Page 303 - Optical Communications Essentials

Optical Networks

Optical Networks 293

a certain path can be switched automatically to an alternate or standby path fol-
lowing failure or degradation of the primary link segment.
Three main features, each with two alternatives, classify all SONET/SDH rings,
thus yielding eight possible combinations of ring types. First, there can be
either two or four fibers running between the nodes on a ring. Second, the oper-
ating signals can travel either clockwise only (which is termed a unidirectional
ring) or in both directions around the ring (which is called a bidirectional ring).
Third, protection switching can be performed via either a line-switching or a
path-switching scheme. Upon link failure or degradation, line switching moves
all signal channels of an entire OC-N channel to a protection fiber. Conversely,
path switching can move individual payload channels within an OC-N channel
(e.g., an STS-1 subchannel in an OC-12 channel) to another path.
Of the eight possible combinations of ring types, the following architectures
have become popular for SONET and SDH networks:

■ Two-fiber, unidirectional, path-switched ring (called two-fiber UPSR)
■ Two-fiber or four-fiber, bidirectional, line-switched ring (called two-fiber or
four-fiber BLSR)

The common abbreviations of these configurations are given in parentheses.
They also are referred to as a unidirectional or a bidirectional self-healing ring
(USHR or BSHR).
Figure 17.9 shows a two-fiber UPSR network. By convention, in a unidirec-
tional ring the normal working traffic travels clockwise around the ring, as indi-
cated by the heavy arrows. For example, the connection from node 1 to node 3 uses
links 1 and 2, whereas the traffic from node 3 to node 1 traverses links 3 and 4.
Thus, two communicating nodes use a specific bandwidth capacity around the
entire perimeter of the ring. If nodes 1 and 3 exchange information at an OC-3
rate in an OC-12 ring, then they use one-fourth of the capacity around the ring
on all the primary links.
In a unidirectional ring the counterclockwise path is used as an alternate
path for protection against link or node failures. To achieve this, the signal from
a transmitting node is dual-fed into both the primary and protection fibers.
This establishes a designated protection path on which traffic flows counter-
clockwise, that is, from node 1 to node 3 via links 4 and 3 (in that order), as
shown in Fig. 17.9a. A heavy line and a dashed line indicate the primary and
protection paths, respectively.
Consequently, two identical signals from a particular node arrive at their des-
tination from opposite directions, usually with different delays, as denoted in
Fig. 17.9b. The receiver normally selects the signal from the primary path.
However, it continuously compares the fidelity of each signal and chooses the
alternate signal in case of severe degradation or loss of the primary signal.
Thus, each path is switched individually based on the quality of the received sig-
nal. For example, if path 2 breaks or equipment in node 2 fails, then node 3 will
switch to the protection channel to receive signals from node 1.

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