Chapter5
Layer1-Media, Connections, and Collisions
Overview
Like any good house, a network must be built on a solid foundation. In the OSI reference model,
this foundation is called Layer 1 or the physical layer. The terms used in this chapter describe
how network functions are linked to Layer 1 of the OSI reference model. The physical layer is
the layer that defines the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between end systems.
In this chapter, you will learn about the network functions that occur at the physical layer of the
OSI model. You will learn about different types of networking media that are used at the
physical layer, including shielded twisted-pair cable, unshielded twisted-pair cable, coaxial cable,
and fiber-optic cable. In addition, you will learn how network devices, cable specifications,
network topologies, collisions and collision domains can help determine such things as how
much data can travel across the network and how fast.
Note: In this chapter, you may need to convert units of measurements. A small utility to help you
make the conversions is available here. You can access it from anywhere in this chapter via the
Index button below.
5.1 Most Common LAN Media
5.1.1 STP
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 4 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, both within the cable (pair to pair
coupling, or crosstalk) and from outside the cable (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 or 120 Ohm cable.
The metallic shielding materials in STP and ScTP need to be grounded at both ends. If
improperly grounded (or if there are any discontinuities in the entire length of the shielding
material, for example due to poor termination or installation), STP and ScTP become
susceptible to major noise problems, because they allow the shield to act like an antenna
picking up unwanted signals. However, this effect works both ways. Not only does the foil
(shield, screen) prevent incoming electromagnetic waves from causing noise on our data wires,
but it minimizes the outgoing radiated electromagnetic waves, which could cause noise in other
devices. STP and ScTP cable cannot be run as far as other networking media (coaxial cable,
optical fiber) without the signal being repeated. More insulation and shielding combine to
considerably increase the size, weight, and cost of the cable. And the shielding materials make
terminations more difficult and susceptible to poor workmanship. However STP and ScTP still
have their role, especially in Europe.
Web Links
Twisted Pair Cabling
5.1.2 UTP
Unshielded twisted-pair cable (UTP)
is a four-pair wire medium - composed of pairs of wires -
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 are 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 cable.
When used as a networking medium, UTP cable has four pairs of either 22 or 24 gauge copper
wire. UTP used as a networking medium has an impedance of 100 ohms. This differentiates it
from other types of twisted-pair wiring such as that used for telephone wiring. Because UTP has
an external diameter of approximately .43 cm, its small size can be advantageous during
installation. Since UTP can be used with most of the major networking architectures, it
continues to grow in popularity.
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 its real advantage is its 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.
Also, when UTP cable is installed using an RJ 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.
While UTP was once considered slower at transmitting data than other types of cable. However,
this is no longer true. In fact, today, UTP is considered the fastest copper-based media.
Web Links
Twisted Pair Cabling
5.1.3 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 it 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, can help reduce the amount of outside
interference. Covering this shield is the cable jacket.
For LANs, coaxial cable offers several advantages. It can be run, without as many boosts from
repeaters, for longer distances between network nodes than either STP or UTP cable.
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 all types of data communication. Can you think of another type of
communication that utilizes coaxial cable?
When working with cable, it is important to consider its size. As the thickness, or diameter, of
the cable increases, so does the difficulty in working with it. You must remember that cable must
be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in
a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable
because it had historically 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, because of its thickness, can be too rigid to install easily in some situations. The rule of
thumb is: "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, coaxial cable with an outside diameter of only .35 cm (sometimes referred to as
thinnet) was used in Ethernet networks. It was especially useful for cable installations that
required the cable to make many twists and turns. Since it was easier to install, it was also
cheaper to install. This led some people to refer to it as cheapernet. However, because the
outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care
must be taken to ensure that it is properly grounded. This is done by ensuring that there is a
solid electrical connection at both ends of the cable. Frequently, installers fail to do this. As a
result, 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. It is for this reason that, despite its small diameter,
thinnet is no longer commonly used in Ethernet networks.
5.1.4 Optical fiber
Fiber-optic cable is a networking medium capable of conducting modulated light transmissions.
Compared to other networking media, it is more expensive; however, it is not susceptible to
electromagnetic interference and is capable of higher data rates than any of the other types of
networking media discussed here. Fiber-optic cable does not carry electrical impulses, as other
forms of networking media that employ copper wire do. Instead, signals that represent bits are
converted into beams of light. Even though light is an electromagnetic wave, light in fibers is not
considered wireless because the electromagnetic waves are guided in the optical fiber. The
term wireless is reserved for radiated, or unguided, electromagnetic waves.
Fiber-optic communication is rooted in a number of inventions made in the 19th century. It was
not until the 1960s, when solid-state laser light sources and high-quality impurity-free glasses
were introduced, that fiber-optic communication became practical. Its use on a widespread
basis was pioneered by telephone companies, who saw its benefits for long-distance
communication.
Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If
viewed in cross section, you would see that each optical fiber is surrounded by layers of
protective buffer material, usually a plastic such as Kevlar, and an outer jacket. The outer jacket
provides protection for the entire cable. Usually made of plastic, it conforms to appropriate fire
and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection
for the fragile hair-thin glass fibers. Wherever buried fiber-optic cables are required by codes, a
stainless steel wire is sometimes included for added strength.
The light-guiding parts of an optical fiber are called the core and the cladding. The core is
usually very pure glass with a high index of refraction. When the core glass is surrounded by a
cladding layer of glass or plastic with a low index of refraction, light can be trapped in the fiber
core. This process is called total internal reflection, and it allows the optical fiber to act like a
light pipe, guiding light for tremendous distances, even around bends.
Web Links
Optical Fiber Cabling
5.1.5 Wireless communication
Wireless signals are electromagnetic waves, which can travel through the vacuum of outer
space and through media such as air. Therefore, no physical medium is necessary for wireless
signals, making them a very versatile way to build a network. Figure
represents an
electromagnetic wave.
Figure
illustrates one of the most important charts in all of science and technology, the
Electromagnetic Spectrum chart. You might be amazed that even tough all of the waves - power
waves, radio waves, microwaves, Infrared light waves, visible light waves, ultraviolet light waves,
x-rays, and gamma rays - look seemingly very different, they share some very important
characteristics:
1. All of these waves have an energy pattern similar to that represented in Figure
.
2. All of these waves travel at the speed of light, c = 299, 792, 458 meters per second, in
vacuum. This speed might more accurately be called the speed of electromagnetic
waves.
3. All of these waves obey the equation (frequency) x (wavelength) = c.
4. All of these waves will travel through vacuum, however, they have very different
interactions with various materials.
The primary difference between the different electromagnetic waves is their frequency. Low
frequency electromagnetic waves have a long wavelength (the distance from one peak to the
next on the sine wave), while high frequency electromagnetic waves have a short wavelength.
The interactive calculator in Figure
doing the following activities:
allows you to experiment. Try the interactive calculator by
1. Enter a frequency and you will notice that the calculator displays the wavelength.
2. Enter a wavelength you will notice that the calculator displays the frequency.
In either case, the calculator display the electromagnetic wave associated with the calculation.
A common application of wireless data communication is for mobile use. Some examples of
mobile use includes:

people in cars or airplanes

satellites

remote space probes

space shuttles and space stations

anyone/anything/anywhere/anytime that requires network data

communications, without having to rely on copper or optical fiber tethers
Another common application of wireless data communication is wireless LANs (WLANs), which
are built in accordance with the IEEE 802.11 standards. WLANs typically use radio waves (for
example, 902 MHz), microwaves (for example, 2.4 GHz), and Infrared waves (for example, 820
nanometers) for communication. Wireless technologies are a crucial part of the future of
networking.
5.2 Cable Specification and Termination
5.2.1 Purpose of LAN media specifications
By the mid-1980s, growing pains from expansion in the field of networking were felt, especially
by those companies that had instituted many different network technologies. It became
increasingly difficult for networks that used different specifications and implementations to
communicate with each other. An organization, called the International Organization for
Standardization (ISO), researched various networks and created a network model, called the
OSI Reference Model. (Note: Do not confuse the name of the model [OSI] with the name of the
organization [ISO].) It was designed to help vendors create networks that would work
compatibly and interoperably. By creating the OSI model, the ISO provided vendors with a set of
standards.
Standards are sets of rules or procedures that are either widely used, or officially specified, and
that serve as the gauge or model of excellence. The OSI model standards ensured compatibility
and interoperability between the various types of network technologies that were produced by
the many companies around the world. The early standards that were developed for networking
media were largely proprietary. They were developed for use by various companies. Eventually,
many more organizations and government bodies joined the movement to regulate and specify
the types of cable that could be used for specific purposes or functions. Until recently, there has
been a somewhat confusing mix of standards governing networking media. Standards have
ranged from fire and building codes to detailed electrical specifications. Others have focused on
tests to ensure safety and performance.
As you begin designing and building networks, make certain that you comply with all applicable
fire codes, building codes, and safety standards. You should also follow any established
performance standards in order to ensure optimal network operation and, because of the wide
variety of options available today in networking media, to ensure compatibility and
interoperability. Your work in this curriculum will focus on the standards for networking media
that have been developed and issued by the following groups:

