SCALANCE W788-2PRO WDS Configuration: Bridging S7 Networks

1. Overview

Bridging two Ethernet subnets that physically span a moving asset (such as a crane) requires an industrial-grade WLAN solution that is transparent at Layer 2. The SCALANCE W788-2PRO is a Siemens 802.11 a/b/g/h/n access point designed for harsh industrial environments, IP65-rated, and rated for –40 °C to +70 °C ambient operation with conformal-coated PCBs. Its dual-radio design (two independent WLAN interfaces, often referred to as WLAN 1 and WLAN 2 or interface 1 and interface 2) makes it well suited to redundancy schemes where a single radio failure or antenna shadowing must not drop the link.

When two SCALANCE W788-2PRO devices must extend a single IP subnet across a wireless span, three operating modes are available in the Web Based Management (WBM):

• Access Point (AP) mode – clients associate, but AP-to-AP bridging is not enabled.
• WDS (Wireless Distribution System) – AP-to-AP bridging at Layer 2, transparent to all higher-layer protocols including PROFINET, S7 communication, and standard ICMP.
• Client mode – the device acts as a wireless client (station) that associates to an upstream AP.

For the S7-400 ↔ S7-300 plant-to-crane scenario where both sides must remain on the same subnet (192.24.4.0/24 in the example), the correct mode is WDS on each of the two W788-2PRO units, configured symmetrically as a point-to-point WDS link. The WDS link passes all Ethernet frames, including broadcast ARP, between the two wired segments, so a ping from a PC on the plant side should reach a drive, ET200, or HMI on the crane side once the WDS link is up.

See the official SCALANCE W788-2PRO operating instructions and the SCALANCE W700 Web Based Management configuration manual for the parameter reference used throughout this article.

2. Prerequisites

Before commissioning the wireless link, verify the following prerequisites are satisfied.

3. Network Architecture and IP Planning

The target architecture is two physical Ethernet segments joined by a single WDS bridge. No IP changes are required on the S7-400 or S7-300 stations; the SCALANCE pair becomes the wireless segment of the unified subnet.

        Plant Segment (S7-400)              Crane Segment (S7-300)
  ┌─────────────────────────────┐      ┌─────────────────────────────┐
  │   S7-400 CPU          .10   │      │   S7-300 CPU          .20   │
  │   ET200S PN/PN       .11   │      │   SINAMICS drive      .21   │
  │   Comfort Panel TP   .12   │      │   KTP Mobile          .22   │
  └──────────┬─────────────┘    │      └──────────┬─────────────┘
             │ LAN (CAT6)                          │ LAN (CAT6, festoon)
             ▼                                     ▼
     ┌──────────────────┐                ┌──────────────────┐
     │ SCALANCE W788-2PRO│  ◄── WDS ──►   │ SCALANCE W788-2PRO│
     │  IP 192.24.4.2    │  (WLAN1 5 GHz  │  IP 192.24.4.3    │
     │  WLAN1 ch 36,     │   or 2.4 GHz)  │  WLAN1 ch 36,     │
     │  WLAN2 ch 149     │               │  WLAN2 ch 149     │
     └──────────────────┘                └──────────────────┘

Key addressing rules:

• Both SCALANCE W788-2PRO modules must have static IPs in the same subnet as the PLCs. The factory default of 192.168.1.1 should be changed to 192.24.4.2 and 192.24.4.3 (or other free addresses in the 192.24.4.0/24 segment) before commissioning the WDS link.
• Set the subnet mask to 255.255.255.0 on both SCALANCE units.
• Disable the DHCP server on both units. With WDS bridging, the SCALANCE acting as a DHCP server on one side would lease addresses that the wired side on the other segment cannot reach, causing intermittent IP conflicts.
• Set the default gateway only if management access from another subnet is required; for the bridging function, no default gateway is needed and no routing is performed.

4. WDS Versus Client/AP Mode

Two W788-2PRO devices will not forward frames to each other simply because they share the same SSID and channel. SSID is a Layer-2 identifier used for client association; AP-to-AP bridging must be explicitly enabled via the WDS table in the WBM. The diagnostic symptom of this misconfiguration is exactly the user's observation: each AP serves its own wired clients, but ping fails across the wireless span.

5. Step-by-Step WDS Configuration

1. Connect and identify each unit. Power one W788-2PRO at a time, connect your service laptop to its LAN port, and open https://192.168.1.1 (factory default). The default user is admin with password admin – change this immediately after first login.
2. Set Layer-2/Layer-3 parameters. Navigate to Layer 3 > Subnets > Configuration and assign:

   IP address (unit A, plant):  192.24.4.2
   IP address (unit B, crane): 192.24.4.3
   Subnet mask:                255.255.255.0
   Default gateway:             0.0.0.0 (no gateway required for bridging)

   Click Set Values, then Restart for the change to take effect.
3. Set the country code. In WLAN > Country, select the deployment country. The unit will automatically restrict channels and maximum transmit power to the regulatory domain. This is mandatory – the radio will not transmit until the country is set.
4. Configure WLAN interface 1 for WDS. Navigate to WLAN > Interfaces > WLAN1:

   Operation mode:              Access Point
   SSID:                       CRANE_LINK_5G (example)
   Channel:                    36 (5 GHz, DFS-free)
   Channel width:              20 MHz (start with 20 MHz; widen later if RSSI healthy)
   802.11 mode:                802.11n (mixed) or 802.11a
   WDS mode:                   Enabled
   WDS peer MAC (unit A):      MAC of WLAN1 on unit B
   WDS peer MAC (unit B):      MAC of WLAN1 on unit A

   The WDS peer MAC is not the MAC printed on the housing label – that is the wired-port MAC. The WLAN-interface MAC must be read from Information > WLAN > Interfaces in the WBM of the remote device. Record the WLAN1 MAC of both units before configuring the WDS table.
5. Save and restart both units. Configuration of WDS peer MACs typically requires a reboot to take effect on V6.x firmware.
6. Verify association. On the WBM home page, the WLAN tile should show the WDS link as Connected with an RSSI value. Typical healthy link values for an industrial crane application are RSSI ≥ –65 dBm. RSSI in the –75 dBm to –85 dBm range will work for light traffic but will not sustain PROFINET IRT cycle times below 4 ms.

