The Physical Layer The Data Link Layer

www.pragsoft.com Chapter 1: Introduction 7 Each of the seven layers of the OSI model is described below in more detail. Subsequent chapters examine the layers in greater depth and discuss their main protocols. It should be pointed out that the OSI model is not the only model in use. It is, however, the most-widely respected model and has become a standard benchmark for comparing other network architectures against.

1.3.1. The Physical Layer

The physical layer is concerned with the transmission of raw data bits over communication lines. Physical layer standards and protocols are concerned with issues such as the following: • How a physical circuit is established between communicating devices. • How the circuit is terminated when no longer needed. • The physical form e.g., voltages, frequencies, timing in which data bits binary values 0 and 1 are represented. • Whether transmission of data can take place in one or both directions over the same physical connection. • Characteristics of the physical media that carry the signals e.g., copper wire, optical fiber, radio waves. • Characteristics of the connectors used for connecting the physical media. • How data from a number of sources should be multiplexed before transmission and demultiplexed upon arrival, and the type of multiplexing technique to be used. • The type of modulation to be used for transmitting digital data over analog transmission lines. The physical layer accounts for much of the tangible components of a network, including cables, satellites, earth stations, repeaters, multiplexers, concentrators, and modems. Physical layer protocols and standards are of mechanical, electrical, functional, and procedural nature. The physical layer hides the above details from the higher layers. To the data link layer, it appears as a logical communication channel which can send a stream of bits from one point in the network to another but not necessarily reliably.

1.3.2. The Data Link Layer

The data link layer is concerned with the reliable transfer of data over the communication channel provided by the physical layer. To do this, the data link layer breaks the data into data frames, transmits the frames sequentially over the channel, 8 Communication Networks Copyright © 2005 PragSoft and checks for transmission errors by requiring the receiving end to send back acknowledgment frames. Data link protocols are concerned with the following issues: • How to divide the data into frames. • How to delimit frames by adding special bit patterns to the beginning and end of each frame. This allows the receiving end to detect where each frame begins and where it ends. • Error detection. Some form of error check is included in the frame header. This is constructed by the transmitting end based on the contents of the frame, and checked for integrity by the receiving end. A change in the frame bits can be detected in this way. • Error correction. When a frame arrives corrupted or is for any reason lost in the network, it is retransmitted. Lost acknowledgment frames may result in duplicate frames, which need to be detected and corrected as well. • Flow control. In general, not all communication devices in a network operate at the same speed. Flow control provides a means of avoiding a slow receiver from being swamped by data from a fast transmitter. The data link layer hides the above details from the higher layers. To the network layer, it appears as a reliable communication channel which can send and receive data packets as frames.

1.3.3. The Network Layer