Transcript Network Topologies - Nassau Community College

Network Topologies
An introduction to Network
Topologies and the Link Layer
Direct Point to Point
Communication

Advantages
 Exclusive
connections between computers
 Useful over long distances
 Easy to enforce security since only two
computers have exclusive access to the one
connection
 Every computer has a direct connection with
each other.
Direct Point-to-Point
Communication

Disadvantages
 Expensive
to implement over a Local Area
network
 The more computers connected, the more
direct connections that are required
increasing the cost of the network.

For LANS shared communications are
more feasible.
LAN
Local Area Network
 Use Shared Communication channels
where each computer take a “turn” to
communicate over a single medium or
network.
 Using shared communication channels are
a cost effective way to implement a LAN

LAN Topologies
Star Topology
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All computers on the network are connected to a
central point
The central point is usually a Hub or switch
The logical layout may resemble a star while the
physical layout may not.
The Hub or switch is often located in a separate
room or closet where networking staff have
exclusive access to them.
Example of a Star Topology
Star Topology Continued
Advantage: If one computer fails
communication on the network remains
intact
 Disadvantage: More wiring is required for
a Star Topology and this increases the
cost of the network.

Example of a Star network
(ATM)

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Uses an ATM switch in the center of the network
Used for Fiber Communication
 Fiber
communication is in one direction only.
 There is a strand for sending and a strand for
receiving

The switch will receive the data from the sender
and transmit it directly to the intended receiver.
Ring Topology
Each computer is connected to each other
forming a logical ring
 Communication of the ring is in one
direction
 The Logical structure of the network is a
ring, but computers may be located
anywhere in a given building.

Example of a Ring Topology
Ring Topology Cont.
Advantages: Makes it easier for many
computers to coordinate communication
with each other on the Network.
 Disadvantage: If there is a break in the
line, i.e. a computer failure,
communication on the network stops.

Self Healing Token Ring
Networks

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Fixes the disadvantages of Ring networks
(Breaks in the line)
The Ring contains two Rings
 An
outer ring where most of the communication takes
place
 An Inner ring – only used when there is a break in the
communication line

When there is a break in the communication line,
all traffic is diverted to the Inner Ring.
Example of a Self Healing Token
Ring Network SONET
Bus Topology
Each computer is attached logically in a
straight path.
 The physical location of the computers are
independent from the topology (each
computer can be located on separate
floors).
 The ends of a bus network must be
terminated or echo noise will result.

Example of a Bus Topology
Bus Topology Cont.
Advantage: requires less wiring than star
topology networks (often the reason why it
was used in earlier LAN’s)
 Disadvantage: Like the ring, if there is a
break in a wire or a computer is down, the
network communication is broken.

Ethernet Standards and
Protocols
Manchester Encoding
As signals are transmitted on the
transmission media, they are timed with
specific time slices.
 Each one and zero that is sent over the
network are interpreted by the receiver in
the middle of each time slice.

Manchester Encoding Cont.

Considered to be edge trigered.
 The
 The

rising edge of a signal will be interpreted as a 1.
falling edge of a signal will be interpreted as a 0.
The sender and receiver are synchronized by
the use of a preamble.
 64
alternating ones and zero’s are sent to
synchronize the sender and receiver.
 Once received, the receiver can accurately determine
the time slices for the message sent.
CSMA (Carrier Sense on Multiple
Access Networks)
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Uses electrical activity on the network to
determine status.
Each message that is sent is sent in the form of
a frame (packet)
Each sender will check the network for a “clear”
(No Carrier signals are present) opportunity to
send a message.
Checking for a Carrier signal is called Carrier
Sense (Where CSMA)
CSMA/CD (Collision Detection)
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CSMA/CD uses the same technology as CSMA,
but provides a means to detect multiple
computers sending at the same time (collisions).
Collisions cause the original message to
become changed.
Detecting the changes in the original messages
detects the collision that occurred.
CSMA/CD Cont.
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When a collision is detect retransmission is
needed.
Each computer will retransmit after waiting a
random amount of time and then retransmit their
original messages.
If a collision occurs again, each computer again
will pick a greater amount of time to wait and
then retransmit their messages.
CSMA/CA (Collision Avoidance)
Used primarily in Wireless networks and
APPLETALK
 With wireless networks it is difficult to
“Detect” a collision on the network due to
distance.
 The sender sends a control message to
the receiver.

CSMA/CA
Receiver will send an acknowledgement if
it is ready to receive a message.
 There can still be collisions with the control
messages sent.
 In the case of collisions, both senders can
back off a random amount of time before
resending the control message.

AppleTalk (Local Talk)
A wired implementation of the CMSA/CA
Standard.
 Similar to the wireless version, a sender
sends a small signal to reserve the BUS
(network)
 All other computers refrain from using the
network until the sender transmits its
frame.

AppleTalk (CMSA/CA) Cont.

Advantages:
 Cheap
to implement in hardware.
 The control message is relatively small so
there is little overhead created for
transmission of data.

Disadvantages: Even with the low
overhead, the transmission speed tops out
at 230.4 kbps
Token Ring Network Standards
The use of Tokens
Sometimes called Token Passing
Networks
 An electronic signal is passed around the
ring to each computer.
 If a Sender wishes to send a message,
that sender will reserve the “token” and
then send its Frame

The use of Tokens Cont.
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When the frame is sent the token is released
Each computer on the ring then has an equal
chance to get the token and send a frame.
If no computer reserves the “token”, the “token”
is just passed from computer to computer on the
ring until it is picked up.