HP 600 Series, Services And Applications

Page: 243 / 316

There are several limitations of optical bypass that a network designer must
be aware of:
When the bypass switch bypasses a station, the station is effectively
removed, possibly allowing the maximum segment length to exceed
2 km.
The integrity of the ring is only as good as the mechanical integrity
of the bypass switch.
Optical bypass switches can produce up to 2.5 dB optical power loss.
This loss must be considered when calculating attenuation.
(Attenuation and optical budget are discussed below.)
Dual Homing
Dual homing is a fault tolerance technique used for redundancy in
concentrator environments where continuous uptime is crucial. Dual
homing allows a DAS to use one link as a backup link for redundancy
purposes. Thus, one of the two attachments is active at any given time.
Figure 6 shows a dual-homed device attached to concentrators.
Figure 6. Dual-Homed Station
A station can be dual-homed to the same concentrator or to two different
concentrators. When it is homed to two different concentrators and the
primary concentrator fails, the DAS automatically enables the backup link to
the secondary concentrator.
Dual Counter-Rotating
FDDI Ring
Dual-Attached
Concentrator
Dual-Homed
Station
Dual-Attached
Concentrator
A
p
p
li
c
a
ti
o
n
N
o
te
s
/
C
a
s
e
S
tu
d
ie
s
3
Shining a Light on FDDI
How FDDI Works
3-49