I85PREP
⌘ K
Nokia SRA Composite study materials
Nokia SRA Composite study resources

Nokia SRA Composite practice questions

One question at a time. Build your Nokia SRA Composite understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Fixed networks

In a Nokia ISAM (7360) access node, which protocol is used between the access node and the BNG/BRAS to establish subscriber sessions and carry per-subscriber QoS policy for PPPoE wholesale deployments?

  1. RADIUS authentication and authorization with per-session policy attributes Correct answer.
  2. RADIUS accounting only, with no session control protocol
  3. PPPoE Intermediate Agent insertion combined with per-subscriber DHCP option 82
  4. L2TP access concentrator tunnels as the primary session control mechanism

Explanation

Subscriber session setup and per-subscriber policy on ISAM-based fixed broadband deployments is driven by RADIUS authentication/authorization, where the access node or BNG exchanges Access-Request and Access-Accept messages carrying policy attributes. Session control is not achieved with L2TP as a primary mechanism, and DHCP option 82 is a DHCP-based access-line identification tool, not the policy framework used for PPPoE sessions.

Question 2Fixed networks

Which Nokia fixed access product family is designed primarily for point-to-point Ethernet business services and mobile backhaul in a ring or linear topology with carrier-grade protection?

  1. 7360 ISAM FX
  2. 7705 SAR Correct answer.
  3. 7210 SAS
  4. 7750 SR

Explanation

The 7705 Service Aggregation Router is a compact, temperature-hardened platform designed for cell site, business, and aggregation roles, including ring and linear topologies with protection switching. The 7750 SR is a larger edge router, the 7210 SAS is a service access switch, and the 7360 ISAM FX is an access node for residential broadband.

Question 3Fixed networks

A GPON deployment uses a 1:32 split ratio and the OLT is configured for 2.488 Gbit/s downstream. Which statement about the upstream direction is correct?

  1. Upstream uses 1.244 Gbit/s and all ONTs share it via TDMA with OLT-assigned grants Correct answer.
  2. Upstream uses 622 Mbit/s and is scheduled by the ONT with the lowest serial number
  3. Upstream uses 2.488 Gbit/s and ONTs transmit continuously without scheduling
  4. Upstream uses 1.244 Gbit/s and each ONT receives a dedicated wavelength

Explanation

GPON uses 2.488 Gbit/s downstream and 1.244 Gbit/s upstream. Because all ONTs share the upstream wavelength, the OLT schedules transmission using grants (TDMA) to avoid collisions, and ONTs only transmit in their allocated timeslots.

Question 4Fixed networks

In XGS-PON, which combination of line rates applies?

  1. 10 Gbit/s downstream and 10 Gbit/s upstream, symmetric Correct answer.
  2. 10 Gbit/s downstream and 1.244 Gbit/s upstream, asymmetric
  3. 2.5 Gbit/s downstream and 10 Gbit/s upstream, asymmetric
  4. 10 Gbit/s downstream and 2.5 Gbit/s upstream, asymmetric

Explanation

XGS-PON is defined as a symmetric 10 Gbit/s PON, with 10 Gbit/s in both downstream and upstream directions. Asymmetric variants such as XG-PON1 provide 10 Gbit/s downstream but only 2.5 Gbit/s upstream.

Question 5Fixed networks

An ONT in a GPON network is ranging and the OLT measures an equalization delay. What is the purpose of this delay?

  1. It sets the downstream encryption key rotation interval for each ONT
  2. It aligns each ONT's upstream bursts to the correct timeslot at the OLT despite differing fiber distances Correct answer.
  3. It limits the maximum downstream bandwidth available to the ONT
  4. It determines the downstream wavelength assigned to the ONT

Explanation

Ranging measures the round-trip delay for each ONT, and the OLT assigns an equalization delay so that all upstream bursts arrive aligned to their assigned timeslots regardless of physical distance. This prevents upstream collisions between ONTs at different distances.

Question 6Fixed networks

Which statement best describes the role of the ONT management and control interface (OMCI) in a GPON deployment?

  1. It defines the OLT-to-ONT management channel used to configure and monitor managed entities such as GEM ports and T-CONTs Correct answer.
  2. It specifies the Ethernet OAM loopback messages used for link fault detection
  3. It provides the RADIUS interface for subscriber authentication
  4. It defines the physical layer wavelength plan for the PON

Explanation

OMCI is the management protocol between OLT and ONT in GPON. It operates over a dedicated GEM channel and configures managed entities such as T-CONTs, GEM ports, and VLAN tagging, as well as monitoring performance counters.

