What is a Topology?
The physical topology of a network refers to the
configuration of cables, computers, and other peripherals. Physical topology
should not be confused with logical topology which is the method used to pass
information between workstations. Logical topology was discussed in the
Protocol chapter.
linear Bus
Now that you know how topologies can affect networks, it's
time to learn more about the different types of typologies. The term
"linear" refers to a line, and the linear bus is a network
configuration in which each computer is connected to the next in a (more or
less) straight line. A small bus network is shown in the graphic below
One important characteristic of the linear bus is that it
has a beginning and an end. When computers are cabled in a line, the signal
travels down the cable. When it reaches the end, it can "bounce" back
(this is called signal bounce), interfering with network communications. To
prevent this, a bus network must be terminated; that is, small devices called
terminators are placed at both ends of the bus.
Function of terminator:
- To prevent signals from being echoed back through the network
- small devices called terminators are placed at both ends of the bus
- Simple set-up
- Least expensive layout
- Uses inexpensive thin coax cable
- Uses smallest amount of cable
- Doesn't require a hub
- Appropriate for small networks
- For instance, a temporary training room that is set up and torn down quickly
disadvantages
- Slow
- Vulnerable to attenuation, or the loss of signal strength over distance
- Inappropriate for large networks
- Less fault tolerant
STAR
A star topology is designed with each node (file server,
workstations, and peripherals) connected directly to a central network hub,
switch, or concentrator.
Data on a star network passes through the hub, switch, or
concentrator before continuing to its destination. The hub, switch, or
concentrator manages and controls all functions of the network. It also acts as
a repeater for the data flow. This configuration is common with twisted pair
cable; however, it can also be used with coaxial cable or fiber optic cable.
advantages
- Easy
to install and wire.
- No
disruptions to the network when connecting or removing devices.
- Easy to detect faults and to remove parts.
- Requires
more cable length than a linear topology.
- If the
hub, switch, or concentrator fails, nodes attached are disabled.
- More expensive than linear bus topologies because of the cost of the hubs, etc.
STAR-WIRED
The standard for the Token Ring protocol is Institute of
Electrical and Electronics Engineers (IEEE)
802.5. The Fiber Distributed-Data Interface (FDDI) also
uses a Token Ring protocol.
A
star-wired ring topology may appear (externally) to be the same as a star
topology. Internally, the MAU (multistation access unit) of a
star-wired ring contains wiring that allows information to pass from one device
to another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring
topology.
- Uses
physical layout of a star topology with ring topology data transmission
method
- Data
is sent around the star in a circular fashion (dotted orange in figure)
- This
hybrid technology has the fault tolerance of star topology and, ...
- in Token
Ring networks, reliability of token passing
- Groups of nodes are connected to hubs wich are then networked on a single bus
- Used to cover longer distances
- Also used to interconnect, or isolate, different network segments
- Expensive, requires more cable and more hubs
- Modern Ethernet, and its faster cousins, are set up using this topology
TOKAN RING
A Token Ring network is a local area network (LAN) in which
all computers are connected in a ring or star topology and a bit- or
token-passing scheme is used in order to prevent the collision of data between
two computers that want to send messages at the same time. The Token Ring
protocol is the second most widely-used protocol on local area networks afterEthernet.
The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5.
Both protocols are used and are very similar. The IEEE 802.5 Token Ring
technology provides for data transfer rates of either 4 or 16 megabits
per second. Very briefly, here is how it works:
- Empty
information frames are continuously circulated on the ring.
- When a
computer has a message to send, it inserts a token in an empty frame (this
may consist of simply changing a 0 to a 1 in the token bit part of the
frame) and inserts a message and a destination identifier in the frame.
- The
frame is then examined by each successive workstation. If the workstation
sees that it is the destination for the message, it copies the message
from the frame and changes the token back to 0.
- When
the frame gets back to the originator, it sees that the token has been
changed to 0 and that the message has been copied and received. It removes
the message from the frame.
- The
frame continues to circulate as an "empty" frame, ready to be
taken by a workstation when it has a message to send.
The standard for the Token Ring protocol is Institute of
Electrical and Electronics Engineers (IEEE)
802.5. The Fiber Distributed-Data Interface (FDDI) also
uses a Token Ring protocol.
lTREE
A tree topology combines characteristics of linear bus and
star topologies. It consists of groups of star-configured workstations
connected to a linear bus backbone cable (See fig. 3). Tree topologies allow
for the expansion of an existing network, and enable schools to configure a
network to meet their needs.
advantages
- Point-to-point wiring for individual segments.
- Supported by several hardware and software venders.
- Overall
length of each segment is limited by the type of cabling used.
- If the
backbone line breaks, the entire segment goes down.
- More
difficult to configure and wire than other topologies.
| Physical Topology | Common Cable | Common Protocol |
|---|---|---|
| Linear Bus | Twisted Pair Coaxial Fiber | Ethernet |
| Star | Twisted Pair Fiber | Ethernet |
| Tree | Twisted Pair Coaxial Fiber | Ethernet |
