After get understanding on E1 Carier System, Frame Relay is part of the related topic which play a big role on E1 Careir System

Frame Relay is a simplified form of connection-based, packet-switching service in which synchronous frames of data are routed to destinations indicated on the header information. Frame Relay assumes an error-free physical link and therefore does not guarantee data integrity. Error detection and correction responsibility is left with the end devices.

Frame Relay uses the synchronous High-level Data Link Control (HDLC) frame format up to 4096 octets in length. Each frame contains a start flag, two octets that contain the information required for multiplexing across the link, the data information (payload), two octets generated by a cyclic redundancy check (CRC) of the rest of the octets between the flags, and the end flag.

Permanent virtual circuits (PVCs) are used to form a connection between any two devices attached to a Frame Relay cloud. Virtual circuits are logical, bidirectional, end-to-end connections that appear to the user as dedicated links. Each PVC is given a unique number on each physical circuit along the path between the two devices. This unique number is called a data link connection identifier (DLCI). The DLCI is automatically changed to the PVC number of the next physical circuit as it passes through each switch along the path. A DLCI is different from a network address in that it identifies a circuit in both directions, not a particular endpoint. A frame contains only one DLCI, not a source and destination.

In general, the only DLCI numbers you see are those numbers assigned to the physical circuits on the perimeter of the Frame Relay cloud. DLCIs only have local significance and represent end-to-end virtual connections that have a permanently configured switching path to a certain destination. Thus, by having a system with several DLCIs configured, you can communicate simultaneously with several different sites.