LTTS_Week4 LTTS_Week4 The number of attempts remaining is 1 1 / 30 1. A SIMD LDPC decoder passes static bit-exact tests but shows a BLER spike only at 256-QAM. Low-MCS RFsim works, while 256-QAM RFsim reproduces the fault. What is the strongest inference? a. SSB periodicity must be wrong b. The MME is rejecting the UE c. A modulation-dependent numerical/software issue such as LLR scaling or saturation is likely d. The antenna is definitely misaligned 2 / 30 2. Which statement best distinguishes a 5G logical channel, transport channel, and physical channel? a. PBCH is logical and PDSCH is transport b. BCCH is logical, BCH is transport, PBCH is physical c. BCCH and BCH are both physical d. BCCH is physical, BCH is logical, PBCH is transport 3 / 30 3. A distributed LTE implementation suffers intermittent service loss whenever fronthaul latency increases. Why can this happen even though LTE itself does not mandate a CU/DU architecture? a. RRC cannot operate remotely b. LTE requires all traffic to use X2 c. S1-U becomes an RF interface d. Distributed implementation introduces transport dependencies between separated processing functions 4 / 30 4. A UE completes RRCReconfiguration successfully, but the newly created DRB carries zero packets. Which conclusion is strongest? a. The AMF cannot send NAS b. The problem must be RF c. RRC success proves the DRB user plane is operational d. Control-plane completion does not prove CU-UP, F1-U, or lower-layer bearer state is consistent 5 / 30 5. An LTE engineer places the MME and S-GW inside the eNB because both appear adjacent to the eNB in an architecture diagram. Why is this interpretation incorrect? a. S-GW belongs to X2 b. MME is part of PDCP c. MME and S-GW are PHY modules d. MME and S-GW are EPC functions connected through S1; they are not eNB submodules 6 / 30 6. An OAI-based RAN shows healthy RF synchronization, but no UE can complete registration. Logs show the radio stack running normally, while the AMF connection is absent. Which conclusion is most appropriate? a. PHY must be reconfigured b. The failure can exist at the core-facing control interface even when PHY is healthy c. UE MAC must be disabled d. RF health proves registration should succeed 7 / 30 7. A 5G UE has completed authentication, but the CU-CP has not successfully established the required DU UE context through F1AP. Which statement is most accurate? a. Authentication guarantees radio data service b. PHY automatically creates the F1 UE context c. F1AP is optional for every CU/DU deployment d. Core authentication can succeed while RAN-side UE resource/context establishment remains incomplete 8 / 30 8. An OAI deployment is upgraded by changing the RAN tag but retaining an older RIC and Core version. The RAN starts, but E2 service-model interactions fail. Which architectural lesson best explains the problem? a. E2 always uses N3 b. Cross-project version and interface compatibility can determine integration behavior c. PHY cannot communicate with RIC d. OAI components are independent and require no version coordination 9 / 30 9. Which mapping correctly separates the 5G protocol responsibilities? a. CU-CP: PHY; CU-UP: RRC; DU: NAS b. CU-CP: GTP-U only; CU-UP: RRC; DU: AMF c. CU-CP: RRC/NGAP; CU-UP: SDAP/UP-PDCP; DU: RLC/MAC/PHY-related processing d. CU-CP: RF; CU-UP: SSB generation only; DU: NAS 10 / 30 10. Why are N3 and F1-U not interchangeable even though both commonly use GTP-U over UDP/IP? a. F1-U carries NAS signaling b. N3 carries only RRC c. One is always SCTP d. They belong to different logical interfaces with different endpoints, contexts, and tunnel state 11 / 30 11. An LTE cell broadcasts correctly, UEs establish RRC connections, but no UE can complete core registration. Which observation most strongly prevents the engineer from blaming PHY? a. Successful RRC only establishes the radio-side control relationship; S1-MME still must work b. RRC establishment proves all EPC procedures succeeded c. PHY controls the MME d. RRC is carried over S1-U 12 / 30 12. A developer claims that LTE user-plane PDCP and control-plane PDCP must have identical security behavior. Which statement identifies the flaw? a. PDCP exists only for user data b. LTE control-plane PDCP applies integrity protection, while normal LTE user-plane PDCP does not c. PDCP performs RF conversion d. PDCP is below PHY 13 / 30 13. In LTE downlink user-plane processing, a packet arrives from the EPC through S1-U. Which sequence correctly represents its radio-side processing? a. GTP-U → MAC → RRC → PHY b. GTP-U → PDCP → RLC → MAC → PHY c. S1AP → RRC → PHY → MAC d. PHY → MAC → RLC → PDCP → GTP-U 14 / 30 14. Which component is primarily responsible for translating higher-level control decisions into RRC, F1AP, and E1AP configurations? a. PHY accelerator b. CU-CP c. O-RU d. UPF 15 / 30 15. A PHY decoder reports incorrect CRC results even though the received RF signal has high SNR. Which software-level issue becomes particularly suspicious? a. S1AP stream allocation b. Xn handover signaling only c. PLMN subscriber database d. LLR sign/scaling, DM-RS configuration, scrambling, HARQ/RV association, or rate matching 16 / 30 16. A PHY troubleshooting team finds healthy timing/RF, then reproduces a failure using 256-QAM with RFsim, and finally discovers an optimized decoder differs from the