Ref: 99980071
Title: Token Ring Technology
Date: 10/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

Introduction

Bridge Token Ring technology is based on the IEEE 802.5 physical
and data link specification.  This standard uses a token passing
access mechanism in a closed ring topology and supports 4Mb/s
transmission on twisted-pair cable and 4 or 16Mb/s on fiber optic
(current implementation from IBM and other vendors implement 4Mb/
s only).  Physically, the wiring used is a star topology.  IBM's
Token Ring is a superset of the IEEE 802.5 standard.  Bridge
produts use the Texas Instruments chip set that was jointly
developed by Texas Instruments and IBM and thus are compatible
with the additonal network management featrues of the IBM Token
Ring.  The following sections describe the operation of a Token
Ring LAN.

.h1;Token Ring Media

The basic media used by IBM's token passing LAN is shielded
twisted-pair cable and optical fiber, as specified by the IBM
cabling scheme.  There are several types of cables defined by IBM
that support the LAN including:

Type 1

Braided cable shield around two twisted paris of #22 AWG
conductors for data communications.  This cable type is typically
used between faceplates in working areas and wiring closets in
the same building or different buildings.  The maximum cable
length from the wiring closet to the faceplate is 300 meters.

Type 1 Outdoor

Corrugated metallic cable shield around two twisted pairs of #22
AWG conductors for data communication.  This cable type is
suitable for serial installation or placement in conduit
underground.

Type 2

Same as Type 1 with the addition of four twisted pairs of #22 AWG
telephone conductors.  This cable type is used in the same
environement as Type 1 cable described above, but it is used for
voice transmission.

Type 3

Twisted-pair of #22 AWG or #24 AWG telephone conductors with a
minimum of two twists per foot.  This cable type is used between
faceplates in working areas and wiring closets in the same of
different building.  Type 3 cable requires Type 3 media filters
to interface to Token Ring network adapters.  It supports a
maximum of 72 stations on the Token Ring and the distances of up
to 100 meters between a Token Ring adapter and the wiring closet.
Note that Bridge does not support Type 3 wiring due to concerns
about system reliability resulting from the lack of shielding and
reliable cable verification.

Type 5

Two optical fiber conductors.  This cable type is used between
wiring closets in the same building or different buildings.

Type 6

Two twisted-pair of #26 AWG stranded conductors for data
communication.

Type 9

Braided cable shield around two twisted pairs of #26 AWG
conductors for data communication.  This cable type is typically
used between faceplates in working areas and wiring closets in
the same building of different buildings.  This cable type is
designed as a lower cost alternative to Type 1 plenum cable.  It
supports up to two-thirds of the distance specified for Type 1
cable.

.h1;Token Ring Tranmission Speed

The IEEE 802.5 standard specifies spees of 4 Mb/s for twisted-
pair on the network and speeds of 4 or 16 Mb/s for optical fiber.
IBM's initial products are based on a 4Mb/s Token Ring.  16Mb/s
products are being proposed for backbone networks between wiring
closets.

.h1;Token Ring Operation

Token Ring operation in the Bridge implementation is based on the
Texas Instruments (TI) chip set.  The chip set provides data
transfer as well as error recovery and network monitoring.  Most
of the Token Ring functions discussed below are implemented in
the TI chip set.

If token-access control mechanism for regulating the flow of data
in a ring topology is based on the principle that permission to
use the communications link, in the form of a "free" token, is
passed sequentially from node to node around the ring.  With the
token-access control scheme, a token is in circulation in the
ring at all times.  The token is set either as a free token, if
the path is available for transmission, or as a busy token, if a
message is already on its way from a sending node to a receiving
node.  The token is set to busy or reset to free by nodes on the
ring.

All nodes receive the token and the data packet accompanying it
(if any), regenerate the entire message and token received, and
retransmit it to the next node  This is typically done at the
hardware level so as to impose minimal delay.

When a node wants to transmit, it waits for a free token to
arrive.

.h1;Frame Format

The format for a frame includes a token-indicator bit, a monitor-
count bit used for error recovery, priority bits that specify
priority levels among the stations on the network, and an address-
recognized bit and error-detected bit for use in detecting
faults.

The priority bits in the frame are particularly important for
accommodating synchronous devices and others that must transmit
at fixed time intervals or transmit high-priority data.

.h1;Token Ring Error and Fault Detection

When faulty data, a faulty address, or an unset copied bit
returns to the transmitting node, the message is transmitted.  If
a node repeatedly returns faulty frames, it is isolated from the
system.  The frame also contains a frame-check algorithm that is
checked by the receiving node to guard against noise-induced
errors.  When a received frame check does not match the node's
computer frame check, the node "knows" there is an error in the
data.

.h1;Token Ring Failure

A faulty node may fail to remove its data from the ring or fail
to reset the token free.  To prevent this from disabling the
system, one node is designated as the active monitor.  If the
active monitor does not see a free token pass within a specified
time limit, it purges any circulating information from the ring
and generates a free token.

The monitor also checks for a persistently circulating busy
token, caused by a node failing to purge its message from the
ring.  If the monitor sees the same message pass more than once,
it purges the message from the ring and generates a free token.
In addition, the monitor looks for intermittent errors.  If
node's error rate exceeds a specified number, the monitor
disconnects the node from the ring.

The ring can also protect itself against failure of the active
monitor.  Every node has a timeout feature that is reset by free
tokens.  Failure of a free token to appear before timeout
indicates that the active monitor node is faulty, so another node
becomes the active monitor.  Every node is capable of becoming
the active monitor.

.h1;Token Ring Wiring Concentrators

Wiring concentrators, or Multistation Access Units (MAUs),
represent points of the star physical topology.  They are
responsible for the following features of the Token Ring scheme:

*   Flexibility and adaptability in the network layout

*   Simplified adds and deletions of network nodes

*   Addressability for network management funcitons

*   Fault isolation

Each wiring concentrator can bypass any node connected to it and
can select either the primary or the alternate ring wire.  If the
primary ring is broken, the network is automatically reconnected
to the alternate ring at the nearest wiring concentrator on
either side of the break.

If a faulty node is detected, the ring is reconnected to bypass
the node.  In either case, the LAN can continue normal operation
after the bypass in exactly the same sequence as before the fault
occurred.

.h1;Token Ring Bridges

Rings can be connected by a bridge to form a single, logical
continuous ring.  Systems can use multiple bridges to group
devices by speed, data transfer needs, or other characteristics.
These bridges are protocol independent and operate using routing
information in the Token Ring protocol header.

.h1;Token Ring Architecture Relative to XNA

The same higher-level communication protocols (for example, XNA)
that control data transfer across Ethernet networks are also
applicable to data transfer across Token Ring networks.





