Chapter: Optical Communication and Networking : Optical Networks


SONET is the TDM optical network standard for North America



ü   SONET is the TDM optical network standard for North America


ü   SONET is called Synchronous Digital Hierarchy (SDH) in the rest of the world


ü   SONET is the basic phycal layer standard


ü   Other data types such as ATM and IP can be transmitted over SONET


ü   OC-1 consists of 810 bytes over 125 us; OC-n consists of 810n bytes over 125 us


ü   Linear multiplexing and de-multiplexing is possible with Add-Drop-Multiplexers


ü   The SONET/SDH standards enable the interconnection of fiber optic transmission equipment from various vendors through multiple-owner trunk networks.


ü   The basic transmission bit rate of the basic SONET signal is


ü   In SDH the basic rate is 155.52 Mb/s.

Common values of OC-N and STM-N:


ü   OC stands for optical carrier. It has become common to refer to SONET links as OC-N links.


ü   The basic SDH rate is 155.52 Mb/s and is called the synchronous transport module—level 1 (STM 1).

SONET Add Drop Multiplexers:

SONET ADM is a fully synchronous, byte oriented device, that can be used add/drop OC sub-channels within an OC-N signal


Ex: OC-3 and OC-12 signals can be individually added/dropped from an OC-48 carrier



ü   SONET and SDH can be configured as either a ring or mesh architecture


ü   SONET/SDH rings are self-healing rings because the traffic flowing along a certain path can be switched automatically to an alternate or standby path following failure or degradation of the link segment


ü   Two popular SONET and SDH networks:


–  2-fiber, unidirectional, path-switched ring (2-fiber UPSR)


–  2-fiber or 4-fiber, bidirectional, line-switched ring (2-fiber or 4-fiber BLSR)

Ex: Total capacity OC-12 may be divided to four OC-3 streams, the OC-3 is called a path here 

2-Fiber UPSR Protection:

ü   Rx compares the signals received via the primary and protection paths and picks the best one


ü   Constant protection and automatic switching

BLSR Recovery from Failure Modes:


ü   If a primary-ring device fails in either node 3 or 4, the affected nodes detect a loss-of-signal condition and switch both primary fibers connecting these nodes to the secondary protection pair


ü   If an entire node fails or both the primary and protection fibers in a given span are severed, the adjacent nodes switch the primary-path connections to the protection fibers, in order to loop traffic back to the previous node.

4-Fiber BLSR Basics:

BLSR Fiber-Fault Reconfiguration:

In case of failure, the secondary fibers between only the affected nodes (3 & 4) are used, the other links remain unaffected


BLSR Node-Fault Reconfiguration:

If both primary and secondary are cut, still the connection is not lost, but both the primary and secondary fibers of the entire ring is occupied

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