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5.12] What is the process for inserting into a ring? (Token Ring Data Link Layer)




Description

This article is from the Token Ring FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.

5.12] What is the process for inserting into a ring? (Token Ring Data Link Layer)

This information is derived from the TMS380 Second-Generation Token
Ring User's Guide from Texas Instruments published in 1990.

In order for any token ring adapter to insert successfully into a
ring, the adapter must successfully complete 5 steps known as the
phases of insertion. These phases are described as follows:

Phase 0 - Media Lobe Check,
Phase 1 - Physical Insertion,
Phase 2 - Address Verification,
Phase 3 - Participation in Ring Poll, and
Phase 4 - Request Initialization.

Phase 0: Media Lobe Check

The first step for any token ring device initialization is known as
the Lobe Media Check. This phase actually tests the transmitter and
receiver of the adapter and the cable between the adapter and the
Multistation Access Unit (MAU). A MAU physically wraps the
connection cable's transmit wire back to its receive wire. The
effect is that the adapter can transmit media test Media Access
Control (MAC) frames up the cable to the MAU (where it is wrapped)
and back to itself. The adapter will send lobe media test MAC frames
to destination address 00-00-00-00-00-00 (with the source address of
the adapter) and a Duplication Address Test (DAT) MAC frame
(containing the address of the adapter as both the source and
destination) up the cable during this phase. 2047 test MAC frames
and 1 DAT frame must be successfully transmitted in order to
complete phase 0. The adapter will only attempt this phase 2 times
before reporting a failure.

Phase 1: Physical Insertion

In phase 1, the adapter attempts to open the relay on the MAU by
sending a DC current (4.1-7.0 V for current less than 1mA or 3.5-7.0
V for current of 1-2 mA, in either case known as phantom since it is
transparent to any signals being transmitted on the same wires) up
the transmit wire pair. Once the phantom is applied and the relay on
the MAU opens (hopefully), the adapter waits to see one of the
following: an Active Monitor Present (AMP) MAC frame, a Standby
Monitor Present (SMP) MAC frame, or a ring purge MAC frame. Any one
of these frames indicates that there is an Active Monitor (AM)
present on the ring, which indicates successful completion of phase
1. If an AM is not detected within 18 seconds, the adapter initiates
the monitor contention process. The monitor contention process
determines a new AM based on the highest address of those contending
for AM status. Not all stations contend for AM every time contention
is initiated. If contention is not completed within one second, the
adapter fails to open. If the adapter becomes AM and initiates a
purge and the purge process does not complete within one second, the
adapter fails to open. If the adapter receives a beacon MAC frame or
a remove station MAC frame, the adapter fails to open.

Phase 2: Address Verification

This phase is also referred to as the duplicate address test. This
phase insures that the address of this adapter is unique to the
local ring. Since token ring allows Locally Administered Addresses
(LAAs), you could end up with two adapters with the same MAC address
if this check was not done. The adapter sends out a series of DAT
MAC frames like the ones used in phase 0. If there is no other
adapter on the local ring with the same address as the adapter in
phase 2, then it will receive all of its DAT frames back with the
ARI (Address Recognized Indicator) and FCI (Frame Copied Indicator)
bits set to zero. At this time, the adapter would enter phase 3. If
the adapter in phase 2 receives 2 frames with either the ARI or FCI
bits set to 1, then it de-inserts from the ring and reports a
failure to open. If phase 2 does not complete within 18 seconds, the
adapter reports a failure and de-inserts.

Phase 3: Participation in Ring Poll.

This process is where a station learns its upstream neighbor's
address and informs its downstream neighbor of the inserting
adapter's address. It is this process which creates a station list
or ring map. The adapter must wait until it receives an AMP or SMP
frame with the ARI/FCI bits set to zero. Upon receipt of an AMP or
SMP frame with the ARI/FCI bits set to zero, the station flips both
bits (ARI and FCI) to one (if enough resources are available) and
queues an SMP frame for transmission. If no such frames are received
within 18 seconds, the adapter reports a failure to open and
de-inserts from the ring. If the adapter successfully participates
in a ring poll, it proceeds into the final phase of insertion.

Phase 4: Request Initialization

The adapter sends four request initialization MAC frames to the
functional address of the Ring Parameter Server (RPS). If there is
no RPS present on the ring, the adapter uses its own default values
and reports successful completion of the insertion process. If the
adapter receives one of its four request initialization MAC frames
back with the ARI/FCI bits set to one, it waits 2 seconds for a
response. If there is no response, it re-transmits up to four times.
At this time, if is no response, it reports a request initialization
failure and de-inserts from the ring.

 

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