bài giảng mạng máy tính căn bản chương 3 môi trường truyền dẫn cho networking

– Describe coaxial cable and its advantages and disadvantages over other types of cable.. – Describe the type of connectors and equipment used with fiber-optic cable... • How far can a s

• 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

• 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

• 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

• 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

– Discuss the characteristics of through, crossover, and rollover cables and where each is used

straight-– 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

3.1 Đường truyền cáp đồng

3.1.1 Các đặc tả cáp

Cables have different specifications and expectations pertaining to performance:

• 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

• 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

• Some examples of Ethernet specifications which relate to cable type include:

• 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

• 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

3.1.2 Cáp đồng trục (Coaxial cable )

• 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

• 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

• 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.

• 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

• 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.

3.1.3 Cáp STP (Shield Twisted-Pain)

• 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)

• 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.

• 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

3.1.4 Cáp UTP (Unshield Twisted-Pair)

• 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 foot of

• 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

• 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 considered the fastest copper-based media

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

• LAN switch is

connected to a

computer The

connects from the

switch port to the

port is called a

straight-through

cable.

• The cable that

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

• Radio, microwaves, radar, visible light, rays, and gamma rays seem to be very different things However, they are all types

x-of electromagnetic energy If all the types x-of electromagnetic waves are arranged in order from the longest wavelength down to the shortest wavelength, a continuum called the electromagnetic spectrum is created

• 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

• 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

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

• 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

• 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

3.2.3 Sự phản xạ (Reflection)

3.2.4 Sự khúc xạ (Refraction )

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