Packet Layer Protocol or PLP is the Network Layer

Network Layer

The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...

 protocol for the X.25

X.25

X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...

 protocol suite. PLP manages the packet exchanges between DTE (data terminal) devices across VCs (virtual calls). PLP also can be used on ISDN using Link Access Procedures, D channel

Link Access Procedures, D channel

Link Access Procedures on the D channel , specified in ITU-T Q.920 and ITU-T Q.921, is the second layer protocol on the ISDN protocol stack in the D channel.It is heavily based on HDLC.-External links:*http://www.protocols.com/pbook/pdf/isdn.pdf...

 (LAPD).

There are 5 modes of PLP: call setup, data transfer, idle, call clearing, and restarting.

• Call setup mode is used to create VC
  VC
  VC may refer to:* Vehicle code, a motor vehicle traffic legal system* Vanadium carbide, an inorganic compound* Vancouver Canucks, a NHL hockey team* Vanier College* Vassar College...
  
  s (virtual calls) between DTE devices. A PLP uses the 14-digit X.121
  X.121
  X.121 is the ITU-T address format of the X.25 protocol suite used as part of call setup to establish a switched virtual circuit between Public Data Networks , connecting two network user addresses...
  
   addressing scheme to set up the virtual call.
• Data transfer mode is used to send data between DTE devices across a virtual call. At this level PLP handles segmentation and reassembly, bit padding, error control and flow control
  Flow control
  In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
  
  .
• Idle mode is used when a virtual call 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 call 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.

External links

• Cisco X.25 Documentation