Lesson #3: Exploring Network Topologies
 Topology
The word topology is commonly used to discuss the properties
of various types of networks. Topology is the branch of mathematics that
examines the characteristics of geometric shapes. Networks have shapes,
and the shape a network takes has much to do with the way it functions.
Two different definitions of topology are as follows:
physical and
logical. You encounter the distinction between physical
and logical frequently in networking. The distinction is necessary because the
way something looks can be quite different from the way it functions.
A motion picture may look like a natural moving scene, but it actually
consists of many snapshots flashed on the screen so quickly that your eye
interprets the individual pictures as moving. The logical, moving picture
is much different from the physical reality.
So it is with computer networks. Your eye may tell you one thing as
you examine the cabling, but the network may operate very differently at the
invisible level of electrical signals in the wire.
NOTE: The following rule of thumb
distinguishes physical from logical:
- If you can see it and touch it, it is physical.
- If you cannot see it or touch it, it is logical.
The physical topology of a network is the layout or
actual appearance of the cabling scheme used on a network.
The logical topology of a network describes how the data
flows through the physical topology. You soon discover that similar-looking
networks can have quite different logical topologies.
A network's logical topology is closely related to the mechanism used to
manage the way stations access the network. Because a given cable in a
baseband network services only one station at a time, it is necessary
to have procedures in place that manage network access so that all stations
have access without conflicting with each other. These procedures are
called access control methods
and will be discussed in the next lesson.
Physical Topologies
All physical topologies are variations of two fundamental methods of
connecting devices: point-to-point and
multipoint. After examining these two
fundamental topologies, several variations will be examined.
Point-to-Point Topology
Point-to-point (PTP) topology connects two nodes directly together.
The following examples are pure point-to-point links:
- Two computers communicating via modems
- A mainframe terminal communicating with a front-end processor
- A workstation communicating along a parallel cable to a printer
In a point-to-point link, two devices monopolize a communication medium.
Because the medium is not shared, a mechanism is not needed to identify
the computers. Therefore, a simple, two-device point-to-point network has no
need for addressing.
Point-to-point links can be simplex, half-duplex, or
full-duplex. When devices must engage in bi-directional
communication on a half-duplex link, some turnaround mechanisms
must be in place to switch the roles of the sending and receiving devices.
Multipoint Topologies
  
Multipoint topologies link three or more devices together through a single
communication medium. Multipoint topologies work much like a party-line
telephone service where several subscribers are connected to the same telephone
line.
Because multipoint topologies share a common
channel, each device needs a way to identify itself and the
evice to which it wants to send information. The method used to identify
senders and receivers is called addressing.
The following four types of physical topologies are frequently
used in computer networking:
- Bus
- Star
- Ring
- Hybrid
NOTE: Most of the preceding
topologies are discussed in greater detail in later lessons.
Bus Topology
In a bus topology, all devices attach to the same transmission medium. The
medium has a physical beginning and end. All buses are implemented using
electrical cable, usually coax, and the ends of the cable must be terminated
with a terminating resistor that matches the
impedance of the cable. The terminating
resistor prevents data reflections from coming across as data corruption.
The bus is considered a multipoint system because all devices tap into the
same backbone cable.
An important characteristic to remember in bus topologies is that all data
signals are broadcast throughout the bus structure.
In the following diagram, if Station B sends a signal to Station C, the signal propagates
for the length of the cable and is seen by Stations A and D as well. It is necessary to
have an addressing mechanism so that each node understands which messages it is to
receive and which to ignore.
Benefits of Bus topology
Bus topology has the following advantage:
- Cabling costs are minimized because of the common trunk.
Disadvantages of Bus topology
Disadvantages of bus topology are as follows:
- Difficult to trouble shoot because no central distribution points exist.
- Cable breaks can disable the entire segment because they remove the
required termination from each of the two cable fragments.
