What are Synchronous Optical Networks?

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Fiber optic transmission media are the media of choice when it comes to “long haul” applications such as intercontinental, cross-continental and oceanic (submarine) backbone links. It is also the preferred medium for tier one ISP backbone links. This means that new WAN implementations and applications are now predominantly fiber optic cable based with public wireless networking rollouts being the major exception.

Synchronous Optical Networking

Synchronous Optical Networking (SONET) is an established high-speed WAN alternative for communicating digital information using lasers or Light-Emitting Diodes (LEDs) over optical cable offered by several telecommunications companies. SONET was originally developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic as well as providing the mechanisms that allow for interoperability between equipment from different vendors. The result is that there are multiple, very closely related standards that describe synchronous optical networking including:

Synchronous Digital Hierarchy (SDH) – The SDH standard was developed by the International Telecommunication Union (ITU) and is documented in standard G.707 and its extension G.708. SDH is used throughout the world but not in North America

Synchronous Optical Networking (SONET) – The SONET standard as defined by GR-253-CORE from Telcordia™. Primarily used exclusively in Canada and the USA where SDH has not been implemented, although it can be found in other countries.

Synchronization is Key

Through the use of atomic clocks synchronous networking data transport rates are very tightly regulated which allows for entire inter-country networks to operate synchronously while greatly reducing the amount of buffering required between elements in the network. This reduction in buffering translates into reduced transmission administrative overheads and thus greater effective net data throughput rates.

Synchronization and the Enterprise – Synchronization is also a key benefit for enterprises that cover large geographic even global areas as it does allow for data at location A to be current and appropriate with data at all other locations more or less simultaneously. This makes truly globally video conferencing a practical reality and aides with development projects. This is particularly important for cooperative enterprise class applications.

For example; distributed cherry-picking development and compilation techniques; such as used in major software product application development becomes more realistically efficient and effective on a global scale.

Another area to benefit is the capacity for ISPs to distribute their processing and storage requirements on a global scale far more efficiently. The ISP is the transporter for such enormous product databases as the open source community (e.g. sourceforge.net) where “mirror” sites are able to offer potential downloads as required from a more geographically local site to the end user rather than expending valuable routing resources and international, transcontinental and transoceanic backbone links bandwidth.

Many tier 1 ISPs; such as Optus in Australia, maintain these mirrors. The reason that they do so is simply one of economics since they know from experience that these sites generate constant high user demand and by maintaining their “local” mirror they do not have to enter into major peering arrangements with overseas ISPs who may demand some form of IP transit charges.

All in all this makes for a far more effective and efficient end-user cost-effective means of digital product distribution as synchronous optical networks are built upon protocols based around a generic transport container philosophy; pretty much akin to the standardized shipping containers we all know so well. 

Generic Transport Containers

SDH and SONET are generic all-purpose transport containers for moving voice and data; rather than just communications protocols per sec, with encapsulation being the key to this capability. Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET/SDH (POS) networking. This reduces the amount of infrastructure restructuring necessary to upgrade major networking and communications infrastructures both locally and globally.

SDH and SONET Frame Structures

Standard packet or frame oriented data transmission frames usually consist of a header and a payload with the header of the frame being transmitted first, followed by the payload and a trailer (e.g. CRC). With synchronous optical networking both the header, which is referred to as the overhead and the payload still exist but the big difference is that the overhead is not all transmitted before the payload, rather the transmission is interleaved.

Interleaved Transmission – The difference between the preceding standard conversation transmission process and synchronous optical interleaved transmissions is that a synchronous optical transmission conversation goes like this: First of all, a portion of the overhead (header) is transmitted. This is followed by part of the payload. After which the next part of the overhead is transmitted. This in turn is followed by the next part of the payload and so on until the entire frame has been transmitted.

SONET Frame Size and Transmission Sequence – SONET frames are 810 octets in size, transmitted as 3 octets of overhead, followed by 87 octets of payload repeated nine times over until 810 octets have been transmitted. The total frame transmission time is 125 microseconds.

SDH Frame Size and Transmission Sequence – SDH frames are 2430 octets in size transmitted as 9 octets of overhead, followed by 261 octets of payload this too is repeated nine times over until all 2430 octets have been transmitted. Importantly the total frame transmission time is also 125 microseconds. It doesn’t take much brain power to see that SDH is capable of an effective data throughput rate three times that which the North American implementation of SONET can achieve.

Security – Synchronous optical communications and networking do deliver superior data confidentiality and data integrity whilst permitting ready access to said data for authorized and duly authenticated entities (both human and otherwise).


With its capacity to transport multiple data types including those of older communications and networking synchronous optical communications and networking protocols and technologies can form the foundation infrastructure of such widely used internetworks as the Internet; both now and for some considerable time into the future.

Scalability and extensibility are two aspects of synchronous optical networking which in combination with flexible encapsulation, high bandwidth capabilities, multiple concurrent conversation transmission and the thoroughly documented and readily accessible open standards they embody are all major pluses that will ensure that these technologies will continue to expand into general wide-spread community usage.

Yet even to this day; it is the relative compactness, speed of transmission, compatibility and immunity to noise, interference and interception that remain the coup de grace for synchronous fiber optical technologies.


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