Semester 1 Chapter 5

1
Semester 1 Chapter 5

• Layer 1 - Media, Connections, and Collisions

2
Shielded twisted-pair cable (STP) 100 ohm
combines the techniques of shielding (each group
of wires are surrounded by a shield),
cancellation, and twisting of wires. Resistance
to both EMI and RFI without significantly
increasing the weight or size of the cable.
Another type STP cable is for token-ring
installations. In this type of STP cable, known
as 150 ohm STP, not only is the entire cable
shielded to reduce EMI and RFI but each pair of
twisted wires is shielded from each other in
order to reduce cross-talk. requires an increased
amount of insulation and a larger amount of
shielding. These factors combine to increase the
size, weight, and cost of the cable. It also
requires the installation of large wiring closets
and large wiring ducts, luxuries that many older
buildings cannot provide.
3
Shielded twisted-pair cable has all of the
advantages and disadvantages of unshielded
twisted-pair cable. However, STP affords greater
protection from all types of external
interference and is more expensive than
unshielded twisted-pair cable. The cable need
only be grounded at one end. Maximum cable
length is 100 meters. Usually, installers ground
STP at either the wiring closet or the hub, If
improperly grounded, STP can become major source
of problems, because it allows the shield to act
like an antenna, absorbing electrical signals
from other wires in the cable and from
electrical noise sources outside the cable.
Finally, shielded twisted-pair cable cannot be
run as far as other networking media without
being boosted.
4
Unshielded twisted-pair cable (UTP) is a
four-pair wire medium - composed of pairs of
wires - used in a variety of networks. Each pair
of wires is insulated from the others. This
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
unshielded twisted-pair cable, the number of
twists in the wire pairs varies. Like shielded
twisted-pair cable, UTP cable must follow precise
specifications as to how many twists or braids
are permitted per foot of cable.
5
When used as a networking medium, unshielded
twisted-pair 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 .17", 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. advantages. It is easy to
install is less expensive its real advantage is
its size. disadvantages is more prone to
electrical noise and interference once
considered slower at transmitting data than other
types of cable. today, UTP is considered the
fastest copper-based media. The distance
between signal boosts is shorter for unshielded
twisted-pair than it is for coaxial cable.
6
Coaxial cable consists of a hollow outer
cylindrical conductor that surrounds a single
inner wire made of two conducting elements.
advantages. It can be run, without as many
boosts from repeaters, for longer distances
between network nodes than either shielded or
unshielded twisted-pair cable Coaxial cable is
less expensive than fiber-optic cable,
technology is well known.. When working with
cable consider its size. Coaxial cable comes in a
variety of sizes.
7
The largest diameter is specified for use as
Ethernet backbone cable because it is stiff with
shielding and its jacket is a distinctive yellow
color. Thicknet. The rule of thumb is the more
diffucult the network media is to install, the
more expensive it is to install. Coaxial cable
is more expensive to install than twisted-pair
cable. coaxial cable with an outside diameter of
only .18" (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
lead some people to refer to it as cheapernet.
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. Frequently, installers fail
to do this. poor shield connection is one of the
biggest sources of connection problems
electrical noise that interferes with signal
transmittal on the networking media. despite its
small diameter, thinnet is no longer commonly
used in Ethernet networks.
8
Fiber-optic cable is a networking medium capable
of conducting modulated light transmissions. It
is more expensive 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,
signals that represent bits, are converted into
beams of light. light in fibers is not
considered wireless because the electromagnetic
waves are guided in the optical fiber.
9
1960s, when solid-state laser (laser.html)
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, Fiber-optic cable used for
networking consists of two fibers encased in
separate sheaths. If viewed in cross section, you
would see that each fiber is surrounded by layers
of reflective cladding, a plastic coating made of
Kevlar, and an outer jacket. 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 (refract.html). 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 (tir.html), and
it allows the optical fiber to act like light
pipe, guiding light for tremendous distances,
even around bends.
