Physical Layer Concepts PDF

# Unit-IV ## Physical Layer ### What is the Physical Layer The physical layer is the lowest layer of the OSI model. Before sending any data on the network, the physical layer on the local node must process the raw data stream, translating frames received from the data link layer into electrical,...

# Unit-IV ## Physical Layer ### What is the Physical Layer The physical layer is the lowest layer of the OSI model. Before sending any data on the network, the physical layer on the local node must process the raw data stream, translating frames received from the data link layer into electrical, optical, or electromagnetic signals representing 0 and 1 values or bit frames. It will incorporate both the data and control information. The local physical layer is responsible for transmitting these bit sequences through the network medium to the physical layer of the remote node, where frames are reconstructed and passed to the remote node data link layer. The transmission medium used for data communications, including both wired and wireless environments, is defined by physical layer protocols and specifications. The type of cables or connectors used, the electrical signals associated with each pin and connectors called pin outs and pin assignments, and the manner in which bit values are converted into physical signals. An example of physical layer specifications is the EIA RS-232C, which defines the electrical and physical characteristics used in several communications. - RS-232 C specifies the 25-pin data bus connector that serves as an interface between a computer referred to as the DTE (data terminal equipment). - Later version is RS 232 C standard is RS 423 which defines a 9-pin DB connector. ### Physical Layer Concepts #### DB-9 | Source | Description | |---|---| | DCE | Data Carrier Detector (DCD) | | DCE | Received Data (RD) | | DTE | Transmitted Data (TD) | | DTE | Data Terminal Ready (DTR) | | Common | Ground (GND) | #### DB-25 | Source | Description | |---|---| | DCE | Secondary Clear to Send | | DCE | Secondary Received Line Signal Detector | | DCE | Not Defined | | DCE | Not Defined | | DCE | Not Defined | | DCE | Received Line Signal Detector| | Common | Signal Ground | | DCE | Data Set Ready | | DCE | Clear to Send | | DCE | Request to Send | | DCE | Received Data | | DTE | Transmitted Data | | Common | Protective Ground (shield) | ### The physical and electrical characteristics of a wire: All physical media regardless of their type share three physical elements: - **Conductor**: The conductor serves as a medium for the physical signal. The conductor is composed of copper wire or glass or plastic fiber. In the case of copper wire, it can be stranded (composed of several thin wires). We can measure thickness in terms of gauges. The lower the gauge the thicker the wire. The 22 gauge wire is more thicker than 24 gauge wire. We can measure in terms of AWG (American wire gauge). - **Insulator**: The insulating material surrounding the conductor. It serves as a barrier to the conductor by preventing the signal from escaping and preventing electrical interference in entering. Finally, the conductor and insulator are encased in an outer sheath or jacket. PVC and Teflon are the materials used as insulating materials. Teflon is fire resistant; it takes much time to get into a burning point. The below diagram shows the physical composition of two commonly used network cables: Unshielded twisted pairs and Shielded twisted pairs. ![]() - **Figure 4.2**: A UTP cable (a) and an STP cable (b). Pairs of wires are twisted around each other. One pair is used to transmit data; a second pair is used to receive data. Note the extra shielding in the STP cable. ### Twisted pairs The twisted pairs consist of at least two insulated copper wires that have been twisted together. #### Shielded Twisted Pairs - In STP because of braided shield and foil metal shield, it is less susceptible to electrical interference and noise. - Twisted-pair is a type of cabling that is used for telephone communications and most modern Ethernet networks. - A pair of wires forms a circuit that can transmit data. The pairs are twisted to provide protection against crosstalk, the noise generated by adjacent pairs. - There are two basic types, shielded twisted-pair (STP) and unshielded twisted-pair (UTP). #### Unshielded Twisted Pairs - Consists of four pairs (eight wires) of insulated copper wires typically about 1 mm thick. - The wires are twisted together in a helical form. - Twisting reduces the interference between pairs of wires. - High bandwidth and High attenuation channel. - Flexible and cheap cable. - Category rating based on the number of twists per inch and the material used. - CAT 