The Synchronous Optical Network (SONET) is a standard for optical transport that was formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI) which sets standards for telecommunications and other industries in the United States. The standard was originally iniated by Bellcore labs in 1984 on behalf of the Regional Bell Operating Companies (RBOC’s) for the following key purposes:

• "Mid-span meet" which is the compatibility of equipment by all vendors who manufacture to a certain standard.
• Synchronous networking
• Enhanced operations, administration, maintenance and provisioning (OAM&P)
• Efficient add/drop multiplexing (ADM)
• Standards-based survival rings
• Transport of new services, such as Asynchronous Transfer Mode (ATM)

Synchronous versus Asynchronous.  Traditionally, transmission systems have been asynchronous with each terminal in the network running on its own clock. Since the clocks are free running large variations can occur in the data stream. In an asynchronous system a DS-3 signal can have a variation of up to 1789 bps. [2] In order for the signal to be multiplexed, a process known as bit stuffing must occur where extra bits are added to the signals so that they can be stacked together. A similar process must take place for the signal to be multiplexed. (Multiplexing is a process of taking packetized data and sending it over a shared wide area connection along with other packetized data form other sources. Demultiplexing breaks this process down and sends the packets to their destination.) [1] [2]

In digital transmission, clocking is one of the most important considerations. Clocking means using a series of repetitive pulses to keep the bit rate of data constant and to indicate where the ones and zeros are located in a data stream.

In a Synchronous system such as SONET, the average frequency of all clocks in the system will be the same (synchronous) or almost the same (plesiochronous). Since every clock in the system can be traced back to a stable reference supply an STS-1 rate will always be 51.84 Mbps with no variation. This allows STS-1 signals to easily be stacked together when multiplexed. Therefore, the process is much more efficient since the method of bit stuffing is not required. [1]

Fiber Optics.  Fiber optics give the following advantages over other media such as coaxial cable, twisted pair and microwave networks for data transport: [2]

• Fiber optics offer a much higher bandwidth.
• Fiber optics can be carried over longer distances without repeaters (regenerators)
• Fiber is immune to interference
• Fiber is very secure
• Although fiber is expensive, it is cheaper to maintain in the long run
• Even a small strand of fiber can offer enormous bandwidth
• Bandwidth upgrades can be made on the same fiber
• Fiber has very low bit error rates

Currently many separate networks provide the communication services available to subscribers. Plain old telephone service (POTS) is provided over a voice channel connected to a circuit-switched network. A totally independent packet-switched network might be used for data communication, offering faster methods for data delivery. [1]

An Integrated Services Digital Network (ISDN) provides a single network that can handle voice, data, and video. Whereas ISDN has traditionally applied to the narrowband telephony world, a broadband ISDN network consists of broadband ISDN switches and terminals which can tie high-speed local area networks (LANs), digital TV and other video services, data communication devices, telemetry equipment (for example automatic electrical reading), and voice into one digital network.

SONET provides the necessary bandwidth to transport information from one BISDN switch (or terminal) to another. For example, an OC-# (155 Mbps) rate may be used to transport an H4 digital broadband channel carrying a broadcast quality TV signal. Broadband ISDN will use asynchronous transfer mode (ATM) technology.

In the future, public and private networks will be based on ATM/cell-relay technology. ATM is the CCITT standard that supports cell-based voice, data, video, and multimedia communication in a public network under Broadband ISDN. SONET provides sufficient payload flexibility that it will be used as the underlying transport layer for BISDN ATM cells. [2] [3]

Technical Background. SONET is a grouping of physical layer specifications based on a signaling speed hierarchy called STS or synchronous transport signals. SONET also defines sub levels of the STS-1 format. It is possible for STS-1 signals to be subdivided into segments called virtual tributaries. Virtual tributaries are synchronous signals that are used for the transport of lower-speed transmissions. Table 1 below contains a listing of virtual tributaries and their sizes.

