| 单词 | Figure P-4 |
| 释义 | Figure P-4 • General Format Identifier (GFI): A 4-bit field that identifies packet parameters, including payload type (e.g., user data or control data), windowing information, and whether or not delivery confirmation is required. • Logical Channel Identifier (LCI): A 12-bit field that identifies the logical channel group and channel number of the virtual circuit that connects to the destination data terminal equipment (DTE). • Packet Type Identifier (PTI): An 8-bit or 16-bit field that identifies the PLP packet type, of which there are 17. Packet types include various call setup and call clearing, data and interrupt, flow control and reset, restart, and diagnostic packets. • User Data or Control Data: The payload of a data packet comprises encapsulated higher-layer application information.The payload of a control packet comprises various information relating to call setup and clearing, flow control and reset, and so on.The default maximum payload size is 128 octets, which every network must support. Public X.25 networks variously accommodate packets with maximum payloads of 16, 32, 64, 128, 256, 512, and 1,024 octets. Custom networks can accommodate packet sizes of up to 4,096 octets. See also block, channel, DTE, error control, flow control, ISDN, LAN, LAPB, LAPD, Network Layer, OSI Reference Model, packet, padding, PVC, SVC, virtual circuit, window, and X.25. |
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