Understanding the network.pdf - Back to Home

In a ring topology, a token is passed around the ring from node to node in one
direction. When a node needs to transmit data, it removes the token from the
network and transmits its packet. The packet, like the token, is passed from node to
node, until it reaches its destination. At the destination node, the packet is copied,
marked as received, and sent on to the next node around the ring. When the packet
reaches the node that transmitted it, the node removes the packet, releases a new
token, and passes it on to the next node.
When used in a bus topology, token passing operates in a manner similar to
operating on a ring. The difference is that on a ring, since there is only one incoming
and one outgoing connection, addressing information is not required. This is known
as implicit token. On a bus, where all nodes are physically adjacent, explicit token is
used. Explicit token passing uses a logical ring created by using the addresses of the
connected nodes. The nodes are logically ordered from highest address to lowest.
Each host passes the token to its next logical successor. This logical ring is created
when the network is first activated. Each node at activation starts a countdown
based on its node address. When a station's timer expires and there is no token on
the network, it generates a token. This is the first station on the logical ring. After
the "ring" is initialized by token generation, the other stations are invited to join the
"ring" at periodic intervals.
With token passing, "fairness" is maintained by forcing each node to release the
token after transmitting a single packet. The design ensures that each node on the
ring gets a chance to send a packet. IEEE 802.x Ethernet and Apple's LocalTalk are
CSMA/CD-based transmission protocols. IBM's Token Ring and ANSI's Fiber
Distributed Data Interface (FDDI) are token passing based protocols. These
transmission protocols will be covered in depth in Chapter 4, "LAN Internetworking
Technologies."
Network Communication Protocols
Transmission protocols are used to send information across the transmission media.
Network protocols are used to format and deliver information between computers or
hosts.
Think of a transmission protocol as your voice; you expel air from your diaphragm
and across the larynx to make sound. That sound is carried across a room as a
sound wave. It reaches someone's ear, and the tympanic nerves and bones
translate the sound waves into nervous impulses that are interpreted by your brain.
This sequence is much like the way electrical signals are sent across a network
cable.
Network protocols are similar to language. Language provides the rules and means
of expression that allow us to transport our ideas to others. Also, like languages,