SELF-HEALING SONETSDH RINGS 37
• Direction of transmission
: A SONETSDH ring can be unidirectional or bidirectional. In a unidirectional ring, signals are only transmitted in one direction of the ring; in a
bidirectional ring, signals are transmitted in both directions. •
Line or path switching : Protection on a SONETSDH ring can be at the level of a line
or a path. Recall from Section 2.3.1 that a line is a link between two SONETSDH devices and might include regenerators. A path is an end-to-end connection between the
point where the SPE originates and the point where it terminates. Note that Section 9.2 refers to line protection as link protection. Line switching restores all of the traffic that
pass through a failed link, and path switching restores some of the connections that are affected by a link failure.
Based on these three features, we have the following two-fiber or four-fiber ring archi- tectures: unidirectional line switched ring ULSR, bidirectional line switched ring BLSR,
unidirectional path switched ring UPSR , and bidirectional path switched ring BPSR. Of
these rings, the following three are currently used: two-fiber unidirectional path switched ring 2F-UPSR, two-fiber bidirectional line switched ring 2F-BLSR
, and four-fiber bidi- rectional line switched ring 4F-BLSR
These three ring architectures are discussed below in detail.
2.6.1 Two-fiber Unidirectional Path Switched Ring 2F-UPSR
This ring architecture, as its name implies, consists of two fibers with unidirectional transmission and path switching. Figure 2.17 shows an example of this ring architecture
type. The working ring consists of fibers 1, 2, 3, and 4; the protection ring consists of fibers 5, 6, 7, and 8. The ring is unidirectional, meaning that traffic is transmitted in the
same direction. That is, A transmits to B over fiber 1 of the working ring, and B transmits over fibers 2, 3, and 4 of the working ring. Protection is provided at the path level using
a scheme similar to the 1 + 1 described above. That is, the signal transmitted by A is split into two; one copy is transmitted over the working fiber fiber 1, and the other copy
is transmitted over the protection fibers fibers 8, 7, and 6. During normal operation, B receives two identical signals from A and selects the one with the best quality. If fiber
1 fails, then B will continue to receive A’s signal over the protection path. The same applies if there is a node failure.
This is a simple ring architecture; it is used as a metro edge ring to interconnect PBXs and access networks to a metro core ring. Typical transmission speeds are OC-3STM-
1 and OC-12STM-4. The disadvantage of this ring architecture is that the maximum amount of traffic it can carry is equal to the traffic it can carry over a single fiber.
ADM 1
ADM 2
ADM 3
ADM 4
5 2
6 4
8 3
7 A
Protection ring Working ring
1 B
Figure 2.17 An example of a 2F-UPSR.
38 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP
2.6.2 Two-fiber Bidirectional Line Switched Ring 2F-BLSR
This is a two-fiber ring with bidirectional transmission and line switching. It is used in metro core rings. As shown in Figure 2.18, fibers 1, 2, 3, 4, 5 and 6 form a ring Ring
1, on which transmission is clockwise. Fibers 7, 8, 9, 10, 11, and 12 meanwhile form another ring Ring 2, on which transmission is counter-clockwise. Unlike the 2F-UPSR,
both Rings 1 and 2 carry working and protection traffic. This is done by dividing the capacity of each fiber on Rings 1 and 2 into two parts. One part is used to carry working
traffic, and the other part to carry protection traffic. For instance, let us assume that the transmission speed of each fiber is OC-12STM-4. Then, two OC-3STM-1s are allocated
to working traffic and the other two to protection traffic. Since only two OC-3STM-1s can be used for working traffic, the maximum capacity that the 2F-BLSR can carry over
both Rings 1 and 2 is OC-12STM-4. The capacity allocated to protection traffic on either Rings 1 and 2 can be used to carry low priority traffic. This traffic can be preempted in
case of failure of a fiber.
The ring is bidirectional, which means that a user can transmit in either direction. That is, it can transmit on either Ring 1 or Ring 2, depending on the route of the shortest path
to the destination. In view of this, under normal operation, A transmits to B over the working part of fibers 1 and 2 of Ring 1, and B transmits to A over the working part of
fibers 8 and 7 of Ring 2.
Assume that fiber 2 fails. Since the ring provides line switching, all of the traffic that goes over fiber 2 will be automatically switched to the protection part of Ring 2. That
is, all of the traffic will be rerouted to ADM 3 over the protection part of Ring 2 using fibers 7, 12, 11, 10, and 9. From there, the traffic for each connection will continue on
following the original path of the connection. For instance, let us consider a connection from A to C, as indicated in Figure 2.18 by the solid line. When fiber 2 fails, the traffic
from A will be rerouted, as shown in Figure 2.18 by the dotted line. At ADM 3, the traffic will continue along the same path as the original connection. That is, it will be
routed to back to ADM 4 over fiber 3.
If both fibers 2 and 8 fail, then the traffic transmitted on fiber 2 from ADM 2 will be switched to the protection part of Ring 2, and the traffic transmitted from ADM 3 on
fiber 8 will be switched to the protection part of Ring 1.
2.6.3 Four-fiber Bidirectional Line Switched Ring 4F-BLSR
This is a four-fiber ring with bidirectional transmission and line switching. There are four fibers between two adjacent SONETSDH devices; they form two working rings
ADM 1 ADM 2
ADM 3
ADM 4 7
3 9
6 12
5 11
A B
1 8
4 2
10 ADM 5
ADM 6 C
Figure 2.18 An example of a 2F-BLSR.
SELF-HEALING SONETSDH RINGS 39