Question 7Fixed networks

A service provider wants to offer a Layer 2 wholesale service where the access node forwards subscriber traffic to a remote BNG over an L2TP tunnel. Which model does this describe?

  1. IPoE with local breakout at the access node
  2. PPPoE relay, where the access node terminates PPPoE and re-originates it
  3. DHCP relay with option 82 insertion to a local BNG
  4. L2TP LAC model, where the access node acts as LAC and the BNG acts as LNS Correct answer.

Explanation

In the L2TP LAC model, the access node acts as the L2TP Access Concentrator, tunneling subscriber PPP sessions to a remote L2TP Network Server (the BNG). This allows wholesale models where the retail provider terminates sessions remotely.

Question 8Fixed networks

Which Nokia ISAM feature is used to prevent a rogue ONT from disrupting upstream traffic by transmitting outside its assigned grants?

  1. T-CONT priority scheduling
  2. Downstream AES encryption key rotation
  3. Upstream ONT rogue detection with automatic disabling of the offending ONT Correct answer.
  4. GEM port interleaving

Explanation

Rogue ONT detection monitors upstream traffic for ONTs transmitting outside their assigned grants and can automatically disable the offending ONT to protect the shared upstream. Encryption, interleaving, and scheduling do not address rogue upstream transmissions.

Question 9Fixed networks

In a VDSL2 vectoring deployment, what is the primary purpose of vectoring?

  1. To bond multiple copper pairs into a single logical link
  2. To cancel crosstalk between copper pairs in the same binder group and thereby increase achievable bit rates Correct answer.
  3. To extend the reach of the copper loop by reducing the upstream band
  4. To increase the downstream transmit power above the regulatory spectral mask

Explanation

Vectoring coordinates signals across multiple lines in a binder group so that crosstalk from neighboring lines can be cancelled, significantly increasing achievable rates on short and medium loops. Bonding and reach extension are separate techniques.

Question 10Fixed networks

In a G.fast deployment, which factor most directly limits the achievable bit rate compared with VDSL2?

  1. The wider bandwidth available at very short loop lengths Correct answer.
  2. The use of discrete multi-tone modulation
  3. The use of time-division duplexing
  4. The use of copper pairs instead of fiber

Explanation

G.fast uses a much wider bandwidth (up to 106 MHz or more) and achieves higher rates than VDSL2, but only over very short loops. The limiting factor is loop length and the resulting attenuation at high frequencies, so the wider bandwidth at short reach is the key enabler.

Question 11Fixed networks

Which protocol is used between an OLT and an ONT for downstream encryption key exchange in GPON?

  1. PLOAM messaging for AES data-encryption key exchange Correct answer.
  2. TLS 1.3
  3. IPsec IKEv2
  4. MACsec

Explanation

GPON uses AES for downstream traffic encryption, with the basic data-key exchange carried in PLOAM messages. The OLT requests a key, the ONT returns it, and PLOAM signaling coordinates activation. OMCI can support encryption configuration or enhanced security management, but it does not carry this basic data-key exchange.

Question 12Fixed networks

Which statement correctly describes the difference between a GEM port and a T-CONT in GPON?

  1. A GEM port is a traffic-bearing entity identified by a GEM port ID, while a T-CONT is an upstream bandwidth allocation entity identified by an Alloc-ID Correct answer.
  2. A GEM port is an upstream bandwidth allocation entity, while a T-CONT is a downstream traffic container
  3. A GEM port defines the physical wavelength, while a T-CONT defines the encryption key
  4. A GEM port and a T-CONT are the same entity referenced by different names in different standards

Explanation

In GPON, GEM ports carry encapsulated Ethernet frames and are identified by GEM port IDs, while T-CONTs represent upstream bandwidth allocations identified by Alloc-IDs. Multiple GEM ports can be mapped to a single T-CONT.

Question 13Fixed networks

A fixed network operator wants to use a single management platform to provision both PON access nodes and residential gateways. Which Nokia solution addresses this?