IEEE - Institute of Electrical and Electronics Engineers

UL - Underwriters Laboratories

EIA - Electronic Industries Alliance

TIA - Telecommunications Industry Association
The latter two organizations, jointly, issue a list of standards that you will frequently see listed as
the TIA/EIA standards. In addition to these groups and organizations, local, state, county, and
national government agencies issue specifications and requirements that can impact the type of
cabling that can be used in a local area network.
The IEEE has outlined cabling requirements in its 802.3 and 802.5 specifications for Ethernet
and Token Ring systems, and the standards for FDDI. Underwriters Laboratories issues cabling
specifications that are primarily concerned with safety standards, however, they also rate
twisted-pair networking media for performance. The Underwriters Laboratories established an
identification program that lists markings for shielded and unshielded twisted-pair networking
media in order to simplify the job of ensuring that materials used in LAN installations meet
specifications.
Web Links
Industry Standards for Structured Cabling
5.2.2 TIA/EIA standards
Of all of the organizations mentioned here, the TIA/EIA has had the greatest impact on
networking media standards. Specifically, TIA/EIA-568-A and TIA/EIA-569-A, have been, and
continue to be, the most widely used standards for technical performance of networking media.
The TIA/EIA standards specify the minimum requirements for multi-product and multi-vendor
environments. They allow for the planning and installation of LAN systems without dictating the
use of specific equipment, thus giving LAN designers the freedom to create options for
improvement and expansion.
Upcoming Changes in Cabling Standards
Web Links
Electronics Industries Alliance
Industry Standards for Structured Cabling
5.2.3 Explain the details of TIA/EIA-568-A
The TIA/EIA standards address six elements of the LAN cabling process. These are:

horizontal cabling

telecommunications closets

backbone cabling

equipment rooms

work areas

entrance facilities
This lesson will focus on TIA/EIA-568-A standards for horizontal cabling, which defines
horizontal cabling as cabling that runs from a telecommunications outlet to a horizontal crossconnect. It includes the networking medium that runs along a horizontal pathway, the
telecommunications outlet or connector, the mechanical terminations in the wiring closet, and
the patch cords or jumpers in the wiring closet. In short, horizontal cabling includes the
networking media that is used in the area that extends from the wiring closet to a workstation.
TIA/EIA-568-A contains specifications governing cable performance. It calls for running two
cables, one for voice and one for data, to each outlet. Of the two cables, the one for voice must
be four-pair UTP. The TIA/EIA-568-A standard specifies five categories in the specifications.
These are category 1 (CAT 1), category 2 (CAT 2), category 3 (CAT 3), category 4 (CAT 4), and
category 5 (CAT 5) cabling. Of these, only CAT 3, CAT 4, and CAT 5 are recognized for use in
LANs. Of these three categories, CAT 5 is the one most frequently recommended and
implemented in installations today.
The networking media that are recognized for these categories are the ones you have studied:

shielded twisted-pair

unshielded twisted-pair

fiber-optic cable

coaxial cable
For shielded twisted-pair cable, the TIA/EIA-568-A standard calls for two pair 150 ohm cable.
For unshielded-twisted pair, the standard calls for four pair 100 ohm cable. For fiber-optic, the
standard calls for two fibers of 62.5/125 multi-mode cable. Although 50 ohm coaxial cable is a
recognized type of networking media in TIA/EIA-568-A, it is not recommended for new
installations. Moreover, this type of coaxial cable is expected to be removed from the list of
recognized networking media the next time this standard is revised.
For the horizontal cabling component, TIA/EIA-568A requires a minimum of two
telecommunications outlets or connectors at each work area. This telecommunications
outlet/connector is supported by two cables. The first is a four-pair 100 ohm CAT 3 or higher
UTP cable along with its appropriate connector. The second can be any one of the following:

four-pair 100 ohm unshielded twisted-pair cable and its appropriate connector

150 ohm shielded twisted-pair cable and its appropriate connector

coaxial cable and its appropriate connector

two-fiber 62.5/125 µ optical fiber cable and its appropriate connector
According to TIA/EIA-568-A, the maximum distance for cable runs in horizontal cabling is 90
meters (m). This is true for all types of CAT 5 UTP recognized networking media. The standard
also specifies that patch cords or cross-connect jumpers located at the horizontal cross-connect
cannot exceed 6 m in length. TIA/EIA-568-A also allows 3 m for patch cords that are used to
connect equipment at the work area. The total length of the patch cords and cross-connect
jumpers used in the horizontal cabling cannot exceed 10 m. A final specification for horizontal
cabling contained in TIA/EIA-568-A requires that all grounding and bonding must conform to
TIA/EIA-607 as well as to any other applicable codes.
The latest industry standards being developed are for Cat 5e, Cat 6, and Cat 7 cabling, all of
which offer improvements over Cat 5.
Upcoming Changes in Cabling Standards
Web Links
Industry Standards for Structured Cabling
5.2.4 Networking media and terminations
Cables
must eventually be terminated in order to provide connectivity. This process
involves much transition and innovation as far as computer networking is concerned. This
presents a tremendous challenge for students, who must learn a wide variety of networking
medium standards, properties, and terminations.
Explore the following Web sites to examine the variety of cable and termination products that
are available.
Web Links
AMP
ANIXTER
Belden
Panduit Wiring and Communication Products
Industry Standards for Structured Cabling
Cable Termination and Installation
5.3 Making and Testing Cable
5.3.1 Testing Ethernet 10BASE-T patch cables with a cable tester
Lab Activity
In this lab, instructor will give you functional, intermittent, and bad cables (Ethernet
10BASE-T RJ-45 568-B) to test for continuity, using the Fluke 620 cable tester (or its
equivalent).
---------------------------------------------------------------------------------------------------------Lab 5.3.1 Basic Cable Tester
Estimated time: 30 min.
Objectives:

Use a cable tester to verify that a straight-thru or crossover cable is
good or bad
Background:
UTP Ethernet Cabling: Cabling is one of the most critical areas of network
design and implementation. The cabling is expected to last from 10 to 15 years.
The quality of cable and connections is a major factor in reducing network
problems and time spent troubleshooting. Unshielded Twisted Pair (UTP)
copper cable is the most common cable used in Ethernet networks. There are
various Categories (CAT 3, CAT 5, CAT 5e etc.) but all of them contain 8 wires
or conductors and use RJ-45 connectors. A UTP patch cable in a network is
usually wired as a straight-thru or crossover. In order to follow proper
specifications, all 8 conductors must be used even though with most earlier
versions of Ethernet, not all 8 conductors were used. You will create these
cables in the future labs. In this lab you will work with several cables that have
already been made and will test them for basic continuity (breaks in wires) and
shorts (2 or more wires touching) using a basic cable tester. (refer to the lab on
resistance measurements).
Basic Cable Testers: There are a number of very simple and inexpensive
basic cable testers available (less than $100). They usually consist of one or
two small boxes with RJ-45 jacks to plug the cables to be tested into. Many of
these are designed specifically to test only Ethernet UTP type of cable. The
testers will have more than one jack to allow for testing of straight thru or
crossover cable. Both ends of the cable are plugged in to the proper jacks and
the tester will test all 8 wires and indicate whether the cable is good or bad. If
any of the eight wires has a break or is shorted to any of the other wires, the
cable is bad. The simple testers may just have a single light to indicate this;
others may have eight lights to tell you which wire is bad. These testers have
internal batteries and are doing continuity checks on the wires.
Advanced Cable Testers: Advanced cable testers, such as the Fluke 620 LAN
CableMeter, are sophisticated cable testers which have basic cable testing
functions and much more. You will use an advanced cable tester in future labs
to do wire maps etc. If an inexpensive basic cable tester is unavailable, the
Fluke (or equivalent) is more than adequate. Advanced cable tests can cost
from hundreds to thousands of dollars.
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have several basic
cable testers available (one for each team of students) or several Fluke Cable
meters and various lengths of wire with induced problems. Work in teams of
two. The following resources will be required:

