SI-4(15): Wireless to Wireline Communications

SI-4(15) requires you to deploy intrusion detection that specifically monitors wireless traffic at the boundary where it transitions onto your wired (wireline) network. Operationally, that means placing IDS/IPS sensors, WIDS/WIPS, or equivalent telemetry at wireless controllers, AP uplinks, or wireless gateways, then alerting, investigating, and retaining evidence for those boundary detections. ¹

Key takeaways:

• Monitor the transition point from wireless to wired; endpoint-only EDR does not satisfy this requirement by itself. ¹
• Scope includes enterprise Wi‑Fi, OT/IoT wireless bridges, and any guest/BYOD SSIDs that touch your wireline network segments.
• Treat this as a “sensor placement + detection content + evidence” control, with clear ownership and recurring artifacts.

The si-4(15): wireless to wireline communications requirement is narrow but commonly missed because teams monitor “wireless” as a configuration problem (WPA2/3, 802.1X, segmentation) rather than as a detection problem at a specific chokepoint. SI-4(15) asks for intrusion detection coverage as wireless communications traffic passes from wireless to wireline networks. ¹

For a Compliance Officer, CCO, or GRC lead, the fastest path to readiness is to (1) identify every location where wireless traffic is bridged, tunneled, or routed into wired networks, (2) confirm there is a detection control positioned to observe that traffic, and (3) produce evidence that the monitoring is operating and generating actionable alerts. You are not proving that Wi‑Fi is “secure.” You are proving that wireless-to-wired transitions are monitored for intrusion activity and that the organization can respond.

This page gives requirement-level implementation guidance you can hand to network/security engineering, then audit against concrete artifacts: network diagrams, sensor placement, alert rules, and tickets that show the control runs in production.

Regulatory text

“Employ an intrusion detection system to monitor wireless communications traffic as the traffic passes from wireless to wireline networks.” ¹

Operator meaning: you must have intrusion detection capability positioned where wireless traffic enters the wireline environment. The monitoring needs to be capable of detecting malicious or unauthorized activity in that transition path (for example: suspicious flows from a guest SSID into corporate segments, rogue AP bridging, unexpected protocols traversing a wireless bridge, or anomalous traffic patterns exiting a WLAN controller). ¹

Plain-English interpretation

• What counts as “wireless” here: enterprise Wi‑Fi, wireless bridges, mesh networks, cellular gateways, and wireless-connected IoT/OT that forward traffic into Ethernet or other wired segments.
• What counts as “wireline” here: your wired LAN, data center networks, core switching, or any routed/segmented environment that receives traffic from wireless infrastructure.
• What counts as “intrusion detection system”: a network IDS/IPS sensor, a WIDS/WIPS with visibility into the wired side, a sensor integrated with a wireless LAN controller that produces IDS-grade detections, or equivalent security monitoring that inspects and alerts on the boundary traffic.

A clean mental model: wireless access is the entry door; SI-4(15) requires a guard at the doorframe where the hallway begins.

Who it applies to (entity and operational context)

This control is typically assessed in:

• Federal information systems and
• Contractor systems handling federal data (for example, environments aligned to NIST SP 800-53). ¹

Operationally, it applies anywhere you operate wireless that connects to business systems, production networks, regulated data environments, or administrative networks, including:

• Headquarters and branch office Wi‑Fi connected to enterprise routing.
• Guest/BYOD Wi‑Fi that shares infrastructure with internal routing/switching.
• Warehouses, labs, plants, and clinics with wireless scanners, telemetry, or OT wireless bridges.
• Cloud-managed wireless where traffic returns via VPN/tunnel to your network.

What you actually need to do (step-by-step)

1) Define the in-scope wireless-to-wireline transition points

Ask network engineering for a list of all places where wireless traffic becomes wired traffic, such as:

• WLAN controllers (on-prem or cloud-managed) and their uplinks
• APs directly bridged into switching (autonomous or distributed architectures)
• Wireless bridges that connect remote segments into core switching
• Cellular/LTE/5G routers that provide WAN and drop into wired LAN
• Any SD-WAN or VPN concentrator terminating wireless-originated tunnels

Deliverable: a scoped inventory and a diagram showing “wireless domain → transition device → wireline segment(s).”

