NET 102 - Networking Essentials II
Chapter 1, Networking Basics; Chapter 2, Cables and Connectors
This lesson introduces the student to networking concepts and to network
cabling. Objectives important to this lesson:
- Basic network concepts, network types, network signaling
There is a good deal of repetition in this lesson from concepts in NET
101, so the parts that are familiar should be easier. It begins with a
very general discussion of communications. The lesson should mention:
- devices that send communications
- the signals are picked up by devices capable of retransmitting
them across some medium
- the signals are sent across the medium
(and maybe forwarded in several
steps to a semi-final destination)
- the signals are received by
other devices like the ones in the second bullet (such as those at a
- those devices distribute the
signals to other communications devices
that the signals were meant for
A definition of networking should be provided. Networking can be defined
with five features:
(like printers or files)
- across a common medium (like copper wire or fiber
- by way of specific rules
(like TCP/IP or other network protocols)
The discussion on pages 418 and 419 adds a bit, pointing out that a network
may be set up to provide a specific purpose,
like access to one's bank account.
Some commonly used three letter acronyms are used for classifying networks
based on their physical size (as opposed to how many hosts they have).
People also tend to make up new items for this list, regardless of the
lack of need for them.
- A Personal Area Network (PAN)
may cover a work area for one person, as the book states, or it may
be the network formed by your phone, your Bluetooth headset, your tablet,
and other personal devices. For this one to be useful, you need to connect
to a larger network as well, making the designation a bit lame.
- A Local Area Network (LAN) covers a small area, like a building
or a campus.
- A Metropolitan Area Network (MAN) covers a city, linking computers
at various locations.
- A Wide Area Network (WAN) covers an area larger than a MAN.
This may be a network between cities or countries.
- A Virtual Local Area Network (VLAN) is a subset of a
LAN. It will not be physically separate from the rest of the LAN, but
it will work as though is it a different LAN.
You should have discussed client-server and peer-to-peer networks previously.
Let's review the ideas:
If entities on a network act as peers, then
this is Peer-to-Peer Networking. If entities act in strictly defined
roles, as either servers or clients, but not as peers, then this is Server-Centric
(Client-Server) Networking. Most PC networks are this type.
Most networks follow a client/server
model. Clients typically perform some or most of the processing on the
network, while servers provide services like data storage, instead of
providing all the computing power. Client/server networks are typically
easier to upgrade, both on the client side and on the server side.
Let's consider physical topologies. Topology is the study of shapes and
configuration. Physical topology is the way a network is wired (or wireless-ed?)
together. Logical topology is the way it works, regardless of the wiring.
Network configurations typically fall into one of these types:
- Bus - essentially one continuous cable for each segment,
as in the examples of coaxial cable on page 423; in the image above,
a line is a special case of a bus
- Easy to install
- Moderately difficult to reconfigure. This is because it
is difficult to add new devices or move existing ones without sufficient
room to tap into the bus.
- Difficult to troubleshoot
- Units affected by media failure: All
- Star - using hubs or switches with an individual
cable radiating away from them for each node, like the picture at the
top of page 423
- Moderately easy to install
- Easy to reconfigure
- Easy to troubleshoot
- Units affected by media failure: One, unless it
is the hub/switch/router, in which case all nodes are affected.
- Ring - like a daisy chain, going from one station
to the next and all the way back to the server
- Moderately simple to install
- More difficult to reconfigure as the number of stations
- Easy to troubleshoot
- Units affected by media failure: All
- Mesh - redundant connections, to survive in case
one cable link is broken; in the image above, the Fully Connected example
is a fully redundant mesh
- Difficult to install
- Difficult to reconfigure
- Easy to troubleshoot
- Units affected by media failure: Few or none
- Tree - more easily understandable if you think about
a collection of related networks, separated by geography, which pass
signals from one region to another, then pass them for delivery within
a region. How about islands, where you can't run cables from one to
another whenever you want?
In the image above, imagine a message originating on Niihau, passing
to a network on Kauai, then to a network on Oahu, then Maui, then Hawaii,
and to an observatory at the top of Mauna Kea. Each of the named nodes
might have branches, but you would have to pass traffic along the main
trunk to get to them, if the network was structured that way. (Don't
know which island is which? Hover over the image. Or click
here for more information.)
