Synchronous Optical Network

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Synchronous Optical Network (SONET) is the North American version of an international standard for a family of high-speed transmission links on fiber-optic cable. SONET takes advantage of the many attractive features available on today’s fiber optic facilities.

The rapid deployment of fiber allowed service providers to deploy equipment that could transmit at very high data rates. But with a lack of standards above 45 Mbps, this equipment used vendor-proprietary multiplexing techniques. Identical vendor equipment was needed at each end of a fiber span, and the addition or drop-off (i.e., add/drop) of various data streams for switching (e.g., a DS-1) necessitated demultiplexing the entire data stream. If identical equipment was not available at both ends of a fiber span, such as in a hand-off between a local carrier and an IXC, hand-off occurred at the lower standard data rates of DS-3 and below.

As fiber was quickly recognized as the most practical method for transmitting very high data rates (above 45 Mbps), SONET, along with the international SDH, was created as the “standard way to use fiber.”

The Case for SONET

T-carrier circuits were substantially limited in data rate capacity, and T-carrier OAM&P capability was limited as well. The older digital signal 1 (DS-1) SF has only a small amount of OAM&P capability. DS-1 ESF has substantial OAM&P capability, and ESF is the preferred framing format. M13, the original DS-3 framing format, has some OAM&P capability. A significant number of legacy M13 links are still in operation. C-bit framing has substantial OAM&P. New DS-3 circuits routinely use C-bit framing.

The major problem with T-carrier OAM&P is that T-carrier is not truly a system, but rather a collection of independent circuits. Centralized monitoring of the health of the circuits requires an “overlay” system, typically vendor-proprietary hardware and software that collects health and status information from the independent circuits and delivers that information to a central location.

The initial (pre-SONET) fiber implementation replicated many of the problems described above for T-carrier and added a new wrinkle. Because of lack of standardization, a vendor X multiplexer would not communicate with a vendor Y multiplexer. In the age of deregulation and multiple communications carriers, this was a problem. Mid-span meets (at data rates above the practical maximum for copper) could exist only if both carriers used multiplexers from the same vendor. (A mid-span meet is a connection between two sites with equipment obtained from different manufacturers, which is not unusual when two sites are owned by different carriers. This connection is logically in the middle of the span but physically in a facility where the carriers are collocated.)

SONET was created to resolve these issues. It exploited the advantages of fiber while standardizing its use. In addition, the OAM&P capability increased significantly over what could be done with T-carrier. Although SONET provides logical point-to-point links, it is truly a system: the SONET architecture incorporates a mechanism for centralized monitoring of OAM&P parameters.

The Mid-Span Meet and Missing Pieces

The strange fact about the T-carrier hierarchy is that it was never completed. Annoying gaps in the original Bell Labs specifications existed. For instance, no method of sending a DS-3 signal over fiber was fully specified and deployed. Therefore, even a simple thing like getting a DS-3 from New York to New Jersey was often an adventure, if the customer could even afford it. DS-4 was in even poorer shape, but even less of it existed at the time.

For these reasons, the carriers that wanted to deploy the T-carrier hierarchy (i.e., all of them) had to fill in the blanks in the specifications with their own proprietary methods. As long as the Bell System remained intact and all of these carriers bought Western Electric Company (WECO) equipment, this was not much of a problem. After all, how many T-1s would start at Illinois Bell and end up at GTE? Not many. If worse came to worst, the signal could be “re-analoged” for the interface. This lack of ability to have one vendor’s equipment or telco on one side of the link and another vendor’s equipment on the other side of the link is known as the lack of the mid-span meet. In a mid-span meet, one carrier is responsible for one end of the link and another carrier is responsible for the other side of the link.

After 1984, however, there were a lot of situations where the two ends of a T-carrier link were under the control of two different organizations. With the equal access rules of divestiture, local callers had the right to use their long distance carrier of choice on a per call basis. This required the use of digital T-carrier trunks from the local exchange carrier’s (LEC) central offices to the long distance carrier’s point of presence (POP). “No problem,” said the new AT&T (which still included WECO), “Everybody just buy WECO.” This philosophy did not sit well with most.

The question then became one of completing the T-carrier standard or trying to come up with something new in the world of divestiture.

Bandwidth Needs and OAM&P

By the mid 1980s, many organizations were experimenting with videoconferencing networks to cut down on employee travel time and expenses. In the days before effective video compression techniques, even a full DS-3 had trouble handling high quality, full motion video. Monday Night Football consumed an appreciable portion of all the DS-3C (90.524 Mbps) bandwidth in the country. And the trend was up.

Multimedia applications, which combined video and audio with text directly to the desktop, were growing in popularity. These applications could easily run over the 10 Mbps Ethernets within an organization, but choked and stumbled when asked to run over the common DS-0 and DS-1 links between LANs.

The process of operation, administration, maintenance, and provisioning (OAM&P) on a T-carrier network was primitive as well (outside North America, especially in Europe, OAM&P is usually just OAM). Most of the OAM&P capabilities in T-carrier systems, from alarms to maintenance signals, robbed bits from the user channels. They really had no choice since the overhead in T-carrier (1 out of 193 bits, or 0.518%) was not adequate for even the simplest OAM&P tasks. Even the simple act of measuring the bit error rate (BER) on a T-1 link (usually because the customer had complained about it) involved taking the link out of service, performing a BER test (BERT) on it, and then returning it to service. The fact was that the OAM&P overhead in the T-carrier network, even at the higher levels of the hierarchy, was totally inadequate for both customer and service provider needs.