Basic cable tester

Advanced cable tester (Fluke 620 or equivalent)

2 lengths of good CAT 5 cable (one crossover and one straight-thru,
use different colors or labels)

2 lengths of bad CAT 5 cable (one with a break and one with a short,
use different colors or labels)
If you are using a basic cable tester, refer to the instructions from the
manufacturer and insert the ends of the cable to be tested into the jacks
accordingly. If you are using the Fluke 620, use the following instructions to test
the four cables. Insert the RJ-45 from one end of the cable into the UTP/FTP
jack on the tester and turn the dial to test. All conductors will be tested to verify
they are not broken or shorted. (Note: this test does not verify that the pins are
connected correctly from one end to the other)
Notes:
Step 1 - For each test, insert the cable into the RJ-45 jack(s) of the cable tester and
record your results below:
Color
Cable #1
Cable #2
Cable #3
Cable #4
Cable #5
Web Links
or Category
Staight-
Cable
Type(CAT 3, through
or
number?
CAT 5 etc?) crossover?
Length
cable
of
Test results
Cable
Termination
and
Installation
5.3.2
Making and testing Ethernet
10BASE-T straight-thru patch
cable
Lab Activity
In this lab you will use the following steps to make
and test a straight-thru patch cable.
----------------------------------------------------------------------------------------------------------------------------- -
Lab 5.3.2 Straight-Thru Cable
Estimated time: 30 min.
Objectives:

Build a straight-thru Ethernet patch cable to T568-B (OR T568-A)
standards for connection from workstation to hub/switch or patch panel
to hub/switch.
Background:
In this lab you will learn how to build a Category 5 (CAT 5) Unshielded Twisted
Pair (UTP) Ethernet network patch cable (or patch cord) and test it for good
connections (continuity) and correct pinouts (correct color of wire on the right
pin). This will be a 4-pair (8-wires) "straight through" cable which means that
the color of wire on pin 1 on one end of the cable will be the same as pin 1 on
the other end. Pin 2 will be the same as pin 2 and so on. It will be wired to
TIA/EIA-568-B or A standards for 10BASE-T Ethernet which determines what
color wire is on each pin. T568-B (also called AT&T specification) is more
common, but many installations are also wired to T568-A (also called ISDN).
This patch cable will conform to the structured cabling standards. It is
considered to be part of the total "horizontal" cabling run which is limited to 99
meters total between workstation and hub or switch. A patch cable can be used
in a workstation area to connect the workstation NIC to the wall plate data jack.
It can also be used in the wiring closet to connect the patch panel (horizontal
cross connect) to an Ethernet hub or switch. Patch cables are wired straight
thru since the cable from the workstation to the hub or switch is normally
crossed over automatically at the switch or the hub. Note that the ports on most
hubs have an X next to them. This means the send and receive pairs will be
crossed when the cabling reaches the switch. The pinouts will be T568-B and
all 8 conductors (wires) should be terminated with RJ-45 modular connectors
(only 4 of the 8 wires are used for 10/100BASE-T Ethernet, all 8 are used for
1000BASE-T Ethernet).
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have a spool of Cat
5 Unshielded Twisted Pair (UTP) cable, RJ-45 (8-pin) connectors, an RJ-45
crimping tool and an Ethernet / RJ-45 continuity tester available. Work
individually or in teams. The following resources will be required:

Two to three foot length of Cat 5 cabling (one per person or one per
team)

Four RJ-45 connectors (two extra for spares)

RJ-45 crimping tools to attach the RJ-45 connectors to the cable ends

Ethernet cabling continuity tester which can test straight-thru or
crossover type cables (T568-A or T568-B).

Wire cutters
Step 1 - Cabling Information.
Explanation: Instructions are provided here for building a T568-A or T568-B
cable. Either can be used as long as all connections (pinouts) from the
workstation to the wiring closet and terminating electronics (hubs or switches)
are consistent. If cables are to be built for an existing network it is important to
keep the same standard as already exists (either T568-A or B). A patch cable
that is wired "straight through" will have the same color of wire on the same pin
(1 – 8) at both ends. A straight through patch cable (T568-A or B) can be used
to connect a PC workstation to a wall plate in a work area or it can be used to
connect from a patch panel in a wiring closet to a hub or a switch. A PC can
also be connected directly to a port on a hub or switch with this cable. If a cable
will be used to connect from an "uplink" port on one hub to a "crossover" front
port on another hub then a straight through cable should be used
Step 2 - Create a T568-B straight-thru patch panel cable.
Task: Use the following tables and diagrams and steps to create a T568-B
patch panel cable.
Explanation: Both cable ends should be wired the same when looking at the
conductors. Only four wires are used with 10BASE-T or 100BASE-TX Ethernet:
T568-B Cabling
Pin#
Pair#
Function
Wire Color
Used with
Used with 100
10/100 BASE-T
BASE-T4 and
Ethernet?
1000 BASE-T
Ethernet?
1
2
Transmit
White/Orange
Yes
Yes
2
2
Transmit
Orange/White
Yes
Yes
3
3
Receive
White/Green
Yes
Yes
4
1
Not used
Blue/White
No
Yes
5
1
Not used
White/Blue
No
Yes
6
3
Receive
Green/White
Yes
Yes
7
4
Not used
White/Brown
No
Yes
8
4
Not used
Brown/White
No
Yes
1. Determine the distance between devices, or device and plug, then add
at least 12" to it. The maximum length for this cord is 3 m; standard
lengths are 6' and 10'.
2. Cut a piece of stranded Cat 5 unshielded twisted-pair cable to the
determined length. You will use stranded cable for patch cables
because it is more durable when bent repeatedly. Solid wire is fine for
cable runs that are punched down into jacks.
3. Strip 2" of jacket off of one end of the cable.
4. Hold the 4 pairs of twisted cables tightly where jacket was cut away,
then reorganize the cable pairs into the order of the 568-B wiring
standard. Take care to maintain the twists since this provides noise
cancellation. (orange pair, green pair, blue pair, brown pair).
5. Hold the jacket and cable in one hand, untwist a short length of the
green and blue pairs, and reorder them to reflect the 568-B wiring color
scheme. Untwist and order the rest of the wire pairs according to the
color scheme.
6. Flatten, straighten, and line up the wires, then trim them in a straight
line to within 1/2" - 3/4" from the edge of the jacket. Be sure not to let
go of the jacket and the wires, which are now in order! You should
minimize the length of untwisted wires because overly-long sections
that are near connectors are a primary source of electrical noise.
7. Place an RJ-45 plug on the end of the cable, with the prong on the
underside and the orange pair to the left side of the connector.
8. Gently push the plug onto wires until you can see the copper ends of
the wires through the end of the plug. Make sure the end of the jacket
is inside the plug and all wires are in the correct order. If the jacket is
not inside the plug, it will not be properly strain relieved and will
eventually cause problems. If everything is correct, crimp the plug hard
enough to force the contacts through the insulation on the wires, thus
completing the conducting path.
9. Repeat steps 3-8 to terminate the other end of the cable, using the
same scheme to finish the straight through cable.
10. Test the finished cable and have the instructor check it. How can you
tell if your cable is functioning properly?
---------------------------------------------------------------------------------------------------------- --------------------
As specified in the 568 standards, your cable may have a maximum length of 3 m.
Cut a length of cable.
Strip off the jacket.
Separate out the 4 pairs of wires.
Untwist the wires.
Organize the wires according to the proper color code and flatten the wires.
Maintain the color order and flatness of the wires, then clip their length so that a maximum
of 1.2 cm of untwisted wire is present.
Insert ordered wires into RJ-45 plug; make sure jackets are inserted into plug.
Push the wires in firmly enough to make sure the conductors are all visible when you look
at the plug from the end.
Inspect the color code and jacket location to be sure they are correct.
Insert the plug firmly into the crimp tool and crimp down completely.
Inspect both ends visually and mechanically.
Use a cable tester to verify the quality of the cable.
Web Links
Cable Termination and Installation
5.3.3 Making and testing Ethernet 10BASE-T console patch cable
In this lab you will make and test a rollover or console cable. This cable is used to connect a PC
to the router for purposes of accessing the router.
Lab Activity
In this lab you will make and test a rollover or console cable.
---------------------------------------------------------------------------------------------------------Lab 5.3.3 Rollover Cable
Estimated time: 30 min.
Objectives:

