Lesson #1: Exploring Networking Essentials
The World of Computer Networking
In the last 40 years,
LANs (Local Area Networks) have gone from being an experimental
technology (like this hand drawn diagram from 1976 by Robert
Metcalfe, putting to paper his ideas for a network connected via the
Ether), to becoming a key business tool used by companies worldwide.
A LAN is a high-speed communications system designed to link computers
and other data processing devices together within a small geographic area such
as a workgroup, department, or a single floor of a multistory building.
Several LANs can also be interconnected within a building -or- campus of
buildings to extend connectivity.
Some Background on LANs
LANs have become popular because they allow
users to share vital computing resources electronically, including
expensive hardware such as printers, CD-ROM drives, application programs,
and, most importantly, the information the users need to do their jobs.
Prior to the development of LAN technology, individual computers were isolated
from each other (sharing was via the sneaker net!) and limited in their range of
applications. By linking these individual computers over LANs, their
usefulness and productivity have been increased enormously. But a LAN
by its very nature is a local network, confined to a fairly small area
such as a building or even a single floor of a building. To realize the
full benefit of computer networking, it is critical to link the individual LANs
together (internetworking) into an enterprise-wide backbone network that
connects all of the company's employees and computing resources, no matter how
geographically dispersed they may be.
Today's LANs and Internetworks
are powerful, flexible, and easy to use, but they incorporate many sophisticated
technologies that must work together flawlessly. For a LAN to really
benefit an organization it must be designed to grow to meet the organization's
changing communications requirements. Building a LAN is a process of
choosing different components and matching them together.
This course is designed to help students understand the fundamentals of how LANs operate, what the different technology choices are for building a LAN,
and the ramifications of choosing one option over another. We will also discuss
the concepts of internetworking which is
connecting disparate and geographically dispersed LANs together to form an enterprise
system; the different technologies and cabling
media available to do so; and the benefits and limitations of each.
To aid students new to much of the networking terminology used, most
of the networking terms in these lessons appear in this Networking Glossary.
The Basics of Local Area Networking
Today local area networking is a shared access
technology. This means that all of the devices attached to the LAN share
a single communications medium, usually a coaxial, twisted pair, or fiber optic
cable. The graphic below illustrates this concept: Several computers
are connected to a single cable that serves as a common communications medium
for all of them. The physical connection to the network is made by
attaching a Network Interface Card
(NIC) to the host computer and connecting to the network, usually
via a network cable.
Once the physical connection is in place,
it is up to the networking software to manage communications.
A Basic LAN, e.g. "Bus" Network
When Station-B sends a frame containing a packet to another station on the LAN,
it passes by all of the other stations connected to that LAN. On the bus
network illustrated here, the electrical signal representing the packet travels
away from the sending station in both directions on the shared cable. All
stations will "see" the packet, but only the station it is addressed
to will pay attention to it.
In a shared media network, when one station wishes to send a message to
another station it uses the networking software in the workstation to put the message
in an "envelope." This envelope, called a packet, or frame, consists of message data surrounded by one or more headers and trailers that carry special
information used by the network software to deliver the message to the destination
station. One of the pieces of information placed in the packet header is the "address" of the destination station.
then transmits a frame containing the packet onto the LAN. The frame/packet is
transmitted as a stream of data bits represented by changes in electrical signals.
As it travels along the shared cable, all of the stations attached to it see the frame
(packet). As it goes by the NIC in each station, the NIC checks the destination address in the
frame header to determine if the frame is addressed to it. When the frame passes
the station it is addressed to, the NIC at that station copies the frame and then takes
the packet out of the frame (envelope) and gives it to the computer.
The diagram above illistrated one source station sending a single message packet (inside
a frame) to one destination station. If the message the source station wants to send
is too big to fit into one frame/packet, it will send the message in a series of frames.
