Bài giảng Mạng máy tính cơ bản - Chương 3: Môi trường truyền dẫn cho Networking - Phan Vĩnh Thuần

Nội dung text: Bài giảng Mạng máy tính cơ bản - Chương 3: Môi trường truyền dẫn cho Networking - Phan Vĩnh Thuần

1. Chương 3: MÔI TRƯỜNG TRUYỀN DẪN CHO NETWORKING Overview • Copper cable is used in almost every LAN. Many different types of copper cable are available, with each type having advantages and disadvantages. Proper selection of cabling is key to efficient network operation. Phan Vĩnh Thuần Computer Network 1
2. • Optical fiber is the most frequently used medium for the longer, high bandwidth, point-to- point transmissions required on LAN backbones and on WANs. Using optical media, light is used to transmit data through thin glass or plastic fiber. Electrical signals cause a fiber- optic transmitter to generate the light signals sent down the fiber. The receiving host receives the light signals and converts them to electrical signals at the far end of the fiber. However, there is no electricity in the fiber-optic cable itself. In fact, the glass used in fiber-optic cable is a very good electrical insulator. Phan Vĩnh Thuần Computer Network 2
3. • Physical connectivity allowed an increase in productivity by allowing the sharing of printers, servers, and software. Traditional networked systems require that the workstation remains stationary permitting moves only within the limits of the media and office area. • The introduction of wireless technology removes these restraints and brings true portability to the computing world. Currently, wireless technology does not provide the high- speed transfers, security, or uptime reliability of cabled networks. However, flexibility of wireless has justified the trade off. Phan Vĩnh Thuần Computer Network 3
4. • Administrators often consider wireless when installing a new network or when upgrading an existing network. A simple wireless network could be working just a few minutes after the workstations are turned on. Connectivity to the Internet is provided through a wired connection, router, cable or DSL modem and a wireless access point that acts as a hub for the wireless nodes. In a residential or small office environment these devices may be combined into a single unit Phan Vĩnh Thuần Computer Network 4
5. • Students completing this module should be able to: – Describe the specifications and performances of different types of cable. – Describe coaxial cable and its advantages and disadvantages over other types of cable. – Describe shielded twisted-pair (STP) cable and its uses. – Describe unshielded twisted-pair cable (UTP) and its uses. Phan Vĩnh Thuần Computer Network 5
6. – Discuss the characteristics of straight- through, crossover, and rollover cables and where each is used. – Explain the basics of fiber-optic cable. – Describe how fibers can guide light for long distances. – Describe multimode and single-mode fiber. – Describe the type of connectors and equipment used with fiber-optic cable. Phan Vĩnh Thuần Computer Network 6
7. 3.1. Đường truyền cáp đồng 3.1.1. Các đặc tả cáp Cables have different specifications and expectations pertaining to performance: Phan Vĩnh Thuần Computer Network 7
8. • What speeds for data transmission can be achieved using a particular type of cable? The speed of bit transmission through the cable is extremely important. The speed of transmission is affected by the kind of conduit used. • What kind of transmission is being considered? Will the transmissions be digital or will they be analog-based? Digital or baseband transmission and analog-based or broadband transmission are the two choices. Phan Vĩnh Thuần Computer Network 8
9. • How far can a signal travel through a particular type of cable before attenuation of that signal becomes a concern? In other words, will the signal become so degraded that the recipient device might not be able to accurately receive and interpret the signal by the time the signal reaches that device? The distance the signal travels through the cable directly affects attenuation of the signal. Degradation of the signal is directly related to the distance the signal travels and the type of cable used. Phan Vĩnh Thuần Computer Network 9
10. • Some examples of Ethernet specifications which relate to cable type include: – 10BASE-T – 10BASE5 – 10BASE2 • 10BASE-T refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The T stands for twisted pair. Phan Vĩnh Thuần Computer Network 10
11. • 10BASE5 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 5 represents the capability of the cable to allow the signal to travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE5 is often referred to as Thicknet. Phan Vĩnh Thuần Computer Network 11
