Active Monitor (AM)
The Active Monitor (AM) is the active node with the highest address: it wins active monitor status by a token claiming process that takes place between all active nodes. Any node can become the AM, all other nodes become Standby Monitors in case the Active Monitor fails or turns off.
The duties of the Active Monitor are:
Maintaining the Master Clock
Ensuring Proper Ring Delay
Initiating Neighbour Notification
Monitoring Neighbour Notification
Monitoring Token and Frame Transmission
Detecting Lost Tokens and Frames
Purging the Ring
Maintaining the Master Clock
The Active Monitor maintains the ring's master clock which controls timing and ensures that all other clocks on the ring are synchronized. The AM beats the drum for the other nodes to follow.
Ensuring Proper Ring Delay
It inserts a latency buffer (delay) to guarantee a minimum ring length. The delay is 24 bits long for 4 Mbps Token Ring and 32 bits long for 16 Mbps Token Ring.
Initiating Neighbour Notification
The Active Monitor periodically broadcasts the Active Monitor Present MAC frame to all ring stations on its ring, allowing each to acquire the address of its Nearest Active Upstream
Neighbour (NAUN). The NAUN address is used during error isolation to determine if there is a failing component in a given ring station's fault domain (next node over).
The nodes on the ring are aware of the MAC addresses of their Nearest Active Upstream Neighbour. Notice the word "active", the neighbour must be connected to the ring.
Monitoring Neighbour Notification
At any time during the neighbour notification cycle, certain events could happen that could affect the Neighbour Notification process. The Active Monitor checks for these conditions and takes appropriate action:
1. The Active Monitor's Neighbour Notification Timer runs out. The ring is taking too long to complete the Neighbour Notification process. The Active Monitor restarts the Neighbour Notification process and reports a Neighbour Notification Incomplete MAC frame to the Ring Error Monitor (REM - just another node on the ring that has the job of monitoring ring errors).
2. The Active Monitor Present Frame takes too long to circle the Ring. The AM initiates token claiming so it can retransmit an Active Monitor Present frame.
3. If a Standby Monitor Present Frame is received after Neighbour Notification is complete, the Neighbour Notification is ignored and restarted. Another station has just connected to the ring and by inserting into the ring, has changed the NAUN order.
4. If another Active Monitor Present Frame is received with a source address different from its own. This means that there is another Active Monitor on the Ring. The receiving Active Monitor shuts down and becomes a Standby Monitor.
5. A hard error (cable fault, student playing with RI and RO ports) causes the ring to go down.
After the hard error is fixed, the Neighbour Notification process is restarted.
Monitoring Token and Frame Transmission
The AM monitors the ring to make sure that Tokens and Frames only circle the ring once. There is a Monitor bit in the MAC frame and whenever a MAC frame is repeated by the AM, the AM sets the Monitor bit to "1". All frames that are received with the Monitor bit set to 1 are not repeated - this means that the frame has already circulated the ring once.
Detecting Lost Tokens and Frames
The AM has a timer to check that there are Tokens and Token Frames circulating the ring. The timer is set for the absolute longest time that it would take for a Token or Frame to circulate the ring. If the timer times out before a new Token or frame is received. The ring is purged and a new Token is released.
Purging the Ring
The AM broadcasts the Ring Purge MAC frame to all ring stations on its ring before originating a new Token. Receipt of the returned frame indicates to the AM that a frame can circulate the ring without incident. The Ring Purge Frame resets the ring stations to Normal Repeat mode.
Standby Monitor (SM)
There is only 1 Active Monitor allowed on the ring at a time, all other stations become Standby Monitors. Standby Monitors determine whether the Active Monitor is functioning properly. If a
Standby Monitor determines that the Active Monitor is not operating properly, the Standby Monitor initiates the Token Claiming process.
Duties of the Standby Monitor
a. The Standby Monitor checks to see if a Token is circulating the ring. It has a Timer called the
"Good_token" timer and knows that a Token has to circulate within that time. If the Token does not go by within this designated period, the Standby Monitor knows that Active Monitor is not doing its job. The Standby Monitor then initiates the Token Claiming process to re-elect an Active Monitor.
b. The Standby Monitor restarts another Timer called "receive notification" whenever an Active Monitor Present frame comes by. If the Timer runs out before another Active Monitor Present frame arrives, the SM assumes that the Active Monitor is not present or has malfunctioned. The SM initiates the Token Claiming process to re-elect an Active Monitor.