  1. Nokia NSP with only IP/MPLS device management
  2. Nokia Altiplano for PON-only management
  3. Nokia Access Management (formerly AMS) for unified access provisioning and management Correct answer.
  4. Nokia Deepfield for DDoS analytics

Explanation

Nokia Access Management provides unified management and provisioning for fixed access nodes and customer premises equipment, including PON and DSL elements. NSP, Altiplano, and Deepfield serve different roles in the portfolio.

Question 14Fixed networks

In a GPON network, what is the function of the PLOAM channel?

  1. It carries physical layer operations, administration, and maintenance messages such as activation, ranging, and key switching between OLT and ONT Correct answer.
  2. It carries the DHCP relay messages for IPoE subscribers
  3. It provides the RADIUS authentication exchange
  4. It carries the Ethernet frames of subscriber traffic

Explanation

The PLOAM channel is a dedicated management channel in GPON used for physical layer OAM functions including ONT activation, ranging, alarm reporting, and key switching. It does not carry subscriber traffic or higher-layer protocols like RADIUS or DHCP.

Question 15Fixed networks

An operator wants to provide a point-to-point Ethernet service over a PON using a dedicated GEM port per subscriber. Which statement about this approach is correct?

  1. All subscribers must share a single GEM port and T-CONT
  2. Each subscriber is mapped to a separate GEM port and T-CONT, allowing per-subscriber QoS and traffic separation Correct answer.
  3. GEM ports cannot be used for point-to-point Ethernet services
  4. Per-subscriber GEM ports require the use of L2TP

Explanation

Mapping each subscriber to a dedicated GEM port and T-CONT provides traffic separation and allows per-subscriber QoS and bandwidth allocation. Sharing a single GEM port would remove that separation, and GEM ports are precisely the mechanism used for Ethernet service delivery over PON.

Question 16Fixed networks

Which statement about the Nokia 7360 ISAM FX chassis is accurate?

  1. It is a fixed-configuration 1RU ONT
  2. It is a core IP/MPLS router
  3. It is a modular access node supporting multiple line card types including GPON, XGS-PON, and VDSL2 Correct answer.
  4. It is a standalone optical splitter

Explanation

The 7360 ISAM FX is a modular access node with multiple slots that support a range of line cards including PON and copper technologies. It is not a fixed ONT, a core router, or a passive splitter.

Question 17Fixed networks

In a fixed broadband network using IPoE, which mechanism allows the BNG to identify the access line of a subscriber for policy and accounting?

  1. Use of L2TP tunnel ID
  2. Insertion of DHCP option 82 by the access node Correct answer.
  3. Use of PPPoE session ID
  4. Use of GEM port ID

Explanation

In IPoE deployments, the access node inserts DHCP option 82 (relay agent information) to identify the access line and subscriber to the BNG, enabling per-line policy and accounting. PPPoE session IDs and L2TP tunnel IDs apply to other access models, and GEM port IDs are internal to the PON.

Question 18Fixed networks

Which statement about the Nokia 7750 SR in a fixed network is correct?

  1. It can act as a BNG providing subscriber termination and policy enforcement for fixed broadband Correct answer.
  2. It is a passive optical splitter
  3. It is an ONT used at the customer premises
  4. It is a copper line card for VDSL2

Explanation

The 7750 SR can be deployed as a Broadband Network Gateway, terminating subscriber sessions and enforcing per-subscriber policy for fixed broadband. It is not an ONT, splitter, or line card.

Question 19Fixed networks

In a GPON deployment, an ONT reports a loss of signal alarm. Which is the most likely first troubleshooting step?

  1. Check the optical receive power at the ONT and verify the fiber connection and splitter path Correct answer.
  2. Reboot the BNG
  3. Reconfigure the DHCP option 82
  4. Change the RADIUS shared secret

Explanation

A loss of signal alarm indicates a physical layer problem, so the first step is to verify optical receive power and the integrity of the fiber and splitter path. BNG, RADIUS, and DHCP configuration changes address higher-layer issues and would not resolve a physical layer signal loss.

Question 20Fixed networks

Which statement best describes the role of the optical distribution network (ODN) in a PON?

  1. It terminates subscriber PPP sessions
  2. It provides the passive optical path between the OLT and ONTs, including splitters and fiber Correct answer.
  3. It provides the RADIUS authentication function
  4. It performs dynamic bandwidth allocation

Explanation

The ODN consists of the passive optical components, including fiber and splitters, that connect the OLT to the ONTs. Session termination, authentication, and DBA are performed by active network elements, not the ODN.