12. • 10BASE2 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 2, in 10BASE2, represents the capability of the cable to allow the signal to travel for approximately 200 meters, before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE2 is often referred to as Thinnet Phan Vĩnh Thuần Computer Network 12
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14. 3.1.2. Cáp đồng trục (Coaxial cable ) Phan Vĩnh Thuần Computer Network 14
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16. • Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield reduces the amount of outside electro-magnetic interference. Covering this shield is the cable jacket. Phan Vĩnh Thuần Computer Network 16
17. • For LANs, coaxial cable offers several advantages. It can be run longer distances than shielded twisted pair, STP, and unshielded twisted pair, UTP, cable without the need for repeaters. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known. It has been used for many years for many types of data communication, including cable television. Phan Vĩnh Thuần Computer Network 17
18. • Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable, because it has a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable can be too rigid to install easily in some situations. Generally, the more difficult the network media is to install, the more expensive it is to install. Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used anymore, except for special purpose installations. Phan Vĩnh Thuần Computer Network 18
19. • In the past, ‘thinnet’ coaxial cable with an outside diameter of only 0.35 cm was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since thinnet was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. Phan Vĩnh Thuần Computer Network 19
20. • The outer copper or metallic braid in coaxial cable comprises half the electric circuit and special care must be taken to ensure a solid electrical connection at both ends resulting in proper grounding. Poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmittal on the networking media. For this reason thinnet is no longer commonly used nor supported by latest standards (100 Mbps and higher) for Ethernet networks. Phan Vĩnh Thuần Computer Network 20
21. 3.1.3. Cáp STP (Shield Twisted-Pain) Phan Vĩnh Thuần Computer Network 21
22. • Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires. Each pair of wires is wrapped in metallic foil. The four pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150-Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable, for example electromagnetic interference (EMI) and radio frequency interference (RFI). Phan Vĩnh Thuần Computer Network 22
23. • Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). STP affords greater protection from all types of external interference, but is more expensive and difficult to install than UTP. Phan Vĩnh Thuần Computer Network 23
24. • A new hybrid of UTP with traditional STP is Screened UTP (ScTP), also known as Foil Twisted Pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. It is usually 100-Ohm or 120-Ohm cable. this effect works both ways. Not only does the shield prevent incoming electromagnetic waves from causing noise on data wires, but it also minimizes the outgoing radiated electromagnetic waves. These waves could cause noise in other devices. Phan Vĩnh Thuần Computer Network 24
25. 3.1.4. Cáp UTP (Unshield Twisted-Pair) Phan Vĩnh Thuần Computer Network 25
26. • Unshielded twisted-pair cable (UTP) is a four- pair wire medium used in a variety of networks. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of 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 UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per Phanfoot Vĩnhof Thuầncable. Computer Network 26
27. • Unshielded twisted-pair cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling. However, the real advantage is the size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when installing a network in an older building. Phan Vĩnh Thuần Computer Network 27
28. • when UTP cable is installed using an RJ-45 connector, potential sources of network noise are greatly reduced and a good solid connection is practically guaranteed. There are disadvantages in using twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables. • UTP was once considered slower at transmitting data than other types of cable. This is no longer true. In fact, today, UTP is Phanconsidered Vĩnh Thuần the fastestComputer Networkcopper-based media28.