Ring Parameter Server (RPS)
The Ring Parameter Server provides 3 main services to the ring:
1. Assigns operational parameters to the station at the time of insertion onto the ring. These are parameters such as: Ring Number, Physical Location and Soft Error Report Timer Value. If there is no RPS present, the ring station uses its default values.
2. Ensures that all stations on the ring have the same operational values.
3. Forwards registration information to the LAN Managers from stations attaching to the ring.
Configuration Report Server (CRS)
A Configuration Report Server accepts commands from the network management software to get station information, set station parameters and remove stations from the ring. It also collects and forwards configuration reports generated by stations on its ring to the LAN manager.
The network management software is a program that monitors the Network and is used by the System Administrator. It can monitor many Rings and may include Ethernet segments and connections to WANs.
Ring Error Monitor
The Ring Error Monitor observes, collects and analyses hard-error and soft-error reports sent by ring stations on a single ring and assists in fault isolation and correction.
Hard Error Processing Function
Hard errors are detected, isolated and bypassed through the use of a Beacon MAC frame. Hard errors are broken cables, failed equipment, improper signal timing, incorrect voltage levels.
Any ring station that detects a hard error can generate a Beacon MAC frame. The frame is addressed to all other stations on the ring. A Beacon Frame contains the address of the station that discovered the Hard Error, its NAUN and a physical location (the RPS gives this
information to the NIC during initialization).
1. Station G hasn't received any frame for a while. Station G starts a Beacon Frame with Station F as its NAUN
2. When Station F receives the Beacon Frame and reads that it is the NAUN of the Fault
Domain. It disconnects from the ring and re-attaches to the ring using the Ring Insertion process.
Note: The cable between Station F and Station G is called the Fault Domain. 3. If the fault still remains on the ring, the Beacon Frame originator Station G, disconnects from the ring and re-attaches using the Ring Insertion process.
A fault that can be cured in this manner is called a Temporary Fault. If the fault cannot be cured than it is called a Permanent Fault.
The Ring Error Monitor monitors the Beacon Frame. It reports the location, NAUN, Beacon Originator address and whether it is a Temporary or Permanent Fault to the network
management program.
Soft Error Processing
Soft Errors are errors in the Bit pattern or encoding. There are 5 Soft Error types:
1. Line Error:
A code violation between the starting and ending delimiters in the MAC frame or a Frame Check Sequence error. The FCS doesn't add up!
2. Internal Error:
The ring station recognizes a recoverable internal error. This can be used for detecting a ring station in marginal operating condition.
3. Burst Error:
The absence of transitions for 5 half-bit times. Manchester encoding is used for Token Ring and a low to high transition is a 1 and a high to low transition is a 0.
4. A/C Error:
The Token Ring Frame has 2 bits called Address-Recognized (A) and Frame-Copied (C). During Neighbour Notification, there should not be 2 Standby Monitor Present frames with AC=00 in a row. This would indicate a copy or framing error.
5. Abort Delimiter Error
A station sends out a special frame called an Abort Sequence or Abort Delimiter when it discovers an error (soft or hard) while transmitting a frame.
The Ring Error Monitor keeps track of the number of Soft Errors, who reported them and the NAUN. The REM has thresholds of acceptable Soft Error levels (can be adjusted) and reports excessive Soft Errors to the LAN Manager.
There are two categories of Soft Errors: Non-Isolating and Isolating.
Non-Isolating means that the Soft Error can only be isolated to the ring.
Isolating means that the Soft Error can be isolated to a station.
The REM keeps track of all stations and the Soft Errors associated with them.
Where are these Monitors?
Active Monitor
Any station can be the Active Monitor - first station on with the highest MAC address.
Standby Monitor
All other stations besides Active Monitor Ring Error Monitor
Usually in a Bridge/Router - something that is always on.
Configuration Report Server
Usually in the same Bridge/Router as the Ring Error Monitor.
Ring Parameter Server
Usually in the same Bridge/Router as the Ring Error Monitor