Question 21Fixed networks

In a VDSL2 deployment, which factor most affects the maximum achievable downstream rate on a given loop?

  1. Loop length and the resulting attenuation at higher frequencies Correct answer.
  2. The RADIUS server response time
  3. The DHCP lease duration
  4. The number of VLANs configured on the BNG

Explanation

VDSL2 rates depend strongly on loop length because higher frequencies attenuate more over distance, reducing achievable bit rates. RADIUS, DHCP, and VLAN configuration affect service delivery but not the physical layer rate.

Question 22Fixed networks

Which Nokia platform is positioned as an SDN-based access and edge controller for fixed and mobile networks?

  1. Nokia Altiplano Access Controller Correct answer.
  2. Nokia 7705 SAR
  3. Nokia 7210 SAS
  4. Nokia 7360 ISAM FX

Explanation

Nokia Altiplano Access Controller provides SDN-based control and automation for access and edge networks, including fixed access. The 7360 ISAM FX, 7705 SAR, and 7210 SAS are network elements rather than controllers.

Question 23Fixed networks

A service provider wants to deploy 10 Gbit/s symmetric PON service while reusing the existing GPON ODN. Which technology coexistence approach is appropriate?

  1. WDM coexistence using a coexistence element to combine GPON and XGS-PON wavelengths on the same ODN Correct answer.
  2. Using DHCP option 82 to separate the technologies
  3. Using L2TP to tunnel GPON traffic to the BNG
  4. Replacing all splitters with active switches

Explanation

GPON and XGS-PON use different wavelengths, so a coexistence element (WDM filter) can combine them on the same ODN, allowing gradual migration. Replacing splitters, L2TP tunneling, and DHCP option 82 do not address wavelength coexistence.

Question 24Fixed networks

In GPON, which mechanism is used to prevent downstream traffic from one ONT being received by another ONT on the same PON?

  1. AES encryption of downstream traffic with per-ONT keys Correct answer.
  2. Ranging and equalization delay
  3. T-CONT priority scheduling
  4. Upstream TDMA scheduling

Explanation

Because the downstream is a shared broadcast medium, GPON uses AES encryption with per-ONT keys so that each ONT can only decrypt its own traffic. TDMA scheduling, ranging, and T-CONT scheduling address upstream coordination and bandwidth allocation, not downstream confidentiality.

Question 25Fixed networks

Which statement about the Nokia 7210 SAS is correct?

  1. It is a residential ONT
  2. It is a RADIUS server
  3. It is an optical line terminal for GPON
  4. It is a service access switch family used for Ethernet service delivery and aggregation Correct answer.

Explanation

The 7210 SAS is a family of service access switches used for Ethernet service delivery and aggregation in fixed and mobile networks. It is not an OLT, ONT, or RADIUS server.

Question 26Fixed networks

In a GPON deployment, which downstream wavelength is used to carry the user traffic from the OLT to the ONTs?

  1. 1625 nm
  2. 1310 nm
  3. 1490 nm Correct answer.
  4. 1550 nm

Explanation

GPON as defined in ITU-T G.984 uses 1490 nm for the downstream direction and 1310 nm for the upstream direction. The 1550 nm and 1625 nm windows are reserved for video overlay and OTDR/monitoring purposes, not for the primary downstream data channel.

Question 27Fixed networks

Which standard defines the VDSL2 access technology used on copper loops in a fixed network?

  1. ITU-T G.984
  2. IEEE 802.3ah
  3. ITU-T G.992.5
  4. ITU-T G.993.2 Correct answer.

Explanation

VDSL2 is specified in ITU-T Recommendation G.993.2, which defines operation over copper pairs up to 30 MHz. G.984 covers GPON, 802.3ah covers Ethernet in the first mile, and G.992.5 covers ADSL2+.

Question 28Fixed networks

In a fixed broadband network, what is the primary function of a BNG (broadband network gateway)?

  1. It aggregates DSL line cards inside the access node
  2. It terminates subscriber sessions and applies per-subscriber policy and IP address assignment Correct answer.
  3. It provides the timing reference for PON synchronization
  4. It converts optical signals to electrical signals at the customer premises

Explanation

The BNG terminates the subscriber's IP session, authenticates the user (for example via PPPoE or IPoE with DHCP), assigns addresses, and enforces per-subscriber QoS and accounting. It sits at the edge of the aggregation network rather than inside the access node or at the customer premises.