reference implementation. Which validation strategy provides the strongest regression protection? a. Remove SIMD optimization permanently b. Test only static low-MCS vectors c. Add MCS-specific bit-exact tests using realistic channel traces and monitor service-level counters d. Test only RF KPIs 17 / 30 17. Which statement correctly distinguishes LTE architecture from a 5G CU/DU architecture? a. LTE eNB is a single logical RAN node, while CU/DU separation is a 5G functional deployment model b. LTE has no MAC scheduler c. LTE mandates separate CU and DU nodes d. 5G eliminates RRC 18 / 30 18. A developer claims that because an OAI CU-UP process is running, the CU-UP can independently create all bearer contexts without CU-CP coordination. Which statement best corrects this claim? a. CU-UP performs SDAP/UP-PDCP and user-plane processing, while bearer lifecycle is controlled/coordinated by CU-CP b. CU-UP owns RRC and NGAP c. CU-UP only performs RF conversion d. CU-UP replaces the AMF 19 / 30 19. MAC provides a PHY with a PDSCH transmission request containing resource allocation, MCS, and HARQ information. What does this interaction fundamentally represent? a. MAC specifying what should be transmitted while PHY performs the radio processing needed to realize it b. PHY replacing MAC scheduling c. RRC directly manipulating IQ samples d. PHY deciding the application's QoS policy 20 / 30 20. An LTE UE has excellent RF quality and successfully establishes RRC, but attachment fails because the eNB cannot establish its signaling association with the MME. Which layer/interface should be investigated first? a. X2-U b. RF antenna gain c. S1-MME/SCTP d. PDSCH 21 / 30 21. An engineer modifies a feature in an OAI RAN repository but discovers that the required functionality actually belongs to a sibling OAI project. What does this primarily demonstrate? a. All OAI features must be implemented in the PHY b. OAI does not support 5G Core integration c. OAI consists of only one monolithic repository d. OAI is an ecosystem of related projects whose features may span repositories 22 / 30 22. A CU-CP successfully completes E1AP bearer setup, but the CU-UP retains stale bearer state after a scaling/rebalance event. What symptom is most consistent with this situation? a. RRC must necessarily fail b. SSB disappears immediately c. PSS becomes undecodable d. Control-plane logs can remain successful while user-plane packets fail to reach the expected path 23 / 30 23. A team observes that a UE's QoS Flow is established, but packets never appear on the intended DRB. Which mapping boundary should be investigated particularly carefully? a. QFI-to-DRB mapping in SDAP/CU-UP b. PCI-to-SSB mapping only c. MME-to-HSS S6a mapping d. PSS-to-PBCH mapping 24 / 30 24. An LTE scheduler operates every 1 ms. A developer concludes that every LTE protocol layer must therefore execute exactly once every 1 ms. Why is this conclusion incorrect? a. LTE has no 1 ms timing b. RRC executes every OFDM symbol c. Only the scheduler's real-time scheduling loop is constrained in that manner; higher-layer procedures have different timing behavior d. PDCP executes only once per second 25 / 30 25. In a 5G deployment, NAS authentication succeeds, but the engineer says the gNB must have decrypted and interpreted the NAS security state. What is the correct response? a. NAS terminates at the DU b. Incorrect; NAS security/state terminate at the AMF, while CU-CP transparently transports NAS c. Correct, because NAS terminates at the gNB d. NAS is handled by PHY 26 / 30 26. In an integrated LTE eNB, which change would represent a deployment change rather than a change to the LTE protocol architecture? a. Moving baseband processing to a centralized host and connecting remote radio equipment b. Changing S1AP into F1AP c. Replacing RRC with NGAP d. Removing MAC scheduling 27 / 30 27. An LTE eNB's MAC scheduler continues issuing grants, but UE uplink CRC indications never arrive from PHY. Which interpretation is most defensible? a. PDCP must generate CRC indications b. S1AP must be retransmitting the CRC c. The failure may be below MAC because PHY feedback is required for HARQ and scheduling decisions d. MAC scheduling automatically proves successful decoding 28 / 30 28. Two OAI deployments use the same nr-softmodem executable name, but one behaves as a CU-related process and another as an integrated RAN process. What is the most technically correct explanation? a. Runtime/build/configuration determine how protocol functions are bound to processing roles b. OAI always uses a fixed process-to-layer mapping c. CU functions are implemented only outside OAI d. The executable name uniquely defines the architecture 29 / 30 29. Why can a fixed LLR saturation clip remain invisible during low-MCS testing but become harmful at 256-QAM? a. 256-QAM does not use LDPC b. Higher-order modulation can produce larger LLR magnitudes, exposing divergence between fixed clipping and MCS-dependent scaling c. Low-MCS signals always bypass LDPC d. 256-QAM eliminates channel coding 30 / 30 30. Which sequence best represents the conceptual 5G user-plane path for downlink traffic in a disaggregated gNB? a. UPF → F1-C → CU-CP → RRC → UE b. AMF → N2 → DU → F1-C → UE c. AMF → N3 → CU-UP → Xn-U → UE d. UPF → N3 → CU-UP → F1-U → DU → PHY/RF → UE Your score is
