Specifications

TOKEN RING OVERVIEW
2-10
Ring De-Insertion
If the station’s hardware adapter experiences a non-recoverable error, the
phantom current will drop causing the relay on the MSAU to open. This
breaks the circuit for that lobe and the station is then bypassed from the
ring. Typical causes for this condition are the station being powered off,
the lobe cable being unplugged, or an adapter hardware failure. While
stations are inserted or removed from the ring, there is a temporary break
in the ring circuit while the relay opens or closes. The Token Ring protocol
allows for recovery from this situation with virtually no impact on
communications among stations still inserted into the ring.
Phantom Current
A DC voltage applied by a station trying to insert into the ring. The
current opens the bypass relay in the TCU allowing the station access. The
voltage is transparent to the signal, hence its name. If a cable failure causes
phantom current to drop, the relay will close thus bypassing the faulty
lobe, offering protection to the whole ring against one faulty cable.
Adjusted Ring Length (ARL)
When a segment of trunk cable fails, the wrap feature connects the main
path to the backup path. This creates a new data path, one that is longer
than the original. The worst case, or longest path, would be brought about
by the failure of the shortest trunk cable segment. This worst case is
known as the ARL and is an important factor calculated into the network
installation composed of passive devices with no repeater functionality.
Physical Layer Signaling Technique
The technique defined by the IEEE for Token Ring signaling is known as
Differential Manchester Encoding on a baseband transmission.
Differential Manchester Encoding uses a signal transition at the start and
center of the bit cell time to represent a 1, 0, J, or K bit, as shown in
Figure 2-3. The bit cell times for 4 Mbps Token Rings is 250 ns and 62.5 ns
for 16 Mbps rings.
In the case of the two data symbols, binary 1s and 0s, a signal element of
one polarity is transmitted for one half of the bit cell time followed by the
transmission of a signal element of the opposite polarity for the remainder
of the bit cell time. This provides two distinct advantages:
• The resulting signal has no DC component and can readily be
inductively or capacitively coupled.
• The forced mid-bit transition conveys inherent timing information on
the channel.