On a shared access LAN (Broadcast network), however, many stations all
share the same cable. Since each individual frame containing a packet is small,
it takes very little time to travel to the ends of the cable where the electrical signal
dissipates. So after a frame/packet carrying a message between one pair of stations
passes along the cable, another station can transmit to whatever station it needs to send a
message. In this way, many devices can share the same LAN medium in a Bus network.
Ethernet (IEEE 802.3)
The most widely used LAN technology in use today is
Ethernet. It strikes a good balance between speed, price, ease of installation, and
supportability. Approximately 90 percent of all LAN installations installed today are one
variety or another of Ethernet. All the images
in this lesson are clickable-links to larger, clearer, or alternate images.
The click-link for the image to the right is an importnt visual supplement to
The Ethernet standard is defined by the Institute of Electrical and Electronics
Engineers (IEEE) in a specification commonly known as
IEEE 802.3 The IEEE 802.3 specification (and modern variants) covers
rules for configuring Ethernet LANs, the types of media that can be used, and how
the elements of the network should interact. The Ethernet protocol provides
the services called for in both the Physical (Layer 1) and Data Link
(Layer 2) Layers of the OSI reference model.
One element of the IEEE 802.3 (or 802.3µ, or 803.3ab, ...)
specification states that Ethernet networks run at a data rate of 10 Million bits
per second (10 Mbps) -or- 100 Million bits per second (100 Mbps) in
the case of Fast Ethernet -or- 1 Billion bits per second
(1 Gbps) in the case of Gigabit Ethernet -or- 10 Billion
bits per second (10 Gbps) in the case of 10-Gig Ethernet (10GbE). This means
that when a station transmits a frame onto the Ethernet medium it travels along that
medium at 10, 100, 1000, 10000 Mbps (or more...).
Another important element defined by the IEEE 802.3 specification is the access method to be used by stations
connected to an Ethernet LAN, called Carrier Sensing, Multiple Access with Collision
Detection (CSMA/CD). In this access
method, each station contends for
access to the shared medium. It is possible for two stations
to try sending frames at the same time, which results in a collision on the LAN medium. In Ethernet networks, collisions
are considered normal events and the CSMA/CD access method is designed to quickly restore the
network to normal activity after a collision occurs.
Ethernet Media and Topologies
An important part of designing and installing a LAN is selecting the appropriate
medium and topology for the environment. Ethernet networks can be configured in
either a star or bus topology and installed using any of three different media.
Coaxial cable was the original LAN
medium and it is what is called (for both physical & logical)
a bus topology. In this configuration, the coaxial cable forms a single
bus to which all stations are attached. This topology is rarely used in new LAN
installations today because it is relatively difficult to accommodate adding new users
or moving existing users from one location to another. It is also difficult to
troubleshoot problems on a bus LAN unless it is very small.
A Physical Star Topology LAN
In a Physical Star
topology all stations are wired to a central hub called a concentrator.
Logically functioning like the (Ethernet) Bus topology, frames received
from one station are repeated to all the other ports on the hub. This allows all
stations to see each frame sent on the network (just like in a Bus topology network),
but only the station the frame is addressed to pays attention to it . Be sure to
click on the image to the right to see multi-port repeater process in action.
image illustrates a Star topology LAN --which is a physically more robust topology than the
Bus topology. In a Star topology, each station is connected to a central wiring hub,
concentrator, or switch by an individual length of mostly unshielded twisted pair (UTP) cable.
This cable is connected to the station's NIC at one end and to a port on the hub at the
other. The hubs are placed in wiring closets centrally located in a building. Click
on the diagram for an illistration.
Ethernet networks can be built using
three different types of media: shielded ¦ screened and unshielded
twisted pair (TP), coaxial cable, and fiber optic cable.
