Packet Layer Protocol (PLP)
Packet-Layer Protocol (PLP) handles call setup, manages data transfer, tracks idle circuits, clears and restarts calls. PLP was designed to manage the exchange of information between two DTE stations across virtual circuits. PLP can also run over LLC2 (IEEE 802.x protocols) and over ISDN interfaces running Link Access Procedure (LAPD) on the D channel.
| MODE | Description | Executed | Used With |
| Call Setup |
Call setup is managed by PLP between two DTE stations using the X.121 addressing scheme to set up the virtual circuit. Each call is independent of other calls, so calls can be in various stages and not affect one another. Call Setup is only done with SVC's, as PVC's are already connected after they are built. | Per Circuit | SVC's |
| Data Transfer |
As the name states, data transfer is handled by PLP between two DTE stations across an SVC or PVC. Since data is transmitted in a constant stream, when there is no data the channel is padded with zeroes. Padding, segmentation and reassembly, error and flow control are handled | Per Circuit | SVC's & PVC's |
| Idle | Intermediate mode between connection and data transfer mode. | Per Circuit | SVC's |
| Call-Clear | Used to tear down a virtual circuit and end a call between DTE stations. | Per Circuit | PVC's & SVC's |
| Restarting | Transmission synchronization between two DTE devices. This is used prior to entering Data Transfer Mode and to re-establish synchronization when lost. | Per DTE Device |
PACKET LAYER PROTOCOL FIELDS
- General Format Identifier (GFI)
- Identifies the packet's parameters such as whether the packet contains Data or Control information, type of windowing performed, and whether to use Delivery Confirmation.
- Logical Channel Identifier (LCI)
- Identifies the local virtual circuit.
- Packet Type Identifier (PTI)
- Identifies one of the following packet types:
| CALL ACC | Call Accept |
| CALL REQ | Call Request |
| CLR CNF | Clear Confirmation |
| CLR REQ | Clear Request |
| DATA | Data Packet |
| DIAG | Diagnostic |
| INT CNF | Interrupt Confirmation |
| INT REQ | Interrupt Request |
| REJ | Reject |
| RES CNF | Reset Confirmation |
| RES REQ | Reset Request |
| RNR | Receive Not Ready |
| RR | Receive Ready |
| RSTR CNF | Restart Confirmation |
| RSTR REQ | Restart Request |
| REG REQ | Registration Request |
| REG CNF | Registration Confirmation |
- User Data
- Encapsulated data from an upper-layer protocol such as TCP/IP. This field only exists in data packets,
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