Examples of Bus topologies
The following networks are examples of bus topology:
Star Topology
 
The star topology is a popular method of connecting the cabling
in a computer network. In a star, each device connects to a central point via
a point-to-point link. Depending on the logical architecture used, several
names are used for the central point including the following:
- Hub
- Multipoint Repeater
- Concentrator
- Multi-Access Unit (MAU)
The central hubs also can be classified in the following manner:
Passive hub - A passive hub
is a simple signal splitter. Its main function is to connect the arms of the
star while maintaining the proper electrical characteristics.
Active hub - An active hub performs
the same function as a passive hub, but contains electronic circuits that
regenerate and retransmit the information. Thus, active hubs can be used
to extend the size of a network.
Intelligent hub - Intelligent hubs perform the same
functions as passive and active hubs; however, they can make informed path
selections and perform some network management. Intelligent hubs route
traffic only to the branch of the star on which the receiving node is
located. If redundant paths exist, an intelligent hub can route
information around normally used paths when cable problems occur.
- Routers, bridges,
&switches are examples of hub devices that can route
transmissions intelligently. Intelligent hubs also can
incorporate diagnostic features that make it easier to troubleshoot
network problems.
Benefits of Stars
Most modern cabling systems are designed in a star physical topology. The
benefits of the star topology are many, including the following:
Each device is isolated on its own cable. This makes it easy
to isolate individual devices from the network by disconnecting them from
the wiring hub.
All data goes through the central point, which can be equipped
with diagnostic devices that make it easy to trouble shoot and manage the
network.
Hierarchical organization allows isolation of traffic on the
channel. This is beneficial when several, but not all, computers place a heavy
load on the network. Traffic from those heavily used computers can be separated
from the rest or dispersed throughout for a more even flow of traffic.
Disadvantages of Star topology
Star topology has the following disadvantages:
- Because point-to-point wiring is utilized for each node, more cable is
required.
- Hub failures can disable large segments of the network.
Examples of Star topology
The following types of networks are examples of star topologies:
- ARCnet
- Ethernet 10Base-T, 100Base-TX, Gigabit Ethernet
- Token Ring which is wired in a physical
star. However, as you will learn later in this lesson, the physical wiring
and the logical characteristics of Token Ring are quite different.
Ring Topology
 
The ring topology is a physical, closed loop
consisting of point-to-point links. In the diagram, you can see how each node
on the ring acts as a repeater. It receives a
transmission from the previous node and amplifies it before passing it on.
Benefits of Ring topology
Ring topology has the following advantage:
- Each repeater duplicates the data signals so that very little signal
degradation occurs.
Disadvantages of Ring topology
Ring topology has the following disadvantages:
A break in the ring can disable the entire network. Many ring designs
incorporate extra cabling that can be switched in if a primary cable
fails.
Because each node must have the capability of functioning as a
repeater, the networking devices tend to be more expensive.
Examples of Ring topology
The following are examples of ring topologies:
Hybrid Topologies
The hybrid topology scheme combines multiple topologies into one
large topology. The hybrid network is common in large wide-area networks.
Because each topology has its own strengths and weaknesses, several different
types can be combined for maximum effectiveness.
Benefits of Hybrid topology
Hybrid topology has the following advantages:
- One company can combine the benefits of several different types of
topologies.
- Workgroup efficiency and traffic can be customized.
Disadvantages of Hybrid topology
The disadvantages of hybrid topology include the following:
- Devices on one topology cannot be placed into another topology without
some hardware changes.
Examples of Hybrid topology
An example of a hybrid topology is as follows:
- A company can place its accounting database users on a ring for better
throughput, and its secretarial staff on a bus for ease of cabling.
STUDY NOTE:
Throughput is the amount of data
transferred per unit of time. It is viewed in terms of bits, bytes, or
packets per second.
Logical Topologies
You have just examined five types of physical topologies. Now you will
examine two types of logical topologies. Logical topologies have
the same names as physical topologies, but keep in mind that the physical
topology describes the network you can see, whereas the logical topology
describes the network from the viewpoint of the data traveling on the
network. Networks can have different physical and logical topologies
(most do!).
