Bus Topology (shared bus)
Signal
propagates along the bus
Sending computer transmits a signal Destination computer receives the signal
Ethernet uses a shared bus topology, therefore multiple computers share access to a single medium. The figure above illustrates the data flow across an Ethernet network. While a computer transmits a frame, it has the exclusive use of the entire bus and all other computers must wait. After the computer finishes the transmission, the shared bus becomes available for others to use.
The question is how does a computer know when it is allowed to transmit (i.e. the bus is free). The Ethernet has an interesting mechanism to determine that. Instead of using a bus controller, all computers attached to an Ethernet participate in a distributed coordination scheme called CSMA (Carrier Sense on Multiple - Access networks). The scheme uses electrical activity on the cable to determine status. When no computer is sending a frame, the ether does not contain electrical signals. However, during frame transmission electrical signals are being sent via the bus (informally called carrier). Therefore, to determine whether the bus is free, the computer checks for a carrier. If no carrier is present, the computer can transmit, otherwise, it must wait for the sender to finish before proceeding.
Because CSMA allows each computer to determine whether the bus is already in use, it prevents computers from interrupting each others’ outgoing transmissions. But it doesn’t solve all the existing problems – see Collision Detection and Backoff algorithm for more details.
Carrier
Sense Multiple Access Networks (current topic)
Collision Detection and Backoff
Algorithm
Ethernet Addressing and Frame Structure
