Network Topology Network Topology


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Network Topology Network Topology

  1. 1. Network Topology SETHALAT RODHETBHAI Updated: 01/11/2002 CONEREL COmputer NEtwork REsearch Laboratory COmputer NEtwork REsearch Computer Science, Silpakorn University CONEREL 2 Physical Topologies The term topology can be confusing. A network’s network’ physical topology is the physical layout of the networking components. There is also a logical topology that is discussed later. The three most common topologies are the bus, star, bus, star, and ring. ring. There are other physical topologies such as the mesh and tree, however, we explores only the three mentioned tree, above. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 1
  2. 2. CONEREL 3 Bus Topology A bus topology connects all stations in a linear fashion. Sometimes it Sometimes is called daisy-chaining. daisy- chaining. The wire on a bus network has two distinct end points, which are capped by terminators. Terminators absorb electronic signals so that they are not reflected reflected on the network. The termination requirement of the bus topology is extremely important and is revisited quite often in discussions of network topology and troubleshooting. On a bus topology, all stations receive the signals transmitted by other stations. Network signals travel in both directions along the bus and each station checks the data frame as it passes by. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 4 Bus Topology (cont.) Although the stations are all on one cable segment, the bus segment, continues to work when stations are off line because the media does does not actually pass through the individual stations. The bus topology has several advantages: It is inexpensive. It is easy to design and implement (you simply daisy-chain the stations daisy- together). The bus topology has its disadvantages: It is difficult to troubleshoot. If a cable break occurs at any location on the bus, the entire bus fails and it can be difficult to locate the point of failure. It requires termination. If the last computers on the chain are not correctly terminated, stations cannot transmit. Traffic problems and collisions are common SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 2
  3. 3. CONEREL 5 Bus Topology (cont.) SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 6 Star Topology The star network configuration is the most popular topology. In a star configuration, all computers or stations are wired directly to a directly central location, which is usually a hub or a MAU. MAU. Depending on the type of networking device used at the center of the star network, collisions can be a problem. A data signal from any station goes directly to this central device, which device, transmits the signal according to the established network access method for the type of network. The star topology has several advantages: A break in one cable does not affect all other stations as in bus technologies, bus because there is only on station per segment. This means the star is generally star more reliable. In a star, problems are easier to locate because symptoms often point to one station. The star is the second-easiest topology to design and install. Only the bus is second- easier. The star does not require manual termination. The media is terminated in the terminated station at the transceiver on the NIC, and in the hub or MAU. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 3
  4. 4. CONEREL 7 Star Topology (cont.) The star topology has its disadvantages: Hubs are more expensive than bus connectors. A failure at the hub can affect the entire configuration. Star topologies use more cable than bus topologies because separate wires run to each individual station. Hubs typically function as repeaters in a star configuration. These hubs are active hubs because they repeat the signal before passing it on. Active hubs, the most common kind in use today, require their own source of electricity. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 8 Star Topology (cont.) SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 4
  5. 5. CONEREL 9 Extended Star Topology (cont.) SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 10 Ring Topology Ring topologies are most often seen in Token Ring and FDDI networks. Stations on a ring are wired to one another in a circle around the entire network. It looks like a bus network except there are no termination points. Data is passed around the ring and unlike a bus network, stations in a ring regenerate the signal before passing it along. This makes the ring topology active, as opposed to the passive bus topology. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 5
  6. 6. CONEREL 11 Ring Topology (cont.) The ring topology has these advantages: It prevents network collisions due to the media access method or architecture required. Each station functions as a repeater, so the topology does not require additional network hardware, such as hubs. However, as with any topology, disadvantages exist: Similar to a bus network, a failure at one point can bring down the network. Since all stations are wired together, to add a station, the network must be temporarily shut down. Maintenance on a ring is more difficult than on a star topology because an adjustment or reconfiguration affects the entire ring. ring. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 12 Ring Topology (cont.) SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 6