Build a rollover cable for connection from a workstation to the console port on a router
or switch
Background:
In this lab you learn how to build a Category 5 (CAT 5) Unshielded Twisted Pair
(UTP) console rollover cable and test it for good connections (continuity) and
correct pinouts (correct wire on the right pin). This will be a 4-pair (8-wires)
"rollover" cable.
This cable should be approximately 10 feet in length but can be as long as 25
feet. It can be used to connect a workstation or dumb terminal to the console
port on the back of a router or Ethernet switch in order to be able to configure
the router or switch. This cable uses an asynchronous serial interface to the
router or switch (8 data bits, No parity and 2 Stop bits). Both ends of the cable
you build will have RJ-45 connectors on them. One end plugs directly into the
RJ-45 console management port on the back of the router or switch and the
other end plugs into an RJ-45-to-DB9 terminal adapter. This adapter converts
the RJ 45 to a 9-pin female D connector that plugs into the DB9 serial port male
adapter on the back of a PC running terminal emulation software such as
HyperTerminal. A DB25 terminal adapter is also available to connect with a
dumb terminal which has a 25 pin connector.
A rollover cable uses 8 pins but is different from the straight-thru cable or
crossover cable that you will build in other labs. With a rollover cable, pin 1 on
one end connects to pin 8 on the other end. Pin 2 connects to pin 7, pin 3
connects to pin 6 and so on. This is why it is referred to as a rollover since the
pins on one end are all reversed on the other end as though one end of the
cable was just rotated or rolled over.
A flat black rollover cable comes with each new router or switch along with the
terminal adapters for both DB9 and DB25 connections to terminals or PC serial
ports. It is approximately 8 feet long. This lab will enable you to build another
cable if the one that comes with the router or switch is damaged or lost. It will
also allow you to connect to routers or switches from workstations that are
greater than 8 feet away by building your own longer cables.
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have a spool of Cat
5 Unshielded Twisted Pair (UTP) cable, RJ-45 (8-pin) connectors, an RJ-45
crimping tool and a continuity tester available. Work individually or in
teams. The following resources will be required:

10 to 20 foot length of Cat 5 cabling (one per person or one per team)

Four RJ-45 connectors (two extra for spares)

RJ-45 crimping tools to attach the RJ-45 connectors to the cable end

An RJ-45 to DB9 female terminal adapter (available from Cisco)

Cabling continuity tester

Wire cutters
Step 1. Review Cable Connections and Pin Locations.
Use the table as a reference to answer the questions below and to help you
create a rollover console cable.
Questions:
1. Which signal on the Router port (column 1 of the table) will be used to
transmit data to the PC when the PC is first connected and
HyperTerminal is started (this is what displays the router prompt on the
workstation.)?
2. Which pin is this connected to on the router end of the RJ-45 cable?
3. Which pin is this connected to on the other end of the RJ-45 cable?
4. Which pin is this connected to in the DB9 connector?
5. Which console device signal does this connect to?
6. What would happen if pin 3 on the left cable end were attached to pin 3
as with a straight-thru cable?
Rollover Console Cable Table
For connecting from a router or switch console port to a PC workstation running
HyperTerminal terminal emulation software. Console port signaling and cabling
using an RJ-45 rollover and DB9 Adapter.
Router
switch
Console
or RJ-45 to RJ-45 RJ-45 to RJ-45 RJ-45 to DB9 Console
Rollover Cable Rollover Cable Adapter
port (left end)
(right end)
Device
(PC
workstation
(DTE)
Signal
serial port)
From
RJ-45 To RJ-45 Pin DB9 Pin No.
Signal
Pin No.
No.
RTS
1
8
8
CTS
DTR
2
7
6
DSR
TxD
3
6
2
RxD
GND
4
5
5
GND
GND
5
4
5
GND
RxD
6
3
3
TxD
DSR
7
2
4
DTR
CTS
8
1
7
RTS
Signal Legend: RTS = Request To Send, DTR = Data Terminal Ready, TxD =
Transmit Data, GND = Ground (One for TxD and one for RxD), RxD = Receive
Data, DSR = Data Set Ready, CTS = Clear To Send.
Step 2. Use the following steps to build the rollover console cable.
1. Determine the distance between devices, then add at least 12" to it.
Make your cable about 10 feet unless you are connecting to router or
switch from a greater distance. The maximum length for this cable is
about 8m (appx 25 feet).
2. Strip 2" of jacket off of one end of the cable.
3. Hold the 4 pairs of twisted cables tightly where jacket was cut away,
then reorganize the cable pairs and wires into the order of the 568-B
wiring standard. You can order them in any sequence but use the 568B sequence to become more familiar with it.
4. Flatten, straighten, and line up the wires, then trim them in a straight
line to within 1/2" - 3/4" from the edge of the jacket. Be sure not to let
go of the jacket and the wires, which are now in order!
5. Place an RJ-45 plug on the end of the cable, with the prong on the
underside and the orange pair to the left side of the connector.
6. Gently push the plug onto wires until you can see the copper ends of
the wires through the end of the plug. Make sure the end of the jacket
is inside the plug and all wires are in the correct order. If the jacket is
not inside the plug, it will not be properly strain relieved and will
eventually cause problems. If everything is correct, crimp the plug hard
enough to force the contacts through the insulation on the wires, thus
completing the conducting path.
7. Repeat steps 2-6 to terminate the other end of the cable, but reversing
every pair of wires as indicated in the table above. (pin 1 to pin 8, pin 2
to pin 7, pin 3 to pin 6 and so on. Alternate Method - Arrange the wires
into the order of the 568-B wiring standard. Place a RJ-45 plug on the
end with the prong on the top side of the connector. This method will
achieve the proper reversing of every pair of wires.
8. Test the finished cable and have the instructor check it. How can you
tell if your cable is functioning properly?
---------------------------------------------------------------------------------------------------------Web Links
Cable Termination and Installation
5.3.4 Making and testing Ethernet 10BASE-T crossover cable
Lab Activity
In this lab you will make and test a crossover cable. Crossover cables are used to
connect one device to another.
-------------------------------------------------------------------------------------------------Lab 5.3.4 Crossover Cable
Estimated time: 30 min.
Objectives:

Build a crossover Ethernet patch cable to T568-B (or T-568-A) standards for connection
from workstation to workstation or from switch to switch.
Background:
In this lab you will learn how to build a Category 5 (CAT 5) Unshielded Twisted
Pair (UTP) Ethernet crossover network cable and test it for good connections
(continuity) and correct pinouts (correct color of wire on the right pin). This will
be a 4-pair (8-wires) "crossover" cable which means that pairs 2 and 3 on one
end of the cable will be reversed on the other end. It will be wired to TIA/EIA568-B and A standards for 10BASE-T Ethernet which determines what color
wire is on each pin. The pinouts will be T568-A on one end and T568-B on the
other end. All 8 conductors (wires) should be terminated with RJ-45 modular
connectors.
This patch cable will conform to the structured cabling standards and, if it is
used between hubs or switches, is considered to be part of the "vertical" cabling
also know as backbone cable. A crossover cable can be used as a backbone
cable to connect two or more hubs or switches in a LAN or to connect 2 isolated
workstations to create a mini-LAN. This will allow you to connect two
workstations together or a server and a workstation without the need for a hub
between them. This can be very helpful for training and testing. If you want to
connect more than two workstations you will need a hub or a switch.
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have a spool of Cat
5 Unshielded Twisted Pair (UTP) cable, RJ-45 (8-pin) connectors, a RJ-45
crimping tool and an Ethernet / RJ-45 continuity tester available. Work
individually or in teams. The following resources will be required:

Two to three foot length of Cat 5 cabling (one per person or one per
team)

Four RJ-45 connectors (two extra for spares)

RJ-45 crimping tools to attach the RJ-45 connectors to the cable ends

Ethernet cabling continuity tester which can test crossover type cables
(T568-A to T568-B).