2) Choose the monitoring pattern per transition point

Common valid patterns (pick based on architecture):

• Sensor on the wireline side of the WLAN controller/uplink (SPAN/TAP into NIDS).
• Inline IPS between wireless aggregation and core (where feasible).
• Controller-integrated detections exported to SIEM plus evidence that detections are intrusion-focused and monitored.
• WIDS/WIPS + wired-side correlation when wireless management data alone cannot see post-bridge behavior.

Decision criteria you can enforce as GRC:

• Can the control observe traffic crossing the boundary?
• Can it alert on suspicious activity (not just log associations)?
• Can you retain evidence to show it operated over time?

3) Document control ownership and operating procedure

Assign a control owner (usually Network Security or SOC) and write a short runbook that answers:

• Where sensors are placed (device names, interfaces, or logical collection points)
• What data is collected (packet metadata, flow, signatures, wireless events)
• Alert routing (SOC queue, paging/on-call rules)
• Triage steps and escalation thresholds

This maps cleanly to an “implementation procedure and recurring evidence artifacts” approach recommended in practice for SI-4(15). ¹

4) Implement detections that are specific to the boundary

Auditors will push past “we have an IDS” and ask what it detects at this chokepoint. Make sure you have boundary-relevant detections, such as:

• Traffic from guest/BYOD SSIDs attempting to reach restricted internal subnets
• Unexpected east-west movement initiated from wireless VLANs
• Known-bad signatures or command-and-control patterns originating from wireless client networks
• New/unknown MAC-to-IP behavior correlated with wireless authentication events
• Rogue AP or evil twin indicators (if your tooling supports it), tied to wired-side consequences

Practical tip: build at least one detection that keys on “source zone = wireless” and “destination zone = restricted wireline segment,” then validate it fires in test.

5) Validate with a test and keep the proof

Run a controlled test from a wireless client (in a non-production window) that should generate an alert (for example, a blocked port scan toward a protected segment from the wireless VLAN). Record:

• the test plan and approval
• sensor logs showing the event
• the SIEM/SOAR alert
• the incident ticket and closure notes

6) Operationalize: monitoring, triage, and review cadence

Set expectations that the SOC:

• reviews and triages alerts generated from the wireless-to-wireline boundary
• tunes noisy rules (with documented rationale)
• reviews coverage when wireless architecture changes (new SSIDs, controllers, uplinks, sites)

For GRC, the key is repeatable evidence: you want a monthly or quarterly evidence bundle you can produce without heroics.

Required evidence and artifacts to retain

Keep evidence that proves placement, operation, and response:

Design / configuration

• Network diagram highlighting wireless-to-wireline transition points
• Sensor placement documentation (interfaces, SPAN/TAP configs, or controller export settings)
• Data flow description: what telemetry goes to SIEM and how it is labeled (wireless zone tagging helps)

Operational

• Sample alerts from the boundary with timestamps and triage disposition
• Incident tickets tied to those alerts (even if benign)
• Tuning/change records for detection rules affecting the boundary
• Access control evidence showing who can change sensor configs and detection rules

Governance

• Control narrative for SI-4(15) (scope, tools, roles, frequency)
• Ownership (RACI) and on-call coverage statement
• Exception register entries if some sites cannot be monitored, with compensating controls and target remediation

If you use Daydream to run compliance operations, store these as recurring evidence requests tied to SI-4(15), with a fixed artifact list (diagram, sensor config export, sample alerts, tickets). That reduces “missing implementation evidence” risk, which is a common assessment failure mode for this enhancement. ¹

Common exam/audit questions and hangups

Expect these questions and pre-answer them in your control narrative:

1. “Where exactly is the sensor?”
   They want device/interface specificity, not “in the network.”

2. “Does your IDS see the traffic after it leaves the AP/controller?”
   If traffic is tunneled and decrypted only at the controller, show how the IDS sees it there.

3. “How do you know this isn’t just wireless logging?”
   Show intrusion detections, not only association/authentication events.

4. “How do you separate guest from corporate wireless?”
   Provide segmentation proof plus boundary detection showing attempted crossings are detected.

5. “What happens when you open a new site?”
   Demonstrate a standard build pattern: new controller/uplink requires sensor onboarding and SIEM parsing.

