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The International Standards Organization (ISO) has developed OSI standard in order to provide some level of uniformity among network vendors. OSI reference model consists of 7 layers. Each layer provides a service for the layer immediately above it:
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(1) Physical: passes bit stream between computer and network; defines voltage used, the data transmission timing, handshaking requirements; EIA-RS232, RS449, ethernet, token ring, FDDI
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(2) Data Link: provides reliable data transfer between computer and network; packages data into data frames; HDSL, SDSL, ATM, FR, NDIS, ODI
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