Wire cutters
Step 1 – Create a crossover patch panel cable.
Use the following tables and diagrams and steps to create a crossover cable.
One end of the cable should be wired to the T568-A standard and the other end
to the T568-B standard. This crosses the transmit and receive pairs (2 and 3) to
allow communication to take place. Only four wires are used with 10BASE-T or
100 BASE-TX Ethernet:
T568-A Cabling
Pin#
Pair#
Function
Wire Color
Used with
Used with 100
10/100 BASE-T
BASE-T4 and
Ethernet?
1000 BASE-T
Ethernet?
1
3
Transmit
White/Green
Yes
Yes
2
3
Transmit
Green/White
Yes
Yes
3
2
Receive
White/Orange
Yes
Yes
4
1
Not used
Blue/White
No
Yes
5
1
Not used
White/Blue
No
Yes
6
2
Receive
Orange/White
Yes
Yes
7
4
Not used
White/Brown
No
Yes
8
4
Not used
Brown/White
No
Yes
Function
Wire Color
Used with
Used with 100
10/100 BASE-T
BASE-T4 and
Ethernet?
1000 BASE-T
T568-B Cabling
Pin#
Pair#
Ethernet?
1
2
Transmit
White/Orange
Yes
Yes
2
2
Transmit
Orange/White
Yes
Yes
3
3
Receive
White/Green
Yes
Yes
4
1
Not used
Blue/White
No
Yes
5
1
Not used
White/Blue
No
Yes
6
3
Receive
Green/White
Yes
Yes
7
4
Not used
White/Brown
No
Yes
8
4
Not used
Brown/White
No
Yes
1. Determine the distance between devices, or device and plug, then add
at least 12" to it. The maximum length for this cord is 3 m; standard
lengths are 6' and 10'.
2. Cut a piece of stranded Cat 5 unshielded twisted-pair cable to the
determined length. You will use stranded cable for patch cables
because it is more durable when bent repeatedly. Solid wire is fine for
cable runs that are punched down into jacks.
3. Strip 2" of jacket off one end of the cable.
4. Hold the 4 pairs of twisted cables tightly where jacket was cut away,
then reorganize the cable pairs into the order of the 568-B wiring
standard. Take care to maintain the twists since this provides noise
cancellation. (orange pair, green pair, blue pair, brown pair)
5. Hold the jacket and cable in one hand, untwist a short length of the
green and blue pairs, and reorder them to reflect the 568-B wiring color
scheme. Untwist and order the rest of the wire pairs according to the
color scheme.
6. Flatten, straighten, and line up the wires, then trim them in a straight
line to within 1/2" - 3/4" from the edge of the jacket. Be sure not to let
go of the jacket and the wires, which are now in order! You should
minimize the length of untwisted wires because overly-long sections
that are near connectors are a primary source of electrical noise.
7. Place an RJ-45 plug on the end of the cable, with the prong on the
underside and the orange (green on the 586-A end) pair to the left side
of the connector.
8. Gently push the plug onto wires until you can see the copper ends of
the wires through the end of the plug. Make sure the end of the jacket
is inside the plug and all wires are in the correct order. If the jacket is
not inside the plug, it will not be properly strain relieved and will
eventually cause problems. If everything is correct, crimp the plug hard
enough to force the contacts through the insulation on the wires, thus
completing the conducting path.
9. Repeat steps 3-8 to terminate the other end of the cable, using the 568A scheme to finish the crossover cable.
10. Test the finished cable and have the instructor check it. How can you
tell if your cable is functioning properly?
----------------------------------------------------------------------------------------------------Web Links
Cable Termination and Installation
5.3.5 Features of an advanced cable tester
Lab Activity
In this lab you will learn the advanced features of the Fluke 620 LAN CableMeter (or its
equivalent).
Lab 5.3.5 Cable Tester - Wire Map
Estimated time: 45 min.
Objectives:

Demonstrate skill with a cable tester more advanced than a simple continuity/pin-out
tester

Use the wire mapping features of the tester to check for opens and shorts with UTP
cable

Perform wire mapping on cables, detecting faults that are not detectable with simple
continuity measuring devices such as crossed and split pairs
Background:
In this lab you will learn the wire mapping features of the Fluke 620 LAN
CableMeter (or its equivalent). Wire maps can be very helpful in troubleshooting
cabling problems with UTP cable. Wire maps are not used with coaxial cable
since there is only one wired and no map is needed. A wire map allows the
network technician to verify which pins on one end of the cable are connected
to which pins on the other end. With an understanding of the proper wiring
connections, you can determine when a cable is wired improperly, depending
on its intended use. You will learn how to use a cable tester to check for the
proper installation of Unshielded Twisted Pair (UTP) Category 5 (CAT 5)
according to TIA/EIA-568 cabling standards in an Ethernet network. You will
test different cables using all four pairs to determine some problems that can
occur from incorrect cabling installation and termination.
The cabling infrastructure (or cable plant) in a building is expected to last at
least 10 years. Cabling related problems are one of the most common causes
of network failure. The quality of cabling components used, the routing and
installation of the cable and quality of the connector terminations will be the
main factors in determining how trouble-free the cabling will be.
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have several
correctly wired CAT 5 cables (both straight thru and crossover) to test. There
should also be several CAT 5 cables created with problems such as poor
connections and split pairs to test. Cables should be numbered to simplify the
testing process and to maintain consistency. A cable tester should be available
that can test at least continuity, cable length and wire map. Work individually or
in teams. The following resources will be required:

CAT 5 straight-wired cables of different colors.

CAT 5 crossover-wired cable (T568A on one end and T568B on the
other )

CAT 5 straight-wired cables with open wire connections in the middle or
one or more conductors shorted at one end of different colors and
different lengths.

CAT 5 straight-wired cable with a split pair mis-wire.

