BASIC TOKEN RING NETWORKS
Page A-4
Early Token Release
Early Token Release (ETR) is a second protocol option presented in
the IEEE 802.5 Standard. ETR is an optional protocol that may be
used with 16 Mbit/sec 802.5 token ring networks. It has the
advantage of increasing the efficiency of the ring by allowing
transmissions from more than one station to occupy the ring at the
same time. This protocol is similar to the basic token ring protocol in
that possession of the token determines the transmitting station, but
with ETR the token is returned to the ring immediately following the
message frame or upon THT expiration. ETR is a function of the
network software and, when used, is usually invoked dynamically in
response to increased network loads.
Expanding the Token Ring
Concentrators, repeaters, converters and bridges are found
throughout token ring networks. They are used to create ring
topologies to meet the specific needs of many different network
applications. Some of these topologies are discussed in Appendix B.
Together with Figure A-3, the following descriptions provide a brief
introduction to these components. The network functions provided by
the following devices are often combined in a single device.
A concentrator is a device that provides multiple trunk coupling
unit ports, bounded by externally accessible Ring-In and Ring-Out
trunk ports. The primary function of a concentrator is to serve as a
hub, providing trunk coupling units for attaching stations and
controlling access to the ring. Each trunk coupling unit port can be
electrically shorted to bypass the attached lobe when a station is
disabled or when the lobe cable is disconnected. A concentrator is
referred to as an Active Concentrator when the Ring-In/Ring-Out
trunk ports provide regeneration and retiming of ring signals.
Passive Concentrators rely on the drive from the transmitting
station to carry a message to its destination. Multiple concentrators
are often linked via their trunk connections to form a single larger
ring.
Repeaters are used when the length of the the main ring must be
extended beyond the drive distance of other components on the ring.
The repeater’s primary role is to regenerate and retime the signals on
the ring. They are often used to connect concentrators together to
form a larger ring. Active concentrators provide the same
regeneration and retiming function as a repeater and some, as in the