  9. 9. CONEREL 17 Logical Topology Logical topology defines how the signal travels, not the physical layout. A logical bus means that the signal will go out from the sending station in all directions to every host. The physical topology may be a bus or a star. In a physical star that uses a logical bus, the central device will be a hub. This may be referred to as a star- star- bus topology. The first part (star) represents the physical topology and the second part (bus) represents the logical topology. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 18 IEEE 802 IEEE 802 covers issues concerning all types of networks: local area, metropolitan, and wide area. As originally written, it only addressed issues related to the first two layers of the OSI networking model: Physical and Data Link. Several different specifications in the 802 standard are of particular interest in basic networking architectures. Ex. 802.2 - Logical Link Control 802.3 - Ethernet 802.4 - Token Bus 802.5 - Token Ring 802.12 - 100VG-AnyLAN; Demand Priority 100VG- SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 9
  10. 10. CONEREL 19 Network Media Modern networks can utilize several types of network media. Most networks use some type of cable for the connection media however, air is sometimes used. Wireless technologies, such as infra-red and satellite infra- communications, will become increasingly important at present. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 20 Media Degradation In general, media is susceptible to degradation. The sources of the degradation can be internal or external. When signals degrade overover distance, it is called attenuation. Three internal factors can cause attenuation. attenuation: Resistance: Opposition to the flow of electrons in a wire Inductance: Opposition to the changes of electrical current in the wire the Capacitance: Opposition to voltage changes in the wire When these internal opposition forces are combined and measured, the measure is called impedance. impedance. Each type of cabling has its own impedance rating, which is part of its specification. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 10
  11. 11. CONEREL 21 Media Degradation (cont.) If all cabling on a network does not have the same impedance, the network can suffer from transmission errors or a reduction in the total usable segment length. In addition to the internal opposition of the wire to the signal, external forces can act upon network signals. These external forces come in two main categories: Electromagnetic Interference (EMI) Radio Frequency Interference (RFI) Both types of interference can degrade and corrupt network signals as they travel through the wire. Electrical lighting, generators, elevators, machinery, and other electromagnetic equipment can be sources of EMI. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 22 Media Degradation (cont.) Crosstalk is a form of EMI. Radio transmitting devices cause RFI. In order to reduce EMI/RFI on network cabling, you can do the following: Keep network media away from sources of EMI, such as lights, generators, and high voltage electrical wiring. Ensure that network media is properly installed. Use shielded cabling, which is more resistant to external interference. Use repeaters, which amplify and clean-up networking signals. clean- Ensure that you install good quality cabling. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 11
  12. 12. CONEREL 23 Twisted-Pair Cabling Most networks are connected with twisted-pair cabling. twisted- The cable that runs from telephone to the wall is probably twisted-pair. twisted- Different types and categories of twisted-pair cable exist, twisted- but they all have two things in common: The wires come in pairs. The pairs of wires are twisted around each other. The twisting of wires reduces crosstalk, which is the bleeding of a signal from one wire to another and which can corrupt signals and cause network errors. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 24 Cancellation Transmitting cables have a naturally created magnetic field around them. When two wires are placed in close proximity, their electronically created magnetic fields cancel each other out. This cancellation actually insulates the signal from the effects of signal bleeding. When the wires are twisted around one another, the magnetic cancellation is enhanced. The twisting of the wires not only protects the signal inside from internal crosstalk, but it also protects it from other external forms of signal interference. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 12