Cable Tester (Fluke 620 LAN CableMeter or similar) to test cable length,
continuity, wire map
Related Research Web Sites:

http://www.bicsi.org/techsem

http://www.fluke.com/
Step 1 - Set the Advanced Cable Tester for the Desired Cable.
These instructions pertain to the Fluke 620 LAN CableMeter. Turn the rotary
switch selector on the tester to the WIRE MAP position. Press the SETUP
button to enter the setup mode and observe the LCD screen on the tester. The
first option should be CABLE: Press the UP or DOWN buttons until the desired
cable type of UTP is selected. Press ENTER to accept that setting and go to
the next one. Continue pressing the UP/DOWN arrows and pressing ENTER
until the tester is set to the following cabling characteristics.
Tester Option
Desired Setting
CABLE:
UTP
WIRING:
10BASE-T or EIA/TIA 4PR
CATEGORY:
CAT 5
WIRE SIZE
AWG 24
CAL to CABLE?
NO
BEEPING:
ON or OFF
LCD CONTRAST
From 1 thru 10 (brightest)
Step 2. Setup the Cable to be Tested
For each cable to be tested use the following procedure, place the near end of
the cable into the RJ-45 jack labeled UTP/FTP on the tester. Place the RJ-45RJ-45 female coupler on the far end of the cable and then insert the Cable
Identifier into the other side of the coupler. The coupler and the cable identifier
are accessories that come with the Fluke 620 LAN CableMeter.
Step 3 – Perform Wire Map Testing
Using the tester Wire Map function and a Cable ID Unit, you can determine the
wiring of both the near and far end of the cable. The top set of numbers
displayed on the LCD screen is the near end and the bottom set is the far end.
Perform a Wire Map test on each of the cables provided and fill in the following
table based on the result for each CAT 5 cable tested. For each cable, write
down the number and color, whether the cable is straight thru or crossover, the
tester screen test results and what you think the problem is.
Cable
Cable
How cable is Tester
No.
Color
wired (straight- Results (Note: refer to the Description
thru
crossover)
Displayed
or Fluke manual for detailed
description of test results
for wire map)
1
2
3
4
Test Problem
Top:
Bot:
Top:
Bot:
Top:
Bot:
Top:
Bot:
5
Top:
Bot:
Web Links
Cable Termination and Installation
5.3.6 Cable identification experiments using an advanced cable tester
Lab Activity
In this lab you will perform cable identification experiments using the Fluke 620 LAN
CableMeter (or its equivalent).
Lab 5.3.6 Straight-Thru Cable
Estimated time: 45 min.
Objectives:

Demonstrate skill with a cable tester more advanced than a simple
continuity/pin-out tester

Use the Test feature of the tester to check for opens and shorts with
coax and UTP cable

Understand the use of the Cable ID feature
Background:
In this lab you will learn the Cable Test – Pass / Fail features of the Fluke 620
LAN CableMeter (or its equivalent). Basic cable tests can be very helpful in
troubleshooting cabling problems with UTP and coaxial cable. You will learn
how to use a cable tester to check for the proper installation of Unshielded
Twisted Pair (UTP) and Coaxial (Thinnet) for an Ethernet network. You will test
different cables to determine some problems that can occur from incorrect
cabling installation and termination.
The cabling infrastructure (or cable plant) in a building is expected to last at
least 10 years. Cabling related problems are one of the most common causes
of network failure. The quality of cabling components used, the routing and
installation of the cable and quality of the connector terminations will be the
main factors in determining how trouble-free the cabling will be.
Tools / Preparation:
Prior to starting the lab, the teacher or lab assistant should have several
correctly wired CAT 5 cables (both straight thru and crossover) to test. There
should also be several CAT 5 cables created with problems and a coaxial cable
to test. Cables should be numbered to simplify the testing process and to
maintain consistency. A cable tester should be available that can do basic cable
test for UTP and coax. Work individually or in teams. The following resources
will be required:

CAT 5 straight-thru and crossover wired cables of different colors.
(some good and some bad)

CAT 5 straight-thru and crossover wired cables with open wire
connections in the middle or one or more conductors shorted at one
end of different colors and different lengths

Coax cable with a short in it

Cable Tester (Fluke 620 LAN CableMeter or similar) to test cable length,
continuity, wire map
Related Research Web Sites:
Here are several web sites where you can get additional information on cabling
standards:

http://www.bicsi.org/techsem

http://www.fluke.com/
Step 1 - Set the Advanced Cable Tester for the Desired Cable (UTP or
COAX).
These instructions pertain to the Fluke 620 LAN CableMeter. Turn the rotary
switch selector on the tester to the TEST position. Press the SETUP button to
enter the setup mode and observe the LCD screen on the tester. The first
option should be CABLE: Press the UP or DOWN buttons until the desired
cable type of UTP or COAX (thinnet) is selected. Press ENTER to accept that
setting and go to the next one. Continue pressing the UP/DOWN arrows and
pressing ENTER until the tester is set to the following cabling characteristics
depending on the type of cable you will be testing.
Tester Option
Desired
UTP
Setting
- Desired
COAX
Setting
-
CABLE:
UTP
COAX
10BASE-T or EIA/TIA 10BASE2
WIRING:
4PR
(thinnet)
CATEGORY:
CAT 5
N/A
WIRE SIZE
AWG 24
N/A
CAL to CABLE?
NO
NO
BEEPING:
ON or OFF
ON or OFF
From
LCD CONTRAST
1
(brightest)
thru
10 From
1
or
RG58
thru
10
(brightest)
Step 2. Setup the Cable to be Tested (UTP or COAX)
For each cable to be tested use the following procedure, place the near end of
the cable into the RJ-45 jack labeled UTP/FTP on the tester. Place the RJ-45RJ-45 female coupler on the far end of the cable and then insert the Cable
Identifier into the other side of the coupler. The coupler and the cable identifier
are accessories that come with the Fluke 620 LAN CableMeter. Multiple Cable
Id's with different numbers can be purchased to help in identifying which cable
you are working with. For coax cables, insert one end of the BNC connector
into the jack labeled COAX on the tester. Coax cables should not have a
terminating resistor.
Step 3 – Perform Basic Cable Test – Pass/Fail Function
Using the tester’s Test function and a Cable ID Unit (for UTP), you can
determine the functionality of the cable. Perform a basic cable test on each of
the cables provided and fill in the following table based on the result for each
cable tested. For each cable, write down the number and color, whether the
cable is straight-through, crossover or coaxial. Include also the tester screen
test results and what you think the problem is. The Cable ID can be used to
identify a particular cable by moving it to another cable.
Cable
Cable
How
cable Tester Displayed Test Problem
No.
Color
is
wired Results (Note: refer to Description
(UTP
or the Fluke manual for
coax)
detailed description
of
test results)
1
2
3
4
Web Links
Cable Termination and Installation
5.3.7 Length experiments using an advanced cable tester
5.4 Layer 1 Components and Devices
5.4.1 Ethernet 10BASE-T
In this curriculum you will be introduced to three LAN technologies: Ethernet, Token Ring, and
FDDI. All three have a wide variety of Layer 1 components and devices. The focus of this
chapter will be Ethernet 10BASE-T technologies.
When it was developed, Ethernet was designed to fill the middle ground between long distance,
low-speed networks, and specialized computer room networks that carried data at high speeds
for very limited distances. Ethernet is well-suited to applications in which a local communication
medium must carry sporadic, occasionally heavy, traffic at high-peak data rates.
The Ethernet 10BASE-T technologies carry Ethernet frames on inexpensive twisted-pair wiring.
You will study four components and three devices that are related to these technologies. The
first four components are passive, meaning they require no energy to operate. They are:

patch panels

plugs

cabling

jacks
The last three are active. They require energy to do their jobs. They are:

