. The bytes serve as a flag.

Section Parity byte (B1): Byte B1 is for bit interleaved parity. Its value is calculated overall bytes of the previous frame. In other words, the ith bit of this byte is the parity bit calculated overall ith bits of the previous STS-n frame. The value of this byte is filled only for the first STS-1 in an STS-n frame.

Identification byte (C1): Byte C1 carries the identity of the STS-1 frame. This byte is necessary when multiple STS-1s are multiplexed to create a higher rate STS (STS-3, STS-9, STS-12, etc..). Information in this byte allows the various signals to be recognized easily upon demultiplexing. For example, in an STS-3 signal, the value of the C1 byte is 1 for the first STS-1, it is 2 for the second, and it is 3 for the third.

Management bytes (D1, D2 and D3): Bytes D1, D2 and D3 together form a 192-kbps channel (3 X 8000 X 8) called the data communication channel. This channel is required for operation, administration and maintenance (OA&M) signalling.

Order wire byte (E1): Byte E1 is the order wire byte. Order wire bytes in consecutive frames form a channel of 64 kbps. This channel is used for communication between regenerators or between terminals and regenerators.

User’s byte (F1): The F1 bytes in consecutive frames form a 64- kbps channel that is reserved for user needs at the section level.

Line Overhead Line overhead consists of 18 bytes. The labels, functions and arrangement of these bytes are shown in Figure 1.10

Figure 1.10

Line Parity byte (B1): Byte B2 is for bit interleaved parity. It is for error checking of the frame over a line (between two multiplexers). In an STS-n frame, B2 is calculated for all bytes in the previous STS-1 frame and inserted at the B2 byte for that frame. In other words, in a STS-3 frame, there are three B2 bytes, each calculated for one STS-1 frame. Contrast this byte with B1 in the section overhead.

Data communication channel bytes (D4 and D12): The line overhead D bytes (D4 and D12) in consecutive frames form a 576-kbps channel that provides the same service as the D1-D3 bytes (OA&M), but at the line rather than the section level (between multiplexers).

Order wire byte (E2): The E2 bytes in consecutive frames form a 64-kbps channel that provides the same functions as the E1 order wire byte, but at the line level.

Pointer bytes (H1, H2 and H3): Bytes H1, H2 and H3 are pointers. The first two bytes are used to show the offset of SPE in the frame; the third is used for justification.

Automatic protection switching bytes (K1 and K2): The K1 and K2 bytes in consecutive frames form a 128-kbps channel used for automatic detection of problems in line terminating equipment.

Growth bytes (Z1 and Z2): The Z1 and Z2 bytes are reserved for future use.

Synchronous Payload Envelope

The synchronous payload envelope(SPE) contains the user data and the overhead related to the user data(path overhead). One SPE does not necessarily fit it into one STS-1 frame; it may be split between two frames. The Path overhead, the leftmost column of an SPE, does not necessarily align with the section or line overhead. The path overhead must be added first to the user data to create an SPE can be inserted into one or two frames. Path overhead consists of 9 bytes. The labels, functions and arrangement of these bytes are shown in Figure 1.11

Figure 1.11

Path parity byte(B3): Byte B3 is for bit interleaved parity, like bytes B1 and B2, but calculated over SPE bits. It is actually calculated over the previous SPE in the stream.

Path signal label byte (C2): Byte C2 is the path identification byte. It is used to identify different protocols used at higher levels (such as IP or ATM) whose data are being carried in the SPE.

Path user channel byte (F2): The F2 bytes in consecutive frames, like the F1 bytes, form a 64-kbps channel that is reserved for user needs, but at the path level.

Path status byte(G1): Byte G1 is sent by the receiver to communicate its status to the sender. It is sent on the reverse channel when the communication is duplex.

Multiframe indicator(H4): Byte H4 is the multiframe indicator. It indicates payloads that cannot fit into a single frame For Example: Virtual tributaries can be combined to form a frame that is larger than an SPE frame and needs to be divided into different frames.

Path trace byte(J1): The J1 bytes in consecutive frames form a 64-kbps channel used for tracking the path. The J1 byte sends a continuous 64 byte string and verifies the connection. The choice of the string is left to the application program. The receiver compares each pattern with the previous one to ensure nothing is wrong with the communication at the path layer.

Growth bytes (Z3, Z4 and Z5): Bytes Z3, Z4 and Z5 are reserved for future use.