Question 29Fixed networks

Which statement correctly describes a VLAN cross-connect in a fixed access node?

  1. It encrypts all upstream frames between ONT and OLT
  2. It maps a VLAN on an uplink port to a VLAN on a subscriber port Correct answer.
  3. It reserves upstream bandwidth for a T-CONT
  4. It assigns IP addresses to residential gateways

Explanation

A VLAN cross-connect (or VLAN translation) rewrites or maps the VLAN identifier of frames as they pass between an uplink and a subscriber-facing port, allowing service separation across the access node. It operates at Layer 2 and does not perform IP addressing, encryption, or bandwidth reservation.

Question 30Fixed networks

An operator wants to deliver a business Ethernet service with guaranteed symmetric bandwidth over a GPON access network. Which combination best supports this requirement?

  1. A single best-effort T-CONT shared by all subscribers
  2. A dedicated T-CONT with fixed bandwidth plus a dedicated GEM port per service Correct answer.
  3. A single GEM port carrying all services with no T-CONT allocation
  4. Downstream-only bandwidth reservation with no upstream allocation

Explanation

Guaranteed symmetric service requires reserving upstream bandwidth through a T-CONT configured with fixed or assured bandwidth, and separating the service into its own GEM port for isolation and identification. Sharing a best-effort T-CONT or omitting upstream allocation cannot provide a symmetric guarantee.

Question 31Fixed networks

A technician observes that a VDSL2 line synchronizes but delivers a much lower rate than expected for its loop length. Which factor is the most likely cause of reduced rate while still achieving sync?

  1. A completely severed copper pair
  2. High crosstalk from other active pairs in the same binder Correct answer.
  3. A missing GPON upstream wavelength
  4. An incorrect ONT serial number

Explanation

Crosstalk from other active pairs in the same cable binder is a dominant impairment in VDSL2 and reduces the achievable rate even when the line still trains. A severed pair would prevent sync entirely, and the other options relate to PON rather than copper line performance.

Question 32Fixed networks

In a fixed network, which mechanism allows a BNG to identify the access line of a subscriber when using IPoE without PPPoE?

  1. The T-CONT identifier included in the IP header
  2. The DHCP Option 82 relay agent information inserted by the access node Correct answer.
  3. The GPON ONT serial number carried in the Ethernet header
  4. The PPPoE session identifier negotiated with the CPE

Explanation

With IPoE, the access node acts as a DHCP relay and inserts Option 82 (circuit ID and remote ID) so the BNG can map the request to a specific access line. PPPoE is not used in this scenario, and PON identifiers such as ONT serial or T-CONT are not carried in the IP header.

Question 33Fixed networks

An operator plans to overlay a 10G PON service on an existing GPON ODN. Which approach allows both services to coexist on the same fiber and splitter plant?

  1. Disable upstream transmission on the GPON OLT
  2. Replace all splitters with active amplifiers
  3. Reduce the split ratio to 1:2 for the entire ODN
  4. Use different wavelengths for GPON and 10G PON with wavelength-combining filters Correct answer.

Explanation

Coexistence of GPON and 10G PON on a shared ODN is achieved by assigning distinct wavelength bands and combining or separating them with WDM filters at the OLT and ONT. Replacing passive splitters with amplifiers or disabling upstream would break the passive ODN model and the existing service.

Question 34Fixed networks

In a fixed access network, what does the term 'vectoring' require between the line cards of a VDSL2 access node?

  1. Sharing of GPON upstream grants
  2. Distribution of DHCP leases among line cards
  3. Exchange of IP routing tables between line cards
  4. Exchange of crosstalk cancellation coefficients synchronized across the lines Correct answer.

Explanation

Vectoring cancels crosstalk by coordinating signals across all lines in a binder, which requires the line cards to exchange precoder and canceller coefficients with tightly synchronized processing. It is a physical-layer coordination function and does not involve routing tables, PON grants, or DHCP.

Question 35Fixed networks

Which statement best describes the purpose of a split ratio in a PON ODN?