transceivers

repeaters

hubs
For more information on other Ethernet, Token Ring, and FDDI components and devices, go to
the Web site.
Web Links
Blackbox Network Services
5.4.2 Connectors
The standard 10BASE-T termination (end point, 0 plug, connector) is the registered jack-45
connector (RJ-45).
It reduces noise, reflection, and mechanical stability problems, and
resembles a phone plug, except that it has eight conductors instead of four. It is considered a
passive networking component because it only serves as a conducting path between the four
pairs of Category 5 twisted cable and the prongs of the RJ-45 jack. It is considered a Layer 1
component, rather than a device, because it serves only as a conducting path for bits.
5.4.3 Cabling
The standard 10BASE-T cable is CAT 5 twisted-pair cable, which is composed of four twisted
pairs that reduce noise problems. CAT 5 is thin, inexpensive, and easy to install. The function of
CAT 5 cable is to carry the bits, therefore, it is a Layer 1 component.
5.4.4 Jacks
RJ-45 plugs fit into RJ-45 jacks or receptacles. The RJ-45 jack has eight conductors
snap together with the RJ-45 plug. On the other side of the RJ-45 jack
, which
is a punch down block
where wires are separated out and forced into slots with a fork-like tool called a punch-down
tool. This provides a copper-conducting path for the bits. The RJ-45 jack is a Layer 1 component.
5.4.5 Patch panels
Patch panels are convenient groupings of RJ-45 jacks. They come in 12, 24, and 48 ports, and
are typically rack-mounted. The front sides are RJ-45 jacks
; the back sides
are punch-
down blocks that provide connectivity or conducting paths. They are classified as Layer 1
devices.
5.4.6 Transceivers
A transceiver is a combination of transmitter and receiver. In networking applications, this
means that they convert one form of signal to another form. For example, many networking
devices come with an auxiliary unit interface and a transceiver that allows a 10BASE2,
10BASE5, 10BASE-T, or 10\100 BASE-FX to be connected to the port. A common application is
the conversion of AUI ports to RJ-45 ports. They are Layer 1 devices. They transmit from one
pin configuration and/or media to another. Transceivers are often built into NICs, which are
typically considered Layer 2 devices. Transceivers on NICs are called signaling components,
which means they encode signals onto the physical medium.
5.4.7 Repeaters
Repeaters
regenerate, and retime signals, which then enables cables to extend farther to
reach longer distances. They only deal with packets at the bit level, therefore they are Layer 1
devices.
Repeaters are internetworking devices that exist at the physical layer (Layer 1) of the OSI
model. They can increase the number of nodes that can be connected to a network, and thus,
the distance over which the network can extend. Repeaters re-shape, regenerate, and retime
signals before sending them on along the network.
The disadvantage of using repeaters is that they cannot filter network traffic. Data (bits) that
arrive at one port of a repeater are sent out on all other ports. The data gets passed along to all
other LAN segments of a network regardless of whether or not it needs to go there.
5.4.8 Multiport repeaters (hubs)
Multiport repeaters combine connectivity with the amplifying and re-timing properties of
repeaters. It is typical to see 4, 8, 12, and up to 24, ports on multiport repeaters. This allows
many devices to be cheaply and easily interconnected. Multiport repeaters are often called hubs,
instead of repeaters, when referring to the devices that serve as the center of a star topology
network. Hubs are very common internetworking devices. Since the typical unmanaged hub
only requires power and plugged-in RJ-45 jacks, they are great for setting up a network quickly.
Like the repeaters on which they are based, they only deal with bits, and are Layer 1 devices.
5.4.9 OSI Layer 1 components and devices
All of these devices - passive and active - create or act on the bits. They recognize no
information patterns in the bits, no addresses, and no data. Their function is simply to move bits
around. Layer 1 is fundamental to troubleshooting networks, and should never be
underestimated. Many network problems are traceable to bad RJ-45 terminations, jacks, punchdowns, repeaters, hubs, or transceivers.
5.5 Collisions and Collision Domains in Shared Layer Environments
5.5.1 Shared media environment
Some networks are directly-connected; all hosts share Layer 1. Examples are:

shared media environment - occurs when multiple hosts have access to the same
medium. For example, if several PCs are attached to the same physical wire, optical
fiber, or share the same airspace, they all share the same media environment.
Occasionally you may hear someone say "all the computers are on the same wire" . It
means that they all share the same media - even though the "wire" might be CAT 5
UTP, which has four pairs of wire.

extended shared media environment - is a special type of shared media environment in
which networking devices can extend the environment so that it can accommodate
multiple-access, or more users. There are, however, negative aspects to this as well as
positive aspects.

point-to-point network environment - is most widely used in dial-up network connections,
and is the one with which you are most likely familiar. It is a shared networking
environment in which one device is connected to only one other device via a link, such
as you connecting to internet service provider by phone line.
Some networks are indirectly-connected, meaning that some higher layer networking devices
and/or some geographical distance is between the two communicating hosts. There are two
types:

circuit-switched - an indirectly-connected network in which actual electrical circuits are
maintained for the duration of the communication. The current telephone system is still,
in part, circuit-switched, although the telephone systems in many countries is now
concentrating less on circuit-switched technologies.

