Transport Networks Shift From SDH To OTN

Transport protocols have evolved over a very long time, and each generation has inherited many attributes and behaviours from its predecessors. The dominant protocol in the transport network over much of the past two decades has been the synchronous digital hierarchy (SDH). Recently, however, the optical transport network (OTN) has taken hold as the protocol of choice.

SDH was originally designed to efficiently transport DS1, DS3, and E1 by defining containers of 1.5, 2, and 50 Mbits/s. This fine granularity was well suited for the typical client bandwidths of that time, but it prevented SDH from scaling to efficiently carry larger payloads such as 10-Gigabit Ethernet (10GE).

Initially, SDH network elements were connected directly by fibre-optic cables and served as the photonic and physical layers of the open systems interconnect (OSI) protocol stack. Later, the need for increased bandwidth over a single fibre led to the deployment of wavelength division multiplexing (WDM) technology to create an underlying transport network to the existing SDH network infrastructure. This resulted in service providers needing to operate two separate transport layer networks.

Dense wavelength division multiplexing (DWDM) networks transport client data transparently. While SDH technology transparently transports plesiochronous digital hierarchy (PDH) signals, it requires adaptation or partial termination for data signals and for multiplexing lower-rate SDH signals. This causes issues when transporting one service provider’s SDH signals through another service provider’s network due to the lack of overhead transparency.

The definition of OTN came at a time when all of these issues were well understood.  OTN was consequently expressly defined to focus on the transport of larger-bandwidth signals, encompass both DWDM and time division multiplexing (TDM) transport network layers, and provide transparent transport of client signals.

SDH Compared To OTN

It’s no surprise that OTN has many similarities to SDH, as many of its characteristics were taken from previous technologies when SDH was defined. The similarities include:

  • Framing and scrambling
  • Layers (path, section)
  • Bit-interleaved parity 8 (BIP-8) error monitoring
  • Forward and backward error and alarm indications
  • Communication channels
  • Automatic protection switch (APS) protection signaling
  • Byte multiplexing

Despite all the obvious similarities, there are also some significant differences that result from some lessons learned in the many years of deployment and operation of SDH equipment.

Layers

SDH is defined to have three layers—regenerator section, multiplex section, and path—whereas OTN includes only the section and path. The multiplex section was defined to facilitate fault isolation and protection. The tandem connection monitoring (TCM) functionality in OTN provides more flexible network fault monitoring and protection and makes the line layer unnecessary.

Frame Structure And Signal Bit Rates

Like SDH, OTN has row- and column-oriented frame structures with framing bytes, overhead bytes, and payload areas. Unlike the fixed frame rate and different frame sizes of the various SDH signals, however, the OTN signals have fixed frame sizes and different frame rates. 

Also, each SDH signal rate is a multiple of four of the next lower rate in the hierarchy (e.g., STM-16 = 4xSTM-4). In the OTN hierarchy, each higher rate is defined so the payload area can carry multiple (usually four) of the next lower signals including all overhead. When overhead is added to this payload area, the resulting multiple is not exactly four.

Bit Error Detection

BIP-8 monitoring from SDH is largely carried over to OTN. But because of the differences in frame structure, OTN does not suffer from the effect of a single BIP-8 count covering progressively larger numbers of bytes for bigger path signals (e.g., VC-4-4c, VC-4-16c, etc.).

Transparency

One of the key properties of DWDM networks is their ability to transparently transport clients, including OTN client signals. This means it’s possible to multiplex OTN signals into higher-rate signals without sacrificing transparency for data, overhead, and timing. SDH transports PDH signals transparently, but it cannot transport other SDH signals without terminating timing and certain overhead.

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.

Deployment Of OTN

The deployment of OTN has primarily been for the transport of SDH and 10GE signals. Many recent changes to the OTN standards have broadened functionality to support 40- and 100-Gbit/s operation and added better capability for Gigabit Ethernet and other protocols such as Fibre Channel and video.

While there is certainly a market for multiplexing and transporting many low-rate signals (<10 Gbits/s), most OTN deployment will be in the core of the transport network and continue to focus on large-bandwidth pipes.

The introduction of OTN switching through development of P-OTS equipment delivers the true networking aspect of OTN and enables a wider set of deployment scenarios and protection options in the network. Despite the linear and ring orientation of SDH networks, it is likely that deployment of OTN equipment will be more focused around a meshed network approach. The planned integration of packet switching capability with OTN switching in P-OTS equipment may also have a significant effect on how OTN equipment is deployed.

The Future

While OTN has taken many elements from previous network technologies such as PDH and SDH, it is certainly a significant evolutionary step in transport technology. Service providers around the world have committed to OTN as their transport technology of choice, and much time and energy is being spent on developing new equipment to enable a greatly expanded rollout of OTN into the service provider networks.

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