MÔI TRƯỜNG TRUYỀN dẫn CHO NETWORKING (MẠNG máy TÍNH cơ bản SLIDE)

Chương 3: MƠI 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 Computer Network network operation • Optical fiber is the most frequently used medium for the longer, high bandwidth, pointto-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 fiberoptic cable itself In fact, the glass used in fiber-optic cable is a very good electrical insulator Computer Network • 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 highspeed transfers, security, or uptime reliability of cabled networks However, flexibility of wireless has justified the trade off Computer Network • 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 Computer Network • Students completing should be able to: this module – 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 Computer uses Network – Discuss the characteristics of straightthrough, 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 Computer Network 3.1 Đường truyền cáp đồng 3.1.1 Các đặc tả cáp Cables have different specifications and expectations pertaining to performance: Computer Network • 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 areComputer theNetwork two choices.  • 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 Computer Network • 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 Computer Network 10 Computer Network 96 3.3.4 Sự xác thực gắn (Authentication and association) kết • WLAN authentication occurs at Layer 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 Computer Network 97 • 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 Computer Network 98 • 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 and receive data through the access point Computer Network 99 3.3.5 Phổ radio 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 Computer Network 100 • 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 Computer Network 101 • 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 Computer Network 102 • 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 Computer Network 103 – 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 Computer Network 104 Computer Network 105 • 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 Computer Network 106 3.3.6 An tịan cho mạng khơng 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 Computer Network 107 • 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 Computer Network 108 – 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 Computer Network 109 – 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 Computer Network 110 ... Describe the type of connectors and equipment used with fiber-optic cable Computer Network 3.1 Đường truyền cáp đồng 3.1.1 Các đặc tả cáp Cables have different specifications and expectations pertaining... or baseband transmission and analog-based or broadband transmission areComputer theNetwork two choices.  • How far can a signal travel through a particular type of cable before attenuation of... 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