10
The medium for wireless communication is the
atmosphere. Wireless signals are electromagnetic
waves, were proven to travel through space, empty
of all matter. This feature means that networks
can be built in places where it would normally be
difficult, or impossible to install wires or
cables. The most common application of
wireless data communications is for mobile users.
people in cars or airplanes
satellites remote space probes
space shuttles anyone/anything that
requires network data communications,
without having to rely on copper optical
fiber tethers
11
International Organization for Standardization
(ISO), researched various networks and created a
network model, called the OSI model. It was
designed to help vendors create networks that
would work compatibly and interoperably.
12
When determining what type of network media to
use in a LAN, you should make certain that you
comply with all applicable fire codes, building
codes, and safety standards You should also
follow any established performance standards and
- because of the wide variety of options
available, today, in networking media - to ensure
compatibility and interoperability.
13
Institute of Electrical and Electronics Engineers
(IEEE) Underwriters Laboratories (UL)
Electrical Industries Association (EIA)
Telecommunications Industry Association (TIA)
The latter two organizations, jointly, issue a
list of standards that you will frequently see
listed as the EIA/TIA 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.
14
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.
15
But no cabling job is complete unless it is done
to standards and documented as such.
EIA/TIA has had the greatest impact on networking
media standards. EIA/TIA-568, EIA/TIA-568B, an
EIA/TIA-569 the most widely used standards for
technical performance of networking media. The
EIA/TIA standards specify the minimum.
16
The EIA/TIA standards address six elements of
the LAN cabling process. These are
horizontal cabling telecommunications
closets backbone cabling equipment
rooms work areas entrance
facilities
17
horizontal cabling horizontal cabling includes
the networking media that is used in the area
that extends from the wiring closet to a
workstation. EIA/TIA-568B 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 category 3 or higher
unshielded twisted-pair cable along with its
appropriate connector. The second can be any
one of the following a four-pair 100 ohm
unshielded twisted-pair cable, a 150 ohm shielded
twisted-pair cable, a coaxial cable, or a
two-fiber 62.5/125 µ optical fiber cable and
its According to EIA/TIA-568B, the maximum
distance for cable runs in horizontal cabling is
90 meters or 295 feet. This is true for all types
of category 5 UTP recognized networking media.
Patch cords or cross-connect jumpers located at
the horizontal cross-connect cannot exceed six
meters or 20 feet in length. EIA/TIA-568B also
allows three meters or 9.8 feet 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 ten meters or 33 feet.
18
The student should be able to identify all of
the major cable types by sight.
19
Show the student how functional cables test.
Then the students will be exposed to the common
failure modes for cables, so that they can begin
to recognize the symptoms of a intermittent and
faulty cables. Demonstrate how to make a
straight-through patch cable (Ethernet 10Base-T
RJ-45 568-B). The maximum length for this cord is
3 m standard lengths are 6' and 10'.
Demonstrate how to make a console patch cable
(Ethernet 10Base-T RJ-45 568-B). Creating a
console patch cable, also known as a rollover
cable. These cables are useful in semesters 2, 3,
and 4, as router and switch console cables.
Stress this to the students, different cable,
different wiring standard, different color code,
different pin-out. Demonstrate how to make a
cross-connect cable (Ethernet 10Base-T RJ-45
568-B). These cables are useful for connecting
networking device to networking device PC to PC,
hub to hub, hub to switch, switch to switch. Thus
they too are useful in semesters 2, 3, and 4.
Stress this to the students different cable,
different wiring standard (568-A on one end and
568-B on the other), different color code,
different pin-out.
20
10Base-T terminations with RJ-45 jacks can be
used for 100Base-TX (Fast Ethernet) and
1000Base-T (Gigabit Ethernet) - hence this skill
has a migration path. Also, the installed base of
10Base-T is huge its popularity is still
growing. This is not to diminish the importance
of learning other cable terminations, especially
optical fiber, but we had to choose one and that
one is 10Base-T. Ethernet, Token Ring, and
FDDI. All three technologies have a wide variety
of Layer 1 components and devices. We will focus
on Ethernet 10BaseT technologies.
