Transaction Language 1 (TL1) is a widely used management protocol in telecommunications. It is a cross-vendor, cross-technology man-machine language, and is widely used to manage optical (SONET) and broadband access infrastructure in North America. TL1 is used in the input and output messages that pass between Operations Support Systems (OSSs) and Network Elements (NEs). Operations domains such as surveillance, memory administration, and access and testing define and use TL1 messages to accomplish specific functions between the OS and the NE. TL1 is defined in Telcordia Technologies (formerly Bellcore) Generic Requirements document GR-831-CORE. History TL1 was developed by Bellcore in 1984 as a standard man-machine language to manage network elements for the Regional Bell Operating Companies (RBOCs). It is based on Z.300 series man machine language standards. TL1 was designed as a standard protocol readable by machines as well as humans to replace the diverse ASCII based protocols used by different Network Element (NE) vendors. It is extensible to incorporate vendor specific commands. Telcordia OSSs such as NMA (Network Monitoring and Analysis) used TL1 as the element management (EMS) protocol. This drove network element vendors to implement TL1 in their devices. Language overview TL1 Messages The TL1 language consists of a set of messages. There are 4 kinds of messages: Input message - This is the command sent by the user or the OSS. Output/Response message - This is reply sent by the NE(Network Element) in response to an input message. Acknowledgment message - This is an acknowledgment of the receipt of a TL1 input message and is sent if the response message will be delayed by more than 2 seconds. Autonomous message - These are asynchronous messages (usually events or alarms) sent by the NE. TL1 message structure TL1 messages follow a fixed structure, and all commands must conform to it. However, the commands themselves are extensible and new commands can be added by NE vendors. These are some of the message components: Target identifier (TID) & Source identifier (SID) - TID/SID is a unique name assigned to each NE. TID is used to route the message to an NE, SID is used to identify the source of an autonomous message. Access identifier (AID) - AID identifies an entity within an NE. Correlation tag (CTAG) & Autonomous correlation tag (ATAG) - CTAG/ATAG are numbers used to correlate messages. TL1 input message Example: Structure: TL1 output message Example: Structure: TL1 acknowledgment