Specifications

2-1
Chapter 2
TOKEN RING OVERVIEW
This chapter introduces Token Ring features and describes characteristics that distinguish Token Ring
from other Local Area Network (LAN) technologies such as Ethernet and FDDI.
INTRODUCTION
The Token Ring network protocol is non-contention based, which means
that because only one station on the network can send data at one time,
stations do not have to compete for access to the transmission medium.
This is controlled by token passing. The token is a unique set of bits that is
recognized by each station on the ring. Only when holding the token may
a station initiate data transfer. A timer (Token Holding Timer) controls the
maximum time that a station can use the network before releasing the
token. The token is passed from one station to another, providing each
station in turn with an opportunity to transmit. A station obtains a token
from its upstream neighbor, and, when finished with the token, passes it to
its downstream neighbor. A station will always pass the token to the next
active station physically in line on the ring. After receiving the token, a
sending station sets the token to busy and sends out a data message. The
data is passed from station to station around the ring, being copied by the
station for which it is addressed, the destination address. After being
copied by the destination station, the data circulates until it arrives back at
the source station, where it is deleted. The station then releases the token
onto the ring for the next station to claim.
The rules of a Token Ring network are monitored by the Active Monitor
(AM). The AM ensures that implementational rules of the Token Ring
protocol are being followed and that the network can overcome violations.