FDDI. There is a bias toward questions about Ethernet, which is reasonable given the installed
base in the marketplace. For this reason, this chapter concentrates on Ethernet, with some
comments on FDDI and Token Ring, as appropriate.
these two specifications, a bus is shared among all devices on the Ethernet, using the carrier
sense multiple access with collision detection (CSMA/CD) algorithm for accessing the bus. The
CSMA/CD algorithm works like this: The sender is ready to send a frame. The device listens
to detect whether any frame is currently being received. When the Ethernet is silent, the device
begins sending the frame. During this time, the sending device listens to ensure that the frame
it is sending does not collide with a frame that another station is sending. If no collisions occur,
the bits of the sent frame are received back successfully. If a collision has occurred, the device
sends a jam signal and then waits a random amount of time before repeating the process, again
listening to hear whether another frame is currently being received.
under higher loads. In fact, two particular negative features of the CSMA/CD algorithm are as
follows:
frames. This wastes time on the bus and increases the latency for delivering the collided
frames.
frames. Devices must wait before sending a frame if another frame is already being sent
by another station. This increases latency while waiting for the incoming frame to
complete.
repeaters; they extend the bus concept of 10Base2 and 10Base5 by regenerating the same
electrical signal sent by the original sender of a frame out every other port. Therefore, collisions
can still occur, so CSMA/CD access rules continue to be used. (This is true of shared Ethernet--
switched Ethernet is covered later.) Knowledge of the operation of Ethernet cards and the
attached hub is important to have a complete understanding of the congestion problems and the
need for full-duplex Ethernet. Figure 4-2 outlines the operation of half-duplex 10BaseT with
hubs.