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Physical Layer. Topic 3. Outline. Circuit & Media Digital Transmission of Digital Data Analog Transmission of Digital Data Digital Transmission of Analog Data Multiplexing. Circuits & Media. Network Configuration. Network configuration is the basic physical layout of the network.
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Physical Layer Topic 3
Outline • Circuit & Media • Digital Transmission of Digital Data • Analog Transmission of Digital Data • Digital Transmission of Analog Data • Multiplexing
Network Configuration Network configuration is the basic physical layout of the network. There are two fundamental network configurations: • Point-to-point configuration (or two-point) - sometimes called dedicated circuits. • Multipoint configuration (or multidrop). Most complex computer networks have many circuits, some of each type.
Network Configuration Point-to-point configuration
Data Flow Circuits can be designed to permit data to flow in one or both directions. There are three ways to transmit: • Simplex - One way transmission • Half-duplex -Two way communications link, but only one system can talk at a time. • Full duplex -Transmit in both directions simultaneously.
Communication Media The medium is the matter or substance that carries the voice or data transmission. There are two basic types of media: • Guided media - those in which the message flows through a physical media. • Radiated media (unguided) - Those in which the message is broadcast through space. Circuits sold by the common carriers are called communications services.
Guided Media • Twisted Pair Wire - insulated pairs of wire, twisted to minimize electromagnetic interference between wires. • Coaxial Cable - wire with a copper core and an outer cylindrical shell for insulation. • Fiber Optic Cable - high speed streams of light pulses from lasers or light-emitting diodes (LEDs) carry information inside hair-thin strands of glass or plastic called optic fibers.
Fiber Optic Cable The earliest fiber optic systems were multimode, (light could reflect inside the cable at many different angles). Single mode fiber optic cables transmit a single direct beam of light through a cable that ensures the light only reflects in one pattern. Fiber optic technology is a revolutionary departure from the traditional message-carrying systems of copper wires.
Radiated Media Radio (wireless) data transmission uses the same basic principles as standard radio transmission. Infrared Transmission uses low frequency light waves to carry data through the air on a direct line-of-sight path between two points.
Radiated Media A microwave is an extremely high frequency radio communication beam that is transmitted over a direct line-of-sight path between two points. Transmission via satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space.
Radiated Media One disadvantage of satellite transmission is the delay that occurs because the signal has to travel out into space and back to Earth (propagation delay, about 0.5 second, 186,000 miles/second). One problem associated with some types of satellite transmission is raindrop attenuation (some waves at the high end of the spectrum are so short they can be absorbed by raindrops).
Media Selection Guided Media Radiated Media Network Transmission Error Media Type Cost Distance Security Rates Speed Twisted Pair LAN Low 100-300M Good Low 1-100Mbps Coaxial Cable LAN Mod. 200-500M Good Low 1-100Mbps Fiber Optics any High up to 75Mile V. Good V.Low 10Gbps Network Transmission Error Media Type Cost Distance Security Rates Speed Radio LAN Low Short Poor Mod 1-4Mbps Infrared LAN, BN Low Short Poor Mod 1-4Mbps Microwave WAN Mod Long Poor Low-Mod 50 Mbps Satellite WAN Mod Long Poor Low-Mod 50 Mbps
Data There are two fundamentally different types of data: • Digital -Computer produced signals that are binary, either on or off. • Analog - Electrical signals which are shaped like the sound waves they transfer.
Bandwidth on a Voice Circuit Every sound wave has two parts, half above the zero point (positive), and half below (negative) and three important characteristics. • The height of the wave is called amplitude. • The length of the sound wave is expressed as the number of waves per second or frequency, expressed in Hertz (Hz). • The phase is the direction in which the wave begins. Bandwidth refers to a range of frequencies.
