SDLC (Synchronous Data Link Control):
A communications line discipline associated with the IBM systems network architecture (SNA).

Segment:
Each line item in a transaction set.

Session:
The act of (at least) signing on and signing offone's Mailbox. A session may or may not include sending and/or receiving data to/from the System. A session MUST end with the transmission (by the customer of a Signoff record.)

SNA (Systems Network Architecture):
An architecture for interchanging data trough an SNA network in a sort-and-forward fashion.

SNA/DS (System Network Architectural Distribution Services):
an E-mail messaging protocol that is used with many mainframe based messaging systems. It operates over a dedicated line using the SNA communications protocol.

Store and Forward:
Service where a message carried by the network may be temporarily sorted or routed in the network prior to delivery to the recipient.

Subset:
A subset of a national standard for ease-of-use within one industry. The subset usually indicates only those segments, elements, and code values needed by the industry.

Synchronous Transmission:
Data communications in which characters or bits are sent at a fixed rate, with the transmitting and receiving devices synchronized; eliminates the need for start and stop bits basic to asynchronous transmission and significantly increases data throughput rates.

Syntax:
The rules governing the structuring of the user data and of associated data in the interchange of messages.