By far the most common is unshielded twisted pair (UTP) because it is
associated with the more popular Physical Star topology. UTP is inexpensive, and very easy to
install, troubleshoot, and repair. Twisted pair cable comes as unshielded, screened, and/or
shielded like coax.¦ Unshielded twisted pair (UTP) cable used for LANs is similar to telephone
cable but has somewhat more stringent specifications regarding its susceptibility to outside
electromagnetic interference (EMI) than common telephone cable. Shielded
(STP) ¦ Screened twisted pair (ScTP), as its name implies, comes with a shielding
of foil wrap (Sreened) or a metalic braid (Shielded) around the cable to provide more protection against EMI.
Of the two types
of twisted pair cable, UTP is by far the most commonly used. The original specification
for running Ethernet on UTP is called 10Base-T. The 10Base-T name
stands for 10 Mbps, baseband signaling (the signaling method used by Ethernet networks), over
twisted pair cable.
Other Ethernet specifications include 10Base5, which
uses a thick coaxial cable, 10Base2, which uses a thin
coaxial cable media, and faster (higher bandwidth) UTP based Ethernet specifications
like 100Base-TX Fast Ethernet (100 Mbps) and 1000Base-T Gigabit Ethernet (1000 Mbps). Today,
10Base5 is obsolete and 10Base2 is seldom installed in new Ethernet networks and
usually seen only in in high EMI areas. Fiber optic standards like
10Base-FL, 100Base-FX, 1000Base-LX, and 1000Base-SX
allow Ethernet to run over fiber optic links.
An extension of the popular 10Base-T Ethernet standard, Fast Ethernet
transports data at 100 Mbps. With rules defined by the
IEEE 802.3µ standard, Fast Ethernet leverages the familiar
Ethernet technology and retains the CSMA/CD protocol of 10 Mbps Ethernet.
Three types of Fast Ethernet are available: 100Base-TX,
which runs over Category 5 UTP; 100Base-T4 which runs over existing
Category 3 UTP; and 100Base-FX, which operates over
multimode fiber optic cabling.
Token Ring LANs
Another major LAN technology in use is Token Ring. Token Ring
rules are defined in the IEEE 802.5 specification.
Like Ethernet, the Token Ring protocol provides services at the Physical and Data Link
Layers of the OSI model. Token Ring networks can be run at two different data
rates, 4 Mbps or 16 Mbps.
The access method used on Token Ring networks is called
token passing. Token passing is a deterministic access method
in which collisions are prevented by assuring that only one station can transmit at
any given time.
This is accomplished by passing a special packet called a
token from one station to another around a ring.
A station can only send packets when it gets the free token. When a station
gets a free token and transmits a packet, the packet travels in one direction around
the ring, passing all of the other stations along the way. As with Ethernet,
the packet is usually addressed to a single station, and when it passes by that station
the packet is copied. The packet continues to travel around the ring until it
returns to the sending station, which removes it and sends a free token to the next
station around the ring.
Token Ring Topology and Media
Token Ring networks use what is called a logical ring topology.
However, it is actually implemented in what can best be described as a star
wired ring that implements a logical ring on a physical star topology.
Basic Ring Topology LAN
The ring topology used in Token Ring networks is a logical
ring topology that physically is a star topology. Each station is connected
to a Token Ring wiring concentrator (MAU) by a shielded twisted pair
(STP) cable with two wire pairs. One pair serves as
the "inbound" portion of the ring (also known as the receive pair) and the other
pair serves as the "outbound" or transmit pair.
In Token Ring LANs, each station is connected to a Token Ring wiring concentrator,
called a Multistation Access Unit (MAU), using an individual
run of shielded twisted pair cable. Like Ethernet hubs, MAUs are located in
Fiber Distributed Data Interface, commonly known as FDDI,
provides data transport usually at 100 Mbps. Fiber is preferred in many
networks because it can be used over much greater distances than UTP cable.
Like Token Ring, FDDI uses a token passing media access method. It is also
usually configured in a collapsed ring, or physical star, topology. FDDI is
used primarily as a backbone, a segment of network that links several
individual workgroup or department LANs together in a single building. It is
also used to link several building LANs together in a campus environment.
