- #THE PPP HEADERS AND TRAILERS USED TO CREATE A PPP FRAME SERIAL#
- #THE PPP HEADERS AND TRAILERS USED TO CREATE A PPP FRAME MAC#
The RLC retransmission mechanism is also responsible for providing error-free delivery of data to higher layers. Upon receiving the scheduling grant, the device includes the amount of data needed to fill up the transport block and updates the header to indicate it is a segmented SDU.
To completely fill up the transport-block size, the last RLC PDU may contain a segment of an SDU.
#THE PPP HEADERS AND TRAILERS USED TO CREATE A PPP FRAME MAC#
By removing the concatenation from RLC, the RLC PDUs can be assembled in advance and upon receipt of the scheduling decision the device only has to forward a suitable number of RLC PDUs to the MAC layer, the number depending on the scheduled transport-block size. In the case of concatenation in LTE, the RLC PDU cannot be assembled until the scheduling decision is known, which results in an additional delay until the uplink transmission and cannot meet the low-latency requirement of NR. As part of the overall low-latency design of NR, the scheduling decision in case of an uplink transmission is known to the device just before transmission, in the order of a few OFDM symbols before. Depending on the scheduler decision, a certain amount of data, that is, certain transport-block size, is selected. Included in the figure is also the corresponding LTE functionality, which also supports concatenation. Segmentation, one of the main RLC functions, is illustrated in Fig. This also helps reduce the overall latency as discussed in Chapter 13. Another difference is the removal of concatenation from the RLC protocol to allow RLC PDUs to be assembled in advance, prior to receiving the uplink scheduling grant. Removing in-sequence delivery from the RLC reduces the overall latency as later packets do not have to wait for retransmission of an earlier missing packet before being delivered to higher layers but can be forwarded immediately. One major difference compared to LTE is that the RLC does not ensure in-sequence delivery of SDUs to upper layers. Unacknowledged mode supports segmentation and duplicate detection, while acknowledged mode in addition supports retransmission of erroneous packets. Transparent mode is, as the name suggests, transparent and no headers are added. Depending on the type of service, the RLC can be configured in one of three modes-transparent mode, unacknowledged mode, and acknowledged mode-to perform some or all of these functions. It also handles retransmission of erroneously received PDUs, as well as removal of duplicate PDUs. 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 HEADERS AND TRAILERS USED TO CREATE A PPP FRAME 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.
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