COMS 113-Data Communications

Chapter 8- OSI Layer

This chapter introduces the overall functions of the Physical layer in addition with the standards and protocols that manage the transmission of data across the local media. The role of the OSI Physical layer is to encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media - copper wires, optical fiber, and wireless - that connect network devices. It has a job also to retrieve these individual signals from the media, restore them to their bit representations, and pass the bits up to the Data Link layer as a complete frame. The three basic forms of network media in which data is represented are copper cable, fiber, and wireless. The representation of the bits - that is, the type of signal - depends on the type of media. For copper cable media, the signals are patterns of electrical pulses. For fiber, the signals are patterns of light. For wireless media, the signals are patterns of radio transmissions.

The three fundamental functions of physical layer are as follows: the physical components, data encoding, and signaling. The physical components are the electronic hardware devices, media and connectors that transmit and carry the signals to represent the bits.

Encoding is a method of converting a stream of data bits into a predefined code. Using predictable patterns helps to distinguish data bits from control bits and provide better media error detection. In addition to creating codes for data, encoding methods at the Physical layer may also provide codes for control purposes such as identifying the beginning and end of a frame. The transmitting host will transmit the specific pattern of bits or a code to identify the beginning and end of the frame.

            In Signaling the Physical layer must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media. Signaling methods represents the bits.  The Physical layer standards must define what type of signal represents a "1" and a "0".

Data transfer can be measured in three ways such as:  Bandwidth, Throughput, and Goodput. Bandwidth of the media is the capacity of the medium to carry data and is typically measured in kilobits per second (kbps) or megabits per second (Mbps). Throughput is the measure of the transfer of bits across the media over a given period of time. Goodput is the measure of usable data transferred over a given period of time, and is therefore the measure that is of most interest to network users.Advantages using code groups include:

·         Reducing bit level error

·         Limiting the effective energy transmitted into the media

·         Helping to distinguish data bits from control bits

·         Better media error detection