How to leverage SMS in marketing to increase return visits

Executive Summary

Physical venues face a persistent challenge in matching the customer retention capabilities of digital spaces. While e-commerce platforms track, retarget, and re-engage visitors with precision, physical locations often operate in an informational vacuum. Enterprise WiFi infrastructure, when integrated with analytics and communication engines, bridges this gap. By utilising the captive portal as a data collection point, venues can capture verified mobile numbers and link them to unique device identifiers.

This guide details how to utilise SMS marketing - triggered by real-time WiFi analytics - to systematically increase return visits. SMS remains an exceptionally effective channel, boasting open rates near 98%, with the majority of messages read within three minutes of delivery. By tying these messages to physical behaviour, such as dwell time, visit frequency, or lapsed attendance, organisations can deliver highly contextual communications that prompt action. This document provides the technical architecture, implementation steps, and best practices required by IT managers, network architects, and operations directors to deploy a reliable, compliant, and automated SMS marketing system.

Technical Deep-Dive

To build an automated SMS marketing system based on physical presence, you must integrate several distinct layers: the physical wireless network, the AAA (Authentication, Authorization, and Accounting) server, the WiFi analytics engine, and the external SMS gateway.

The Data Capture and Authentication Flow

When a visitor enters a venue and attempts to connect to the guest WiFi, the process begins at the Access Point (AP) or Wireless LAN Controller (WLC). The WLC redirects the user's HTTP traffic to a captive portal hosted by the Purple platform.

1. Association and Redirection: The user's device associates with the guest SSID. The WLC intercepts the initial browser request and redirects the user to the captive portal URL, appending the client's MAC address and the AP's MAC address (called the Called-Station-ID) to the query string.
2. Data Collection and Consent: The captive portal presents a registration form. To enable SMS marketing, the form must collect the user's mobile phone number with an explicit, active opt-in checkbox that complies with local regulations (such as GDPR in Europe or TCPA in the United States). The country code must be automatically detected or explicitly selected to ensure correct routing.
3. RADIUS Authentication: Once the user submits the form, the Purple platform communicates with the network's RADIUS server to authorise internet access. The RADIUS server logs the session start time, associating the authenticated MAC address with the user profile in the database.

Presence Detection and Behavioral Tracking

To trigger SMS messages based on return behaviour, the system must distinguish between active connections (users logged into the WiFi) and passive presence (devices with WiFi enabled but not logged in).

• Active Connection Tracking: This relies on RADIUS accounting packets (Start, Interim-Update, and Stop). When a user connects, the RADIUS Start packet registers their presence. Interim-Update packets, sent at configured intervals (typically 15 minutes), confirm ongoing dwell time. A RADIUS Stop packet registers their departure.
• Passive Presence Tracking: This utilises probe requests sent by mobile devices searching for known networks. Access Points capture these probe requests, recording the device's MAC address, timestamp, and Received Signal Strength Indicator (RSSI). If the device has previously registered through the captive portal, the system can identify the user's physical presence near the venue even if they do not log into the WiFi during that specific visit. To protect privacy, MAC addresses are cryptographically hashed (using SHA-256) immediately upon capture.

Integration Architecture and Webhooks

To initiate an SMS, the WiFi analytics engine must transmit data to an SMS gateway (such as Twilio, Sinch, or Link Mobility) in real time. This is achieved using webhooks or REST APIs.

+-------------------+       RADIUS       +---------------------+
|  Wireless Network | <----------------> |   Purple Platform   |
|  (APs / WLC)      |                    | (Analytics Engine)  |
+-------------------+                    +---------------------+
         |                                          |
         | Redirect                                 | Webhook (JSON)
         v                                          v
+-------------------+                    +---------------------+
|  Captive Portal   |                    |     SMS Gateway     |
|  (User Opt-in)    |                    |  (Twilio / Sinch)   |
+-------------------+                    +---------------------+
                                                    |
                                                    | SMPP / HTTP
                                                    v
                                         +---------------------+
                                         |   User Handset      |
                                         +---------------------+

When a behavioral rule is met - for example, a registered user has not been detected in the venue for 30 days - the Purple analytics engine generates an event. This event triggers a webhook that sends a POST request containing a JSON payload to the SMS gateway. The payload includes the recipient's phone number, the message body (populated with dynamic fields like first name and last visited location), and tracking parameters.