- Hybrid - combination of more than one type, such
as a series of bus networks, connected together in a ring, or a combination
of networks running different Network Operating Systems.
- Can be Easy or Difficult to install
- Easy to reconfigure
- Easy to troubleshoot
- Units affected by media failure: All, if the central
hub fails. One, if a workstation fails.
Operational standards for networks have been created by many organizations,
including the Institute of Electrical and Electronic Engineers (IEEE)
and the American National Standards Institute (ANSI). Several are listed
in most texts.
Standards are often referred to by number. Some of the IEEE 802.x
standards (there are about a dozen and a half, currently) are LAN
- 802.3 - specifies the CSMA/CD access method, so this
is often thought to be the Ethernet standard. More data appears
802.3 was based on Ethernet, but it is a more general standard. Ethernet
can be thought of as one implementation of the 802.3 standard.
- 802.5 - specifies a token passing system based on IBM's token
ring standard. IBM's standard specifies a physical ring, but 802.5
does not, so we often see physical stars that are logical rings by this
- 802.11 - specifies how wireless LANs work, like spread
spectrum, infrared, and short range Gigahertz radio
So , what's the deal about contention? Contention
systems work by letting each device try to send a message on the net as
needed, contending or competing with all the other devices for
the bandwidth. Two examples of methods that support such
systems are CSMA/CD (Carrier Sense, Multiple Access, with Collision
Detection) and CSMA/CA (Carrier Sense, Multiple Access,
with Collision Avoidance). A collision occurs when two signals
collide on the medium, causing signal loss. These protocols best support
intermittent transmissions. Time sensitivity is good, as users do not
often have to wait for media access.
In a CSMA/CD system (example: Ethernet), the collision is detected and
the devices that caused it each wait a random number of seconds before
sending again. This usually results in one device going ahead of the other.
In a CSMA/CA system, devices can be assigned time slices or can be required
to ask permission to send, avoiding collisions. Apple LocalTalk is an
example of this.
involves passing a token, a small data frame, from station to station.
When a station has the token, it is that station's turn to access the
medium. Examples of this method are token ring, FDDI, and token bus. This
type of media access is predictable and consistent, allowing
large or small transmissions. It is not the best for time sensitive data
since waits are built in, but it will support more devices than contention.
Contention is best when the load is light, token passing is better with
heavier loads, and both schemes crash under too much load.
The chapter changes gears to discuss the TCP/IP
protocol suite (lots of programs in it) which can run on either
of the two types above. TCP/IP, in fact, is the name of two
protocols: Transmission Control Protocol (TCP)
and Internet Protocol (IP). They are
used with several other protocols to enable some kinds of network communications.
IP is used by routers, devices that
find communication paths to other computers. Finding currently valid paths
is necessary because any path may be available or unavailable at any given
time. TCP is used to assure that messages we send are actually
delivered. (There is a lot more to it, but this is an overview.)
The two concepts make the Internet work: find a way,
and make sure it gets there.
Another concept that the book mixes up a bit concerns equipment used
on networks. A good way to think about such equipment is to answer a question:
is the device used to attach something to a network, or is it used to
connect networks together? It is important to know which kind of job a
given device does. A previous text stated that "network
connectivity devices connect individual devices to a single
network", and that "internetwork connectivity
devices connect multiple independent networks together to provide access
to remote resources". Those are valuable statements that you should use
to sort out devices. Most devices fall into one category or the other.
- Network Interface Card (NIC) - the
most common device used to directly connect a computer to a network.
(It is called a NIC, not a NIC card.)
Sometimes, a motherboard may have this kind of device built into it.
Sometimes users connect by other methods, such as by using modems. A
NIC is an example of basic hardware needed by most
network devices. Contrary to the impression you may have from the text,
most networked devices are not wireless.
- Hub and Switches - An advantage to UTP cable is that
networks using it are usually wired as stars, which means that wire
run from nodes to hubs or switches.
The text makes a strange statement on page 439, saying that hubs are
usually on large networks, switches usually on small networks. This
is backwards, and no one really
uses hubs any more. Why?
A hub is a device that has several
RJ-45 ports. You can plug in
as many devices as you have ports, then every signal that is transmitted
by any device that is plugged in to that hub will be passed on to the
rest of the devices plugged in to that hub. Some hubs can retransmit
(amplify) the signals, but none of them decide where to send a signal.
