Synchronous optical network

The synchronous optical network (in English Synchronous Optical Network, whose acronym is SONET) is a standard for the transport of telecommunications in fiber optic networks.

Origins

The decision to create SONET was taken by the E.C.S.A. (Exchange Carriers Standard Association) in the United States to enable the standardized connection of fiber optic systems among themselves, even if they were from different manufacturers. In the last stages of development of SONET, the CCITT (International Telephone and Telegraph Consultative Committee), predecessor of the current ITU-T, also entered the ITU (International Telecommunication Union, telecommunications standardization sector) so that a standard could be developed. that would make possible the interconnection by fiber of telephone networks worldwide.
From this stage starts the development of the so-called Synchronous Digital Hierarchy, popularly known as SDH (Synchronous Digital Hierarchy). In the late 1990s, it is estimated that SONET/SDH standards will be able to provide the transport infrastructure for the global telecommunications network for the next two to three decades.
Even though they have points of compatibility, the SONET standard is practically only applied in the United States and Canada while the SDH is applied in the rest of the world.

Basic Signal and SONET Network Elements

The Basic SONET Signal

SONET defines a technology for transporting many signals of different capacities through a flexible synchronous optical hierarchy. This is accomplished by means of a byte interpolation multiplexing scheme. Byte interpolation simplifies multiplexing and offers end-to-end network management.
The first step in the SONET multiplexing process involves the generation of the lower level signals of the multiplexing fabric. In SONET the basic signal is known as a level 1 signal or also STS-1 (Synchronous Transport Signal level 1). It is made up of a set of 810 bytes distributed in 9 rows of 90 bytes. This set is transmitted every 125 microseconds, corresponding to the speed of the basic telephone channel of 64 kbit/s, so the bit rate of the STS-1 signal is 51.84 Mbit/s.

Figure 1.- STS-1 signal frame

The signals of higher levels are formed by the multiplexing of different signals of level 1 (STS-1), creating a family of signals STS-N, where the N indicates the number of signals of level 1 that compose it. Table 1 indicates the denominations of the electrical signals and optical carriers, as well as their speeds and the points of coincidence with those of the Synchronous Digital Hierarchy.

Sonet Network Elements

1.- Terminal multiplexer

It is the element that acts as a concentrator of tributary DS-1 (1.544 Mbit/s) signals as well as other signals derived from it and performs the transformation of the electrical signal into optical and vice versa.
Two terminal multiplexers linked by a fiber with or without an intermediate regenerator make up the simplest of SONET links.

2.- Regenerator

We need a regenerator when the distance separating two terminal multiplexers is very large and the optical signal received is very low. The regenerator clock is turned off when the signal is received, and the regenerator in turn replaces part of the signal frame header before retransmitting it. The traffic information found in the frame is not altered.

3.- Add/Drop Multiplexer (ADM)

The drop-add multiplexer (ADM) allows extracting part of the traffic carried at an intermediate point of a route and in turn injecting new traffic from that point. At the points where we have an ADM, only those signals that we need will be downloaded or inserted into the main data stream. The rest of the signals that we do not have to access will continue through the network.
Although network elements are compatible with the OC-N level, there may be differences in the future between different vendors of different elements. SONET does not restrict the fabrication of network elements. For example, one vendor may offer an ADM with access only to DS-1 signals, while another may offer simultaneous access to DS-1 (1.544 Mbit/s) and DS-3 (44.736 Mbit/s) signals.

SONET network configuration

1.- Point to point

The point-to-point network configuration is made up of two terminal multiplexers, linked by means of an optical fiber, at the ends of the connection and with the possibility of a regenerator in the middle of the link if necessary. In the future, point-to-point connections will traverse the entire network and will always originate and terminate at a multiplexer.

2.- Point to multipoint

A point-to-multipoint architecture includes ADM network elements along its path. The ADM is the only network element specially designed for this task. This avoids cumbersome network architectures of demultiplexing, cross-connecting, and then multiplexing again. The ADM is placed along the link to facilitate access to channels at intermediate points in the network.

3.- Red Hub

The hub network architecture is prepared for unexpected growth and changes in the network in a simpler way than peer-to-peer networks. A hub concentrates the traffic in a central point and distributes the signals to several circuits.

4.- Ring architecture:

The main element in a ring architecture (Figure 2) is the ADM. Multiple ADMs can be placed in a ring configuration for bidirectional or unidirectional traffic. The main advantage of the ring topology is its security; if a fiber cable breaks or is cut, multiplexers have the intelligence to divert traffic through other nodes on the ring without interruption.
The demand for security services, route diversity in fiber installations, flexibility to change services to alternate nodes, as well as automatic restoration in a few seconds, have made the ring architecture a very popular topology in SONET.

Figure 2.- Architecture in ring

Benefits of the SONET Network

The key to SONET is that it allows interfaces with asynchronous sources so that existing equipment can be replaced or supported by the SONET network. In this way transitions can be made gradually.

Here we can see the advantages that SONET presents over other systems:

• The growing configuration flexibility and the availability of SONET bandwidth provides significant advantages against other older telecommunication systems.
• Reduction of equipment needed for multiplexing and the extraction-insertion of traffic at intermediate points of the major routes.
• Increased network reliability as a result of the lower number of equipment involved in connections.
• It provides header bytes that facilitate the administration of information bytes and the maintenance of the equipment itself.
• Definition of a synchronous format of multiplexation for the transport of digital signals from the Plesiócrone or PDH Digital Hierarchy, at its various levels (such as DS-1, DS-3) and a synchronous structure that greatly simplifies the interface of digital switches, as well as connectors and multiplexers.
• The existence of a wide range of generic standards that allow the interconnection of products from different manufacturers.
• The definition of a flexible architecture capable of incorporating future applications, with a wide variety of transmission speeds.

Other advantages are:

• Centralized, integrated and remote interface for transport and multiplexing equipment.
• Quick bug isolation.
• Extreme to extreme performance monitoring.
• Support for new high-speed services.
• Allows private VIRTUAL REDES.
• The possibility of creating network structures distributed in a very economic way thanks to the ADD/DROP (ADM) multiplexers
• Double ring structure for greater immunity to failures.