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

Phan Vĩnh Thuần Computer Network 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.
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•
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.
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•
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 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 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.
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–
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.
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3.1. Đường truyền cáp đồng
3.1.1. Các đặc tả cáp
Cables have different specifications
and expectations pertaining to
performance:
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•
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 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 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 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 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
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3.1.2. Cáp đồng trục (Coaxial cable )
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•
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.
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•
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 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 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 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.
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3.1.3. Cáp STP (Shield Twisted-Pain)
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•
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 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 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.
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3.1.4. Cáp UTP (Unshield Twisted-Pair)