6. Channel Selection, Antenna Placement, and Link Budget

The user's crane-swing application introduces dynamic antenna shadowing as the boom rotates. Two strategies apply:

1. Spatial diversity with two physical antennas on one radio – the W788-2PRO has two antenna ports per WLAN interface; the radio uses diversity switching on a per-packet basis. Mount antennas at least one wavelength apart (≈ 6 cm at 2.4 GHz, ≈ 3 cm at 5 GHz) and with orthogonal polarization where practical.
2. Radio diversity with both WLAN1 and WLAN2 – for the most demanding cases, use iFeatures redundancy across the two radios on different channels (see Section 8). Do not stack two WDS bridges across the same two APs on different channels; this creates a Layer-2 loop even with RSTP.

Channel selection rules:

• Avoid DFS channels (52–144) on 5 GHz when possible. DFS requires a 1–10 minute channel availability check and radar-detection silence; on a swinging crane the link will not survive a radar event gracefully.
• Prefer non-overlapping 2.4 GHz channels: 1, 6, 11 (5 GHz channels 36, 100, 149 are also commonly used).
• Run a site survey with the SCALANCE WBM WLAN > Spectrum Analyzer page (V6.4+ firmware) to identify interference from neighboring APs, microwave links, or variable-frequency drives.

Link budget for crane spans up to 200 m, 2.4 GHz, 6 dBi omnidirectional antennas at both ends:

Tx power (regulatory max)        : +20 dBm (set per country code)
Tx antenna gain                  :  +6 dBi
Free-space path loss (200 m, 2.4): –86.4 dB   =  32.44 + 20*log10(200) + 20*log10(2400)
Rx antenna gain                  :  +6 dBi
Cable loss (LMR-400, 5 m, 2.4)   :  –2.5 dB
Rx input level (estimated)       : –56.9 dBm
Required Rx sensitivity (54 Mb)  : –72 dBm
Margin                           : +15.1 dB  (link is healthy)

For 5 GHz, expect 8–12 dB less margin at the same distance. Re-run the calculation using FSPL (dB) = 32.44 + 20*log10(d_km) + 20*log10(f_MHz) with d in km and f in MHz.

7. RSTP and Loop Avoidance

Because WDS is a transparent bridge, enabling two WDS links between the same pair of SCALANCE W788-2PRO units creates a Layer-2 loop, which without protection will broadcast-storm the wireless segment within seconds. The correct protections are:

• Enable RSTP (Rapid Spanning Tree Protocol, IEEE 802.1w) on the WBM Layer 2 > Spanning Tree page. Set one SCALANCE as the root bridge (lowest Bridge Priority, default 32768; set the plant-side unit to 4096 to force it to root). The W788-2PRO supports RSTP with typical convergence of < 2 s on a wired segment, but on a wireless link the convergence can be 6–10 s depending on retransmits.
• Set Bridge Priority on the desired root: Layer 2 > Spanning Tree > General → Bridge Priority = 4096 for the plant unit, = 8192 for the crane unit.
• Enable STP Forwarding on the WLAN port in the port role table so RSTP BPDUs pass over the WDS link. Without this, BPDUs may be filtered and the loop will not be detected.
• Do not enable RSTP on a WDS link that is itself one half of a redundancy pair; instead use iFeatures redundancy (Section 8) which manages failover without relying on STP.

8. iFeatures Redundancy Mode (Preferred for Crane Applications)

The cleanest answer to the user's antenna-shadowing problem is to abandon dual-WDS and use Siemens' iFeatures redundancy, available on the W788-2PRO with V6.x firmware. This mode binds two physical WLAN interfaces (WLAN1 + WLAN2) into a single redundant bridge; only one link is active at a time, and failover occurs in < 500 ms.

Configuration steps:

1. In iFeatures > Redundancy, enable redundancy mode on both APs. Each AP must be told about the other AP by either System Name or WLAN-interface MAC.
2. Disable WDS on both radios. iFeatures and WDS are mutually exclusive per the firmware.
3. Set the partner identification. Use System Name (recommended – easier to maintain than MAC) and ensure each W788-2PRO has a unique, descriptive System Name under System > General.
4. Configure WLAN1 on channel 36 (5 GHz) and WLAN2 on channel 1 (2.4 GHz). The two different bands provide frequency diversity in addition to spatial diversity.
5. Mount antennas for WLAN1 on one side of the crane boom and antennas for WLAN2 on the opposite side. As the crane swings, one radio's LOS is preserved while the other may be shadowed; iFeatures keeps the link up on the healthier radio.

Verification: in the WBM Information > iFeatures > Redundancy page, the active and passive radios are listed along with the partner's RSSI. Manually shadow one antenna set with a metal plate and confirm the active radio switches over within 1 s; verify ping continuity with a continuous ping from the service laptop to a crane-side device during the switch.

9. Verification Procedures

After WDS is configured, perform these checks in order. Each must pass before the next is attempted.

If step 2 fails, the WDS bridge is not up. Go to Section 10.

10. Troubleshooting Matrix