# Overview

Unchannelized T3 (DS3) and E3 interfaces are supported by the modules described in this chapter.

Throughout this chapter, interfaces on modules that provide ATM support are called T3/E3 ATM interfaces. Similarly, interfaces on modules that provide frame (HDLC) support are called T3/E3 frame interfaces.

This section describes the features of unchannelized T3/E3 interfaces. For information about configuring unchannelized T3 (DS3) interfaces over SONET/SDH, see Configuring Channelized OCx/STMx Interfaces.

## MDL Support

T3 interfaces on some line modules support maintenance data link (MDL) messages. For a list of the line modules that support MDL, see ERX Module Guide, Appendix A, Module Protocol Support.

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

### MDL Standards

You can configure 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.

### Timeout of Received MDL Messages

When a line module receives an MDL 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 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 5 briefly describes each MDL message string and indicates, with a checkmark (✓), the types of messages in which it can be sent.

Table 5: MDL 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 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 will erase the local copy of the generator string.

## Higher-Level Protocols

See ERX Module Guide, Appendix A, Module Protocol Support for information about the higher-level protocols that T3 and E3 interfaces support.