3, CAT 4, CAT 5, Enhanced CAT 5, and now CAT 6. ### Coaxial Cables - Coaxial cable is a copper-cored cable surrounded by a heavy shielding and is used to connect computers in a network. - Outer conductor shields the inner conductor from picking up stray signal from the air. - High bandwidth but lossy channel. - Repeater is used to regenerate the weakened signals. ### Fibre Optic Cables - Fibre optic cable consists of a glass fibre covered by a plastic buffer coating and surrounded by Kevlar fibres. The Kevlar fibre gives the cable its strength. These are used for bullet proof vests and combat helmets. - Optical fibers use light to send information through the optical medium. - It uses the principal of total internal reflection. - Modulated light transmissions are used to transmit the signal. ![]() - **The electrical characteristics of a wire:** The performance of the wired network is greatly dependent on the electrical characteristics of the cable used. The characteristics are: Capacitance, Impedance and Attenuation. #### Capacitance - Capacitance is the property of a circuit that permits it to store an electrical charge. - The capacitance of a cable determines its ability to carry a signal without distortion, which is rounding of the waveform due to stored charge between the conductors of a cable. - The more distorted the signal becomes the more likely a receiving node will be unable to distinguish between 0's and 1's. - High quality cable has low capacitance, the lower the capacitance the longer the distance a signal can travel before signal distortion becomes unacceptable. - Network cables can have low characteristicts capacitance per meter; the overall capacitance of a cable increases as the cable gets longer. - Because of noise and other problems in the transmission, a maximum cable length of about 100m exists for for unshielded twisted pairs network cable. ![]() - **Figure 4.5**: Capacitance eventually will distort a transmitted signal. Source: adapted & Chorey, 1991a. #### Impedance - Impedance is a measure of the opposition to the flow of electrical current in an alternating current circuit. - It is measured in ohms. - Impedance is a function of capacitance, resistance and inductance. - Impedance mismatches, caused by mixing cables of different types with different characteristicts impedances, can result in signal distortion. - For example token ring network cable requires 150 Ω of impedance. - Ethernet and twisted pair networks want 85-111 Ω. #### Attenuation - Attenuation is a decrease in signal strength, which occurs as the signal travels through a circuit or along a cable. - The longer the cable the greater the attenuation. - The higher the frequency the greater the attenuation. - Different types of cables are also subject to different amounts of attenuation. - In the twisted pairs the attenuation rises sharply as the signal frequency increases. - In the coaxial cables it rises less sharply as frequency increases. - It is measured in decibels of signal loss. ![]() - **Figure 4.6**: The combined effects of capacitance and attenuation result in a signal that is received distorted and weaker than what it was when transmitted. This can severely impact the performance of a network. Source: adapted froen Leeds & Chimey, 1991. ### Copper Media It consists of: - STP - UTP - IBM CABLE - COAXIAL CABLE ### Unshielded and Shielded Twisted Pair Cable - Twisted pairs are the most popular type of cable used in networks today. - Twisted pair cable consists of two insulated copper wires that have been twisted together. - Data transmission requires four wires. - One pair to transmit data and pair to receive data - Two types of cables are there - Unshielded - Shielded ### Standards of the UTP and STP Standards of the UTP and STP are provided by EIA/TIA-568 which is North American standard used worldwide. | Category | Description | |---|---| | Category 1 | Used for voice transmission; not suitable for data transmission. | | Category 2 | Low-performance cable; used for voice and low-speed data transmission; has capacity of up to 4 Mbps. | | Category 3 | Used for data and voice transmission; rated at 10 MHz; voice-grade; can be used for Ethernet, Fast Ethernet, and token ring. | | Category 4 | Used for data and voice transmission; rated at 20 MHz; can be used for Ethernet, Fast Ethernet, and token ring. | | Category 5 | Used for data and voice transmission; rated at 100 MHz; suitable for Ethernet, Fast Ethernet, Gigabit Ethernet, token ring, and 155-Mbps ATM. | | Enhanced Category 5 | Same as Cat 5 but manufacturing process is refined; higher-grade cable than Cat 5; rated at 200 MHz; suitable for Ethernet, Fast Ethernet, Gigabit Ethernet, token ring, and 155-Mbps ATM. Also known