Layer 2 Functionalities Framing Error detection & Correction Multiplexing Flow Controller Framing Issues LAPD uses HDLC frame format for framing. Where as LAPDm uses synchronization scheme of radio interface to convey information on frame limits. LAPDm avoids using flags by ready-made blocks of the physical layer. LAPDm(LAPD modified) frame has a maximum length of 23 octets on all TCH channels and 21 octets on SACCH blocks(remaining 2 octets are for Timing Advance and Transmission Power Control). But the actual frame length is less than 23/21 octets, so Length Indicator(LI) is included in each frame to identify the length of the frame. Segmentation & Reassembly There is no need of segmentation & Reassembly in LAPD(the length of the Abis LAPD frame is 264 octets excluding flags, which corresponds to 260 octets of upper layer information). Where as LAPD requires segmentation& reassembly because maximum frame length of 21 or 23 octets is not sufficient for most signalling ...
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