  13. 13. CONEREL 25 Unshielded Twisted Pair (UTP) Unshielded twisted-pair cable (UTP) is a four-pair wire medium - composed of pairs of wires - used in a variety of networks. Each pair of wires is insulated from the others. This cable relies solely on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in unshielded twisted-pair cable, the number of twists in the wire pairs varies. Like shielded twisted-pair cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 26 Unshielded Twisted Pair (UTP) (cont.) UTP cabling is used for a variety of electronic communications. UTP cable supports data transmissions of 4, 10, 16, 100 and 1000 Mbps. The maximum segment length for UTP is 100 meters. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 13
  14. 14. CONEREL 27 Unshielded Twisted Pair (UTP) (cont.) When used as a networking medium, unshielded twisted-pair cable has four pairs of twisted- either 22 or 24 gauge copper wire. UTP used as a networking medium has an medium impedance of 100 ohms. This differentiates it from other types of twisted-pair wiring twisted- such as that used for telephone wiring. Because UTP has an external diameter of external approximately .17", its small size can be advantageous during installation. Since installation. UTP can be used with most of the major networking architectures, it continues to architectures, grow in popularity. The advantages of UTP cable include: Easy installation: It is a thin, flexible cable that is easy to string between walls. Pre-installation: Most modern buildings come with CAT 5 UTP already wired into the wall Pre- outlets, or at least run between the floors. Size: Since UTP is small, it does not quickly fill up wiring ducts. ducts. Inexpensive: UTP costs less per foot than any other type of LAN cable. Using UTP does have its disadvantages: Susceptibility to interference: It is more susceptible to interference than most other types of interference cabling. The pair twisting does help, but it does not come close to making the cable impervious to electrical noise. Limited segment length: It is limited to segments of 100 meters. The distance between signal boosts is shorter for unshielded twisted-pair than it is for coaxial cable. twisted- Once considered slower at transmitting data than other types of cable. Today, UTP is considered the fastest copper-based media. copper- SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 28 Shielded Twisted Pair (STP) Shielded twisted-pair cable (STP) 100 ohm combines the techniques of shielding (each group of wires are surrounded by a shield), cancellation, and twisting of wires. Resistance to both EMI and RFI without significantly increasing the weight or size of the cable. Another type STP cable is for token-ring installations. In this type of STP cable, known as 150 ohm STP, not only is the entire cable shielded to reduce EMI and RFI but each pair of twisted wires is shielded from each other in order to reduce cross- talk. requires an increased amount of insulation and a larger amount of shielding. These factors combine to increase the size, weight, and cost of the cable. It also requires the installation of large wiring closets and large wiring ducts, luxuries that many older buildings cannot provide. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 14
  15. 15. CONEREL 29 Shielded Twisted Pair (STP) (cont.) STP is similar to UTP in that the wire pairs are twisted around each other inside the cable. STP also has shielding around the cable to further protect it from external interference. The shielding of the individual pairs of wires further reduces the chance of crosstalk. Shielding the individual wire pairs increases the overall diameter and weight of the cable. The maximum segment length of STP cable is 100 meters. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 30 Shielded Twisted Pair (STP) (cont.) The advantage of STP over UTP is: Greater protection from interference and crosstalk due to shielding. The disadvantages to STP include the following: Cost: STP has a higher cost per foot. Grounding: The STP Shield must be grounded at both ends; if grounded improperly, it can cause serious interference. Difficulty in installation: Heavier and less flexible, STP is more more difficult to install. Thickness: Because of its thickness, STP may not fit down narrow cable ducts. No segment length advantage: Despite the heavier and thicker cable, there is no increased segment length. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 15
  16. 16. CONEREL 31 RJ45 Connectors Twisted-pair wiring is fitted with a plastic connector and Twisted- inserted into a wall connector. These connectors are known as Registered Jacks (RJ). (RJ). Typically, RJ-45 connectors connect UTP networks. RJ- The RJ-45 connectors look much like the typical RJ-11 RJ- RJ- connectors that connect telephone to the wall. The difference between the RJ-45 connectors and the RJ- RJ-11 connectors is that the former has 8 wire traces (4- RJ- (4- pair) and the latter has only 4 (2-pair). (2- SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 32 Coaxial Cabling Coaxial cable consists of a solid inner core, usually copper, or wire strand conductor that is surrounded by insulation. Just outside of the insulation is a shield, which is a foil or copper braid that protects the inner conductor. A protective jacket surrounds the outside of the cable. Coaxial cable supports data transmission rates of 10 Mbps. The two most commonly used types of coaxial cable are Thicknet and Thinnet. Thinnet. Thicknet is a thicker form of coaxial cable than Thinnet. Thinnet. Outside diameter of Thinnet is 0.18“. 0.18“ Thicknet segments can be up to 500 meters long while Thinnet segments are limited to 185 meters. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 16