as Category SE. Became a TIA standard in late 1990. | | Category 6 | Not yet a TIA standard, but general specifications are expected to include: 250-MHz rating; suitable for Ethernet, Fast Ethernet, Gigabit Ethernet, token ring, and 155-Mbps ATM. Should also be able to handle 550-MHz broadband video and 622-Mbps, 1.2-Gbps, and 2.4-Gbps ATM. | | Category 6 (Class E) | Similar to Category 6 but is a proposed international standard to be included in ISO/IEC 11801. | | Category 6 (STP) | Shielded twisted-pair cable; rated at 600 MHz; used for data transmission; suitable for Ethernet, Fast Ethernet, Gigabit Ethernet, token ring, and high-speed ATM. | | Category 7 | Not yet a TIA standard, but general specifications are expected to include: 600-MHz rating; capable of achieving higher speeds than Category 6. Will probably require new connectors instead of current RJ-45 connectors. | | Category 7 (Class FF) | Similar to Category 7 but is a proposed international standard to be included in ISO/IEC 11801. | **Categories 3 and 5 mostly used for voice transmission** UTP poses two main problems in data transmission at the higher frequencies: - Cross talk - Attenuation The combined effects of cross talk and distortion results in the irregular variation in the shape or timing of a signal. This irregular variation is called jitter. Jitter is mainly caused by shielded and unshielded cable. ### IBM CABLE IBM has its own classification cable, the IBM cable system which specifies nine cable types. ### TYPE It is a grouping of categories and fiber optic cables in a bundle, based on which type is being conducted. A category is an EIA specification for the cables construction. | Type | Description | |---|---| | Type 1 | 2-pair STP, 22-gauge solid wire; used for token ring networks. | | Type 2 | Contains UTP and STP; 4-pair UTP, 22-gauge solid wire used for voice. | | Type 3 | 2-pair UTP, 22-gauge solid wire used for data; 2-, 3-, or 4-pair UTP cable with 22- or 24-gauge solid wire; pairs must have a minimum of 2 twists/foot; voice-grade only.| | Type 4 | Not defined. | | Type 5 | Fiber-optic; 2 glass fiber cores at 100/140 micron; 62.5/125 micron fiber also allowed and is recommended by IBM; used as main ring of a token ring network. | | Type 6 | 2-pair STP, 26-gauge stranded wire; used mostly as a patch cable to connect a node to a network. | | Type 7 | Not defined. | | Type 8 | 2-pair STP, 26-gauge flat solid wire; designed for under-carpet installations. | | Type 9 | 2-pair STP, 26-gauge solid or stranded wire; contains a plenum outer jacket; used for between-floor runs. | ### Coaxial cables ![]() | Category | Impedance | Use | |---|---|---| | RG-59 | 75 W | Cable TV | | RG-58 | 50 W | Thin Ethernet | | RG-11 | 50 W | Thick Ethernet | Another type of copper cable is coaxial cable. In computer networking, coax is described as either thick or thin. - Thick coaxial is used as the medium for thick Ethernet, which is knows as IEEE 802.3 10 Base5. - Thin coaxial cable is used as the medium for "Thin Ethernet," which is known as IEEE 802.3 10 Base2. In analog coaxial networks such as residential cable television networks, cable such as RG-9 may be used. - RG-59 with an impedance of 75 OHMS is used for home TV cable, but looks almost the same as RG-58. All the cables are not the same; we should select the right one for the types of network equipment being considered for use. - A base band network transmits the digital signals directly without modulating their transmission. - A base band network is capable of transmitting only a single stream of data. That means the transmission medium uses the entire bandwidth to carry a single signal. - It doesn't mean, however, that the channel cannot be shared. - Using multiplexing techniques such as TDM, nodes connected to a base band network can share the medium, but they can only transmit when the channel is not busy. - The transmission media of a Base band network can include twisted pair cable, coaxial cable, and fiber optic cable. - Various topologies are also available including star, ring, and bus. **Three examples of Base Band networks are:** - 10 Base 5 > 500 M - 10 Base 2 > 200 M - 10 Base T > UTP - **Base means Base band LAINS** - **The 10 refers to 10 Mbps speed** **Broad Band network, it uses FDM (Frequency Division Multiplexing)** - To divide the channels band width into smaller and distinct channels, which can be used concurrently to transmits different signals. - It is capable of transmitting voice data and video signals over the same cable. ### Fibre optic media - It carries data signals in the form of modulated light beams. The electrical signals from the sending computer to the receiving computer are converted into optical signals by a light