Physical Layer and Media – Signal Encoding
By the end of this unit, you should be able to:
- Define signals and discuss the analog and digital signals
- Define modulation and discuss the different modulation techniques
- Define multiplexing and explain the different and
- Discuss the various network topologies
- Discuss the different ways that analog/digital data is converted to analog/digital signals and vice versa.
- Explain the various guided and unguided transmission media and their characteristics
- Discuss the use of the concept of multimedia in coding pictures or videos
3.1 Data Transmission
Data transmission refers to the movement of encoded information from one point to another by means of electronic transmission system. It can also be defined as the exchange of data between two devices via some form of transmission medium which can be wired or wireless. Another definition for data communications simply mean the transferring of digital information (usually in binary form) between two or more points (terminals). At both the source and destination, data are in digital form; however, during transmission, they can be in digital or analog form Information is carried by signal, which is a physical quantity that changes with time. The signal can be a voltage proportional to the amplitude of the voice like in simple telephone, a sequence of pulses of light in an optical fiber, or a radio-electric wave radiated by an antenna.
The fundamental purpose of data communication is to exchange information which is done by following certain rules and regulations called protocols and standards.
Network topology refers to how various nodes, devices, and connections on your network are physically or logically arranged in relation to each other.
There are two approaches to network topology: physical and logical. Physical network topology, as the name suggests, refers to the physical connections and interconnections between nodes and the network—the wires, cables, and so forth. Logical network topology is a little more abstract and strategic, referring to the conceptual understanding of how and why the network is arranged the way it is, and how data moves through it.
Types of Network Topologies
Figure 1. Network Topologies
Types of Topologies
3.3 Signals – analog, digital
The entire world is full of signals, both natural and artificial. Signals can be analog or digital. Figure 2, illustrates an analog signal. The term analog signal refers to signal that is continuous and takes continuous value. Most phenomenon’s in the world today are analog. There are an infinite amount of colours to paint an object (even if the difference is indiscernible to the eye), it is possible for us to hear different sounds and also smell different odours. The common theme among all of these analog signals is their infinite possibilities.
Figure 3.2 shows a typical representation of analog signal. Because the signal varies with time, time is plotted on horizontal (x-axis), and voltage on the vertical (y-axis). While this signals may be limited to a range of maximum and minimum values. There are still an infinite number of possible values within that range. For example the analog voltage that light the bulbs is clamped between -220V and +220V, but as you increase the resolution more and more, you discover an infinite number of values that the signal can be. For example, pure audio signals are analog. The signal that comes out of a microphone is full of analog frequencies and harmonics, which combine to beautiful music.
A digital signal is a physical signal that is a representation of a sequence of discrete values. The signal must have a finite set of possible values, the number of set which can be anywhere between two and very large number that is not infinity. Digital signal is one of two voltage value (0V or 5V) timing graphs of these signals look like square waves as shown in figure 3
Figure 3. Digital Signal
3.4 Analog transmission – modulation and multiplexing
Modulation is the process where a Radio Frequency or Light Wave’s amplitude, frequency, or phase is changed in order to transmit intelligence. Modulation Techniques are methods used to encode digital information in an analog world. Additionally, digital signals usually require an intermediate modulation step for transport 130across wideband, analog-oriented networks. Digital information changes the carrier signal by modifying one or more of its characteristics (amplitude, frequency, or phase). This kind of modification is called modulation (shift keying).
Modulation is the process of changing the parameters of the carrier signal, in accordance with the instantaneous values of the modulating signal.
Figure 4. Modulations
Definition of Multiplexing
Multiplexing is the process of combining multiple signals into one signal, over a shared medium. If the analog signals are multiplexed, then it is called as analog multiplexing. Multiplexing was first developed in telephony. A number of signals were combined to send through a single cable. The process of multiplexing divides a communication channel into several number of logical channels, allotting each one for a different message signal or a data stream to be transferred. The device that does multiplexing can be called as Multiplexer or MUX.
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic. We can accommodate this increase by continuing to add individual links each time a new channel is needed; or we can install higher-bandwidth links and use each to carry multiple signals. In a multiplexed system, n lines share the bandwidth of one link. Figure 5 shows the basic format of a multiplexed system. The lines on the left direct their transmission streams to a multiplexer (MUX), which combines them into a single stream (many-to-one).At the receiving end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) and directs them to their corresponding lines. In the figure, the word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels. Multiplexingis the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer, etc.
Figure 5. Categories of multiplexing
3.5 Digital transmission – Signal encoding
Digital transmission is employed in order to achieve high reliability and because the cost of digital switching systems is much lower than the cost of analog systems. In order to use digital transmission, however, the analog signals that make up most voice, radio, and television communication must be subjected to a process of analog-to-digital conversion.
