Chapter 4 Signal Transmission Network Media

Signal Transmission Signaling—the way data is transmitted across the media Digital signaling Two discrete states 0 or 1, on or off Analog signaling Constantly changing electromagnetic waves

Digital Signaling Two different voltages are used. One voltage to represent on and another voltage to represent off

Analog Signaling Constantly changing electromagnetic wave Characteristics: Amplitude Strength of signal (height of wave) Frequency Amount of time needed to complete one cycle of the wave Phase Relative state of one wave in reference to another wave Can be divided into multiple channels to allow greater bandwidth

Attenuation/Noise Loss of signal strength as signal travels across media Signal must be amplified or regenerated to ensure that data is transmitted correctly Noise on media can disrupt data signal Static, cross-talk, etc. Electromagnetic interference (EMI) Radio frequency interference (RFI) Amplifiers and regenerators are used to increase the distance that signals can travel

Converting Analog to Digital Modems At sending computer, convert computer digital signal to analog signal for transmission via media At receiving computer, convert analog signal to digital signal Modulator/Demodulator

Transmission Direction Simplex—transmission of data in one direction only (television) Half-duplex—transmission of data in both directions but only one direction at a time (CB radio) Full-duplex—simultaneous sending and receiving of data (telephone) Full-duplex is used on data networks by using multiple channels one wire used for sending and another wire for receiving

Multiplexers Enables multiple signals to travel simultaneously by combining two or more separate signals and transmitting them together. Multiplexer (mux) at sending end combines signals and demultiplexer (demux) at receiving end separates signals Example: Cable TV--numerous signals travel through coaxial cable; circuitry in the TV, VCR, or cable box separate the signals into different channels

Multiplexing Methods Time Division Multiplexing Divides channel into time slots--each device is allotted a time slot Statistical Multiplexing Number of time slots allotted for each device varies depending on priority and need Wavelength Division Multiplexing Used for fiber-optic (light); different wavelength used for each channel

Throughput/Bandwidth Throughput is the amount of data that can be sent across the network media in a given time. Measured in bits per second Bandwidth is the range of frequencies that the media can transmit Frequently throughput and bandwidth are used interchangable Two methods for allocating bandwidth: Baseband Broadband

Baseband Transmission signals use the entire media bandwidth. Commonly used for digital signaling. Most LANs use baseband signaling

Broadband Media bandwidth is divided into multiple channels. Each channel can carry different analog signals Broadband networks support multiple simultaneous signals over a single transmission medium Cable TV is an example of broadband transmission. Many channels are broadcast over the same media all the time. You select which channel you want to watch.

Network Adapters (NICs or NACs) Provide the physical connection between your computer and the network media Transmit and receive data Prepare data for the network cable Send the data to another computer or device Control the flow of data between the computer and the network media

Media Characteristics Throughput/bandwidth Cost Installation—cable cost, ease of installation Maintenance—troubleshooting, repairing, replacing Scalability Node Capacity—max # of node, Attenuation—max length of segment, max number of segments Noise immunity electromagnetic interference, radio frequency interference, and eavesdropping Connectors

Network Media Anything that carries the message through the network Copper Coaxial cable Twisted-pair cable Glass Fiber-optic cable Wireless (Air) Radio waves Microwaves Infrared Transmission media—Physical path through which computers send and receive signals

Coaxial Cable (Coax) Copper core surrounded by insulation and a sheath Shielding makes it more resistant to interference Two types of coax cable used in networks Thinnet (10Base2)—RG 58 A/U Thicknet (10Base5)—RG 62 Thinnet Thicknet

Coaxial cable showing various layers Outer shield Insulation (PVC, Teflon) Conducting core Copper wire mesh or aluminum sleeve

Thicknet cable transceiver with detail of a vampire tap piercing the core Drop Cable

Coaxial Cable Terminators used at both ends of network to prevent signal bounce back Used with bus topology BNC cable connector used for Thinnet cable; n-connectors are used for Thicknet BNC T-connector BNC cable connector BNC Terminator

Coaxial Characteristics

Twisted-Pair Cable Similar to telephone wiring Color-coded pairs of wire; twisted and encased in plastic coating Unshielded twisted-pair and shielded twisted-pair cables UTP Twisted to reduce crosstalk (signal traveling one pair being hear by another pair or from another cable) More twisted the higher quality of wire (twist ratio). Shielded twisted pair has added layer of insulation such as a foil or metal shield STP Shielding