29. 3.1.5. Kết nối cáp UTP When communication occurs, the signal that is transmitted by the source needs to be understood by the destination. This is true from both a software and physical perspective. The transmitted signal needs to be properly received by the circuit connection designed to receive signals. The transmit pin of the source needs to ultimately connect to the receiving pin of the destination. The following are the types of cable connections used between internetwork devices. Phan Vĩnh Thuần Computer Network 29
30. • LAN switch is connected to a computer. The cable that connects from the switch port to the computer NIC port is called a straight-through cable. Phan Vĩnh Thuần Computer Network 30
31. • Two switches are connected together. The cable that connects from one switch port to another switch port is called a crossover cable Phan Vĩnh Thuần Computer Network 31
32. • The cable that connects the RJ- 45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable. Phan Vĩnh Thuần Computer Network 32
33. 3.2. Đường truyền cáp quang (Optical media) 3.2.1. Phổ điện từ (The electromagnetic spectrum) • The light used in optical fiber networks is one type of electromagnetic energy. When an electric charge moves back and forth, or accelerates, a type of energy called electromagnetic energy is produced. This energy in the form of waves can travel through a vacuum, the air, and through some materials like glass. An important property of any energy wave is the wavelength. Phan Vĩnh Thuần Computer Network 33
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35. • Radio, microwaves, radar, visible light, x- rays, and gamma rays seem to be very different things. However, they are all types of electromagnetic energy. If all the types of electromagnetic waves are arranged in order from the longest wavelength down to the shortest wavelength, a continuum called the electromagnetic spectrum is created Phan Vĩnh Thuần Computer Network 35
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37. • Human eyes were designed to only sense electromagnetic energy with wavelengths between 700 nanometers and 400 nanometers (nm). Electromagnetic energy with wavelengths between 700 and 400 nm is called visible light. The longer wavelengths of light that are around 700 nm are seen as the color red. The shortest wavelengths that are around 400 nm appear as the color violet. Phan Vĩnh Thuần Computer Network 37
38. • Wavelengths that are not visible to the human eye are used to transmit data over optical fiber. These wavelengths are slightly longer than red light and are called infrared light. Infrared light is used in TV remote controls. The wavelength of the light in optical fiber is either 850 nm, 1310 nm, or 1550 nm. These wavelengths were selected because they travel through optical fiber better than other wavelengths. Phan Vĩnh Thuần Computer Network 38
39. 3.2.2. Mô hình tia sáng (Ray model of light) • When electromagnetic waves travel out from a source, they travel in straight lines. These straight lines pointing out from the source are called rays. • When a light ray called the incident ray, crosses the boundary from one material to another: from air to glass, for instance, some of the light energy in the ray will be reflected back. The light that is reflected back is called the reflected ray Phan Vĩnh Thuần Computer Network 39
40. • The light energy in the incident ray that is not reflected will enter the glass. The entering ray will be bent at an angle from its original path. This ray is called the refracted ray. How much the incident light ray is bent depends on the angle at which the incident ray strikes the surface of the glass and the different rates of speed at which light travels through the two substances. Phan Vĩnh Thuần Computer Network 40
41. • The bending of light rays at the boundary of two substances is the reason why light rays are able to travel through an optical fiber even if the fiber curves in a circle. • The ratio of the speed of light in a material to the speed of light in a vacuum is called the Index of Refraction Phan Vĩnh Thuần Computer Network 41
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43. 3.2.3. Sự phản xạ (Reflection) Phan Vĩnh Thuần Computer Network 43
44. 3.2.4. Sự khúc xạ (Refraction) Phan Vĩnh Thuần Computer Network 44
45. 3.2.5. Sự phản xạ hòan tòan vào trong (Total internal reflection) • A light ray that is being turned on and off to send data (1s and 0s) into an optical fiber must stay inside the fiber until it reaches the far end. The ray must not refract into the material wrapped around the outside of the fiber. The refraction would cause the loss of part of the light energy of the ray. A design must be achieved for the fiber that will make the outside surface of the fiber act like a mirror to the light ray moving through the fiber. If any light ray that tries to move out through the side of the fiber were reflected back into the fiber at an angle that sends it towards the far end of the fiber. Phan Vĩnh Thuần Computer Network 45
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47. • The laws of reflection and refraction illustrate how to design a fiber that guides the light waves through the fiber with a minimum energy loss. The following two conditions must be met for the light rays in a fiber to be reflected back into the fiber without any loss due to refraction: – The core of the optical fiber has to have a larger index of refraction (n) than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding. – The angle of incidence of the light ray is greater than the critical angle for the core and its cladding. Phan Vĩnh Thuần Computer Network 47
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49. • When both of these conditions are met, the entire incident light in the fiber is reflected back inside the fiber. This is called total internal reflection, which is the foundation upon which optical fiber is constructed. Total internal reflection causes the light rays in the fiber to bounce off the core-cladding boundary and continue its journey towards the far end of the fiber. The light will follow a zigzag path through the core of the fiber. Phan Vĩnh Thuần Computer Network 49