Signal Encoding is the process of turning thoughts into communication. The encoder uses a ‘medium’ to send the message — a phone call, email, text message, face-to-face meeting, or other communication tool. The level of conscious thought that goes into encoding messages may vary. The encoder should also take into account any ‘noise’ that might interfere with their message, such as other messages, distractions, or influences.
3.6 Transmission media
Definition of Data Transmission Media
Transmission media is a pathway that carries the information from sender to receiver. We use different types of cables or waves to transmit data. Data is transmitted normally through electrical or electromagnetic signals. An electrical signal is in the form of current. An electromagnetic signal is series of electromagnetic energy pulses at various frequencies. These signals can be transmitted through copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different properties like bandwidth, delay, cost and ease of installation and maintenance. Transmission media is also called Communication channel. Transmission media is broadly classified into two groups. Wired or Guided Media or Bound Transmission Media and Wireless or Unguided Media or Unbound Transmission Media. (See the diagram below).
Figure 7. Communication Media
3.7 Multiplexing of signals
Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic. We can accommodate this increase by continuing to add individual links each time a new channel is needed; or we can install higher-bandwidth links and use each to carry multiple signals. In a multiplexed system, n lines share the bandwidth of one link. Figure 6.1 shows the basic format of a multiplexed system. The lines on the left direct their transmission streams to a multiplexer (MUX), which combines them into a single stream (many-to-one).At the receiving end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) and directs them to their corresponding lines. In the figure, the word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels. Multiplexing is the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer, etc. There are three basic multiplexing techniques: frequency-141 division multiplexing, wavelength-division multiplexing, and time-division multiplexing
Figure 8. Multiplexing Signals
Types of Multiplexing Techniques
- Frequency Division Multiplexing
Frequency Division Multiplexing (FDM) is an analog technique where each communications channel is assigned a carrier frequency. To separate the channels, a guard-band would be used. This is to ensure that the channels do not interfere with each other. For example, if we had our 3 terminals each requiring a bandwidth of 3 kHz and a 300 Hz guard-band, Terminal 1 would be assigned the lowest frequency channel 0-3 kHz, Terminal 2 would be assigned the next frequency channel 3.3 kHz-6.3 kHz and Terminal 3 would be assigned the final frequency channel 6.6 kHz-9.6 kHz. The frequencies are stacked on top of each other and many frequencies can be sent at once. The downside is that the overall line bandwidth increases. Individual terminal requirement were 3 kHz bandwidth each, in the above example: the bandwidth to transmit all 3 terminals is now 9.6 kHz.
Figure 9. Frequency Division Multiplexing
FDM does not require all channels to terminate at a single location. Channels can be extracted using a multi-drop technique; terminals can be stationed at different locations within a building or a city. FDM is an analog and slightly historical multiplexing technique. It is prone to noise problems and has been overtaken by Time Division Multiplexing which is better suited for digital data.
- Time Division Multiplexing
Time Division Multiplexing is a technique where a short time sample of each channel is inserted into the multiplexed data stream. Each channel is sampled in turn and then the sequence is repeated. The sample period has to be fast enough to sample each channel according to the Nyquist Theory (2x highest frequency) and to be able to sample all the other channels within that same time period. It can be thought of as a very fast mechanical switch, selecting each channel for a very short time then going on to the next channel. Each channel has a time slice assigned to it whether the terminal is being used or not. Again, to the send and receiving stations, it appears as if there is a single line connecting them. All lines 143 originate in one location and end in one location. TDM is more efficient, easier to operate, less complex and less expensive than FDM.
Figure 10. Time Division Multiplexing
- Wavelength Division Multiplexing
This technique is used in optical fiber. It is useful to increase the information carried by single optical fiber WDM can be view as an optical domain version of FDM in which multiple information signal modulate optical signals at different optical wavelength (colors). The resulting signals are combine and transmitted simultaneously over the same optical fiber as in the diagram below.
Figure 11. Wavelength Division Multiplexing
Various optical devices such as prisms and diffraction grating can be used to combine and split color signal. For instance early WDM systems combine 16 wavelengths at 2.5Gbps to provide an aggregate signal of 16 x 2.5Gbps. WDM systems 144with 32 wavelengths at 10gbps have a total bit rate of 320Gbps and are widely deployed. Systems that carry 160 wavelengths at 10Gbps are also available and achieved at amazing bit rate of 1.6 terabit/second. The attraction of WDM is that a huge increase in available bandwidth is obtained with less investment associated with deploying additional optical fiber. The additional bandwidth can be used to carry more traffic and can also provide the additional protection bandwidth refined by self-healing network topologies.
Yekini N. Asafe, Adebari F. Adebayo & Bello Olalekan. (2015). Data Communication & Networking. Lagos: YEKNUA ICT & Educational Research-Publication Centre No. 07.
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