Layered protocols in distributed system

Due to the absence of shared memory, all communication in distributed systems is based on message passing. When process A wants to communicate with process B, it first builds a message in its own address space. Then it executes a system call that causes the operating system to fetch the message and send it over the network to B. Although this basic idea sounds simple enough, in order to prevent chaos, A and B have to agree on the meaning of the bits being sent.

Many different agreements are needed. How many volts should be used to signal a 0-bit, and how many volts for a 1-bit? How does the receiver know which is the last bit of the message? How can it detect if a message has been damaged or lost, and what should it do if it finds out? How long are numbers, strings, and other data items, and how are they represented? In short, agreements are needed at a variety of levels, varying from the low-level details of bit transmission to the high-level details of how information is to be expressed. To make it easier to deal with the numerous levels and issues involved in communication, the International Standards Organization (ISO) has developed a reference model that clearly identifies the various levels involved, gives them standard names, and points out which level should do which job. This model is called the Open Systems Interconnection Reference Model, usually abbreviated as ISO OSI or sometimes just the OSI model.

The physical layer is concerned with transmitting the Os and 1 s. How many volts to use for 0 and 1, how many bits per second can be sent, and whether transmission can take place in both directions simultaneously are key issues in the physical layer.

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Originally published at https://pywix.blogspot.com.