1 nd semester King Saud University College of Applied studies and Community Service 1301CT

Outline Communication system Communication system model Mode of communication Data transmission mode Effectiveness of a communications system. Channel bandwidth. Signal power. Communication system resources. 2

Long Distance Communication Communication is the transmission of information. Some examples of past communication methods: 3

Communication System These methods of communication have been superseded by electrical communication systems, where the communication is by using electrical signal. Communication system is a combination of circuits and devices put together to accomplish the transmission of information from one point to another. i.e. the purpose of a communication system is to carry information from one point to another. 4

Examples of Communications Systems 5

Communications System Model A typical communication system can be modeled as Input Transducer Transmitter ChannelReceiver Output Transducer Distortion And Noise Source Input message Input signal Transmitted signal Received signal Output signal Output message Destination Sender/ Transmitter Receiver 6

Source Source: Produces an input message( voice, picture, computer data etc ). There are many different types of sources and there are different forms for messages. In general, input messages may be classified as analog or digital. Analog messages (such as voice, music, temperature, …) are represented by continuous-time variables while discrete messages (such as text or numeric data) are represented by discrete symbols 7

Input Transducer If the input message is nonelectrical ( e.g. voice), it must be converted by an input transducer to an electrical signal (the Baseband signal / Message Signal) A transducer, in simple terms is a device that converts one form of energy into another. In the communication system, it convert the output of a source into an electrical signal that is suitable for transmission; e.g., a microphone and a camera. 8

Transmitter The transmitter converts the electrical signal (the Baseband signal/Message Signal) into a form that is suitable for transmission through the transmission medium or channel by a process called modulation. 9

Channel Channel: medium used to transfer signal from transmitter to receiver Channel can be wired or wireless. While the signal is travelling through the channel(the medium) it is always attenuated (and the level of attenuation increases with distance). Also, the signal shape may be changed during the transmission i.e. become ‘distorted’. 10

Noise The signal is not only distorted by a channel, but it is also contaminated along the path by undesirable signals lumped under the broad term noise, which are random and unpredictable signals from causes external ( such interference from signals transmitted on nearby channels) and internal ( such noise resulted from thermal motion of electrons in conductors). 11

Receiver The function of the receiver is to recover the message signal contained in the signal received from the channel (received signal). The received signal is a corrupted version of the transmitted signal. So, the receiver reconstruct a recognizable form of the original message signal. It reprocess the received signal by undoing the signal modifications ( demodulation) made at the transmitter and the channel. 12

Output Transducer The receiver output is fed to the output transducer, which convert the electrical signals that are received into a form that is suitable for the final destination; e.g., speaker, monitor, etc. 13

Mode of Communication There are two basic mode of communication: 1- Broadcasting  single transmitter and numerous receiver. 2- Point to point communication  single transmitter and single receiver. 14 Broadcasting Point to point

Data Transmission Mode Data transmission between two devices can be: Simplex Half- duplex Full- duplex 15

Data Transmission Mode In simplex mode, the communication is unidirectional. Only one of the two devices on a link can transmit; the other can only receive. In half-duplex mode, each device can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. In full-duplex mode (also called duplex), both devices can transmit and receive simultaneously. 16

Effectiveness of Communications System The effectiveness of a communications system depends on four fundamental characteristics: Delivery AccuracyTimelinessJitter 17

Effectiveness of a Communications System Delivery: The system must deliver information to the correct receiver. Information must be received by the intended device or user and only by that device or user. Accuracy: The system must deliver the information accurately. Information that have been altered in transmission and left uncorrected are unusable. 18

Effectiveness of a Data Communications System Timeliness: The system must deliver information in a timely manner. Information delivered late are useless. In the case of video and audio, timely delivery means : delivering data in the same order as they are produced, and without significant delay. This kind of delivery is called real-time transmission. 19

Effectiveness of a Data Communications System Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 3 ms. If some of the packets arrive with 3 ms delay and others with 4 ms delay, an uneven quality in the video is the result. 20

The Rate and Quality of Data Transmission The fundamental parameters that control the rate and quality of data transmission in the communication system are: Channel bandwidth. Signal power. 21

Channel Bandwidth The channel can transmit a range of frequencies with reasonable fidelity. The bandwidth of a channel (B) is the width of the frequency band used to transmit the data. It is the difference between the highest and lowest frequencies which the channel can carry. 22

Channel Bandwidth If a channel can carry a signal which its frequency range from 0 to 5000 Hz (5 khz), the channel bandwidth B is 5 khz. If a channel can carry frequencies between 200Hz and 4kHz, its bandwidth (the difference between those two frequencies) is 3.8kHz. If a channel can carry frequencies between 10MHz and 100MHz, what is the channel bandwidth? 23

Channel Bandwidth The rate of data transmission is directly proportional to B. 24

Signal Power The power of a signal is defined as the average energy over time. The signal power is related to the quality of transmission. Increasing the signal power ( S ), reduces the effect of channel noise, and the information is received more accurately. We measure noise relative to a signal in terms of the signal-to-noise ratio (SNR). 25

Signal Power The SNR is the ratio of the average signal power to the average noise power. A larger SNR also allows transmission over a longer distance. In any event, a certain minimum SNR is necessary for communication. 26

Communication System Resources So, in a communication system there are tow primary resources : Channel bandwidth. Power A general system design objective is to use these resources as efficiently as possible. 27

Communication System Resources Based on the channel bandwidth, the communication system can be classified as: Baseband communication system  channel bandwidth 0 to f Bandpass communication system  channel bandwidth f 1 to f 2 28

Communication System Resources There are two types of power in a communication system: Transmit power : is the average power of the transmitted signal Received power : the average power of the received signal 29

Communication System Resources Generally, system performance will be better if there is high transmitted power. Practical constraints on cost, properties of the transmission medium, battery life imply low transmitted power. The received power is a function of the transmitted power and the channel. A larger distance between the transmitter and the receiver  lower received power 30