Synchronous Optical Network

Synchronous Optical Network (SONET) is an industry standard for highspeed transmission over optical fiber. SONET networks offer many advantages for large corporations as well as carriers, including equipment interoperability between the two environments, virtually unlimited bandwidth, fault recovery, and network management.

The importance of SONET has been underscored in recent years with the rapid increase of data traffic on the public networks, fueled by Internet usage. Data now accounts for about 50 percent of the traffic on the telephone networks in North America. Today, all the major long distance carriers, RBOCs, national Internet service providers, and next generation carriers—and their competitors—have SONET networks in place. Of course, many large corporations as well as state and federal government agencies have deployed SONET as well.

Transmission Rates

The SONET standard specifies a hierarchy of rates and formats for optical transmission, ranging from 51.84 Mbps to 13.271 Gbps. The following table summarizes the standard optical carrier (OC) transmission rates:

OC Level

Line Rate


51.84 Mbps


155.520 Mbps


466.560 Mbps


622.080 Mbps


933.120 Mbps


1.244 Gbps


1.866 Gbps


2.488 Gbps


9.95 Gbps


13.271 Gbps

Network Elements

SONET network infrastructures consist of various types of specialized equipment including ADM, BDCS, WDCS, digital loop carriers, regenerators, and SONET CPE (customer premises equipment).

ADD-DROP MULTIPLEXER The ADM provides an interface between network signals and SONET signals. It is a single-stage multiplexer/demultiplexer that converts DS-n signals into OC-n signals. The ADM can be used in terminal sites and intermediate (add-drop) sites; at an add-drop site, it can drop lower-rate signals down or pull lower-rate signals up into the higher-rate OC-n signal.

BROADBAND DIGITAL CROSS-CONNECT The BDCS interfaces various SONET signals and legacy DS3s; it accesses the STS-1 signals, and switches at this level. It is the synchronous equivalent of the DS3 digital cross-connect; however, the BDCS accepts optical signals and allows overhead to be maintained for integrated Operations, Administration, Maintenance, and Provisioning (OAM&P). Most asynchronous systems prevent overhead from being passed from signal to signal, but the BDCS makes two-way cross-connections at the DS3, STS-1, and STS-c (concatenated) levels. It is typically used as a SONET hub that grooms STS-1s for broadband restoration purposes, or for routing traffic.

WIDEBAND DIGITAL CROSS-CONNECT WDCS is a digital crossconnect that terminates SONET and DS3 signals, and maintains the basic functionality of a virtual tributary (VT) and/or DS1-level cross-connects. It is the optical equivalent of the DS3/DS1 digital cross-connect and accepts optical-carrier signals as well as DS1s and DS3s. In a WDCS, switching is done at the VT, DS1, or DS0 level. Because SONET is synchronous, low-speed tributaries are visible in VT-based systems and directly accessible within the STS-1 signal; this allows tributaries to be extracted and inserted without demultiplexing. Finally, the WDCS cross-connects constituent DS1s between DS3 terminations, and between DS3 and DS1 terminations.

DIGITAL-LOOP CARRIER Like the DS1 digital-loop carrier, this network element accepts and distributes SONET optical-level signals. Digital-loop carriers allow the network to transport services that require large amounts of bandwidth. The integrated overhead capability of the digital-loop carrier allows surveillance, control, and provisioning from the central office.

REGENERATOR A SONET regenerator drives a transmitter with output from a receiver, and stretches transmission distances far beyond what is normally possible over a single length of fiber.

PROTOCOL STACK The SONET transmission protocol consists of four layers: photonic, section, line, and path.

PHOTONIC LAYER The photonic layer is the electrical and optical interface for the transport of information bits across the physical medium. Its primary function is to convert STS-N electrical signals into OC-N optical signals. This layer performs functions associated with the bit rate, optical-pulse shape, power, and wavelength; it uses no overhead.

SECTION LAYER The section layer deals with the transport of the STS-N frame across the optical cable, and performs a function similar to the data-link layer (layer 2) of bit-oriented protocols such as high-level data-link control (HDLC) and synchronous data-link control (SDLC). This layer establishes frame synchronicity and the maintenance signal; functions include framing, scrambling, error monitoring, and orderwire communications.

LINE LAYER The line layer provides the synchronization, multiplexing, and automatic protection switching (APS) for the path layer. Primarily concerned with the reliable transport of the path layer payload (voice, data, or video) and overhead, it allows automatic switching to another circuit if the quality of the primary circuit drops below a specified threshold. Overhead includes line-error monitoring, maintenance, protection switching, and express orderwire.

PATH LAYER The path layer maps services such as DS3, FDDI, and ATM into the SONET payload format. This layer provides end-to-end communications, signal labeling, path maintenance, and control, and is accessible only through terminating equipment. A SONET ADM accesses the path layer overhead; a cross-connect system that performs section and line-layer processing does not require access to the path layer overhead.