source-LED or a laser. - With the LED, the presence of light represents 1 and the absence of light represents 0. - With a laser source which emits the complete low level of light, a 0 is represented by low level and a 1 is represented by a high intensity pulse. This modulation technique is called as intensity modulation. The light pulses enter one end of the fibre and travel through the fibre and exit at the other end. The received light pulse is converted back to the electrical signals via a photo detector, which is a tiny solar cell. ![]() - Multimode is so named because multiple beams from a light source move through the core in different paths. #### Multimode Step-Index Fibre - In multimode step-index fiber, the density of the core remains constant from the center to the edges. - A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding. - Step index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber. #### Multimode Graded-Index Fibre - A graded-index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge. #### Single-Mode - Single-mode uses step-index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal. - Propagation of different beams is almost identical, and delays are negligible. - All the beams arrive at the destination "together" and can be recombined with little distortion to the signal. ![]() ### Applications - Fiber-optic cable is often found in backbone networks. - Cable TV companies use a combination of optical fiber and coaxial cable, thus creating a hybrid network. - Local-area networks such as 100Base-FX network (Fast Ethernet) and 1000Base-X also use fiber-optic cable. ### Wireless communications - In wireless communications, signals travel through space instead of through a physical cable. There are two general types of wireless communications: radio transmission and infrared transmission. ![]() - **Figure 4.9**: The electromagnetic spectrum (in Hz). Higher frequencies support greater bandwidth. Source: adapted from Breidenbach, 1990. #### Propagation Methods - **Ground propagation**: In ground propagation, radio waves travel through the lowest portion of the atmosphere, hugging the earth. - **Sky propagation**: In sky propagation, higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to earth. - **Line-of-sight propagation**: In line-of-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other. #### Wireless Transmission - **Radio waves**: Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are called radio waves. - Radio waves, for the most part, are omni directional. - When an antenna transmits radio waves, they are propagated in all directions. - The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency. - Radio waves, particularly those of low and medium frequencies, can penetrate walls. - Applications: AM and FM radio, television, maritime radio, cordless phones, and paging are examples of multicasting. Radio waves are used for multicast communications, such as radio and television, and paging systems. - **Microwaves**: Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves. - Microwaves are unidirectional. - Sending and receiving antennas need to be aligned. - Microwave propagation is line-of-sight. - Very high-frequency microwaves cannot penetrate walls. - Applications: Microwaves, due to their unidirectional properties, are very useful when unicast (one-to-one) communication is needed. Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. - **Infrared**: Infrared waves, with frequencies from 300 GHz to 400 THz, can be used for short-range communication. - Infrared waves, having high frequencies, cannot penetrate walls. - Applications: Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation. A wireless keyboard to communicate with a PC. #### Wireless LAN standards - Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE. - Intended for home or office use (primarily indoor). - 802.11 standard describes the MAC layer, while other substandards (802.11a, 802.11b) describe the physical layer. - Wireless version of the Ethernet (802.3) standard. ![]() - **Base Station**: All communication through an Access Point (AP) {note hub topology}. Other nodes can be fixed or mobile. - **Infrastructure Wireless**: AP is connected to the wired internet. - **Ad Hoc Wireless**: Wireless nodes communicate directly with one another. - **MANETS (Mobile Ad Hoc Networks)**: ad hoc nodes are mobile. ![]() - **Physical layer conforms to OSI (five options)** - 1997: 802.11 infrared, FHSS, DSSS {FHSS and DSSS run in the 2.4GHz band}. - 1999: 802.11a OFDM and 802.11b HR-DSSS. - 2001: 802.11g OFDM. - **802.11 Infrared** - Range is 10 to 20 meters and cannot penetrate walls. - Does not work outdoors. - **802.11 FHSS (Frequency Hopping Spread Spectrum)** - The