Bandwidth on a Voice Circuit Frequency: 1 Period/Sec = 1 Hertz
Bandwidth on a Voice Circuit Phase Frequency: 1 Period/Sec = 1 Hertz
Bandwidth on a Voice Circuit Human hearing ranges from about 20 Hz to about 14,000 Hz (some up to 20,000 Hz). Human voice ranges from 20 Hz to about 14,000 Hz. The bandwidth of a voice grade telephone circuit is 0 to 4000 Hz or 4000 Hz (4 KHz). Guardbands prevent data transmissions from interfering with other transmission when these circuits are multiplexed using FDM.
Bandwidth on a Voice Circuit It is important to note that the limit on bandwidth is imposed by the equipment used in the telephone network. The actual capacity of bandwidth of the wires in the local loop depends on what exact type of wires were installed, and the number of miles in the local loop. Actual bandwidth in North America varies from 300 KHz to 1 MHz depending on distance.
Data Transmission Devices Data can be transmitted through a circuit in the same form they are produced, or converted from one form into the other for transmission over network circuits. Modem (Modulator/demodulator) translates digital data into analog form for transmission over analog circuits. Codec (Coder/decoder) translates analog voice data into digital form for transmission over digital circuits.
Benefits of Digital Transmission Digital transmission offers five key benefits over analog transmission. • Digital transmission produces fewer errors than analog transmission. • Digital transmission is more efficient. • Digital transmission permits higher maximum transmission rates. • Digital transmission is more secure because it is easier to encrypt. • Finally, and most importantly, integrating voice, video and data on the same circuit is far simpler with digital transmission.
Broadband and Baseband • In an analog system data are represented by measurements on a continuous scale. Analog is also called broadband. • In contrast, digital data can only take on specific discrete values. Digital is also called baseband.
Data Transmissions Analog data Analog Transmission Modem Codec Digital data Digital Transmission We use signals to send data in different transmission methods.
Digital Transmission of Digital Data All computers produce binary data. For this data to be understood by both the sender and receiver, both must agree on a standard system for representing the letters, numbers, and symbols that comprise the messages.
Coding Character: A symbol that has a common, constant meaning. Characters in data communications, as in computer systems, are represented by groups of bits[1’s and 0’s]. The group of bits representing the set of characters in the “alphabet” of any given system are called a coding scheme, or simply a code.
Coding A byte consists of 8 bits that is treated as a unit or character. (Some Asian languages use 2 bytes for each of their characters, such as Chinese.) (The length of a computer word could be 1, 2, 4 bytes.) There are two predominant coding schemes in use today: • United States of America Standard Code for Information Interchange (USASCII or ASCII) • Extended Binary Coded Decimal Interchange Code (EBCDIC)
Transmission Modes Parallel Mode: Data are transferred simultaneously in groups of bits. It is the way the internal transfer of binary data takes place inside a computer. Serial Mode: Data are sent one bit after another. It is the predominant method of transferring information in data communications.
Transmission Modes Serial Mode
Baseband Transmission Digital Transmission: the transmission of electrical pulses. Digital Information: It has only two possible states 1 or 0, or binary values. Baseband Signals: i.e. Digital signals. Data Rate: In order to successfully send and receive a message, both the sender and receiver have to agree how often the sender can transmit data.
Baseband Transmission With unipolar signaling technique, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct.
Baseband Transmission Manchester encoding is a special type of unipolar signaling in which the signal is changed from a high to low or low to high in the middle of the signal. Manchester encoding is less susceptible to having errors go undetected, because if there is no transition, the receiver knows that an error must have occurred. Manchester encoding is commonly used in local area networks (Ethernet, token ring).
Manchester Encoding “00001110” in a 10BASE-T circuit
*Differential Manchester • Midbit transition is only for clocking • Transition at beginning of bit period=0 • Transition absent at beginning=1 • Has added advantage of differential encoding • Used in token-ring
Modulation Modulation is the technique that modifies the form of a digital electrical signal so the signal can carry information on a communications media. Three fundamental methods: • Amplitude Modulation (AM)(also called Amplitude Shift Keying, or ASK) • Frequency Modulation (FM)(also called Frequency Shift Keying, or FSK) • Phase Modulation(PM)(also called Phase Shift Keying, or PSK)