Channelized interfaces provide network configuration flexibility and contribute to lower telecommunications costs. For example, channelized T1 delivers bandwidth in economical 56/64K bps DS0 units, each of which can be used for voice or data and routed to different locations within the network or aggregated as needed to support specific applications. Likewise, channelized DS3 delivers economical 1.544 Mbps DS1 units, which can be routed separately or aggregated as needed. Channelized interfaces also apply to the SONET world—channelized OC-48, OC-12, and OC-3 can all be subchannelized down through DS3 speeds. OC-48, for example, can be channelized as follows:

■ Four OC-12 tributaries, all configured for IP (Packet over SONET) framing, or all configured for ATM framing

■ Four OC-12 tributaries, with two configured for IP (Packet over SONET) framing, and the other two for ATM framing

■ Two OC-12, with one configured for IP framing, and the other for ATM framing; 8 OC-3, with four configured for IP framing, and four configured for ATM framing

■ Two OC-12, with one configured for IP framing, and the other for ATM framing; 6 OC-3, with three configured for IP framing, and three configured for ATM framing; 6 DS3, with three configured for IP framing, and three configured for ATM framing

■ Forty-eight DS3s, with 24 configured for IP framing, and 24 for ATM framing

Multiservice channelized SONET is implemented on a single OC-48 line card, which provides IP packet and ATM cell encapsulation to support business data and Internet services from the same hardware platform. One vendor that offers such products is Argon Networks. The company’s GigaPacket Node (GPN) is a native IP router as well as a native ATM switch in a single modular platform, which enables any port to be configured for either Packet over SONET (PoS) or ATM service. The configurations are implemented through keyboard commands at service time, rather than permanently assigned at network build out time.

Survivable Networking

Network failures come in the form of hard failures such as blown circuit packs and soft failures such as degraded optics. SONET offers several ways to recover from both types of failures, including:

AUTOMATIC PROTECTION SWITCHING If a transmission system detects a failure on a working facility, it switches to a standby facility to recover the traffic. One-to-one and one-to-n protection switching are provided.

BIDIRECTIONAL LINE SWITCHING If two fiber pairs exist between each recoverable node and a fiber facility fails, the node preceding the break loops the signal back toward the originating node, where it travels different fiber pairs to its destination.

UNIDIRECTIONAL PATH SWITCHING If one fiber pair is between each node and a signal is transmitted in two different paths around the ring, the network determines and uses the best path at the receiving end. If a fiber facility fails, the destination node switches traffic to the alternate receive path. The switchover occurs in as little as 50 milliseconds.

SYSTEM REDUNDANCY All SONET network elements use circuit pack redundancy, such as crucial optic cards with 1 x 1 backup cards. Other service cards are backed up in a 1 x n scheme. So for four DS1 mapper cards, there will be a spare on “standby” for redundancy.

SONET’s embedded control channels enable end-to-end performance tracking and the identification of elements that cause errors. With this capability, carriers can guarantee transmission performance, and users can verify it without going offline to implement test procedures. These capabilities allow problem identification prior to service disruption. Combined with the self-healing capabilities, SONET’s diagnostic capabilities ensure that properly configured networks experience virtually no downtime.

IP vs. ATM over SONET

Organizations of all types and sizes have an insatiable appetite for more bandwidth. Because most carriers are having difficulty meeting bandwidth demand as corporate data-network usage continues to increase, they are turning to bandwidth-multiplying technologies such as Wave Division Multiplexing (WDM) and Dense Wave Division Multiplexing (DWDM), as well as SONET. Often these technologies are used together as an integrated optical networking system.

The industry is now focusing on the fact that LANs and WANs are now merging, as both are now used to carry voice, data, and video traf-fic—a theme initiated by the Internet under the rubric of “voice-data convergence.” ATM is conducive to switching connections in WANs, but it is still too expensive to implement on LANs.

IP has become prevalent throughout both the Internet and in LANs. Therefore, many experts believe that IP traffic should be the primary protocol. There will be continued debate on IP vs. ATM for building transparent networks. However, both can run on top of SONET in any combination of channels over the same optical fiber. This gives companies the flexibility they need in building large-scale transparent networks capable of supporting any application and bandwidth requirement.

The choice of SONET as the transmission facility—rather than relying exclusively on WDM, DWDM, and emerging proprietary technologies— would give organizations the bandwidth they need now and position them for future growth in virtually any protocol, application, and technology direction they choose to go.

Network planners should evaluate broadband equipment in terms of its migration path to SONET; vendors should be evaluated on the extent to which they plan to implement path/overhead signaling for network management, user-to-network signaling, network synchronization, partitioning, channelization that supports both IP and ATM, remote management and diagnostics, and bandwidth on demand. Compliance with standards and participation in interoperability testing should also be considered.


SONET standards were developed by the Alliance for Telecommunications Industry Solutions (ATIS), formerly known as the Exchange Carriers Standards Association (ECSA). These standards were published and distributed by the American National Standards Institute (ANSI). Bellcore, the research and development arm of the RBOCs, was involved in the development of SONET standards from the beginning, and continues to issue technical specifications that ensure standards compliance. ATIS sponsors the SONET Interoperability Forum (SIF)—a membership organization comprised of equipment vendors, service providers and end users—which drives the industry to implement interoperable SONET products and services based on open industry and international standards.

Last Word

Data transfer on existing phone lines is now doubling in size annually, with the portion of data transferred on lines via the Internet now more than quadrupling each year. Optical internetworking technologies such as WDM and DWDM, coupled with SONET applications and legacy asynchronous multiplexing, could result in as much as a 100-fold increase in available bandwidth. Greater bandwidth translates into easier and more efficient transfer of data—enabling, for example, end users of the Internet to receive information more quickly and take advantage of real-time applications such as IP telephony, videoconferencing, and collaborative computing.

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