Automatic Protection Switching (APS) is one the most important feature of SONET topology protection, capable of less than 50 ms failover.

SONET APS Overview

  • Protection is differs from restoration by reserving a backup resource if primary fails, while restoring is re-establishing a path based on resources available after the failure (IP Fast Restoration is an example of restoration).
  • APS switchover can occur up to 50 ms after failure detection for BLSR ring and less than 1200 km of fiber. 10 ms is set for discovery of failure detection.
  • Two sets of fiber are required for protection. One set (working) is used for TX/RX while the other is used for protection (protect TX/RX).
  • Types of protection consist of the following properties:
    • 1+1
    • 1:1
    • 1:n
    • Unidirectional Switching
    • Bidirectional Switching
    • Revertive
    • Nonrevertive


Protection Types

1+1 Protection

  • Two signals, working and protect are transmitted at the same time. Terminal devices look at both to see which one to pick.
  • Both signals are bridge together.
  • Does not allow for unprotected channels.
  • Switchover occurs locally and doesn't require any coordination with other nodes, since both signals are present.
  • Recover time is much less than 50 ms.

1:1 Protection

  • For each path there is a backup fiber that sits idle.
  • If failure occurs the signal is bridged with the backup fiber.
  • Nonrevertive protection is most common on 1+1, where even after original link come backup, the protect link will be used.

1:n Protection

  • For a number of paths there is a designated backup fiber, not used.
  • There could be n number of paths that are protected by 1 backup fiber.
  • If anyone of them fails, signal is bridged with the backup fiber.
  • If multiple links fail, there must be a priority mechanism to decide which link gets bridged to the backup link.
  • Revertive protection is supported, where if the failed link becomes restored, traffic will automatically switch back to the original fiber. Wait to Restore (WTR) timer is used before switching back.

Unidirectional Switching

  • Unidirectional switching protects a single fiber either TX or RX during failures.
  • For example if TX fails, it will be switched to the protect fiber, while RX will remain on the original fiber.

Bidirectional Switching

  • Bidirectional switching will switch both TX and RX even when only one side fails.
  • For example if RX fails, TX and RX will be switched to the protect fiber.


  • Revertive deals with the action after a failover when the originally failed path recovers. The Revertive options returns to service the original path (working).
  • Common with 1:n protection to free up the 1 protect path. 1:1 can also use revertive.


  • Nonrevertive deals with the action after a failover when the originally failed path recovers. The Nonrevertive options keeps on using the protect path and leaves the original one.
  • It does not revert back.
  • Common with 1+1 protection. 1:1 can also use nonrevertive.

Topology Protection

  • There are two protection topologies using properties from above.

Unidirectional Path-Switched Ring (UPSR)

  • Two fibers connecting all network elements. Traffic is transmitted at the same time using both connections, one pair is used for traffic flow sent in the clockwise direction (working), the other counterclockwise (protect). Best signal is used by receivers.
  • No signalling required for failover. Recovery is local at each node.
  • Uses 1+1 protection.
  • Typically deployed in access networks or metro networks.
  • Because traffic flow is uniderectional ring, the latency is different for each directions this is called Asymmetrical Delay. For example ring a->b->c->d->a, flow from a to b will be faster than back from b to a which will be slower. This should not be an issue for small rings and small spans of distance.

Bidirectional Path-Switched Ring (BPSR)


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