LAN Topologies• There are three main types of LAN systems: – Star Topology – Ring Topology – Bus Topology• Some topologies are actually hybrids of the above!
Star Topology• All computers connect to a centralized point.• The central point is called the hub.
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.
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.
Ethernet• This is the original Ethernet design by Bob Metcalfe in 1976.
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.
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.”
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.
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.
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.
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!
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.