packet-switched - rather than dedicating a link as an exclusive circuit connection
between two communicating hosts, the source sends messages in packets. Each
packet contains enough information for it to be routed to the proper destination host.
The advantage is that many hosts can share the same link; the disadvantage is that
conflicts can occur.
5.5.2 Collisions and collision domains
A situation that can occur, when two bits propagate at the same time on the same network, is a
collision. A small, slow network could work out a system that allowed only two computers to
send messages, each agreeing to take turns. That would mean that they could both send
messages, but there would be only one bit on the system. The problem is that many computers
are connected to large networks, each one wanting to communicate billions of bits every second.
It's also important to remember that the "bits" are actually packets containing many bits.
Serious problems can occur as a result of too much traffic on a network. If there is only one
cable that interconnects all of the devices on a network, the possibility of conflicts with more
than one user sending data at the same time is very high. The same is true if segments of a
network are only connected by non-filtering devices, such as repeaters. Ethernet allows only
one data packet to access the cable at any one time. If more than one node attempts to transmit
at the same time, a collision occurs, and the data from each device suffers damage .
The area within the network, where the data packets originated and collided, is called a collision
domain, and includes all shared media environments. One wire may be connected to another
wire through patch cables, transceivers, patch panels, repeaters, and even hubs. All of these
Layer 1 interconnections are part of the collision domain.
5.5.3 Signals in a collision
When a collision occurs, the data packets that are involved are destroyed, bit by bit. In order to
avoid this problem, the network should have in place a system that can manage the competition
for the medium (contention). For example, a digital system can only recognize two voltage, light,
or electromagnetic wave states. Therefore, in a collision, the signals interfere, or collide, with
each other. Just as two cars cannot occupy the same space, on the same road, at the same
time, neither can two signals occupy the same medium, at the same time.
5.5.4 Collisions as natural functions of shared media environments and collision
domains
Normally, people think that collisions are bad because they decrease network performance.
However, a certain amount of collisions are a natural function of a shared media environment
(i.e. collision domain). This is because large numbers of computers are all trying to
communicate with each other at the same time, by using the same wire.
The history of how Ethernet handles collisions and collision domain dates back to research at
the University of Hawaii. In its attempts to develop a wireless communication system for the
Islands of Hawaii, university researchers developed a protocol called Aloha. This protocol was
instrumental in the development of Ethernet.
5.5.5 Shared access as a collision domain
As a networking professional, one important skill is the ability to recognize collision domains. If
you connect several computers to a single medium that has no other networking devices
attached, you have a shared-access situation, and you have a collision domain. Depending on
the particular technology used, this situation limits the number of computers that can use that
part of the medium, also called a segment.
5.5.6 Repeaters and collision domains
Repeaters regenerate and retime bits, but they cannot filter the flow of traffic that pass through
them. Data (bits) that arrive at one port of a repeater are sent out on all other ports. Using a
repeater extends the collision domain, therefore, the network on both sides of the repeater is
one larger collision domain.
5.5.7 Hubs and collision domains
You have already learned that another name for a hub is a multiport repeater. Any signal that
comes in one port of the hub is regenerated, retimed, and sent out every other port. Therefore,
hubs, which are useful for connecting large numbers of computers, extend collision domains.
The final result is diminished network performance if all the computers on that network are
demanding large bandwidths, simultaneously.
5.5.8 Hubs and repeaters as causes of collision domains
Both repeaters and hubs are Layer 1 devices, therefore they perform no filtering of network
traffic. Extending a run of cable with a repeater, and ending that run with a hub, results in a
larger collision domain.
5.5.9 The four repeater rule
The four repeater rule in Ethernet states, that no more than four repeaters or repeating hubs
can be between any two computers on the network. To assure that a repeated 10BASE-T
network will function properly, the following condition must be true: (repeater delays + cable
delays + NIC delays) x 2 < maximum round-trip delay. Repeater delays for 10BASE-T are
usually less than 2 microseconds per repeater; cable delays are near 0.55 microseconds per
100 m trip; NIC delays are about 1 microsecond per NIC; and the maximum round-trip delay
(the 10BASE-T bit time of 0.1 microseconds times the minimum frame size of 512 bits) is 51.2
microseconds. For a 500 m length of UTP connected by 4 repeaters (hubs) and 2 NIC delays
the total delay would be well below the maximum round-trip delay. Repeater latency,
propagation delay, and NIC latency all contribute to the 4-repeater rule. Exceeding the four
repeater rule can lead to violating the maximum delay limit.
When this delay limit is exceeded, the number of late collisions dramatically increase. A late
collision, is when a collision happens after the first 64 bytes of the frame are transmitted. The
chipsets in NICs are not required to retransmit automatically when a late collision occurs. These
late collision frames add delay referred to as consumption delay. As consumption delay and
latency increase, network performance decreases. This Ethernet rule of thumb is also known as
the 5-4-3-2-1 rule. Five sections of the network, four repeaters or hubs, three sections of the
network are "mixing" sections (with hosts), two sections are link sections (for link purposes), and
one large collision domain.
5.5.10 Segmenting collision domains
Although repeaters and hubs are useful, inexpensive networking devices, they extend collision
domains. If the collision domain becomes too large, this can cause too many collisions and
result in poor network performance.
The size of collision domains can be reduced by using
intelligent networking devices that break up the domains. Examples of this type of networking
device are bridges, switches, and routers. This process is called segmentation.
A bridge can eliminate unnecessary traffic on a busy network by dividing a network into
segments and filtering traffic based on the station address. Traffic between devices on the same
segment does not cross the bridge, and does not affect other segments. This works well as long
as the traffic between segments is not too heavy. Otherwise, the bridge can become a
bottleneck, and actually slow down communication.
5.6 Basic Topologies Used in Networking
5.6.1 Network topologies
The word topology can be thought of as "the study of location." Topology is a subject of study in
mathematics, where maps of nodes (dots) and links (lines) often contain patterns. In this chapter,
you will examine the various topologies used in networking from a mathematical perspective.
Then, you will learn how a physical topology describes the plan for wiring the physical devices.
Finally, you will use a logical topology to learn how information flows through a network to
determine where collisions may occur.
A network may have one type of physical topology
, and a completely different type of
logical topology. For example, Ethernet 10BASE-T uses an extended-star physical topology, but
acts as though it uses a logical bus topology. Token Ring uses a physical star, and a logical ring.
FDDI uses a physical and a logical ring.
5.6.2 Linear bus network topology
Mathematical Perspective
The bus topology has all of its nodes connected directly to one link, and has no other
connections between nodes
Physical Perspective
Each host is wired to a common wire. In this topology, the key devices are those that allow the
host to join or tap into the single shared medium. One advantage of this topology is that all
hosts are connected to each other, and thus, can communicate directly. One disadvantage of
this topology is that a break in the cable disconnects hosts from each other.
Logical Perspective
A bus topology enables every networking device to see all signals from all other devices. This
can be an advantage if you want all information to go to every device.
5.6.3 Ring network topology
Mathematical Perspective
A ring topology is a single closed ring consisting of nodes and links, with each node connected
to only two adjacent nodes.
Physical Perspective
The topology shows all devices wired directly to each other in what is called a daisy-chain. This
is similar to the manner in which a mouse on an Apple PC plugs into the keyboard and then into
the PC.
Logical Perspective
In order for information to flow, each station must pass the information to its adjacent station.
5.6.4 Dual ring network topology
Mathematical Perspective
A dual ring topology consists of two concentric rings, each of which is linked only to its adjacent
ring neighbor. The two rings are not connected.
Physical Perspective
A dual ring topology is the same as a ring topology, except that there is a second, redundant
ring, that connects the same devices. In other words, in order to provide reliability and flexibility
in the network, each networking device is part of two independent ring topologies.
Logical Perspective
A dual ring topology acts like two independent rings, of which, only one at a time is used.
5.6.5 Star network topology
Mathematical Perspective
A star topology has a central node with all links to other nodes radiating from it and allows no
other links.
Physical Perspective
A star topology has a central node with all links radiating from it. Its primary advantage is that it
allows all other nodes to communicate with each other, conveniently. Its primary disadvantage
is that if the central node fails, the whole network becomes disconnected. Depending on the
type of networking device used at the center of the star network, collisions can be a problem.
Logical Perspective
The flow of all information would go through one device. This might be desirable for security or
restricted access reasons, but it would be very susceptible to any problems in the star's central
node.
5.6.6 Extended star network topology
Mathematical Perspective
An extended star topology repeats a star topology, except that each node that links to the center
node is, also, the center of another star.
Physical Perspective
An extended star topology has a core star topology, with each of the end nodes of the core
topology acting as the center of its own star topology. The advantage of this is that it keeps
wiring runs shorter, and limits the number of devices that need to interconnect to any one
central node.
Logical Perspective
An extended star topology is very hierarchical, and information is encouraged to stay local. This
is how the phone system is currently structured.
5.6.7 Tree network topology
Mathematical Perspective
The tree topology is similar to the extended star topology, the primary difference is that it does
not use one central node. Instead, it uses a trunk node from which it branches to other nodes.
There are two types of tree topologies: the binary tree (each node splits into two links); and the
backbone tree (a backbone trunk has branch nodes with links hanging from it).
Physical Perspective
The trunk is a wire that has several layers of branches.
Logical Perspective
The flow of information is hierarchical.
5.6.8 Irregular network topology
Mathematical Perspective
In the irregular network topology there is no obvious pattern to the links and nodes.
Physical Perspective
The wiring is inconsistent; the nodes have varying numbers of wires leading from them. This is
how networks that are in the early stages of construction, or poorly planned, are often wired.
Logical Perspective
There is no obvious pattern to the links and nodes.
5.6.9 Complete (mesh) network topology
Mathematical Perspective
In a complete, or mesh topology, every node is linked directly to every other node.
Physical Perspective
This wiring has very distinct advantages and disadvantages. One advantage is every node is
physically connected to every other node (creating a redundant connection). Should any link fail
to function, information can flow through any number of other links to reach its destination.
Another advantage of this topology is that it allows information to flow along many paths on its
way back through the network. The primary physical disadvantage is that for anything more
than a small number of nodes, the amount of media for the links, and the amount of connections
to the links becomes overwhelming.
Logical Perspective
The behavior of a complete, or mesh topology depends greatly on the devices used.
5.6.10 Cellular network topology
Mathematical Perspective
The cellular topology consists of circular or hexagonal areas, each of which has an individual
node at its center.
Physical Perspective
The cellular topology is a geographic area that is divided into regions (cells) for the purposes of
wireless technology – a technology that becomes increasingly more important each day. There
are no physical links in a cellular topology, only electromagnetic waves. Sometimes the
receiving nodes move (e.g. car cell phone), and sometimes the sending nodes move (e.g.
satellite communication links).
The obvious advantage of a cellular (wireless) topology is that there are no tangible media other
than the earth's atmosphere or the vacuum of off-planet space (and satellites). The
disadvantages are that signals are present everywhere in a cell and, thus, are susceptible to
disruptions (man-made and environmental) and to security violations (i.e. electronic monitoring
and theft of service).
Logical Perspective
Cellular technologies communicate with each other directly (though distance limitations and
interference sometimes make it extremely difficult), or communicate only with their adjacent
cells, which is extremely inefficient. As a rule, cellular-based topologies are integrated with
other topologies, whether they use the atmosphere or satellites.
Summary
In this chapter, you learned that the function of the physical layer is to transmit data. In addition
you learned that the following types of networking media may be used to connect computers:

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single
inner wire conductor.

UTP cable is a four-pair wire medium used in a variety of networks.

STP cable combines the techniques of shielding, cancellation, and twisting of wires.

Fiber-optic cable is a networking medium capable of conducting modulated light
transmissions.
This chapter discussed various criteria, such as rate of data transfer and expense, that help
determine which types of networking media should be used. You learned that TIA/EIA-568-A
and TIA/EIA-569 are the most widely used standards for technical performance of networking
media.
Additionally, this chapter described how bits propagating at the same time on the same network
result in a collision. Lastly, you learned that a network may have one type of physical topology,
and a completely different type of logical topology. In the next chapter, you will learn about
LAN media and the IEEE model and how the data link layer provides reliable transit of data
across a physical link by using Media Access Control (MAC) addresses.