lec3 - ECE 333: Introduction to Communication Networks Fall...

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ECE 333: Introduction to Communication Networks Fall 2002 Lecture 3: Physical Layer I Overview Fundamental Limitations 2 Notes: The last lecture introduced layered network architectures. Layering provides a hierarchical decomposition of a network into functional modules called layers. Each layer provides a service to the next higher layer. In this lecture and the next, we focus on the lowest layer in most protocol stacks – the physical layer. Following the OSI architecture, we assume that the next layer above the physical layer is the Data Link Layer (DLL). The physical layer provides a "virtual bit pipe" service between two nodes in a network that are connected by a communication link (also called a communication channel). In other words, for two nodes connected by a communication channel, the physical layer is responsible for transporting sequences of bits from a DLL peer process at one node to the DLL peer process at the other node. Basic characteristics of the bit pipe service provided by the physical layer include the bit rate (how many bits can be sent per second), the bit error rate (what is the probability a bit is received in eror), the delay from when a bit is sent until it is received, whether or not this bit pipe is synchronous, and if this pipe is full-duplex (two-way), half duplex (two way, but only one way at a time) or simplex (one way). In a synchronous bit pipe, bits are sent and received at a fixed rate (e.g. 1 bit per T second). The sending DLL peer must supply bits at this rate even when it has no data to send and the receiving DLL peer will receive bits at this rate. In an asynchronous bit pipe, characters (groups of bits) are sent asynchronously whenever they are generated, and no bits are sent when no data is available. Asynchronous bit pipes are only used over short
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lec3 - ECE 333: Introduction to Communication Networks Fall...

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