Implementation Guide

Deploying an automated SMS marketing system requires systematic configuration across your network infrastructure, the Purple platform, and your chosen SMS gateway.

Step 1: Configure the Captive Portal for Compliant Data Capture

1. Log into the Purple Portal administration interface.
2. Navigate to Form Builder and select your active splash page.
3. Add a Phone Number field. Configure the field settings:
   • Set the field as Required.
   • Enable International Format Validation to force users to input their country code.
4. Add a Consent Checkbox specifically for SMS marketing. This must be separate from the general terms and conditions checkbox.
   • Label text: "I agree to receive updates and exclusive offers via SMS. Max 2 messages per month. Reply STOP to opt-out."
   • Ensure the checkbox is unchecked by default.
5. Save and publish the splash page changes.

Step 2: Establish the SMS Gateway Integration

This step configures the communication link between Purple and your SMS provider. This example assumes the use of Twilio.

1. Obtain your Account SID, Auth Token, and a dedicated Messaging Service SID or phone number from your Twilio console.
2. In the Purple Portal, navigate to Integrations > Connectors > Add New.
3. Select Twilio from the list of supported SMS providers.
4. Enter your Twilio credentials into the configuration fields.
5. Test the connection by entering your own mobile number and clicking Send Test SMS. Verify that the message arrives and that the delivery status is logged as successful.

Step 3: Define Behavioral Segments and Triggers

To drive return visits, you must target users based on their physical behaviour. Create a segment for "Lapsed Visitors" who have not visited the venue in the last 30 days.

1. In the Purple Portal, navigate to Analytics > Visitor Profiling > Segments.
2. Click Create Segment and name it Lapsed_30_Days.
3. Define the criteria:
   • Last Visit Date is greater than 30 days ago.
   • Total Visits is greater than or equal to 1 (ensuring they are a historical visitor).
   • SMS Opt-in is equal to True.
4. Save the segment.

Step 4: Configure the Automated Campaign and Webhook Trigger

Now, link the segment to an automated action that fires when a user enters this state.

1. Navigate to Marketing > Campaigns > Create Campaign.
2. Select Triggered Campaign and choose the trigger event: Enter Segment (Lapsed_30_Days).
3. Select SMS as the delivery channel.
4. Draft the message template using dynamic placeholders to personalise the content:

   Hi {{visitor.first_name}}, we miss you at {{venue.name}}! Come back this week and show this text for 15% off your next purchase. Opt-out: {{sms.opt_out_link}}
   

5. Configure Quiet Hours to prevent messages from sending during unsociable hours. Set the quiet window from 20:00 to 09:00 based on the venue's local time zone. Messages triggered during this window must be queued and sent the following morning.
6. Set a Frequency Cap of 1 message per 30 days for this specific campaign to prevent over-communication.
7. Activate the campaign.

Best Practices

To maximise return visits while maintaining high opt-in rates and network performance, adhere to the following industry standards.

Data Hygiene and Number Validation

Invalid phone numbers waste marketing budget and skew performance analytics. Implement real-time validation at the point of capture.

• Use HLR Lookups: Before sending high-volume campaigns, configure your SMS gateway to perform Home Location Register (HLR) lookups. This queries the mobile network to verify if the number is active and currently routed, filtering out landlines and deactivated numbers.
• Enforce E.164 Formatting: Ensure all numbers captured are stored in the international E.164 format (e.g., +447700900077). This prevents delivery failures when users travel internationally or when routing through global carriers.

Timing and Contextual Relevance

SMS is an intrusive channel. Sending messages at the wrong time leads to high opt-out rates.