The following two logical topologies are discussed in the
following sections:
- Ring logical topology
- Bus logical topology
Ring Logical Topologies
Ring topologies function by passing data transmissions from one node to
the next. This operation is clearest when the physical topology is also a
ring. Any time data are passed from node-to-node, the network has a ring
logical topology.
Another way to identify a ring is to determine whether each node has
separate receive and transmit circuits. If that is the case, the node is
functioning as a repeater and is probably connected in a logical
ring network.
Bus Logical Topologies
In a bus topology, each data transmission passes by each node on the
network. Essentially, each transmission is broadcast throughout the network,
and the nodes use addresses to determine whether they should pay
attention. Any time all transmissions are available to all nodes on the
network, the network has a bus logical topology.
If the nodes on a network use the same circuits to transmit and receive,
the logical network is a bus.
Topologies of Common Networks:
As mentioned earlier, a network can have a logical topology
different from its physical topology. In this class, you work with the
following four common types of networks:
- Ethernet
- Token Ring
- ARCnet
- FDDI
Ethernet
 An older, common wiring system
for Ethernet (10Base2) and (10Base5) uses coaxial cable in a linear bus topology.
In the most common type of Ethernet, each node connects to the coax through
a T-connector (BNC) that taps into the signals on the coaxial cable.
The nodes both transmit and receive through the same connector. Therefore,
10Base2 Ethernet is a logical as well as a physical bus.
Newer versions of Ethernet,
10Base-T and 100Base-TX, are cabled using wiring hubs
(concentrators), as shown below. Each station is connected to the hub via an
individual UTP twisted pair cable. Within the hub, however, the
individual signals are combined into a bus. Therefor 10Base-T and 100Base-TX
are physical stars, but logical buses.
Token Ring
If you examine the wiring of a Token Ring, it
meets all the specifications of a star. Token Ring uses central
wiring hubs, and each node is wired to the hub with an individual run of
cable.
If you look inside the hubs and wires, however, you can see why this is
called a ring network. The figure above shows the path that a
transmission follows through the network. Starting at the hub, the signal
travels through a pair of wires to the receive circuit on the node's network
interface. The receive circuit passes the signal to the transmit circuit,
which repeats the signal on a separate pair of wires and sends the signal
back to the hub.
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If you follow the signal around the entire network, you can see that it
completes a circuitous path, proving that Token Ring has a ring logical
topology.
Token Ring is wired in a physical star to obtain the advantages of a
central wiring hub. All stations can be connected and disconnected at a
central point, and the wiring hub can be equipped with hub management and
diagnostic systems. We therefor sometimes refer to Token Ring as a
star-wired-ring. Note the dual shielded
twisted pair cable.
ARCnet
ARCnet is wired in a star using coaxial
cable. The diagram shows an extremely simple ARCnet using a single,
passive hub.
Recall from the earlier discussion about passive hubs, that they simply
split the signal and pass it on to all the nodes connected to the hub. This
is an indication that ARCnet is a logical bus: all nodes see all
signals on the network.
That ARCnet is a logical bus also can be determined by examining the
network cards. They both receive and transmit through the same coax
connector.
ARCnet is wired as a physical star, and has some of the
advantages of a star network. However, ARCnet functions as a logical bus
. Because of its signaling characteristics, ARCnet is also called a
token-passing-bus.
FDDI
Fiber Distributed Data Interface (FDDI) can be configured as both
a physical and a logical ring. FDDI is commonly used to connect widely
dispersed areas with a high-speed fiber network. When doing so, a physical
ring is the most cost-effective cabling plan. The diagram above shows an
FDDI network covering a large metropolitan area. FDDI uses the same signaling
mechanism as Token Ring; each node repeats the incoming signal and transmits
it to the next node in the ring.
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Key Terms
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