Understanding LTE Model Internals and Interfaces

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1581 Understanding LTE Model Internals and InterfacesPDCP Features and Related Code• PDCP overhead of 16 bits is added to all higher-layer packets.• PDCP header compression is performed for UDP/IP and TCP/IP headers forall higher-layer packets.• Encapsulation: All packets entering LTE go through PDCP encapsulation• lte_pdcp_pdu• Encapsulation occurs in lte_support_pdcp_higher_layer_to_pdcp_pdu_convert() inlte_support.ex.c• Compression supported conditionally for TCP/IP and UDP/IP• lte_support_pdcp_header_comp_size_compute() in lte_support.ex.c does thecompression job• Compression algorithm: A compression factor generated using aconfigured distribution• Compression reflected by setting the a negative bulk size for lte_pdcp_pdu• Decapsulation:• Simply recovers original packet – its size was never changed• lte_support_pdcp_pdu_to_higher_layer_convert() in lte_support.ex.cCONFIDENTIAL ─ RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc.38
1581 Understanding LTE Model Internals and InterfacesRLC Features• Segmentation and concatenation procedures are performed using a dynamicPDU size that is determined by the scheduler decisions.• The model supports the following RLC modes:• Transparent mode—No RLC header is included in this mode.• Unacknowledged mode—This mode ensures in-sequence delivery of SDUs to thehigher layers.• Acknowledged mode—This mode ensures retransmission of missing SDUs in additionto in-sequence delivery of SDUs to the higher layers.• While transmitting PDUs, an RLC entity in acknowledged mode follows this priorityorder: status report PDU > retransmitted PDU(s) > PDU with new data• While retransmitting RLC AMD PDUs, segmentation of the retransmitted PDUs incases of small maximum allowed PDU sizes is supported• SRBs use RLC UM mode• The RLC mode of the data radio bearers is configurable separately for uplink anddownlink• Default bearer always uses UM• CCCH transmissions use transparent mode, and TM is used only by CCCHtransmissionsCONFIDENTIAL ─ RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. © 2010 OPNET Technologies, Inc.39
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Understanding LTE Model Internals and Interfaces

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