• Align with Historical Behaviour: If analytics show a user typically visits your venue on Friday afternoons, schedule their re-engagement SMS for Friday morning at 10:00. This places the incentive top-of-mind when they are planning their day.
• Dwell Time Verification: Do not trigger "thank you" or feedback SMS messages immediately upon connection. Set a minimum dwell time threshold (e.g., 20 minutes) to ensure the user has actually spent time in the venue, rather than just walking past and briefly associating with the network.

Compliance and Privacy

Regulatory bodies heavily penalise non-compliant SMS marketing.

• Explicit Consent: Never bundle SMS marketing consent with WiFi terms of service. It must be a distinct, affirmative action by the user.
• Simple Opt-Out: Every SMS must contain a clear, free method for opting out. The standard is to support "STOP" replies or provide a shortened, zero-rated URL that processes the opt-out instantly. When a user opts out, their profile in the Purple database must be updated to SMS Opt-in = False within seconds to prevent subsequent sends.

Troubleshooting & Risk Mitigation

Issue 1: High SMS Delivery Failure Rates

• Root Cause: Users entering fake phone numbers to bypass the captive portal and gain internet access.
• Mitigation: Implement SMS Verification (Two-Factor Authentication) for WiFi access. Instead of granting immediate access upon form submission, send a 4-digit PIN via SMS to the entered number. The user must input this PIN into the captive portal to access the internet. This guarantees that only valid, owned mobile numbers are added to your database.

Issue 2: Webhook Latency and Queue Backlogs

• Root Cause: During peak hours (e.g., halftime at a stadium or Saturday afternoon at a shopping centre), thousands of users may trigger events simultaneously, overwhelming the SMS gateway API.
• Mitigation: Configure an asynchronous message queue (such as RabbitMQ or AWS SQS) between the Purple webhook output and the SMS gateway. This buffers the requests, allowing the system to process messages at a controlled rate without dropping payloads or hitting API rate limits.

Issue 3: MAC Randomisation Disrupting Return Metrics

• Root Cause: Modern mobile operating systems (iOS 14+, Android 10+) randomise MAC addresses by default when scanning for networks, making it difficult to track return visits via passive probe requests.
• Mitigation: Rely on authenticated data rather than passive probe data for high-accuracy campaigns. When a user logs into the captive portal, link their verified identity (phone number) to their current MAC address. If they return and log in again, the system matches the phone number, bypassing the limitations of MAC randomisation.

ROI & Business Impact

To justify the investment in WiFi-integrated SMS marketing, you must track specific metrics that demonstrate a direct correlation between SMS delivery and physical return visits.

Key Performance Indicators (KPIs)

1. Return Visit Rate (RVR): The percentage of users who received an SMS and subsequently authenticated on the venue's WiFi within a defined attribution window (typically 7, 14, or 30 days). $$\text{RVR} = \left( \frac{\text{Number of SMS recipients who returned and authenticated}}{\text{Total SMS messages successfully delivered}} \right) \times 100$$
2. Attribution Window Match: The system must correlate SMS delivery logs with RADIUS accounting logs. If a user receives an SMS on Tuesday and their MAC address registers a RADIUS Start packet on Thursday, this is counted as an attributed return visit.
3. Cost Per Return Visit (CPRV): Calculate the total cost of SMS delivery divided by the number of attributed return visits. $$\text{CPRV} = \left( \frac{\text{Total SMS Cost}}{\text{Attributed Return Visits}} \right)$$ For example, if sending 10,000 SMS messages costs £200 (at £0.02 per message) and results in 400 return visits, the CPRV is £0.50. Compare this against the average customer lifetime value (LTV) or average transaction value to determine profitability.

Data-Driven Optimisation

By continuously analysing these metrics within the Purple dashboard, operations teams can run A/B tests on message copy, incentive values, and delivery timing. This iterative process ensures the SMS channel remains a highly efficient driver of footfall and revenue.