  1. It determines the encryption key length used in GPON
  2. It sets how many ONTs share a single OLT PON port Correct answer.
  3. It fixes the maximum number of VLANs per subscriber
  4. It defines the upstream wavelength assigned to each ONT

Explanation

The split ratio describes how many subscriber ONTs are connected through the passive splitters to one OLT PON port, trading per-user bandwidth and reach against fiber and port savings. It does not define wavelengths, encryption, or VLAN limits.

Question 36Fixed networks

A residential gateway in a fixed network is failing to obtain an IP address. Which sequence of checks is most appropriate for isolating the fault?

  1. Verify physical link and ONT status, then check DHCP relay and BNG session state Correct answer.
  2. Reconfigure the GPON encryption keys first
  3. Change the VDSL2 band plan before verifying the link
  4. Replace the BNG before checking the ONT

Explanation

Troubleshooting should proceed from the physical and access layer upward: confirm the link and ONT are operational, then verify the DHCP relay and BNG session state. Replacing core equipment or changing encryption or band plans before confirming basic connectivity is inefficient and may introduce new faults.

Question 37Fixed networks

In a fixed network, which function does an OLT perform during ONT ranging?

  1. It measures each ONT's round-trip delay to align upstream transmission slots Correct answer.
  2. It performs vectoring coefficient calculation for copper lines
  3. It encrypts downstream video broadcast streams
  4. It assigns IP addresses to each ONT management interface

Explanation

Ranging measures the propagation delay between the OLT and each ONT so that upstream bursts arrive in their assigned slots without colliding. IP addressing, video encryption, and copper vectoring are separate functions not performed by OLT ranging.

Question 38Fixed networks

An operator wants to separate residential and business traffic on the same access node while keeping them isolated at Layer 2. Which configuration achieves this?

  1. Use a single GEM port for both services with no VLAN tagging
  2. Assign both services to the same VLAN and rely on QoS marking
  3. Disable VLAN tagging on the uplink and rely on MAC learning
  4. Assign each service class to a distinct VLAN and apply per-VLAN forwarding rules Correct answer.

Explanation

Layer 2 isolation is achieved by placing each service in its own VLAN and enforcing forwarding rules per VLAN, so frames cannot leak between service groups. Sharing a VLAN or disabling tagging removes the separation boundary and allows cross-service flooding.

Question 39Fixed networks

In a GPON network, which statement about the downstream direction is correct?

  1. The OLT broadcasts downstream frames to all ONTs, and each ONT filters by GEM port and ONU-ID Correct answer.
  2. The OLT sends downstream frames only to the ONT that requested them
  3. The OLT unicasts downstream frames using the ONT MAC address at the PON layer
  4. Each ONT receives downstream frames only after winning an upstream grant

Explanation

GPON downstream is a broadcast medium: the OLT transmits frames that all ONTs can physically receive, and each ONT filters on GEM port and ONU-ID to accept only its own traffic. Confidentiality is provided by downstream encryption rather than by physical unicasting.

Question 40Fixed networks

A fixed network operator wants to enforce per-subscriber rate limits on a BNG for IPoE subscribers. Which approach is most appropriate?

  1. Rely on ONT-side traffic shaping alone
  2. Set the GPON split ratio to match the subscriber rate
  3. Apply a global policer on the uplink port shared by all subscribers
  4. Apply a subscriber-aware policer or shaper tied to the session identified via DHCP Option 82 Correct answer.

Explanation

Per-subscriber enforcement requires the BNG to associate traffic with a specific session, which IPoE identifies through DHCP Option 82 relay information, and then to apply a policer or shaper to that session. A global uplink policer cannot differentiate subscribers, and neither ONT shaping nor split ratio provides BNG-side enforcement.

Question 41Mobile networks

In a 5G NR standalone deployment, which network element holds the subscriber's subscription data and authentication credentials that the AMF queries during registration?

  1. The Session Management Function (SMF)
  2. The Network Repository Function (NRF)
  3. The Unified Data Management (UDM) Correct answer.
  4. The User Plane Function (UPF)

Explanation

The UDM stores subscription data and authentication credentials, and the AMF retrieves authentication vectors and subscription information from it during registration. The SMF and UPF handle session and user-plane forwarding, while the NRF is a service discovery registry.

Question 42Mobile networks

Which interface carries control-plane signalling between the eNodeB and the MME in an LTE/EPS network?