21
This may seem obvious, but once you consider what
the signal is doing at a connector -- interfacing
with other networking devices active ports or
passive jacks, its importance increases. A lot of
design goes into the connector so that the signal
insertion loss will be minimized and the
impedance will match that of the NIC cards.
Cables and connectors are said to be "tuned",
that is, impedance matched. Secondly,
terminations are a common point-of-failure for
networks -- improper strain relief and poor
crimping being typical culprits. Every point
along the networks -- including the connectors --
are important. Not to mention that they are not
free! The standard 10Base-T termination (end
point, 0 plug, connector) is the Registered
Jack-45 (RJ-45). It reduces noise, reflection,
and mechanical stability problems. It is
considered a Layer 1 component, rather than a
device, because it serves only as a conducting
path for bits..
22
The standard 10Base-T cable is Category 5 (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 component.
23
RJ-45 plugs fit into RJ-45 jacks or receptacles.
The RJ-45 jacks have eight conductors, which snap
together with the RJ-45 plug. On the other side
of the RJ-45 jack is a punch down block where
wires are separated out and forced into slots
with a fork-like tool. This provides a
copper-conducting path for the bits. The RJ45
jacks are Layer 1 components.
24
RJ-45 plugs fit into RJ-45 jacks or receptacles.
The RJ-45 jacks have eight conductors, which snap
together with the RJ-45 plug. On the other side
of the RJ-45 jack is a punch down block where
wires are separated out and forced into slots
with a fork-like tool. This provides a
copper-conducting path for the bits. The RJ45
jacks are Layer 1 components.
25
Transceiver is an short for transmitter (often
abbreviated Tx) and -receiver (often abbreviated
Rx). Transceivers are typically media converters
-- where one media, say CAT 5, is to be converted
to another form (optical, or AUI electronic are
the two most common conversions. Transceivers are
active layer 1 devices in that they involve the
transfer of energy to the signal (they require
energy to perform their function).
26
10Base2, 10Base5, 10BaseT, or 10\Base FX to be
connected to the port. A common application is to
convert AUI ports to RJ-45 ports. These are layer
one since they transmit from one pin
configuration and/or media to another.
Transceivers are often built into NIC cards,
which are typically considered layer 2 devices.
Transceivers on NICs are called "signaling
components", which means they encode signals onto
the physical medium.
27
Repeaters do not amplify they regenerate and
retime signals. This is because technically
amplification refers to increasing the amplitude
of an analog signal. The repeater does not
amplify incoming bits (which may have distortions
and noise on them), but rather it detects
(hopefully correctly) incoming ones and zeros and
regenerates without any noise or distortion for
the next leg in the journey across the network.
Repeaters are active layer 1 devices because they
involve the transfer of energy to the signal
(they require energy to perform their function).
28
Multiport repeaters combine the amplifying and
re-timing properties of repeaters with
connectivity. It is typical to see four, eight,
12 and up to 24 ports. This allows many devices
to be cheaply and easily interconnected. Since
the typical "unmanaged" hub simply requires
power and plugged-in RJ-45 jacks, they are great
for quickly setting up a network. And like the
repeaters on which they are based, they are
layer one devices, since they only deal with bit
levels. Multi-port repeaters are often called
hubs. Hubs are very common internetworking
devices. The term hub is used instead of repeater
when referring to the device that serves as the
center of a star topology network. The sequence
of graphics summarizes the most important
properties of hubs.
29
The student must understand the common thread
through all of these devices -- they are all
considered Layer 1 devices in the OSI model. As
such, problems with any of these devices
--improperly terminated plug, a bent pin in jack,
an short in the cable, a improperly plugged in
transceiver, a damaged repeater port, or a hub
without its power supply on -- all of these are
layer 1 issues when troubleshooting a network.
All of these issues affect the basic flow of bits
across the medium.
