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# Overview

SONET/SDH interfaces are supported by the modules described in this chapter. This section describes features that are available with SONET/SDH interfaces.

## APS and MSP

E-series routers support Automatic Protection Switching (APS) and Multiplex Section Protection (MSP) on selected I/O modules that provide SONET/SDH connections. This feature provides a redundant connection if a primary SONET/SDH connection fails.

For a list of I/O modules that support APS/MSP, see ERX Module Guide, Appendix A, Module Protocol Support.

| Note: The E120 router and the E320 router do not support APS/MSP.

I/O modules that support APS/MSP have some ports designated for primary operation and other ports designated for redundant operation. For APS/MSP to work correctly, you must provide connections from a primary port and a corresponding redundant port to the remote device. The remote device must also support APS/MSP.

You configure a working interface on the primary port and a corresponding protect interface on the redundant port of the I/O module. The working interface provides the primary connection, and the protect interface provides the redundant connection.

The router sends and receives data through both interfaces; however, in normal operation, only the signal on the working interface is used. If the signal on the primary interface fails, the router can use the signal on the protect interface. The process by which the router switches to the protect interface is called switchover.

When you configure APS/MSP, you must assign a working interface and a corresponding protect interface to a unique group. This group establishes the relationship between the interfaces. Within the group, each interface is identified by an APS/MSP channel number. For information about identifying the channel number, see Numbering Scheme.

You must pair a working interface and its corresponding protect interface on an I/O module to form a valid linear APS 1+1 group. For example, on an I/O module that provides four working (primary) ports and four protect (redundant) ports, the working interface ports are numbered 0–3, and the protect interface ports are numbered 4–7. Table 6 lists the pairings required to form four valid APS 1+1 groups on this I/O module. Each working/protect port pair (for example, port 0 and port 4) forms a valid APS 1+1 group.

Table 6: Sample Pairings for Valid APS/MSP Groups

Pair This Working Port | With This Protect Port
--- | ---
0 | 4
1 | 5
2 | 6
3 | 7

### Automatic Switchover

Provided you have not issued the aps lockout command for the protect interface, the router switches over to the protect interface if it detects signal failure. You can set the SONET/SDH alarms that determine signal failure and signal degradation.

### Manual Switchover

When the router is running and you have configured the I/O module for APS/MSP, you can cause switchover by issuing the aps force or aps manual command.

### Switching Mechanisms

E-series routers support both bidirectional and unidirectional APS switching modes. By default, the router uses bidirectional switching mode.

#### Bidirectional Switching Mode

In bidirectional switching mode, the router switches both ends of an APS pair to the same working interface or to the same protect interface when either end determines that a switch is required.

Possible reasons for initiating a bidirectional switch include:

- Detection of a signal failure
- Receipt of an aps force or aps manual from the local end of an APS pair
- Reversion to the working interface after a failure has been corrected and the timeout value specified in aps revert has expired

The devices at both the local and remote ends of an APS pair must support bidirectional switching for the router to implement bidirectional switching mode. Otherwise, the router implements unidirectional switching mode at both ends of the APS pair.

The router detects support for bidirectional switching by interpreting the values of the K1 and K2 bytes in the SONET/SDH frame. For details about the meanings of the values of K1 and K2 bytes, see Communication Methods.

#### Unidirectional Switching Mode

In unidirectional switching mode, the router switches only one end of an APS pair to the working interface or to the protect interface when that end determines that a switch is required. Possible reasons for initiating a unidirectional switch are the same as those described in Bidirectional Switching Mode for initiating a bidirectional switch.

### Reversion After Switchover

A failed interface automatically reverts from the protect interface to the working interface after the router detects that the working interface is operational and the timeout value specified in aps revert has expired. Reversion applies only to recovery from failures.

You can configure the router to revert to the working interface at a specified time after it recovers. This feature enables you to use the protect interface as a redundant connection that functions only when the working interface is not available.

### Communication Methods

The router communicates with the remote device by using the K1 and K2 bytes in the line overhead of the SONET/SDH frame. The values of these bytes determine the switching and protect actions. Table 7 and Table 8 list the meanings of the values of the K1 and K2 bytes. The bytes are defined in Telcordia document GR-253—Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, Revision 3 (September 2000). See requirement objects R5-56 [179] and R5-58 [181] for information about bit ordering and meaning for the K1 byte; see R5-67 [190v2] for information about the K2 byte.

Table 7: Explanation of K1 Byte

Bit Value (12345678) | Meaning
--- | ---
Bits 1–4 represent a request.
0000 | No request
0001 | Do not revert
0010 | Reverse request
0011 | Not used
0100 | Exercise
0101 | Not used
0110 | Wait-to-restore
0111 | Not used
1000 | Manual switch
1001 | Not used
1010 | Low-priority signal degradation
1011 | High-priority signal degradation
1100 | Low-priority signal failure
1101 | High-priority signal failure
1110 | Forced switch
1111 | Lockout of protection
Bits 5–8 represent the channel number.
0 | Channel number of protect interface
0001–1110 | Channel number of working interface

Table 8: Explanation of K2 Byte

Bit Value (12345678) | Meaning
--- | ---
Bits 1–4 represent the channel number. |
0 | Channel number of protect interface
0001–1110 | Channel number of working interface
Bit 5 indicates the type of redundancy.
0 | 1+1 architecture
Bits 6–8 indicate the switching mode.
000– 011 | Reserved for future use
100 | Unidirectional mode
101 | Bidirectional mode
110 | Line remote defect indication (RDI)
111 | Line alarm indication signal (AIS)

## Higher-Level Protocols

See ERX Module Guide, Appendix A, Module Protocol Support for information about the higher-level protocols that the interfaces described in this chapter support.

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