Page A-1
APPENDIX A
INTRODUCTION TO FOUR-RING
FNB FUNCTIONALITY
A.1 Introduction
This appendix addresses only Token Ring network applications. It
is intended to introduce and explain the closely related concepts of
port switching and the four-ring Flexible Network Bus (FNB).
These concepts reflect recent developments essential to the basic
functionality of a number of Cabletron products. An understanding
of these concepts is essential to the full operation of all port
switching Token Ring Media Interface Modules (e.g., TRXMIM,
TDRMIM) and all Management Modules (e.g., TRMM-2) that
support port switching.
A.2 The Flexible Network Bus (FNB)
The FNB is a data bus which spans the entire width of the
backplane of the Multi Media Access Center (MMAC), providing
Ring In / Ring Out connections between each module and forming
continuous channels for the inter-communication of data between
modules within the MMAC.
A.3 FNB Operations Without Port Switching
Creation of Multiple FNB Rings
In normal Token Ring applications, when using Media Interface
Modules (MIMs) that do not support port switching (such as
TRMIM and TRRMIM), the FNB serves as a single Token Ring
LAN. Each Media Interface Module (MIM) in the MMAC may
attach to this single LAN and operate together; or MIMs with left/