Any incoming signal goes back out all ports except for the one the hub
received the signal on. This means only one of those devices can transmit
at any given time.
A switch learns which devices
are reachable on which ports by noticing the sender's
MAC address on each incoming packet and making a list. If a switch
knows that a message is meant for a device connected to port 7, that's
the only port that signal will be sent through. All other ports are
available for other traffic. This is much more efficient. It should
be clear that the statement in the text is illogical and wrong.
Switches increase the bandwidth inside a network by connecting only
the devices that need to be connected.
- Routers - pass signals from one network to another.
Routers use software addresses instead of hardware
Chapter 2, Cables and Connectors
Networks are often discussed in terms of models, like the ISO Model,
which breaks the subject into layers, topics, and methods. The bandwidth
topic can be split into two methods for using the bandwidth of a medium.
It relates directly to cabling.
- Baseband - this method uses the entire bandwidth of the medium
to send one signal at a time. This means one transmitter at a time,
so users have to take turns or contend for access.
- Broadband - this method uses separate channels in a medium,
such as frequencies, to send multiple signals simultaneously. Users
have little need to wait for access in this kind of system.
Computers use electric currents and various forms of electromagnetic
waves to communicate. We can class networks as being cabled (wired)
or wireless, for obvious reasons. We should consider five attributes
for each type of medium to make a good choice:
- Cost - which media cost more or less than others
- Ease of installation - how easy is it to set up
- Capacity - also called bandwidth, this means how much
data can be on the net at once
- Attenuation - When a signal passes along a medium, it tends
to fade (attenuate) over distance. We compare media to see which ones
have better (longer) attenuation rates.
- Immunity from EMI and RFI - Electromagnetic Interference happens
when your medium picks up static or bad data you don't want. Radio Frequency
Interference means that you are getting the interference from an actual
radio signal source, not just from stray static electricity. Media that
are susceptible to EMI and RFI are also susceptible to eavesdropping
(also called signal capture).
There have been several types of cable
media used in networks over the years. (Follow the link to a Microsoft
TechNet article about media. It is not perfect, but it is pretty good.)
- twisted pair - has been used in two types:
- unshielded - UTP does not have an EMI resistant
- shielded - STP has an EMI resistant sheath, which
can be foil or braided metal
- coaxial - Coax similar to that used for cable TV, and
now used by cable providers for network access as well
- fiber optic - glass or plastic channels that conduct light,
often red laser light
Short physics lesson: In a copper wire, electrons
don't actually flow from one end of the wire to the other. What happens
is more like the movement of a large ripple or wave
in water. Imagine a wave moving toward a shore in a lake or an
ocean. Do specific water molecules make the whole trip? No. The energy
of the wave is passed across a series of molecules. The energy
passes across the medium. For the purists among you, I will note that
the speed of electromagnetic waves through the electrical media varies
with the nature of the conductor. It can be over
90% of c in a UTP wire, and a bit slower in coax. What's c? The speed
of light in a vacuum.
The graphic shown here illustrates several twisted pairs of wires. Each
wire is covered with an insulator, and the two wires in each pair are
meant to be used as a circuit. These wires suffer from crosstalk,
leakage of signal. The twists help cancel out such leaks. The graphic
shows a UTP cable with eight wires in it, making four pairs. As is typical,
there is a green pair, a blue pair, an orange pair, and a brown pair.
Other color schemes are used, but this one is common.
The wires in each pair are twisted around each other.
This type of cable comes in several varieties: two pair, three pair and
four pair were common, but four pair is the current standard. Also, each
variety may be available in grades, such as CAT 1 (Category 1, which is
pretty useless on modern networks) and CAT 5 (Category 5, which has been
a standard for several years). There are several such categories, and
a major difference between them is the number of twists per foot in each
pair. CAT 1 will have less than 5 twists per foot, CAT 5 will have 25
or more twists per foot (so it is better, and costs more). Note that the
better the class of cable, the less leakage, and the more bits per second
can be passed across it.
Connecting a system with
twisted pair wiring is easy. A possible problem is that the wiring
closets in any building are often in need of being "cleaned up". The "closet",
typically on each floor of a building, contains punch-down blocks, patch
panels, and hubs (or switches). Many are disorganized and messy. People
who try to clean them up, however, must be careful not to disconnect circuits
that are needed.