In order to compensate for frequency and phase variations, a concept known as "pointers" is used. Pointers allow the transparent transport of synchronous payload envelopes (either STS or virtual tributaries) across plesiochronous boundaries, which are between nodes with separate network clocks having almost the same timing). Pointers are useful in helping avoid delays and data loss. [1]

SONET and SDH.  When SONET was originally developed by Bellcore Labs in 1984, it was designed for use in domestic U.S. networks. However, SONET has been implemented for private LANs and WANs as well. SONET is a standard for the United States and Canada. It should be pointed out that although SONET and SDH are similar there are some fundamental differences, therefore the two standards don’t really interoperate. [3] SONET is based on the STS-1 at 51.84 Mbps, which makes it an affective carrier of T3 signals. There is no STS-1 level for SDH. SDH starts at STS-3, which is also known as STM-1 (Synchronous Transport Module-1) equal to 155.52 Mbps, which makes SDH more suited for the carrying of E4 signals. The fundamental differences in SONET and SDH are mainly a result of different current rates in Europe and North America. A listing of SONET/SDH hierarchies is presented below in Table 1. [4] [6] [12]

SONET Topologies.  The most common architecture for the deployment of SONET is the ring. Multiple ADMs can be placed in a ring configuration. A primary benefit of the ring architecture is its survivability. For example, if a fiber cut were cut the multiplexers will automatically send the signals through an alternative path through the ring without interruption. Because of the built in redundancy, rings are the most popular architecture for SONET implementation. [1] [2]

The two primary architecture types for the deployment of SONET are Unidirectional path-switched rings (UPSR) and Bidirectional line-switched rings (BLSR). In the UPSR architecture, all users share transmission capacity around a ring. [2] BLSR is a four fiber topology with built in redundancy that protects the network against network failures. In the case of a failure the user’s traffic is rerouted without compromising the data. [11]

Availability of SONET.  SONET services are mostly available only in major metropolitian areas. In order to access SONET services a local carrier must bring the fiber-based ring directly to the specified location and assign dedicated bandwidth to the customer. Most organizations with high bandwidth requirements are currently employing T-3s. SONET can be viewed as a possible upgrade for these services. [2]

GTE Corp is currently involved with a project that will provide SONET services to more cities by the end of 1999. GTE has purchased on $150 million worth of Northern Telecom Inc’s fiber-optic equipment to like more than 100 U.S. cities through a synchronous optical network backbone. Most of the 100 cities will be turned up on the new backbone by the end of 1998. GTE plans to begin the project on the coasts and move in to the Midwest. [13] MCI is also planning to offer a similar SONET service. The service would be offered in numerous metropolitan areas and would be on the OC-3 level. [14]

The research for this paper was conducted for the purpose of explaining the benefits of implementing the Synchronous Optical Network. In short, what are the benefits that SONET provides?

Most of the literature that is currently available to the general public on SONET falls into one of two categories: 1) Highly technical, engineering type documents that explain how and why SONET works, and 2) propaganda distributed by telecommunications companies for the purpose of marketing their products and services. Obviously the latter of the two makes more of an attempt to define the benefits that SONET provides, in most cases that is its sole purpose.

Any type of propaganda should always be taken with a ‘grain of salt,’ therefore it was necessary to attempt to prove the claims of telecommunications companies with the unbiased information provided by the technical documents. Interviews with engineering professionals also helped to add clarity and credibility to the information provided by the telecommunications companies. A good synthesis of these diverse prospectives on the subject of SONET helped to more completely answer the question, ‘what are the benefits that SONET provides?’

A document provided by Nortel, which was used for their implementation of SONET was particularly helpful. The Nortel document made attempts to bridge the gap left by the other documents that were available. The document discussed potential benefits of SONET, and at the same time, it was technical in nature answering how and why this technology works. Overall, the document was helpful in describing SONET as an industry standard. It should be noted that this document was not widely available to the general public. I was able to obtain the document through my contact with an engineering professional who has contacts with Nortel.

The approach used in the SONET benefit study consisted of three basic parts. First it was necessary to gain a fundamental understanding of the technology. After a basic knowledge on the subject was obtained, materials were gathered that more specifically dealt with the question at hand. Interviews were also conducted with individuals knowledgeable on the subject. Finally, analysis of the information was performed and compiled to help explain the benefits that SONET provides.

SONET and Related literature. In order to gain a good understanding of SONET, literature on the subject was obtained and examined. SONET is a highly technical telecommunications subject. SONET literature often uses technical terms when explaining concepts. For this research it was necessary to gain a good understanding of SONET and telecommunications related terms. A listing of telecommunications terms related to SONET can be found in Appendix A.

