# Overview

Channelized OC3/STM1 and OC12/STM4 interfaces are supported by the modules described in this chapter.

This section describes the features of cOCx/STMx interfaces.

## SONET APS and SDH MSP

The router supports 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. For an overview of APS/MSP, see Bidirectional Switching Mode in Configuring Unchannelized OCx/STMx Interfaces .

## MDL/FDL Support

Interfaces on cOCx/STMx line modules support maintenance data link (MDL) messages at the T3 level and facilities data link (FDL) messages at the T1 level. For a list of the line modules that support MDL and FDL, see ERX Module Guide, Appendix A, Module Protocol Support.

You can use MDL and FDL messages to determine the status of a link and to display statistics for the remote end of a connection. MDL and FDL messages do not interfere with other data transmitted over the link.

### MDL Standards

You can configure channelized T3 interfaces to send MDL messages that comply with ANSI T1.107a-1990 Standard for Telecommunications—Digital Hierarchy – Supplement to Formats Specification (August 1990). MDL messages identify a particular link by sharing common codes for data such as the equipment identifier, line identifier, frame identifier, and unit.

### FDL Standards

Similarly, you can configure T1 channels to send FDL messages that comply with either or both of the following standards:

- ANSI T1.403-1989 Standard for Telecommunications—Network and Customer Installation Interfaces – DS1 Metallic Interface – Robbed-bit Signaling State Definitions (1989)

  FDL messages that comply with the ANSI standard identify a particular link by sharing common codes for data such as the equipment identifier, line identifier, frame identifier, and unit.

- AT&T Technical Reference 54016—Requirements for Interfacing Digital Terminal Equipment to Services Employing the Extended Superframe Format (September 1989)

  FDL messages that comply with the AT&T standard identify a particular link by sharing performance data and do not use common codes for data such as the equipment identifier, line identifier, frame identifier, and unit.

### Timeout of Received MDL and FDL Messages

When a line module receives an MDL or FDL message string, it stores the strings for a period of 10 seconds after the last message was received. If the line module does not receive another message of any type containing the same string within 10 seconds, it erases the local copy of the message.

Most MDL and FDL message strings are common to all three types of messages that can be transmitted: path identifications, idle signals, and test signals. Certain message strings, however, are unique to a particular message type. Table 9 briefly describes each MDL/FDL message string and indicates, with a checkmark (✓), the types of messages in which it can be sent.

Table 9: MDL and FDL Message Strings and Message Types

Message String | Description | Path Message | Idle Signal Message | Test Signal Message
--- | --- | --- | --- | ---
eic | Equipment identification code | ✓ | ✓ | ✓
fic | Frame identification code | ✓ | ✓ | ✓
generator | Generator number | – | – | ✓
lic | Line identification code | ✓ | ✓ | ✓
pfi | Facility identification code | ✓ | – | –
port | Equipment port number | – | ✓ | –
unit | Unit identification code | ✓ | ✓ | ✓

As long as another message of any type containing the same string is received within 10 seconds, the line module retains the local copy of the message string and resets the 10-second timer for that string.

For example, if a line module receives an MDL or FDL test signal message containing an eic string, and then receives a idle signal message within 10 seconds that also contains an eic string, it retains the local copy of the most recent eic string received and resets the 10-second timer for that message. However, if 10 seconds pass without the line module receiving a path identification, test signal, or idle signal message containing an eic string, the line module erases the local copy of the eic message string.

For message strings that are unique to a particular message type, the line module must receive another message of the same type containing this string in order to retain the local copy of the string and reset the timer. For example, if the line module receives a test signal message containing a generator string and does not receive another test signal message within 10 seconds, it erases the local copy of the generator string.

### Frequency of FDL Path Messages

E Series routers transmit FDL path identifier messages every second. This behavior complies with the ANSI T1.403 specification (listed in References) and is consistent with the MDL implementation for E Series routers.

## Higher-Level Protocols

See ERX Module Guide, Appendix A, Module Protocol Support for information about the higher-level protocols that cOCx/STMx interfaces support.