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L2/L3 signaling
Below diagram shows example data flow at L3/L2 for DTCH. Refer article on LTE data flow for brief introduction to functions performed by L2/L3.
Fig 1.3.5
We will consider case of maximum sized transport block i.e. 75,376. Maximum size of IP packet is 65,535 (2length field=16), so we take case of two IP packets, totaling value close to one TB size. Overhead of L2/L3 signaling would be = 2 x PDCP header length - 2 x gain due to header compression + RLC header length for two data fields + MAC header length + total length for MAC control elements.
PDCP header length would be 2 octets (RLC AM use 12 bit PDCP SN) [36.323:6.2.3]. RoHC compresses 40 bytes of IP/UDP/RTP headers to 2 bytes (based on few RoHC assumptions). So gain due to compression would be 38 bytes. RLC header length for two data field elements would be 5 octets [36.322:6.2.1.4]. MAC header length would be 3 octets (15 bits length field) [36.321:6.1.2]. Taking case of four MAC control elements - buffer status, C-RNTI, timing advance, and power headroom, - total length for MAC control elements would (1+2+1+1=) 5 octets.
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