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1. LAN TECHNOLOGIES

2. LAN Topologies
• There are three main types of LAN systems:
– Star Topology
– Ring Topology
– Bus Topology
• Some topologies are actually hybrids of the
above!

3. Star Topology
• All computers connect
to a centralized point.
• The central point is
called the hub.

4. Ring Topology
• One computer is connected
to the two computers
adjacent to it.
• In the traditional case, if a
system is disconnected from
the ring, the network fails.

5. Bus Topology
• A bus topology consists of a single cable to which
each network device attaches.
• Bus topology has the same problems as a ring
topology.

6. Ethernet
• This is the original Ethernet design by Bob
Metcalfe in 1976.

7. Ethernet (cont.)
• The Ethernet specification details how devices
are supposed to interact on the segment,
distances between devices, and a whole other
multitude of other things.
• Carrier Sense On Multi-Access Networks (CSMA)
with Collision Detection (CD) was the most
important part of the specification.

8. CSMA on Ethernet
• CSMA indicates that computers wait until the
ether is free. In this case, no electrical signal
denotes when the ether is “free.”

9. CSMA on Ethernet (cont.)
• The “signal” is the carrier of the ether.
• The act of waiting for the opportunity to send on
the ether is carrier sense.

10. Collision Detection
• Since networks are not instantaneous, it is possible for
two stations to check the ether, deem it time to send,
and both send at the same time.
• This is called a collision.
• When a collision occurs, the sender immediately stops
transmission, and waits some random length of time,
and then begins transmission again.

11. Collision Detection (cont.)
• A busy segment will always have collisions.
• Collisions do not damage the equipment, but it
forces the two machines that caused the
collision to wait, and therefore it slows down the
network.
• Fewer machines on a segment, smaller
segments, can improve the collision count.

12. LocalTalk
• LocalTalk is another form of a bus network.
• LocalTalk differs from Ethernet in a few ways:
– When a machine senses the segment is free, it holds
it until the frame transmission is over. All other
machines know to wait for transmission to
complete.
– Bandwidth is quite small at ~230Kbps. That’s 2.3% of
the earliest Ethernet specification!

13. IBM Token Ring
• Access mechanism is called token
passing.
• Once the sent information makes a
complete turn around the ring, the
sender passes the token to the
next machine.
• This is a strict-alternation scheme
where all machines have the same
chance to transmit information.

14. Fiber Distributed Data
Interconnect (FDDI)