RJ-45 connector and jack

Twisted Pair Cable Categories Category 1-2: Voice communications and low speed data communications. Category 3: Suitable for computer networks. Data transmission rates up to 10 Mbps. Currently used for phone installations (home) Category 4: Data transmission rates up to 20 Mbps Category 5: Data transmission rates up to 100 Mbps. Very popular for LANs. Category 5E: Higher speeds are possible (200 Mbps); more twists Category 6: Speeds up to six times faster than Cat 5 Category 7: Speeds up to 1 Ghz (Gigabit)

UTP Characteristics Cost: Relatively inexpensive; depends on grade of copper and any enhancements Installation: Easy to install Bandwidth: 10-100 Mbps (Cat 5) Higher speeds are possible (up to 1000Mbps—Cat5E, Cat 6 and Cat 7) Node Capacity: Two (computer to hub) Attenuation: 100 meters EMI: Very susceptible to EMI and eavesdropping Connector: RJ-45

STP Characteristics Cost: Relatively inexpensive (more UTP, less than Thicknet or fiber-optic. Installation: Slightly more difficult than UTP Bandwidth: same as UTP Node Capacity: Two Attenuation: 100 meters EMI: Susceptible to EMI (but less susceptible than UTP) and eavesdropping Connector: RJ-45

Plenum Cable A plenum is the space between the false ceiling and the floor above. Plenum-grade cable is fire resistant and produces a minimum of smoke More expensive than PVC cable and less flexible May be required by fire code

Fiber-optic cable Contains one or more glass fibers (core) Data transmitted via pulsing light Two categories: Single-mode and multi-mode Optical fiber (core) Glass cladding One or more glass fibers surrounded by glass cladding and an outer sheath. Transmits light pulses -Single-mode cable—carries one frequency of light—faster and longer distances but more expenses -Multi-mode cable—carries several frequencies of light—more often used because its cheaper Fiber is expected to replace UTP in networks over the next decade Currently still more expensive and difficult to work with but prices are coming down. Fiber-optic connectors: ST and SC Protective outer sheath (jacket)

Fiber-optic Characteristics Cost: More expensive than copper cable Installation: More difficult than copper cable Bandwidth: 100 Mbps to 1 Gbps Node: 2 Attenuation: Several kilometers EMI: Not subject to EMI; immune to eavesdropping Connectors: ST and SC are popular

Cable Media Comparison

Wireless Media (Atmospheric) Infrared Radio Frequency (RF) Narrow-band Spread-spectrum Microwaves

Wireless portable computer using an infrared light beam to print

Infrared Transmission Systems Use infrared light to transmit signals Point-to-Point (Direct) or Broadcast (Indirect) Point-to-Point (Direct)—highly focused and directed at a specific target Line of Site Broadcast—spread the signal to cover a wider area and allow reception of signal by several receivers Signal can be bounced off walls and ceilings

Infrared Characteristics Line of site Light must be able to reach target Bandwidth: 100 Kbps to 16 Mbps Tested at up to 100Mbps but slower speeds are currently standard Attenuation: Depends upon the quality of light and atmospheric conditions EMI: Can be affected by intense light. Point-to-Point transmissions are fairly immune to eavesdropping. However, broadcast transmission are more easily intercepted.

Wireless portable computer connecting to a cabled network access point

Radio Frequencies Characteristics

Wireless bridge connecting two LANs

Microwave Transmission Terrestrial Microwave Line of site (max distance ~ 23 miles) Transmission can be affected by atmospheric conditions (rain/fog). Vulnerable to EMI, jamming and eavesdropping Bandwidth 1-10 Mbps Satellite Microwave Can transmit data over vast distances Extremely expensive if you put up your own satellite

Example of Network Wiring Horizontal cabling is the twisted-pair or fiber network media that connects the workstations and the wiring closets. Workstation area cable (or patch cable) connects the workstation to the wall outlet, and the telecommunications connector is found at the wall outlet. Horizontal cable runs between the wiring closet and the wall outlet. Last, horizontal cross connects connect devices in the wiring closet. patch cable

Example Wiring Closet Patch cables used between workstation and wall jack Cable (often plenum grade) runs in walls and ceilings to patch panels Patch cables run from patch panels to hubs, switches, and routers Different color cable is often used to signify different cable runs In a large network, several closets containing the patch panels and hubs may be located throughout the building(s) Often fiber-optic cable is used between floors or buildings to increase speed (backbone)