main issue is multipath fading. - The idea behind spread spectrum is to spread the signal over a wider frequency to minimize the interference from other devices. - 79 non-overlapping channels, each 1 Mhz wide at the low end of 2.4 GHz ISM band. - The same pseudo-random number generator used by all stations to start the hopping process. - Dwell time: minimum time on channel before hopping (400msec). - **802.11 DSSS (Direct Sequence Spread Spectrum)** - The main idea is to represent each bit in the frame by multiple bits in the transmitted signal (i.e., it sends the XOR of that bit and n random bits). - Spreads signal over the entire spectrum using pseudo-random sequence (similar to CDMA see Tanenbaum sec. 2.6.2). - Each bit transmitted using an 11-bit chipping Barker sequence, PSK at 1Mbaud. This yields a capacity of 1 or 2 Mbps. ### Satellite Communications - Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means. - The two stations can use a satellite as a relay station for their communication. - One Earth Station sends a transmission to the satellite. This is called a Uplink. - The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink. - The transponder, a type of repeater, listens to some part of the spectrum. When it hears an incoming signal, it amplifies the signal and then rebroadcasts it at a different frequency. The downward signals can cover a large or narrow area. ![]() ### Types of Satellites - **GEO**: Geostationary Earth Orbit (GEO) - These satellites are in orbit 35,863 km above the earth's surface along the equator. - Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth. - Advantages: - A GEO satellite's distance from earth gives it a large coverage area, almost a fourth of the earth's surface. - GEO satellites have a 24-hour view of a particular area. These factors make it ideal for satellite broadcast and other multipoint applications. - Disadvantages: - A GEO satellite's distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication. - GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions. - **LEO**: Low Earth Orbit (LEO) - LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface. - LEO satellites don't stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass. - A network of LEO satellites is necessary for LEO satellites to be useful. - Advantages: - A LEO satellite's proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication. - A LEO satellite's smaller area of coverage is less of a waste of bandwidth. - Disadvantages: - A network of LEO satellites is needed, which can be costly. - LEO satellites have to compensate for Doppler shifts cause by their relative movement. - Atmospheric drag effects LEO satellites, causing gradual orbital deterioration. - **MEO**: Medium Earth Orbit (MEO) - A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth's surface. - MEO satellites are similar to LEO satellites in functionality. - MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours. - MEO satellites have a larger coverage area than LEO satellites. - Advantage: - A MEO satellite's longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network. - Disadvantage: - A MED satellite's distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite. ### Structured cabling systems A structured cabling system comprises six subsystems: - Building entrance - Equipment room - Backbone cabling - Telecommunications closet - Horizontal cabling - The building entrance provides inter-building connectivity. This is where an organization's overall main network trunk line interconnects with a building communication facilities so that LANs within the building have connectivity throughout the enterprise. - The equipment room is the heart and soul of the building networks infrastructure network. It contains equipment that provides connectivity to other buildings as well as telecommunications closets located on each floor of the building. - A building backbone cabling interconnects the buildings telecommunications closets, equipment rooms, and entrance. Thus the backbone cable serves as the main trunk line for network connectivity. The specified backbone cabling topology is a hierarchical star. - A telecommunications closet, commonly called a wiring closet, houses a building's telecommunications equipment and is where cable is terminated or where cross connects are made. Most buildings have one communication closet for the floor, and they are interconnected by a backbone cable. - The horizontal cable extends from the work area to the telecommunications closet and is based on a star topology. The horizontal cable consists of cable itself the wall outlet (formerly called telecommunications outlet), cable terminations, and cross connections. ![]() *****THE END******