Introduction to SONET (Synchronous Optical Networking)
By Colin Yao
The SONET standards were developed in the mid-1980s to take advantage of low-cost transmission
over optical fibers. It defines a hierarchy of data rates, formats for framing and multiplexing
the payload data, as well as optical signal specifications (wavelength and dispersion), allowing
multi-vendor interoperability.
SONET is sometimes described as "T-1 on steroids". Why is that? As we know, the digital
hierarchy (DS-0, DS-1, DS-2, DS-3 and more) was created to provide cost-effective multiplexed
transport for voice and data traffic from one location in a network to another.
SONET (Synchronous Optical Networking, used in the United States) and SDH (Synchronous
Digital Hierarchy, used in Europe and other parts of the world) are two equivalent multiplexing
protocols for transferring multiple digital bit streams using lasers or LEDs (light-emitting
diodes) over the same optical fiber.
SONET and SDH were designed to replace PDH (Plesiochronous Digital Hierarchy) system
to eliminate the synchronization problems that PDH had. SONET is synchronous, which means that
each connection achieves a constant bit rate and delay. For example, SDH or SONET may be utilized
to allow several Internet Service Providers to share the same optical fiber, without being
affected by each others traffic load, and without being able to temporarily borrow free capacity
from each other.
SONET and SDH are considered to be physical layer protocols since they offer permanent
connections and do not involve packet mode communication. Only certain integer multiples of
64kbits⁄s are possible bit rates.
SONET is actually TDM (time division multiplexing) based and this makes it readily supports
fixed-rate services such as telephony. Its synchronous nature is designed to accept traffic
at fixed multiples of a basic rate (64kbit⁄s), without requiring variable stuff bits or complex
rate adaptation.
The SONET data transmission format is based on a 125us frame consisting of 810 octets,
of which 36 are overhead and 774 are payload data. The basic SONET signal, whose electrical
and optical versions are referred to as STS-1 and OC-1, respectively, is thus a 51.84Mb⁄s
data stream that readily accommodates TDM channels in multiples of 8 kb⁄s.
SONET can be used to encapsulate PDH and other earlier digital transmission standards.
It can also be used directly to support either ATM (Asynchronous Transfer Mode) or Packet over
SONET⁄SDH (POS) networking. So SONET⁄SDH is actually a generic all-purpose transport
container for moving both voice and data traffic. They in themselves are not communications protocols.
SONET brings with it a subset of advantages that makes it stand above competitive technologies.
These include mid-span meet, improved operations, administration, maintenance, and provisioning
(OAM&P), support for multipoint circuit configurations, non-intrusive facility monitoring,
and the ability to deploy a variety of new services.
Improved OAM&P is one of the greatest contributions that SONET brings to the networking
field. Element and network monitoring, management, and maintenance have always been something
of a catch-as-catch-can effort because of the complexity and diversity of elements in a typical
service provider's network. SONET overhead includes error-checking ability, bytes for network
survivability, and a diverse set of clearly defined management messages.
Colin Yao is an expert on fiber optic communication technologies and products. Learn
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