Telecommunications textbooks were a good source for technical information. Another source for information was the Internet. Addresses to potential sites were obtained using various Internet search engines. Keyword searches were conducted using the terms "SONET", "ATM", and "High Speed Connectivity." Searches were further narrowed down using the phrases "Benefits", "Benefits of SONET", and "SONET provides."

Interviews. Many of the concepts involved with SONET are very technical in nature. For this reason, it was necessary to seek out engineering professionals who have a good understanding of SONET or related telecommunications technologies. The interviews served two basic purposes: 1) to obtain a better understanding of the technical aspects of SONET, and 2) to help clarify literature so that the benefits that SONET provides could be better understood. Whenever possible, SONET literature was obtained from the engineering professionals who were interviewed.

Analysis and findings on the benefits of SONET are presented in the following section.

Overall, the networking capabilities of a network using SONET are much more powerful than existing asynchronous systems. The main advantages that SONET provides are listed below in table 3, below:
 

Multi-Point Configurations. Most existing asynchronous systems are only suitable for point-to-point, whereas SONET supports a multi-point or hub configuration. A hub is an intermediate site from which traffic is distributed. The hub allows nodes to communicate as a single network instead of separate networks. Hubbing reduces requirements for back-to-back multiplexing and demultiplexing and helps realize the benefits of traffic grooming. [1][2]

Network providers no longer need to own and maintain customer-located equipment. A multi-point implementation permits optical carrier (OC-n) interconnects of mid-span meet, allowing network providers and their customers to optimize their shared use of the SONET infrastructure.

Grooming.  Grooming can both consolidation and segregation of traffic for the purpose of making facilities operate more efficiently. Consolidation means combining traffic form different locations onto one facility. Segregation refers to the separation of traffic. Grooming also provides segregation of services. An example of this would be an incoming SONET line that contained different types of traffic, i.e., voice, data, and video. A SONET network can conveniently separate the data into switched and non-switched traffic.[1]

Reduced Back-to-Back Multiplexing.  The current asynchronous format requires that care be taken while routing circuits. This is necessary to avoid multiplexing and demultiplexing too many times because electronics and their associated capital cost are required for each DS-1 signal processed. SONET allows DS-1s to be multiplexed directly to the optical carrier (OC) rate. This also means that the entire signal does not have to be demultiplexed, only the STS signals that need to be accessed. [2]

Reduced Cabling and Elimination of DSX Panels.  In asynchronous systems equipment called DSX panels are required to perform the task of grooming. In the synchronous optical network, grooming is performed electronically so DSX panels are not needed except when required to interface with existing asynchronous equipment. [1][3]

Enhanced OAM&P. OAM&P stands for operations, administration, maintenance, and provisioning. OAM&P and network management are major concerns among network providers. This is because the transmission network is growing continually and there are many vendors with different types of equipment. Network providers must be able to administer monitor, provision and control the network from a central location. [3]

SONET makes integrated network OAM&P possible. One connection can reach all network elements. Separate links are not required for each network element. Remote provisioning provides centralized maintenance and reduced travel for maintenance personnel. This translates into money saved. [1] [3]

Enhanced Performance Monitoring.  Substantial overhead information is provided in SONET to allow quicker trouble-shooting and failure detection. For example, if SONET senses a transmission problem, it can switch traffic to an alternate path in as little as 50 msec (100ths of a second). This network survivability is due to SONET’s redundant dual-ring physical architecture. [1] [2]

New Services. A benefit of SONET is its strong capability in concerning services flexibility. High-speed packet switched services, LAN transport, high definition television (HDTV), Video-on-demand, full-motion catalogs, high resolution imaging and high-fidelity sound broadcast are all examples of new SONET supported services. [5][7] These services may use (ATM) asynchronous transfer mode, which is a fast packet switching technique that uses short fixed length packets called cells. SONET is a great carrier for ATM because of its high bandwidth capacity. [1] [8]

For some future services, it is possible that the STS-1 may not have the needed capacity. An advantage of SONET is that it offers the ability of concatenating STS-1s. This method would allow the necessary bandwidth up to STS-3. In order to provide more bandwidth at lower levels, it is also possible to concatenate virtual tributaries. [2]

