Introduction to SONET/SDH/OTN Technology
SONET/SDH networks have stood the test of time and, until recently, they have been primarily used to transport aggregated voice signals and private-line frame-relay and ATM services. The growth of these high-revenue-generating services over the years has resulted in large-scale deployments of metropolitan and long-haul SONET/SDH networks worldwide.
Today, however, service demand and, consequently, service-provider revenue growth has shifted from these legacy-based services to IP/Ethernet-based services. This shift in demand is largely due to two market factors. First, enterprise customers have expressed an increasing need for reliable Ethernet-based transport services and, second, Ethernet is becoming the Layer 2 transport technology of choice for both enterprise connectivity and access aggregation networks. In addition, with the convergence of voice, video and data (e.g., triple-play access networks), Ethernet connections are now being used for delivery of mission-critical services, thus making 99.999% reliability an absolute service requirement for these enterprise customers.
Fig 1: Multiservice SONET/SDH network
This growing demand for Ethernet-based transport services has led to a rebirth of SONET/SDH. Given the fundamental robustness of the technology and the massive capital investment that has been made over the years, service providers have been eager to find ways of using their existing SONET/SDH infrastructure to fulfill the growing market demand for Ethernet.
Packet-aware SONET/SDH add/drop multiplexers, commonly known as multiservice provisioning platforms (MSPPs), are actively being deployed in service-provider networks worldwide, as they offer an efficient means of transporting packet-based client signals, such as Ethernet and Fibre Channel, over existing SONET/SDH infrastructures. This efficiency is provided through their support of recent ITU and ANSI standards; namely, generic framing procedure (GFP), virtual concatenation (VCAT) and link-capacity adjustment scheme (LCAS).
SONET/SDH Basics
Synchronous optical networking commonly known as SONET and synchronous digital hierarchy (SDH) have been around for decades. SONET and SDH standards were developed for communicating digital information over optical fiber. The SONET specifications define optical-carrier (OC) interfaces and their electrical equivalents to allow for transmission of lower-rate signals at a common synchronous rate. These specifications were developed to replace the lower-rate T-Carrier/PDH systems, as they allow for the transport of large amounts of telephony and data traffic.
Table 1: Common SONET/SDH data rates
Both SONET and SDH are widely used today; SONET mainly in North America, and SDH in the rest of the world. One of the benefits of the SONET/SDH signal, as with any standard, is that it facilitates the interoperability between multiple vendors within the same network. Another major advantage of SONET/SDH is that the operations, administration, maintenance and provisioning (OAM&P) capabilities are built directly into the signal overhead to allow for maintenance of the network from one central location.
Next-Generation SONET/SDH
Next-generation SONET/SDH has gained a significant momentum in the industry as its associated technologies have allowed SONET/SDH to evolve with the times and offer an efficient means of transporting packet-based services over the widely deployed SONET/SDH networks. As described above, SONET/SDH standards have their roots in the multiplexing and transport of voice channels, as well as private line ATM and Frame Relay services; therefore, they have not been designed to transport data services such as Ethernet, resulting in several limiting factors when attempting to transport packet-based services.
Table 2. SONET/SDH limitations and solutions
As mentioned above, Ethernet has become the Layer 2 technology of choice for both enterprise connectivity and access aggregation networks. As service providers had invested heavily in their SONET/SDH networks and they found the technology to be reliable, there was great incentive to keep using the existing infrastructure to meet Ethernet demand.
Consequently, in 1999, work was initiated within the ITU-T and ANSI standards bodies to define technologies that would help SONET/SDH evolve with the times and offer efficient means of transporting these packet-based services over the widely deployed SONET/SDH networks.
Their answer: the definition and ratification of three key technologies that form the basis of next-generation SONET/SDH: generic framing procedure (GFP), virtual concatenation (VCAT) and link-capacity-adjustment scheme (LCAS).
Introduction to OTN (G.709)
In todays telecom/datacom environment, network operators are forced to integrate their networks in order to reduce operational expenses (OPEX) and eliminate the additional capital expenditures (CAPEX) generated by multiple parallel networks. In response to such changes, the ITU-T developed a set of standards to meet these emerging needs. ITU-T recommendation G.709, interface for the optical transport network (OTN), is among the latest of these standards, and its aim is to address the transmission requirements of todays wide range of services; namely, it was developed to assist in network evolution to higher bandwidth and improve network performance. Many of the notions in ITU-T G.709 are similar to those in SONET/SDH; e.g., layered structure, in-service performance monitoring, protection and other management functions. However, some key elements have been added to continue the cycle of improved performance and reduced cost. Among these key elements, the ITU-T G.709 provides a standardized way to manage the optical channels in the optical domain without the need to convert the optical signals to electrical signals and apply the forward error correction (FEC) algorithm to improve transmission performance and enable longer optical spans.
Currently, the majority of OTN applications are running on DWDM transport networks. However, products that support OTN standards to various degrees are already available and even more OTN-based product lines and feature sets are expected to hit the market in the very near future. The table below lists the G.709 line rates and their corresponding SONET/SDH interfaces.
Table 1: OTN data rates
The ITU-T G.709 recommendation defines standard interfaces and rates based on the existing SONET/SDH rates. When taking into consideration the additional G.709 overhead and FEC information, the resulting interfaces operate at line rates roughly 7% higher than the corresponding SONET/SDH rates.
Testing SONET/SDH Operation
Before going into a description of the SONET/SDH tests required to qualify today`s networks, it is important to understand that proper testing of any network—whether SONET or Ethernet, fiber or copper—should start from the bottom; that is, at the physical-layer level. A comprehensive series of physical-layer tests should be completed prior to any testing in the digital domain. Although we will not be addressing them in this section, it is essential to know what they are:
- Power loss
- Return loss
- Dispersion (for high-speed optical systems)
- Media profile (splice loss, attenuation,
reflectance, etc)
Fig 2. Configuration for continuity, BER testing, delay and alarm processing
Important tests to verify correct SONET/SDH operation include:
- BERT (for circuit turn-up)
- Circuit continuity tests
- Through-mode analysis
- Service disruption time measurement
- Error performance monitoring: G.821,
G.826, G.828, G.829, M.2100 & M.2101
- Power and frequency measurement
- Synchronization
- Pointer processor generation and monitoring
- Automatic protection-switching operation
SONET/SDH-based networks have proven to be the most robust data-transport technology available and will continue to be the transmission system of choice for carriers who demand a stable and resilient method of delivering broadband services. Now let us take a look at how it is possible to ensure correct operation of next generation features.
- Network Element GFP Conformance
Validation
- Network Element GFP Performance
Characterization
- Network Element VCAT Conformance
- End-to-End EoS Testing and
Troubleshooting
- Ethernet Add/Drop (from GFP)
For More on
Next-Generation SONET/SDH
Next-Generation SONET/SDH Reference Guide
For More on OTN
(G.709)
OTN (G.709) Reference Guide