Standards and Protocols
LANs are complex systems that implement many different services
in order to provide communication between all of the types of devices that can be
connected to them. A communications model called the Open Systems Interconnect
(OSI) reference model was developed by the International
Standards Organization (ISO) to define all of the services a
LAN should provide. This model defines seven layers, each of which provides
a subset of all of the LAN services. This layered approach allows small
groups of related services to be implemented in a modular fashion that makes
designing network software much more flexible. A network software module
that implements services at the Network and Transport Layers of the model can
be paired up with different Physical and Data Link Layer modules depending on
the requirements of the user's application.
But the OSI model doesn't say how these services should
actually be implemented in LAN equipment. The "how to" part has been defined
in a number of different protocols that have been developed by international
standards bodies, individual LAN equipment vendors, and ad hoc groups of interested
parties. These protocols typically define how to implement a group of services
in one or two layers of the OSI model. For example, Ethernet and Token Ring
are both protocols that define different ways to provide the services called for in
the Physical and Data Link Layers of the OSI model. They have both been
approved by the Institute of Electrical and Electronics Engineers
(IEEE), an international communications standards body.
The International Standards Organization (ISO), the primary standard-setting
body in the data communications industry, developed the framework for LAN standards
called the Open Systems Interconnect (OSI) reference model. This reference
model represents a standard approach to communicate information throughout a network
so that a variety of independently developed computer and communications devices can
Because they are approved and published by the IEEE, both the Ethernet
and Token Ring protocols are said to be industry standards. Any company can
acquire the specifications and design Ethernet or Token Ring NICs. Users
can purchase an Ethernet NIC, for example, from any vendor and be assured that
it will operate in a network with Ethernet NICs from other vendors. This
degree of interoperability is highly desirable.
However, there are many more protocols for providing services at the higher layers
of the OSI model and very few of them have been approved by an international
standards bodies. In fact, most upper layer LAN protocols are incorporated
into proprietary network operating systems, such as Novell's NetWare, IBM's
LAN Server, and Microsoft's LAN Manager. A user has to buy only that
vendor's products in order to be assured that they will interoperate on a LAN.
Network Operating Systems
Ethernet and Token Ring technologies are just one part of a complete LAN.
They provide the services specified in the Physical and Data Link Layers of the
OSI model, but several other services must be added on top of the connectivity
of Ethernet or Token Ring. Network operating systems (NOS) are most often
used to provide the additional communications services.
An NOS defines client and server systems. Clients are
individual user workstations (hosts) attached to the network where application
programs are run and data is generated. Servers have shared network resources
that provide hard disk space for users to store files, printer services, and a
number of other network services. The network operating system provides a
set of protocols in software that run on both server and client systems and allow
them to communicate with each other, share files, printers, and other network
LAN technology is evolving. In the early 1980s LANs were strictly
local area networks, linking small groups of computers in company departments.
As workgroup LANs proliferated over the past 20 years, users began
connecting them to form internetworks, first with bridges and later
with routers. Today's networks typically comprise a combination of
workgroup and campus hubs, Wi-Fi access points,
bridges, and routers. Switches
are used to build collission free LANs
and are becoming more prevalent than hubs.
The next few years will see networks evolve to include more sophisticated LAN
switches (and Layer 3 switches) and ever more sophisticated Wi-Fi access points (hubs). Networks will be designed using
several different types of components, both old and new. LANs will be built
with stackable workgroup switches, which, in turn, will be interconnected
by larger modular/blade switches that may incorporate advanced Layer 2/Layer 3 switching functionality. Large
networks will include another layer of consolidation with network center bridges linking workgroup switches
and access points. Routers will continue
to be used as gateways to the wide area network
linking other buildings and remote sites.
For networks to deliver the performance today's users require, their
many components must work together to deliver seamless
connectivity between all of the users computing and networking systems
throughout the enterprise. Flexibility to grow, power to support
applications, and seamless connectivity are what users expect in the products
they choose to build LANs and enterprise networks.