by Dinesh Thakur Category: Network Technologies

SONET (Synchronous Optical Network) is a proposal by Bellcore (Bell Communications Research), SONET is a technique of transport between two nodes, which defines the interface adopted for the NNI (Network Node Interface). She initially concerned that the interconnection of telephone networks of major operators, PTT, carrier, etc. The difficulty of standardization has been to find a compromise between the American, European and Japanese interests to interconnect different operator networks and national networks.

The hierarchy of rates is different on the three continents; we had to agree on a basic level. It is ultimately the rate of 51.84 Mbit / s which was accepted and which forms the first level, called STS-1 (Synchronous Transport Signal, level 1). The levels located above level 1, called STS-N, are multiples of the basic level.

SONET describes the composition of a synchronous frame transmitted every 125 microseconds. The length of this field depends on the speed of the interface. Its various values ​​are listed in table depending on the speed of the optical medium, or OC (Optical Carrier).

  Values of the SONET frame depending on the speed of the optical medium

As illustrated in the figure, the SONET frame comprises in the first three bytes of each row of synchronization and supervisory information. Cells are transmitted in the frame. The start time of the sending of a cell is not necessarily the beginning of the frame, but can be located anywhere in the frame. Supervision bits preceding the beginning so that one does not waste time for the emission of a cell. Basic SONET frame

When the signals arrive in the transport SONET module, they are not copied directly as such but in a virtual container (Virtual Container). This filling is called adaptation. SONET and SDH include several types of virtual containers, called VC-N (Virtual Container level N). In these containers, it is necessary to add management information located in the byte start of each row. By adding this additional information, we define an administrative unit, or AU-N (Administrative Unit-N).

The higher levels still have nine rows, but there is n times 90 bytes per row for the level N. The N-level frame of the SONET hierarchy is illustrated in figure. SONET STS-N frame

The SONET standard is used for the implementation of the physical layer of the ATM cell switched networks and the interconnection of the operator networks. The flow rates used for the ATM access interfaces are respectively 155 Mbit/s, 622 Mbit/s and 2.488Gbit/s for OC3, OC12 and OC48's.

A very important feature of SONET is more reliable communication in the event of breakage or failure of one of its components. SONET found in metropolises have a loop topology. Two paths are thus available to go from one point to another, in particular of the user at the operator's core network. SONET allows changing this path in 50 ms. During a break in communication in one direction of the loop, the reconfiguration can be done in a time resulting in a barely detectable break for two people talking to each other. This ability of reconfiguration is a key strength of SONET / SDH structures. 

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About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.