APS Protection

What Is APS Protection?

APS, which stands for Automatic Protection Switching, is a linear mechanism utilized for network protection. By implementing APS, a dedicated protection link is reserved alongside a working link. In the event of a signal failure or degradation on the working link, the traffic is swiftly switched to the protection link.

Bidirectional protection switching with APS coordinates the source and sink ends to perform simultaneous traffic switchovers, negotiate switchover delays, and enable traffic switchbacks after a specified WTR time. APS protocol packets are transmitted via the backup channel, allowing both transmitting and receiving ends to verify channel configurations. APS protects tunnels (e.g., MPLS, Ethernet, SR-TP, FlexE, FlexE FG) and PWs, offering rapid unidirectional and bidirectional switching within 50 ms with minimal packet loss.

APS Protection Types

During end-to-end Ethernet transmission, APS operates on both the working and protection links. Under normal circumstances, traffic flows through the working link. However, if the working link experiences a failure, the traffic is seamlessly switched to the protection link. The source end of an APS link acts as a bridge, determining the path for outbound traffic, while the sink end acts as a selector, determining the path for inbound traffic.

APS can be categorized as either 1+1 APS or 1:N APS, depending on whether traffic is continuously transmitted on the protection link. The 1:1 APS is a specific variant of the 1:N APS, where one protection link safeguards multiple working links. In the 1+1 APS model, traffic is duplicated and transmitted separately on both the working and protection links. On the other hand, in the 1:1 APS model, the protection link remains idle when the working link is functioning normally. Traffic is only switched to the protection link when a failure occurs on the working link.

1+1 Protection

In the 1+1 protection mode, depicted in the illustration below, services are simultaneously transmitted on both the working and protection links from the source end. At the sink end, services are selectively received based on the link status and external commands. To clarify, the source end transmits services in duplicate, while the sink end chooses which services to receive.

1:1 Protection

In the 1:1 protection mode, illustrated below, services are exclusively transmitted on the working link from the source end, while the protection link remains inactive. Both the source and sink ends selectively transmit services based on the link status and external commands. In other words, the source end chooses which services to transmit, and the sink end selectively receives those services.

APS 1:1 protection

In 1+1 protection mode, switching occurs only at the sink end, making it fast but with lower resource utilization at the source end. In contrast, 1:1 protection mode utilizes only the working link, allowing for high bandwidth utilization, but switching takes place at both ends and is relatively slower.

APS Protection Switching Modes

Unidirectional Switching

In unidirectional switching mode, if there is a signal failure in one direction of the working link, only that direction is switched while the other direction remains unaffected. Services are still selectively received from the working link, and the switching at the source and sink ends are independent of each other.

Bidirectional Switching

In bidirectional switching mode, when there is a signal failure in one direction of the working link, switching occurs in both directions regardless of the status of the signal in the other direction. This means that both the source and sink ends utilize either the working link or the protection link for transmission.

Unidirectional switching is not recommended as it involves transmitting and receiving services along different paths. In bidirectional switching mode, services are transmitted and received along the same path, resulting in similar delays and facilitating maintenance.

Principles of APS Protection Switching

1+1 Unidirectional Protection

In this configuration, the source end consistently duplicates and transmits traffic on both the working and protection links. The source end periodically sends OAM CCMs (Continuity Check Messages), while the sink end detects these messages. If the sink end fails to receive CCMs for three consecutive periods (approximately 10 ms; each period lasts 3.3 ms), it determines that the signal on the link has failed. In such a scenario, the sink end's selector uses the protection link to receive traffic. Since each direction operates independently, the selector only relies on local information and does not require the exchange of APS packets to negotiate information between the two ends.

1+1 Bidirectional Protection

In this configuration, the source end continuously duplicates traffic on both the working and protection links. The source end periodically sends OAM CCMs (Continuity Check Messages), which are detected by the sink end. If the sink end fails to receive CCMs for three consecutive periods (approximately 10 ms; each period lasts 3.3 ms), it identifies a signal failure on the link. In such a scenario, both the source and sink ends are required to perform state switching. This means that the bridges and selectors at both ends must redirect traffic to the protection link. Consequently, APS packets are exchanged to negotiate information between the two ends.

1:1 Unidirectional Protection

1:1 Unidirectional switching involves exchanging APS protocol packets. When CCMs sent by the source end are not received by the sink end for three consecutive periods (around 10 ms), signaling failure is detected. The sink end immediately switches to the protection link, selectively receiving services and instructing the peer end to use the protection link for packet transmission through APS protocol packets. The peer end promptly switches to using the protection link upon receiving the APS protocol packets.

1:1 Bidirectional Protection

1:1 Bidirectional switching requires APS for communication. When CCMs are not received by the sink end for three consecutive periods, signaling failure is detected. The sink end immediately switches to the protection link for both receiving and sending services. It also sends APS protocol packets to instruct the peer end to use the protection link for packet transmission. Upon receiving the APS protocol packets, the peer end promptly initiates switching.

In summary, the key distinction between 1+1 protection and 1:1 protection lies in whether the protection link carries backup traffic when the working link is functioning properly. In 1+1 bidirectional, 1:1 unidirectional, and 1:1 bidirectional protection modes, the transmission of APS packets between the source and sink ends is required for negotiation prior to initiating the switching process.