Specifications

2-13
TOKEN RING OVERVIEW
its transmit clock. This problem is known as Accumulated Jitter and is one
of the main reasons for the limitations on the number of stations and
repeaters on a ring.
To overcome Accumulated Jitter an elastic buffer is set up by the Active
Monitor, in addition to the Latency buffer. For a 4 Mbps ring the buffer is
initiated at 3 bits and can grow to 6 bits or shrink to 0 bits. For 16 Mbps,
the buffer is initialized at 16 bits and can grow to 32 bits or shrink to 0 bits.
Ring Latency
Each station uses a one bit buffer in which to store the incoming signal. As
the next bit comes in the buffer releases the bit to the ring effectively
regenerating and re-timing the signal. Therefore each station induces at
least a one bit latency. As each bit passes through a stations repeater buffer,
a copy may also be made in the stations frame buffer, so gradually the
frame is built up bit by bit. Depending on the information already stored
in the frame buffer, a station may be allowed to modify the current bit in
its repeater buffer. This function will be described in detail later.
TOKEN RING PROTOCOL OPERATION
While looking at the protocol operation it is important to understand that
the IEEE 802.5 standard provides that each station has the following four
(4) Network Management Agents:
• Active or Standby Monitors (AM or SM)
• Ring Error Monitor (REM)
• Configuration Report Server (CRS)
• Ring Parameter Server (RPS)
These agents are responsible for generating the 25 variations (plus an
additional 17 if an IBM application) of MAC frames used for ring
operation. Inherently this provides for a wealth of network statistics that
are used by network management products.
The main functions of the Token Ring Protocol are illustrated, during ring
operation, when looking at how a new station is inserted into a ring. The
following sections describe how a station becomes part of the ring and
how fault diagnosis and recovery provide the ring with a resilient
network.