The RLC protocol is responsible for segmentation of RLC SDUs from the PDCP into suitably sized RLC PDUs. Johan Sköld, in 5G NR (Second Edition), 2021 6.4.3 Radio-Link Control PPP frames may be compressed, field sizes reduced, and used for many specific tasks, as long as the endpoints agree.Įrik Dahlman. For LCP messages, the Protocol field has the value 0xC021 (1100 0000 0010 0001), for NCP the field has the value 0x8021 (1000 0000 0010 0001), and for IP packets the field has the value 0x0021 (0000 0000 0010 0001).įollowing the header is a variable-length Information field (the IP packet), followed by a PPP frame trailer with a 16-bit, frame check sequence (FCS) for error control, and finally an end-of-frame Flag field. The Protocol field identifies the format and use of the content of the PPP frame itself. The UI format is used to indicate that the connectionless IP protocol is in use. The Control field is set to 0x03 (0000 0011), which is the Unnumbered Information (UI) format, meaning that there is no sequence numbering in these frames.
#The ppp frame format serial
Point-to-point links only care about the destination, which is always 0xFF in PPP and essentially means “any device at the other end of this link that sees this frame.” This is one reason why serial interfaces on routers sometimes do not have IP addresses (but many serial interfaces, especially to other routers, have them anyway-this is the only way to make the serial links “visible” to the IP layer and network operations).
Note that none of the other fields in the Point-to-Point Protocol header have a source address for the frame. The Address field is set to 0xFF (1111 1111), which, by convention, is the “all-stations” or broadcast address. The Flag field is 0x7E (0111 1110), as in many other data link layer protocols. The flag bytes (0x7E) essentially form an “idle pattern” on the link that is “interrupted” by frames carrying information.