Optical Interconnect. Different Optical formats among asynchronous vendors products make it impossible to optically connect the fiber of one vendor to another. For example, if one vendor used a 570 Mbps line rate, it would not be possible to connect to another using 565 Mbps. A major strength of SONET is that it allows mid-span meet. SONET standards define fibre-to-fibre at the physical or (photonic) level. The standards determine the optical line rate, wavelength, power levels, pulse shapes, coding, frame structure, overhead and payload mappings. SONET also makes interconnection between network providers regardless of the manufacturer of the equipment being used. [2]

The emerging Synchronous Optical Network standard (SONET) specifies common rules to ensure efficient, high volume transport of digitized voice, image or data communications on fiber-optic networks. This allows communication between different vendors’ equipment in a standard form. [9]

Applications emerging from the convergence of telephony and data communications, such as video-on-demand, HDTV, video teleconferencing and other applications with high bandwidth requirements, coupled with the explosive growth of the Internet and World Wide Web, have created an unprecedented demand for higher bandwidth among ISPs, telephone companies, and even Fortune 500 companies. SONET has the potential to provide high bandwidth speeds of up to 10 Gbps for voice and data transmission. Already the standard for encapsulating and transporting data for telecommunications carrier products, SONET is starting to be adopted by Internet service providers and the cable TV industry. [6]

It can be determined from this body of research that SONET provides the following advantages over existing networks: [1]

• Compatibility of equipment by all vendors who manufacture to the standard often referred to as "mid-span meet"
• Synchronous networking
• Enhanced OAM&P (operations, administration, maintenance, and provisioning)
• More efficient add/drop multiplexing (ADM)
• Standards-based survival rings

Limitations of the research. With few exceptions, the research for this paper consisted of documentation that is available to the public at large. It was also difficult to find individuals who are knowledgeable on technologies such as SONET. Telecommunications firms are reluctant to give up ‘valuable’ information for free that is relevant to their implementations of SONET. This information is only available to those in the general public who are willing to pay large sums of money for it.

Implications for Future Research. This study is somewhat limited in scope. Only a few professionals were contacted who had knowledge of SONET. In some cases, even the professionals’ knowledge of the subject was limited. More useful information could probably be obtained if actual vendors were interviewed.

As SONET becomes more widely available it will become easier to locate individuals with a working knowledge of the technology. It has been predicted that the global market for SONET will grow at 17.4% compounded annually over the next five years. [10] It would be interesting to reconduct this research in three to five years to determine the benefits that SONET provides at that time when more data is available.

[2] Goldman, James, E., Applied Data Communications A Business-Oriented Approach, second edition, 1998 John Wiley & Sons, Inc. pp. 320-323.

[3] SONET, SDH & ATM - Q&A http://www.broadband- guide.com/lw/reports/olr0498.html

[4] What’s the difference between SONET and SDH: http://www.ele.auckland.ac.nz/students/tankh/atm/faq/D15.html

[5] Ameritech SONET: http://www.ameritech.com/products/custom/product/01t.thml

[6] ADV: News: SONET/SDH Technology: http://www.ispo.cec.be/ispo/lists/ispo/0396.html

[7] PACIFIC BELL NETWORK "fasTrak on SONET":
http://www.pacbell.com/prodcuts/business/fastrak/networking/sonet/features.html

[8] ATM Physical Layer Interfaces: http://www.fudan.sh.cn/shnet/ddd/stn/atm002.html.

[9] New AT&T chips for digital SONET networks: http://www.att.com/press/0290.900227.mea.html

[10] SONET: Double Digit Growth, Double Digit Price Drop: http://www.insight-corp.com/2_26_98.html

[11] SPRINT COMPLETES FIRST INTERNATIONAL SONET RING: http://www.sprint.com/sprint/press/releases/9606/9606100264.html

[12] SONET FAQ: http://www.sonet.com/docs/faq.htm

[13] Bernier, Paula, "GTE building nationwide IP, SONET network" http://www.zdnet.com/zdnn

[14] Bernier, Paula, "MCI Goes End-To-End With New SONET Service" http://www.zdnet.com/zdnn/content/inwk/0434/inwk0056.html