50. 3.2.6. Sợi đa mode (Multimode fiber) • The part of an optical fiber through which light rays travel is called the core of the fiber. Phan Vĩnh Thuần Computer Network 50
51. • Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called “multimode” fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Phan Vĩnh Thuần Computer Network 51
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54. • Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B. The second fiber carries data from device B to device A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are Phanattached Vĩnh Thuần Computer Network 54
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57. • Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket. Phan Vĩnh Thuần Computer Network 57
58. • The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass for its core. This glass has a lower index of refraction towards the outer edge of the core. Therefore, the outer area of the core is less optically dense than the center and light can go faster in the outer part of the core. Phan Vĩnh Thuần Computer Network 58
59. • This design is used because a light ray following a mode that goes straight down the center of the core does not have as far to travel as a ray following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together. Then the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse. Phan Vĩnh Thuần Computer Network 59
60. • Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. Standard multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. Phan Vĩnh Thuần Computer Network 60
61. • A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1µ). Phan Vĩnh Thuần Computer Network 61
62. • Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. There are two basic cable designs. They are the loose-tube and the tight-buffered cable designs. Most of the fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. Phan Vĩnh Thuần Computer Network 62
63. • The most practical difference between the two designs is the applications for which they are used. Loose-tube cable is primarily used for outside-building installations, while tight-buffered cable is used inside buildings. Phan Vĩnh Thuần Computer Network 63
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65. • The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests • The final element is the outer jacket. The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color. Phan Vĩnh Thuần Computer Network 65
66. • Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs) are two types of light source usually used with multimode fiber. Use one or the other. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6,560 ft). Phan Vĩnh Thuần Computer Network 66
67. 3.2.7. Sợi đơn mode (Single-mode fiber) • Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode fiber is usually yellow. The major difference between multimode and single-mode fiber is that single-mode allows only one mode of light to propagate through the smaller, fiber-optic core. Phan Vĩnh Thuần Computer Network 67
68. • The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of the single-mode fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter. Phan Vĩnh Thuần Computer Network 68
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70. • An infrared laser is used as the light source in single-mode fiber. The ray of light it generates enters the core at a 90-degree angle. As a result, the data carrying light ray pulses in single-mode fiber are essentially transmitted in a straight line right down the middle of the core. This greatly increases both the speed and the distance that data can be transmitted. Phan Vĩnh Thuần Computer Network 70
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72. • Because of its design, single-mode fiber is capable of higher rates of data transmission (bandwidth) and greater cable run distances than multimode fiber. Single- mode fiber can carry LAN data up to 3000 meters. Multimode is only capable of carrying up to 2000 meters. Lasers and single-mode fibers are more expensive than LEDs and multimode fiber. Because of these characteristics, single-mode fiber is often used for inter-building connectivity. Phan Vĩnh Thuần Computer Network 72
73. • Warning: The laser light used with single-mode has a longer wavelength than can be seen. The laser is so strong that it can seriously damage eyes. Never look at the near end of a fiber that is connected to a device at the far end. Never look into the transmit port on a NIC, switch, or router. Remember to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Be very careful. Phan Vĩnh Thuần Computer Network 73
74. 3.2.8. Đầu nối sợi quang • Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC connector). On single-mode fiber, the Straight Tip (ST) connector is frequently used. Phan Vĩnh Thuần Computer Network 74
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78. 3.3. Môi trường truyền không dây (Wireless Media) 3.3.1. Các tổ chức và tiêu chuẩn Wireless LAN (WLAN) • An understanding of the regulations and standards that apply to wireless technology will ensure that deployed networks will be interoperable and in compliance. Just as in cabled networks, IEEE is the prime issuer of standards for wireless networks. The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC). Phan Vĩnh Thuần Computer Network 78
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80. • A key technology contained within the 802.11 standard is Direct Sequence Spread Spectrum (DSSS). DSSS applies to wireless devices operating within a 1 to 2 Mbps range. Phan Vĩnh Thuần Computer Network 80
81. • 802.11b may also be called Wi-Fi™ or high- speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps. All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points. Phan Vĩnh Thuần Computer Network 81
82. • 802.11a covers WLAN devices operating in the 5 GHZ transmission band. Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ. 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps. Phan Vĩnh Thuần Computer Network 82