Problems with M13 Multiplexing

The M13 multiplexing process has several inherent disadvantages. From a user perspective, the most important of these disadvantages is that there is no workable metallic carrier standard at rates above DS-3. In today’s environment of bandwidth-hungry, distributed applications (e.g., video and imaging), 44.736 Mbps falls short of user requirements, providing a strong impetus for development of standards above the DS-3 rate (e.g., SONET).

From a carrier perspective, the lack of electro-optical standards is equally problematic, especially in the United States. With no optical transmission standard, carriers that purchase equipment from different vendors are unable to interoperate at interface points. This lack of a mid-span meet standard prompted the development of the SONET standard.

Due to the plesiochronous nature of the M13 multiplexing process, full demultiplexing is required to access an individual DS-1 signal within the DS-3 aggregate. Bits that were stuffed in the multiplexing process must be destuffed in the demultiplexing process. In the M13 process, there are two stages of destuffing/demultiplexing required. Provision of enhanced services such as DS-1 cross-connection and DS-1 add/drop are thus rendered difficult and expensive in the M13 environment.

Finally, OAM&P interfaces and systems in the M13 environment are typically vendor-specific. Service technicians might require training on a number of such systems to become proficient in fault diagnosis and repair operations. In addition, even when equipment is purchased from a single vendor, OAM&P systems might not be fully integrated. That is, a service technician performing a diagnostic operation on an M13 multiplexer cannot see through its associated electro-optical converter to another M13 multiplexer.

SONET Features

SONET Features

The drive toward higher capacity transmission systems with low deployment costs helped lead to the development of the SONET standard, which has several key features.

First, the SONET standard defines a transmission hierarchy that begins where the current digital hierarchy ends; in North American T-carrier circuits, that rate is 45 Mbps; in ETSI systems, that rate is 139 Mbps.

The second feature of the SONET standard is the nature of its multiplexing system. Subrate channels can be easily identified, extracted, or inserted by a single device without the need to demultiplex the higher rate SONET signal (i.e., simplified add/drop capability).

The third feature is SONET’s support for OAM&P systems. Bandwidth is allocated for use as a common OAM&P conduit. This allows each element in the transmission system to communicate with a centralized management system or to distribute the management responsibility throughout the elements. This extensive OAM&P support could conceivably redefine communication network management and the way these networks provide services to customers.

There is an apparent dichotomy in that SONET operates synchronously although it carries plesiochronous DS-1s and DS-3s (among other things). Bit stuffing techniques similar to those for T-carrier are used to map DS-1s and DS-3s into SONET. They travel through SONET synchronously; they are returned to plesiochronous when the stuffing is removed and they exit the SONET system.

SONET Terminology

A number of terms are critical to understand when discussing SONET. They include:

  • Optical Carrier (OC-n): refers to a particular SONET transmission rate. This notation is a reference to the optical transmission. This notation is analogous to the terms T-1 and T-3.
  • Synchronous Transport Signal (STS): refers to the SONET frame structure for a particular SONET transmission rate. This notation, however, is a reference to the electrical form prior to conversion to optical. This notation is analogous to the terms DS-1 and DS-3.
  • Synchronous Digital Hierarchy (SDH): this is the international version of SONET. It uses a slightly different notation and differs in other implementation details.
  • Synchronous Transport Module (STM): refers to the international equivalent of the STS notation. The STM rate is three times the equivalent STS rate (e.g., STS-1 is 51.84 Mbps but STM-1 is 155.52 Mbps). In other words, an STS-3 is equivalent to an STM-1.
  • Payload: The user data within a SONET frame is called the payload. Payload can be sub-STS-1 level traffic (e.g., DS-1s, E-1s, DS-2s), STS-1 level traffic (e.g., DS-3s), or super rate traffic (e.g., ATM cells, or IP packets).
  • Synchronous Payload Envelope (SPE): portion of the STS-1 frame carries payload and end system overhead.
  • Overhead: The overhead portion of the STS-n frame carries management data for OAM&P, among other things.
  • Concatenation: Conceptually equivalent to an unchannelized carrier system, concatenation is linking multiple STS-1 frames to form a single SPE that carries super rate payloads. A concatenated SONET link is correctly referred to as an STS-3c, STS-12c, etc., according to the SONET level. Although technically incorrect, the OC designation is commonly used (e.g., OC-3c, OC-12c).
  • Virtual Tributary: the SPE structure used to carry sub-rate payloads (e.g., payloads below 51.84 Mbps).

SONET Layers

Whenever we discuss peer entities that exchange information, we introduce protocols, and protocols are best handled in a layered architecture. It is important to note that SONET deals mainly with the Physical Layer of the OSI Reference Model, similar to T-1 or T-3 services. However, SONET is itself a layered architecture. The defined layers of the SONET model are the Photonic, Section, Line, and Path Layers.

  • The Photonic Layer deals with the electrical to optical conversion and the properties of the signal transmission.
  • The Section Layer defines the link between any two devices and deals with timing, framing, bit scrambling, and error detection
  • The Line Layer is primarily concerned with multiplexing.
  • The Path Layer is concerned with encapsulating end-traffic for transmission across the SONET network. It is at this layer that the Synchronous Payload Envelope (SPE) is created.

SONET Transmission Rates

SONET defines several transmissions rates. These include:

  • OC-1: 51.84 Mbps (seldom deployed, there is an electrical STS-1 interface instead)
  • OC-3: 155.52 Mbps
  • OC-12: 622.08 Mbps
  • OC-48: 2.48832 Gbps
  • OC-192: 9.95328 Gbps
  • OC-768: 39.81312 Gbps

See Also


<mp3></mp3> | Optical Carrier (OC) vs. Synchronous Transport Signal (STS)
<mp3></mp3> | SONET
<mp3></mp3> | SONET (and Cows)