Optical Transport Network

Multi-Operator Networks

The management shortcomings of SDH include poor data integrity and fault isolation methods for multi-operator environments. Whenever a particular end-to-end connection passes through network elements (NEs) in more than one operator network, it is important for each operator to monitor services between the NEs in its own network.

TCM allows the definition of multiple arbitrary pairs of connection monitoring end points so an operator is provided with a single set of alarms and bit-error counts associated with any portion of its network (Fig. 1). Tandem connection was eventually introduced into SDH, but it was cumbersome and not heavily deployed.

Inclusion of FEC

Forward error correction (FEC) is used in transport networks to correct transmission errors that typically occur on long fiber routes. Some SDH equipment with proprietary FEC capabilities (typically for STM-64) has been developed, but deployment is very limited. By contrast, FEC is part of the OTN standard. There also are several proprietary FEC schemes that have better performance than the Reed-Solomon FEC specified in the OTN standard.

Mapping and Multiplexing

When multiplexing SDH containers into higher-rate signals, the payloads of all containers are mapped to a common time base, and a pointer mechanism is used to locate the frame boundary of each payload. In this manner, the section overhead of all SDH containers is aligned and the actual payloads float with respect to each other. Although various administrative group levels are defined in SDH, the multiplexing is effectively single stage.

In OTN, the entire lower-level signal, including overhead and payload, is asynchronously mapped into the payload of the higher-level signal using one of two mechanisms. The first mechanism is the asynchronous mapping procedure (AMP), which allows for small positive or negative frequency offsets of the lower rate signal relative to the higher rate.

The second is the generic mapping procedure (GMP), which allows for almost infinite negative frequency offsets of the lower-rate signal relative to the higher rate. While the OTN originally recommended single-stage multiplexing of containers, multi-stage multiplexing is also now permitted (Fig. 2).

Typical Equipment Platforms

SDH network equipment began primarily with simple terminal multiplexers, which mapped and multiplexed many PDH signals into STM-1, STM-4, and STM-16 transport signals. Add/drop multiplexers (ADMs) were then developed to enable ring topologies and linear add/drop chains.

The multi-service provisioning platform (MSPP) added capability for a larger variety of client signals such as Ethernet and asynchronous transfer mode (ATM). Lastly, these MSPPs evolved into multi-service transport platforms (MSTPs), which typically included DWDM and/or OTN capabilities.

OTN-capable equipment evolved from both the MSPPs of the SDH network as well as the optical ADMs (OADMs) in the DWDM network. The earliest equipment implemented a digital wrapper function where non-OTN signals were simply mapped into OTN prior to transmission to take advantage of the FEC of the OTN protocol.

More recently, the OADM has evolved into a software-reconfigurable version known as the reconfigurable OADM (ROADM). These systems typically have both transponder cards (performing digital wrapper functionality) as well as muxponder cards, which include a multiplexing stage to combine multiple lower-speed signals into a single OTN signal.

The latest transport platform, known as the packet optical transport system (P-OTS), combines transponder and muxponder functions present in the ROADM with OTN container (i.e., ODUj) switching. In many cases, these systems are also capable of packet switching for services such as Ethernet or MPLS to enable a more flexible transport platform designed to natively handle both circuit and packet switching functions.