83. • 802.11g provides the same throughout as 802.11a but with backwards compatibility for 802.11b devices using Othogonal Frequency Division Multiplexing (OFDM) modulation technology. Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate. Phan Vĩnh Thuần Computer Network 83
84. 3.3.2. Các thiết bị không dây và topo • A wireless network may consist of as few as two devices. The nodes could simply be desktop workstations or notebook computers. Equipped with wireless NICs, an ‘ad hoc’ network could be established which compares to a peer-to-peer wired network. Both devices act as servers and clients in this environment. Although it does provide connectivity, security is at a minimum along with throughput. Another problem with this type of network is compatibility. Many times NICs from different manufacturers are not compatible. Phan Vĩnh Thuần Computer Network 84
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86. • To solve the problem of compatibility, an access point (AP) is commonly installed to act as a central hub for the WLAN. The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell. Phan Vĩnh Thuần Computer Network 86
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88. • Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell could greatly vary. Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet). To service larger areas, multiple access points may be installed with a degree of overlap. Although not addressed in the IEEE standards, a 20- 30% overlap is desirable Phan Vĩnh Thuần Computer Network 88
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90. • When a client is activated within the WLAN, it will start "listening" for a compatible device with which to "associate". This is referred to as "scanning" and may be active or passive. – Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response. The authentication and association steps are completed Phan Vĩnh Thuần Computer Network 90
91. • Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes. Phan Vĩnh Thuần Computer Network 91
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93. 3.3.3. Sự truyền thông trên WLAN (How wireless LANs communicate) • After establishing connectivity to the WLAN, a node will pass frames in the same manner as on any other 802.x network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, management, and data. Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500 bytes; however, an Ether frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network Phan Vĩnh Thuần Computer Network 93
94. • WLANs use protocol Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD (Carrier Sense Multiple Access/Collision Detect). • When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps. Phan Vĩnh Thuần Computer Network 94
95. • Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked. The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps. Phan Vĩnh Thuần Computer Network 95
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97. 3.3.4. Sự xác thực và gắn kết (Authentication and association). • WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. This is a critical point to remember when considering WLAN security, troubleshooting and overall management. Phan Vĩnh Thuần Computer Network 97
98. • Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process Phan Vĩnh Thuần Computer Network 98
99. • Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data. • Authentication and Association types – Unauthenticated and unassociated: The node is disconnected from the network and not associated to an access point. – Authenticated and unassociated : The node has been authenticated on the network but has not yet associated with the access point. – Authenticated and associated : The node is connected to the network and able to transmit Phan andVĩnh Thuầnreceive data Computerthrough Networkthe access point. 99
100. 3.3.5. Phổ radio và vi ba (The radio wave and microwave spectrums) • Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna. Phan Vĩnh Thuần Computer Network 100
101. • However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength • Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air Phan Vĩnh Thuần Computer Network 101
102. • These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey. • Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals. Phan Vĩnh Thuần Computer Network 102
103. • In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal. • The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. Phan Vĩnh Thuần Computer Network 103
104. – Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. – Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. • In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter. In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal. Phan Vĩnh Thuần Computer Network 104
105. Phan Vĩnh Thuần Computer Network 105
106. • A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal. Phan Vĩnh Thuần Computer Network 106
107. 3.3.6. An tòan cho mạng không dây (Wireless security) • As previously discussed in this chapter, wireless security can be difficult to achieve. Where wireless networks exist there is little security. This has been a problem from the earliest days of WLANs. Phan Vĩnh Thuần Computer Network 107
108. • A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose. Phan Vĩnh Thuần Computer Network 108
109. – EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest authentication type, which is very similar to password protection on a wired network. – LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP (Wireless Equivalency Protocol) keys, and supports mutual authentication. Phan Vĩnh Thuần Computer Network 109
110. – User authentication – Allows only authorized users to connect, send and receive data over the wireless network. – Encryption – Provides encryption services further protecting the data from intruders. – Data authentication – Ensures the integrity of the data, authenticating source and destination devices. Phan Vĩnh Thuần Computer Network 110