Local Area Networks

Broadcast Networks (e.g., Ethernet)

Broadcast Networks

Broadcast networks (Fig 25.2 are categorised by:

• Bandwidth ranges from 10 to 100 Mbps.

• To send a message, put address of recipient on message, send to everyone. Most will ignore.

• Multiple access to network solved by sensing when network is free (carrier sense) and detecting collisions (collision detection).

  -> CSMA/CD (Carrier Sense, Multiple Access/Collision Detection).

• Each station can be directly connected to a hub (so to make up a star, not a bus), each will then enjoy the full Ethernet bandwidth.

Token rings

Token rings

Token rings (Fig 25.3 are categorised by:

• Data transmitted in one direction around the ring, and can be read by all stations.

• A special packet, called the token, circulates while ring is idle.

  To transmit, a station grabs the token, transmits, releases the token.

  Regulates access to networks by only allowing one unit to broadcast at a time.

• A multiple priority scheme is usually used for access control.

  A station can transmit a frame at a given priority if it can grab a token with an equal or lower priority. Otherwise, make reservation.

FDDI (Fiber Distributed Data Interface)

• A successor of the original Token Ring.

  It has a dual ring topology with primary ring for data transmission and secondary ring for fault tolerance (See Figure 10.3 from the Steinmetz and Nahrstedt book.)

• 100 Mbps over distances up to 100 km with up to 500 stations.

• Supports both synchronous and asynchronous modes.

  The synchronous mode enables bandwidth reservation and the asynchronous mode is similar to the Token Ring protocol.

• FDDI-2 supports an additional mode - isochronous mode in which network is time sliced, each machine gets a fixed piece.

  -> circuit switched services (isochronous services) for delay-sensitive applications, e.g., audio and video; and packet switched services (synch and asynch) for others in the same network.