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Problems synchronous data transmission on fiber optic cable and ways of their solution


D.S. Shishkin – leading соnstructor, JSC «Concern «Automation». E-mail:

In SCS often a need for synchronous transmission data bit stream over long distances via a highly secure channel with the speed of the units of kbit/s to tens of Mbit/s. The optimal way to communicate with such criteria is fiber optic link. And often fiber-optic cables may already be laid at the site of reception and transmission, and to have free and (or) stand optical fiber cable. The most common methods of data transmission standard Ethernet and SDH have several drawbacks. Standard Ethernet (IEEE 802.3) is very popular, has many modifications and a wide variety of equipment, but it was not originally designed for synchronous data transmission, аnd when you try to restore the synchronization signal by means of algorithms based on the continuous accumulation of data on the packet arrival delay is not very accurate. Relatively new standard for transmitting data to be suitable for the task, is a technology SDH. In SDH networks, channel switching occurs from different sources using synchronous multiplexing by time division. This standard has a number of drawbacks: the relatively expensive equipment, the need for additional equipment for the transmission of different types of digital data over a network, the lack of compatibility of equipment from different manufacturers, the inefficient use of bandwidth available (increase communication channels for redundancy in case of failure reduces the rate of transmission data in a dedicated channel and expensive system). The recommended method for multichannel synchronous transmission bit stream over a wide frequency range of the signal sources are encouraged to implement using parallel-serial converter (serializer). Decoding of the signal at the receiving end to make using deserialayzera using control circuitry capture clock serializer and a threshold detector signal loss of the optical receiver. The optical transceivers are encouraged to use compact, protocol independent SFP modules with data rates up to 1 Gbit/s. These optical modules are good mates with the interface ASIC serializer and desserialayzerov by a small number of passive components. Transformation of the structure of data transmitted in a balanced alternating sequence is proposed to using the inverters connected to the odd ranks serializer, with alternating direct connections to even bits of the input data. This transformation provides a data code containing no more than two consecutive logical «0» and «1», that is enough. On the receiving side, the output deserialayzera recommended to connect the D-flip-flop, which would produce «snap» in the event of errors associated with low (or lack thereof) of the optical signal at the receiver input SFP module, or in the case of loss of synchronism with deserialayzerom serializer. This solution will make the system more reliable and interference-free, giving the time needed to restore the system to normal operation. The proposed system is relatively inexpensive, versatile, simple and reliable, with high stability timing of the transmitted information and a wide range of data rates (from zero to a few Mbit/s).

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