2-Fiber BLSRs
Overview
A 2-fiber bidirectional line-switched ring (BLSR) is a self-healing ring configuration in which bidirectional traffic travels over the same set of nodes in each direction. Bidirectional switching makes use of redundant (protection) bandwidth on the bidirectional lines that interconnect the nodes in the ring to provide protection for traffic carried on the working bandwidth. Because the traffic is bidirectional, a single circuit does not necessarily consume bandwidth on every span in the ring. This leaves the spans between other nodes available for additional traffic. Therefore, with distributed switching patterns, a BLSR can carry more traffic than the same facilities could carry if configured for a unidirectional path-switched ring.
Alcatel-Lucent 1665 DMX supports 15 simultaneous BLSR protection groups in a single shelf. The 15 protection groups can be provisioned on any available OC-48 and OC-192 port pairs within the high-speed and low-speed groups.
For example, you can have an application with two high-speed OC-192 2-fiber BLSR and 13 low-speed OC-48 BLSRs. The low-speed OC-48 BLSRs are hosted by OC-48 circuit packs housed in the Function slots and can add and drop traffic with the high-speed OC-192 circuit packs housed in the Main slots. The low-speed OC-48 BLSRs can also add and drop traffic with interfaces in other tributary (low-speed) slots (includes both Function Units and Growth slots).
Traffic capacity
The following figure shows working and protection traffic capacities in an OC-192 2-fiber BLSR. The OC-48 2-fiber BLSR operates in the same way, but has 48 STS-1 time slots in each direction, 24 for working traffic and 24 for protection.
Figure 3-24: Traffic capacity in an OC-192 2-fiber BLSR
Self-healing rings
Alcatel-Lucent 1665 DMX 2-fiber BLSRs are self-healing in that transport is automatically restored after node or fiber failures. Each OC-192 line carries 96 STS-1 equivalent time slots of working capacity plus 96 STS-1 equivalent time slots of protection capacity. Each OC-48 line carries 24 STS-1 equivalent time slots of working capacity plus 24 STS-1 equivalent time slots of protection capacity. In the event of a fiber or node failure, service is restored by switching traffic from the working capacity of the failed line to the protection capacity in the opposite direction around the ring. See the figure below and the figure on the following page. In the event of a node failure, traffic added and dropped from the failed node is not protected by line switching.
2-fiber BLSR traffic flow
The figure below shows normal (non-protection-switched) traffic flow on a 2-fiber BLSR.
Figure 3-25: Normal traffic flow in a 2-fiber BLSR
Protection switching
When a line failure triggers a protection switch, the nodes adjacent to the failure switch traffic on to protection capacity. Traffic heading toward the failure is looped back on to the protection capacity traveling away from the failure to reach its destination by traveling the opposite way around the ring (see the figure below). Service is reestablished on the protection capacity in ≤50 milliseconds after detection of the failure (for catastrophic failures in rings without existing protection switches or extra traffic).
Fiber cut example
The figure below illustrates a 2-fiber BLSR protection switch that results from a fiber cut.
Figure 3-26: Loopback protection switch in a 2-fiber BLSR
Protection traffic flow
In the figure above, traffic going from Node A to Node C that normally passes through Node E and Node D on "working 2" capacity is switched at Node E on to the "protection 2" capacity of the line leaving Node E in the direction of Node A. The traffic passes through Nodes A, B, and C on the protection bandwidth to Node D where it is looped back to Node C. Similarly, traffic going from Node C to Node A that normally passes through Node D and Node E on "working 1" capacity is switched at Node D on to the "protection 1" capacity of the line leaving Node D in the direction of Node C. The traffic passes through Nodes C, B, and A on the protection bandwidth to Node E where it is looped back to Node A. Only the nodes adjacent to the failure perform loopback protection switches. The same approach is used for a node failure. For example, if Node D fails, Nodes C and E perform loopback protection switches to provide an alternate route for ring traffic. However, traffic that is added and dropped at the failed node, Node D, cannot be protected by ring switching and is therefore squelched at the switching nodes.
Extra traffic
Alcatel-Lucent 1665 DMX supports extra traffic
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OC-192 BLSRs (Main and FN) in systems with VLF mains
(OC-192 VLF mains are required in all contiguous nodes participating in the extra traffic part of the network.)
The extra traffic capability allows the protection channels to carry additional low-priority traffic during fault-free conditions. The extra traffic is established by provisioning cross-connections to the high-speed (main) BLSR protection channels. Provisioning cross-connections on the protection channels is only supported on high-speed BLSRs. Pass through cross-connections are provisioned on the protection channels at intermediate nodes. (Protection channels that are not carrying Extra Traffic are terminated at the intermediate nodes.)
If a BLSR protection switch occurs, traffic is switched from the working channels to the protection channels and extra traffic is preempted.
If extra traffic circuits are preempted, AIS-P is used to squelch the circuits. When the BLSR protection switch clears, the extra traffic is restored.
Nonpreemptive unprotected traffic
Non-preemptible unprotected traffic is traffic carried on (working and/or protection) BLSR channels for which protection switching has been provisioned as disabled. As the name implies, NUT is unprotected and not preempted in the event of a protection switch.
BLSR protocols allow the available bandwidth of a BLSR to be partitioned into three types of channels:
Working traffic is protected against failure events via the BLSR APS protocol, while extra traffic is unprotected traffic carried on the protection channels. Any failure event that may require the protection channels for protection purposes shall preempt the extra traffic. NUT carried on non-preemptible unprotected channels affords a higher level of survivability as compared to extra traffic, which is preempted during a protection switch, but a lower level of survivability as compared to working traffic, which is carried on its corresponding protection channel during a protection switch. Note that non-preemptible unprotected traffic is not considered extra traffic.
Important!
Alcatel-Lucent 1665 DMX supports NUT on 2-fiber OC-48 and OC-192 BLSRs.