x.25(page2)


 * X.25 Layers**


 * X.25 protocol specifies three layers:**

- specifies the physical interface between the node (computer, terminal) and the link that   connected to X.25 network. - specifies a protocol called X.21
 * i.	Physical Layer**


 * ii.	Frame Layer**

- provides a reliable data transfer process through data link control which used link access procedure, balanced (LAPB) protocol. - There are 3 categories of frame involved in the LAPB frame format: **I-Frames **– encapsulate PLP packets from the network layer and before being passed to the physical layer ** S-Frames** – flow and error control in the frame layer **U-Frames**- used to set up and disconnect the links between a DTE and a DCE. -	In the frame layer, communication between a DTE and a DCE involves three phases: 1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect

**Four types of PLP packet fields exist:** •** General Format Identifier (GFI)**—Identifies packet parameters, such as whether the packet carries user data or control information, what kind of windowing is being used, and whether delivery confirmation is required. • **Logical Channel Identifier (LCI)**—Identifies the virtual circuit across the local DTE/DCE Interface • **Packet Type Identifier (PTI)**—Identifies the packet as one of 17 different PLP packet types. • **User Data**—Contains encapsulated upper-layer information. This field is present only in data packets. Otherwise, additional fields containing control information are added.
 * iii.	Packet Layer Protocol (PLP)**

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