The factors for UTP:
- Cost - inexpensive
- Installation - cheap and easy
- Capacity - 1 to 100 Megabits per second (Mbps), but 10 Mbps is common
in home networking. Gigabit throughput is possible if the other network
equipment (network cards, switches, routers) support it
- Attenuation - nothing is perfect, so this is high (poor)
- Immunity from EMI - also poor. Recommendation: run UTP lines perpendicular
to fluorescent lights to avoid a constant static buildup.
cables are usually connected to devices with RJ-45
connectors. Your text does not show an RJ-45 connector (or any other)
very well. In the enlarged picture on the right, note the eight gold-colored
conntacts for the eight wires usually found in UTP cables. The wires are
used in pairs to form two to four circuits.
RJ-11 connectors are typically
used for telephone
An STP (Shielded Twisted Pair) cable is very similar to a UTP
cable, except that it has a metal foil sheath around
each pair of wires, or around the entire set of wires. When the sheath
encloses all the wires, it resembles the ground sheath in a coaxial cable,
but its purpose is to shield the cable from interference..
This cable is more expensive than unshielded cable, and is less flexible
due to the stiff shielding. The shield, however, makes it more EMI resistant
The factors for STP:
- Cost - Moderately expensive
- Installation - harder than UTP, needs special connectors (note the
IBM-style Token Ring connector shown here)
- Capacity - 1 to 500 Megabits per second (Mbps) is possible, but 16
Mbps is common (for Token Ring)
- Attenuation - high (poor)
- Immunity from EMI - also poor, but not as bad as UTP.
Data connectors were typically used with Shielded Twisted Pair cable on
a Token Ring network.
Coaxial cable is called that because it has
two conductors, one wire in center and a conductive sheath around it,
that share a common axis, hence coax.
Most people have seen this style of cable used with cable television.
The wiring standards used for network coax have been different from
those used for cable TV. This is a list of cable used through networking
- 50 ohm cable, available as RG-8 and RG-11. Used in
Thick Ethernet, also called "Ether Hose".
- 50 ohm cable, available as RG-58. Used in Thin Ethernet
- 75 ohm cable, available as RG-59.
- 93 ohm cable, available as RG-62. Used in ARCnet.
The number associated with each RG specification tells you the relative
size of the central conductor. Smaller numbers mean thicker
- The coaxial line is essentially a single bus, going from one station
to the next.
- At each end of the line, the cable has to have a terminator
- At one end, it also has to be grounded.
- If using thin Ethernet, T-connectors are used.
- If using thick Ethernet, vampire
taps are used. They are called vampire taps because little teeth
bite into the cable (to contact the shield), and a big tooth bites deeper
to contact the central conductor when you screw the clamp down.
The factors for Coax:
- Cost - Relatively low to Moderately expensive (depending on thickness
of the cable)
- Installation - simple to install, hard to modify
- Capacity - high rates are possible, but 10 Mbps is common
- Attenuation - high, but less than twisted pair
- Immunity from EMI - moderate
The example on the right shows a typical T-connector with BNC
fittings. The fitting on the bottom of the image might
attach to a port on a NIC that looks like the barrel
on either end of the top of the T. Attachment is achieved by pushing the
connector onto the barrel of the port, then twisting the collar of the
connector to lock onto the pin that is part of the port. In other words,
it mounts like a bayonet.
next (enlarged) picture shows a BNC connector attached to a thin Ethernet
cable. Such a connector would be used to attach to one of the T-connector
barrels in the photo above. The other end of the cable would run to the
next node on the network.
optic can be glass or plastic, and is meant to conduct light instead
of electricity. The conductor is called a waveguide, and is covered
with cladding, a material to reflect the signal back into the center
of the conductor. Two configurations exist. Loose configuration
has a liquid filler between the outer sheath and the conductor. Tight
configuration has wire or stiff fibers around the conductor to add strength
to the cable.
Fiber optic comes in two modes: single mode conducts a single
signal, while multi-mode conducts many signals simultaneously.
You may want to know that the most common type used is 62.5 micron
core with 125 micron cladding, multimode.
The factors for fiber optic:
- Cost - Expensive, mostly for installation
- Installation - difficult
- Capacity - 100 Mbps at up to 20 kilometers per segment
- Attenuation - very low
- Immunity from EMI - immune. This is light, not electricity.