30
There are three situations of directly connected
networks shared media extended
shared media point to point There are
two types of indirectly connected networks
circuit-switched packet-switched
31
There are three situations of directly connected
networks shared media --Shared media
environments occur when multiple hosts have
access to the same medium. "all the computer
share are on the same wire" extended
shared media Using networking devices can extend
the medium which is being "shared". Even more
multiple-access, or more users, can be
accommodated. point to point This is most
widely used in WANs where one networking device
is connected to precisely one other device via a
link. There are two types of indirectly
connected networks circuit-switched In a
circuit-switched indirect network, actual
electrical circuits are maintained for the
duration of the communication. The current
telephone system is still, in part,
circuit-switched. packet-switched Rather
than dedicating a link as an exclusive circuit
connection between two communicating hosts, the
source sends messages in packets, which have
enough information that they can be "routed" to
the right destination hosts. Many hosts can share
the same link, though contention problems can
occur.
32
This target indicator introduces the terminology
of shared media environments and equates this
term with collision domain. Control of collision
domains is integral to LAN analysis,
troubleshooting, and design.
33
To deepen the students understanding of what
exactly a collision is at its most fundamental
level. You may also want to note that while
collisions do occur bit by bit, generally we
consider frames -- specially marked bit streams
-- as having collided.
34
The Hawaiian Islands serve as an example of a
shared, broadcast media for Electromagnetic Wave
Signals. In a similar way, the nodes on an
Ethernet share can share a copper media for
voltage pulse signals. There is historical
importance to this analogy -- Hawaii is where the
early networking protocol Aloha was developed.
Aloha evolved into Ethernet!
35
The most basic collision domain occurs when
multiple computers have access to the same
medium. Design goals are to minimize the number
of hosts in a single collision domain and to
minimize the physical extant of collision
domains.
36
This illustrates that networking devices which
solve one problem can help cause another. In this
case, a repeater is shown extending a collision
domain.
37
Since both repeaters and hubs are layer one
devices and therefore perform no filtering of
network traffic, extending a run of cable with a
repeater and terminating that run with a hub
simply results in a larger collision domain.
38
The purpose of this target indicator is to
highlight the importance of the 4 repeater rule,
also known as the 5-4-3-2-1 rule, for Ethernet.
39
Although repeaters and hubs are useful,
inexpensive networking devices, they extend
collision domains, therefore causing network
performance to suffer because of too many
collisions. The size of collision domains can be
reduced, first, by designing, and then, by
installing, complex networking devices that break
up the domains by using bridges, switches, and
routers. This process is called segmentation
40
Explain what the word "topology" means in
networking. topo meaning map and logy the study
of. Then emphasize topology as a branch of
mathematics, which deals with networks of nodes
and links. Find a partner and solve the
following problem Given n nodes, how many links
are required to create a fully complete network,
that is one in which every node has an
independent link to every other node? Give the
students problem of n 11, for which a graphical
solution is very difficult. Have students write
out a chart with nodes and links columns, and see
if they can deduce the pattern .
41
The correct formula is n(n-1) /2.
Two types of topologies will be important in the
curriculum physical and logical. By physical
topology we mean how networking devices are
actually wired together. By logical topology we
mean how data flows, and how access to shared
media is determined. Both types of topological
diagrams are crucial for the networking
professional to be able to draw, read, and
interpret.
42
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 all
networking devices to see all signals from all
other devices, which can be an advantage if you
want all information to go to all devices.
However, it can be a disadvantage because traffic
problems and collisions are common.
43
Mathematical Perspective The bus topology has
all of its nodes connected directly to one link,
and has no other connection 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 all
networking devices to see all signals from all
other devices, which can be an advantage if you
want all information to go to all devices.
However, it can be a disadvantage because traffic
problems and collisions are common.
44
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.
45
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.
46
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 wired.
47
Mathematical Perspective The tree topology is
similar to the extended star topology, the
primary difference being that it does not use one
central node. Instead, it uses a trunk node from
which it, then, 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.
48
Mathematical Perspective 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 that have been poor planned,
are often wired. Extra NIC s in some stations.
Logical Perspective There is no obvious pattern
to the links and nodes.
49
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.
The advantages are that, because 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. Also, this topology allows
information to flow along many paths on its way
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.
50
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 along which nodes can move. Sometimes the
receiving nodes move (car cell phone), and
sometimes the sending nodes move (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
(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.
51
The End