OSI (1) - The Physical Layer
Virtually all networks in use today are based in some fashion on the Open Systems Interconnection(OSI) standard. OSI was developed in 1984 by the International Organization for Standardization, a global federation of national standards organizations representing approximately 130 countries.
The seven layers of the OSI Reference Model
The seven layers can be separated into two sets:
- Transport Set: physical layer, data link layer, network layer, transport layer.
- Application Set: Session layer, Presentation layer, Application layer
The Physical Layer
The Physical layer is the lowest layer of OSI. It provides the means to transport the bits that make up a Data Link layer frame through the network media. This layer accept a complete frame from the Data Link layer and encodes it as a series of signals that are transmitted onto the local media. The encoded bits that comprise a frame are received by either an end device or an intermediate devie.
The purpose of the Physical layer is to create the electrical, optical, or microwave signal that represents the bits in each frame. The delivery of frames across the local media requires the following Physical layer elements:
- The physical media and associated connectors
- A representation of bits on the media
- Encoding of data and control information
- Transmitter and receiver circuitry on the network devices
There are three basic forms of network media on which data is represented:
copper cable: The most commonly used media for data communications is cabling that uses copper wires to signal data and control bits betwwen network devies. Data is transmitted on copper cables as electrical pulses.
fiber media: Fiber-optic cabling uses either glass or plastic to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabiling is capable of very large raw data bandwidth rates.
wireless media: Wireless media carry electromagnetic signals at radio and microwave frequencies that represent the binary digits of data communications.
The Physical layer must generate the electrical, optical, or wireless signals that represent the “1” and “0” on the media. The method of representing the bits is called the signaling method. Bits are represented on the medium by changing one or more of the following characteristics of a singal:
Instead of represeting bits as puses of simple voltage values, in the Manchester Encoding scheme, bit values are represented as voltage transitions.
Encoding is a method of converting a stream of data bits into a predefined code. Codes are grouping of bits used to provide a predictable pattern that can be recognized by both the sender and the receiver. 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.
Singal patterns can indicate: start of frame, end of frame, and frame contents. These signal patterns can be decoded into bits. The bits are interpreted as codes. The codes indicate where the frames start and stop.
Encoding techniques use bit patterns called symbols. The Physical layer may use a set of encded symbols - called code groups - to represent encoded data or control information. A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns.
An example, we will examine a simple code group called 4B/5B. Code groups that are currently used in modern networks are generally more complex.
4, Data Carrying Capacity
Different physical media support the transfer of bits at different speeds. Data transfer can be measured in three ways:
- Bandwidth: The capacity of a medium to carry data is described as the raw data bandwidth of the media. The practical bandwidth of a network is determined by a combination of factors: the properties of the physical media and the technologies chosen for signaling and detecting network singals.
- Throughput: Due to a number of factors, throughput usually does not match the specified bandwidth in Physical layer implementations such as Ethernet. The throughput of each node is degraded as usage of the media increases.
- Goodput: A third measurement has been created to measure the transfer of usable data. That measure is known as goodput. Goodput is the measure of usable data transferred over a given period of time.
As an example, consider two hosts on a LAN transferring a file. The bandwidth of the LAN is 100 Mbps. Due to the sharing and media overhead the throughput between the computers is only 60 Mbps. With the overhead of the encapsulation process of the TCP/IP stack, the actual rate of the data received by the destination computer, goodput is only 40Mbps.