  1. S1-MME Correct answer.
  2. X2-C
  3. S6a
  4. S1-U

Explanation

The S1-MME interface connects the eNodeB to the MME and carries NAS and S1-AP control signalling. S1-U is the user-plane path to the S-GW, S6a links the MME to the HSS, and X2-C is the inter-eNodeB control interface.

Question 43Mobile networks

What is the primary purpose of the NAS (Non-Access Stratum) protocol in LTE and 5G?

  1. To schedule radio resource blocks on the air interface
  2. To perform ciphering of the physical downlink control channel
  3. To route IP packets between the UPF and the data network
  4. To manage mobility and session management signalling between the UE and the core network Correct answer.

Explanation

NAS runs between the UE and the core control node (MME or AMF) and handles attach, authentication, security mode, and bearer/session management. It is transparent to the radio access network, which only relays NAS messages.

Question 44Mobile networks

Which frequency range does 5G NR define as FR1?

  1. 88 MHz to 108 MHz
  2. 410 MHz to 7125 MHz Correct answer.
  3. 57 GHz to 71 GHz
  4. 24.25 GHz to 52.6 GHz

Explanation

FR1 covers roughly 410 MHz to 7125 MHz, commonly called sub-7 GHz, and supports subcarrier spacings from 15 to 60 kHz. FR2 covers the millimetre-wave bands from 24.25 GHz upward.

Question 45Mobile networks

In 5G NR, which subcarrier spacing (SCS) corresponds to numerology μ = 1?

  1. 30 kHz Correct answer.
  2. 120 kHz
  3. 15 kHz
  4. 60 kHz

Explanation

NR defines SCS as 15 kHz × 2^μ, so μ = 1 yields 30 kHz. The base 15 kHz corresponds to μ = 0, and each increment doubles the spacing.

Question 46Mobile networks

What does the term 'beamforming' describe in the context of 5G NR massive MIMO?

  1. Spreading a signal across a wider bandwidth to reduce peak power
  2. Splitting one carrier into multiple orthogonal subcarriers
  3. Directing radiated energy into spatial beams by phase-weighting antenna elements Correct answer.
  4. Encoding data with redundant bits to correct transmission errors

Explanation

Beamforming applies amplitude and phase weights across antenna array elements so energy is concentrated toward a target direction. This improves gain and spatial reuse, and is central to massive MIMO operation in NR.

Question 47Mobile networks

Which node in the 5G core architecture is responsible for selecting and controlling the user-plane path for a PDU session?

  1. AMF
  2. NSSF
  3. SMF Correct answer.
  4. AUSF

Explanation

The SMF establishes, modifies, and releases PDU sessions and configures the UPF to forward user traffic. The AMF handles access and mobility, the AUSF performs authentication, and the NSSF selects network slices.

Question 48Mobile networks

In LTE, which channel carries the system information blocks (SIBs) that a UE reads to camp on a cell?

  1. PDSCH scheduled by PDCCH with SI-RNTI Correct answer.
  2. PRACH
  3. PSS
  4. PUCCH

Explanation

SIBs are transmitted on the downlink shared channel (PDSCH) and scheduled by PDCCH masked with the SI-RNTI. The UE first decodes MIB on PBCH, then acquires SIBs to learn cell access parameters.

Question 49Mobile networks

A UE in 5G NR moves from a cell served by gNB-A to a cell served by gNB-B during an active PDU session. Which procedure keeps the user-plane path continuous?

  1. Service request triggered by paging
  2. PDU session release and re-establishment
  3. Random access channel re-selection
  4. Xn-based handover with path switch at the SMF Correct answer.

Explanation

Inter-gNB handover uses the Xn interface to transfer UE context and forward data, after which the target gNB requests a path switch so the UPF sends traffic to the new node. This preserves session continuity.

Question 50Mobile networks

What is the function of the S-GW in an LTE/EPS network?

  1. Performing DNS resolution for the core network
  2. Anchoring the user plane for inter-eNodeB handover and routing packets toward the P-GW Correct answer.
  3. Managing the radio resource control connection
  4. Storing the subscriber's authentication vectors

Explanation

The Serving Gateway anchors the user plane for mobility between eNodeBs and relays user packets between the E-UTRAN and the P-GW. It also handles downlink packet buffering during idle-to-connected transitions.