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X-25 makes use of the physical layer standard X-21, but in many cases it uses other standards such as EIA 232. The physical layer deals with the physical interface station and switching node with X-25, packet layer data are transmitted as packets over virtual circuits. The link layer provides for the reliable transfer of data across the physical link by transmitting the data as a sequence of frames. The link layer standard is known as LAPB. User data are passed down to X-25 level 3, which appends control information as a header, creating a packet. The entire X-25 packet is then passed down to LAPB entity, which appends control information at the front and back of the packet, forming an LAPB frame. The basic format of an HDLC packet is shown in Fig. 11.10.

The opening flag and closing flag are made up of 8 bit information. Packets are delimited by a starting and an ending flag (01111110). The address field is typically 8 bits long, but can be extended in increments of 8 bits (Here 16 bits).

Bits 8 16 8 4 4 8 8 < 1024* 16 8

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DIY Battery Repair

DIY Battery Repair

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