Packet Layer Protocol orPLP is thenetwork-layer protocol for theX.25 protocol suite. PLP manages the packet exchanges between DTE (data terminal) devices across VCs (virtual circuits). PLP also can be used onISDN using Link Access Procedures, D channel (LAPD).
There are 5 modes of PLP: call setup, data transfer, idle, call clearing, and restarting.
- Call setup mode is used to createVCs (virtual circuits) between DTE devices. A PLP uses the 14-digitX.121 addressing scheme to set up the virtual circuit.
- Data transfer mode is used to send data between DTE devices across a virtual circuit. At this level PLP handles segmentation and reassembly, bit padding, error control andflow control.
- Idle mode is used when a virtual circuit is established but there is no data transfer happening.
- Call clearing mode is used to end sessions between DTE devices and to terminate VCs.
- Restarting mode is used to synchronize the transmission between a DTE device and its locally connected DCE (data communications) device.
There are 4 types of PLP packet fields:
- General Format Identifier (GFI): Identifies packet parameters (whether it is data or control information), what type of windowing is being used, and whether delivery confirmation is needed.
- Logical Channel Identifier (LCI): Identifies the virtual circuit across the local DTE/DCE interface.
- Packet Type Identifier (PTI): Identifies the PLP packet type (17 different types).
- User Data—Contains encapsulated upper-layer information when there is user data present, otherwise additional fields containing control information are added.