Frequent implementation mistakes and how to avoid them

Mistake	Why it fails SI-4(15)	Avoid it by
Relying only on EDR on laptops	EDR does not monitor traffic “as it passes” from wireless to wireline	Deploy network-side monitoring at controller/uplink and feed alerts to SOC 1
Monitoring only the wireless management plane	Association logs don’t equal intrusion detection on boundary traffic	Ensure detections cover flows crossing into wired segments
Assuming “guest network is isolated” without monitoring	Misconfigs happen; SI-4(15) is about detection	Create alerts for guest-to-internal attempts and validate in testing
Sensors exist but no one watches alerts	Control becomes “installed, not operating”	Route alerts to a monitored queue and retain triage tickets
No evidence bundle	You cannot prove operation during an assessment	Predefine artifacts and collect them on a recurring schedule

Enforcement context and risk implications

No public enforcement cases were provided in the source materials for this requirement. In assessments, the practical risk is straightforward: wireless is a common entry path, and the wireless-to-wireline boundary is where lateral movement becomes possible. SI-4(15) is designed to force visibility at that pivot point. ¹

From a program perspective, the biggest avoidable risk is evidence failure: teams may have sensors deployed but cannot demonstrate placement, alerting, and response with artifacts that map cleanly to SI-4(15). ¹

Practical 30/60/90-day execution plan

30 days: establish scope and prove current state

• Build the inventory of wireless-to-wireline transition points and confirm architectures (controller-based, bridged, tunneled).
• Identify existing IDS/WIDS tooling and where it currently sits.
• Draft the SI-4(15) control narrative with owner, scope, and evidence list.
• Collect a first evidence packet (current diagrams, configs, sample logs), even if gaps exist.

60 days: close coverage gaps and validate detection

• Implement sensor placement for any uncovered transition points (SPAN/TAP or equivalent).
• Normalize telemetry in SIEM with clear tagging for “wireless source zone.”
• Stand up initial boundary detections and route to SOC queues.
• Execute a controlled test and retain end-to-end proof (alert + ticket).

90 days: operationalize and make it repeatable

• Tune detections based on SOC feedback; document changes.
• Add SI-4(15) checks to wireless change management (new SSIDs, new sites, new controller uplinks).
• Schedule recurring evidence collection (config export, alert samples, ticket samples).
• Review exceptions and either remediate or document compensating monitoring.

Frequently Asked Questions

Does SI-4(15) require a dedicated wireless IDS product?

The requirement is to employ an intrusion detection system that monitors traffic as it passes from wireless to wireline networks. A dedicated WIDS/WIPS can work, but a network IDS positioned at the wireless-to-wired transition can also satisfy the intent if it produces intrusion detections and alerts. ¹

We have a SIEM collecting WLAN controller logs. Is that enough?

Controller logs help, but SI-4(15) calls for intrusion detection monitoring of communications traffic at the transition. You should be able to show intrusion-focused alerts tied to boundary traffic, not only authentication/association events. ¹

What is the “wireless to wireline” boundary in a tunneled WLAN design?

In many tunneled designs, the boundary is the WLAN controller (or gateway) where client traffic is decapsulated and forwarded into wired segments. Place monitoring there or on the wired uplinks where the traffic exits into the LAN. ¹

How do we handle guest Wi‑Fi that never touches internal networks?

If guest traffic is truly isolated and exits directly to the internet without entering your wireline enterprise segments, document that architecture and the transition points that do exist. If guest traffic traverses shared wired infrastructure, monitor those transition points and alert on attempted access to restricted segments. ¹

What evidence do auditors accept to prove the IDS is “monitoring”?

Provide diagrams showing sensor placement, configurations or exports that show collection is enabled, and operational records like sample alerts plus SOC tickets showing triage. Evidence needs to show the control runs in production, not just that tooling exists. ¹

How do we operationalize this across many sites without creating audit chaos?

Standardize a reference architecture (where the sensor sits, what logs are forwarded, what tags are applied) and collect the same evidence bundle per site or per controller cluster. Tools like Daydream help by turning that bundle into recurring evidence requests tied to SI-4(15). ¹

Footnotes

1. NIST SP 800-53 Rev. 5 OSCAL JSON