There's three types of packet transfer. All packet sizes must be multiple of 8 bits. RLC buffers whole packets and only forms smaller pieces when the MAC layer informs how many bytes can be sent.
| Mode | Transfer mode | Headers | Segm't | Order | ARQ | Logical channels | Upper layer |
|---|---|---|---|---|---|---|---|
| TM | Transparent mode | No headers | No | No | No | BCCH, DL/UL CCCH, PCCH | |
| AM | Acknowledged mode | RLC headers | Yes | Yes | Yes | DL/UL DCCH, DL/UL DTCH | PDCP, also control in PDCP |
| UM | Unacknowledged mode | RLC headers | Yes | Yes | No | DL/UL DTCH, MCCHa MTCHa | PDCP |
a: No reordering for these channels
Segm't: RLC provides segmentation to fit MAC transport blocks and reassembles when received
Order: On reception, packets must be delivered strictly in order and without duplicates
ARQ: Detected lost segments must be re-requested. If no ARQ is used, then
discard packet if not all segments have been received in a timely manner
CCCH is only used for initial connection establishment. Further RRC communication goes on top of PDCP/DTCH/DL-SCH On top of PDCP there's IPv4, IPv6, RRC.
Uses RLC AM over CCCH for initial connection establishment. Further communication is over PDCP.
Physical, logical and transport channel names http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-evolution/physical-logical-transport-channels.php RLC AM and UM headers http://www.3glteinfo.com/lte-rlc-pdu-headers-for-am-um-tm-detail-overview/