Introduction
The dominant protocol in the transport network over much decades has been SONET/SDH.Recently,However OTN has taken hold as the protocol of choice for transport networks of today.All major service providers are committed to deployment of OTN as their central transport technology and the telecom equipment manufacturers have clearly responded with a shift away from development of transport systems based on SONET/SDH in favor of newer systems based on OTN.
Rationale for OTN
Transport protocols have evolved over a very long time and each generation has inherited many attributes
and behaviors from its predecessors. This is very apparent when comparing SONET/SDH and OTN.
SONET/SDH was originally designed to efficiently multiplex the most prevalent telecommunications signals in
the late 1980s which were DS1, DS3, and E1. This was accomplished by defining transport containers of 1.5
Mb/s 2 Mb/s, and 50 Mb/s that were multiplexed into aggregate signals ranging from 155 Mb/s to 10 Gb/s. This
fine granularity was well suited toward the typical client bandwidths of that time, but prevented SONET/SDH
from scaling to efficiently carry larger payloads (e.g. 10 Gb/s).
Initially, SONET/SDH network elements were connected directly by fibre optic cables and served as the
photonic and physical layers of the OSI stack (layers 0 and 1). Beginning in the 1990s, the introduction of
DWDM technology to address the need for increased bandwidth over a single fiber led to deployment of
WDM networks serving as an underlying transport network to the existing SONET/SDH network infrastructure.
This resulted in service providers needing to deploy, provision and maintain two separate transport layer
networks.
One very attractive attribute of DWDM networks is that they transport client data very transparently. While
SONET/SDH technology transparently transports PDH signals, it requires adaptation or partial termination
for data signals and for multiplexing of lower rate SONET/SDH signals. This causes issues when transporting
one service provider’s SONET/SDH signals through another service provider’s network because network
management functions contained in transport overhead channels are unable to transparently traverse
another service provider’s network.
The definition of OTN came at a time when all of these issues were well understood in the deployed SONET/
SDH networks. OTN was therefore expressly defined to focus on transport of larger bandwidth signals,
encompass both DWDM and existing TDM transport network layers and to provide transparent transport of
client signals across an OTN network.
