Introduction to Data Communications
Previous	45m. Token Ring Monitors and Servers (cont'd)	Next

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.

Introduction to Data Communications
Previous	Table of Contents	Next