  17. 17. CONEREL 33 Coaxial Cabling (cont.) The advantages to using coaxial cabling on a LAN include the following: longer segment lengths than UTP or STP built-in shielding built- hubs between stations are not required Coaxial cable does have its disadvantages: not as easy to run as UTP, because it is not as flexible or thin more expensive than UTP requires more room in wiring ducts than UTP Thinnet is sometimes referred to as cheapernet. cheapernet. Thinnet is no longer commonly used in Ethernet networks. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 34 Thinnet Connectors The most common connectors for RG-58 cabling on thinnet RG- networks are barrel connectors, T connectors, and terminators. connectors , connectors, terminators. These connectors are known as British Naval Connectors or just BNC connectors. connectors. A barrel connector connects two sections of coaxial cabling. A BNC- BNC- T connector fits on the end of the network interface card and connects a station to two sections of RG-58 cabling. RG- Terminators are placed on each end of a Thinnet network segment to absorb signals as they reach the end of the wire. This absorption keeps the signals from reflecting and preventing other stations from transmitting. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 17
  18. 18. CONEREL 35 Thinnet Connectors (cont.) SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 36 Thinnet Connectors (cont.) A common problem in the past on Thinnet networks has been the substitution of 75-ohm terminators designed for 75- RG-59 broadband cable for the 50-ohm terminators RG- 50- designed for RG-58 baseband networks (Thinnet). RG- (Thinnet). The terminators look identical but can't be substituted without network problems occurring. Most terminators today have “50-ohm” etched on them 50- ohm” for easier identification. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 18
  19. 19. CONEREL 37 Fiber Optic Cabling Fiber optic cable carries light pulses along its fibers rather than electrical signals. This cable is made of glass or plastic fibers, rather than copper wire like most other network cabling. The core of the cable is usually pure glass. Surrounding the glass is a layer of cladding made of glass or plastic. It traps the light in the core. The cladding is surrounded usually by Kevlar, which is the strong substance used in bulletproof vests. There is an outer protective jacket made of Teflon (plenum grade) or PVC. The light pulses originate from a laser (single-mode cable) or light emitting (single- diode (multimode cable). This type of cable supports data transmission rates of 100 Mbps to 2 Gbps. Fiber optic Gbps. segments can span up to 100 km. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 38 Fiber Optic Cabling (cont.) The advantages of fiber optic cabling include: Can transmit over long distances, farther than any other network media. Is not susceptible to electromagnetic interference. Is immune from crosstalk. Supports extremely high transmission rates. Has a small diameter and can be used in narrow wiring ducts. Is not susceptible to eavesdropping. To its disadvantage, fiber optic cable: Is more expensive than other types of networking media. Is more difficult and expensive to install than other types of network network media. The ends of each cable must be polished perfectly flat and and scratch free before they are inserted into the connection devices. devices. Is fragile and must be installed carefully and protected after installation. installation. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 19
  20. 20. CONEREL 39 Wireless Communication The medium for wireless communication is the atmosphere. Wireless signals are electromagnetic waves, were proven to travel through space, empty of all matter. This feature means that networks can be built in places where it would normally be difficult, or impossible to install wires or cables. The most common application of wireless data communications is for mobile users. SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY CONEREL 40 Criteria for Media Selection Media selection is probably the most important consideration in LAN design. We must consider several factors when determining what type of cable to use in a network installation. These factors include: cost limitations the distance the signal must travel speed considerations the need for security the person who will do the installation potential interference problems SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 20
  21. 21. CONEREL 41 Media Comparisons Type Max Length Bandwidth Install Cost Interference UTP 100 m 100 MBPS easy cheapest high STP 100 m 500 MBPS moderate moderate moderate Thinnet 185 m 10 MBPS easy cheap moderate Thicknet 500 m 10 MBPS hard moderate low Multimode 2 km 100 MBPS very hard expensive none Single-Mode 100 km 2 GBPS very hard expensive none SETHALAT RODHETBHAI COMPUTER SCIENCE ▪ SILPAKORN UNIVERSITY 21