Sizing WAN Bandwidth and Latency for Extended PCS 7 Terminal Bus Multi-Monitor Clients

Extending the SIMATIC PCS 7 Terminal Bus directly over a Wide Area Network (WAN) via VPN to operate remote multi-monitor OS Clients (e.g., two 4-monitor stations located 100 km away) is strongly discouraged by Siemens due to network latency constraints. Native WinCC Terminal Bus communication requires a maximum round-trip latency (ping) under 5 ms to avoid WinCC DataManager timeouts and screen freezing. To achieve 2-to-3 second picture switch times over a 10 Mbps WAN link, deploy virtualized OS Clients hosted locally in the plant data center and stream the multi-monitor desktops to the remote control room using Microsoft Remote Desktop Services (RDP) or Teradici PCoIP, consuming only 1.5 to 3 Mbps per station.

Native Terminal Bus over WAN vs Virtualized Remote Desktop

Industrial operators frequently seek to consolidate operations by controlling distant production facilities from a centralized remote control room connected over corporate WAN or leased telecommunication lines (e.g., 10 Mbps VPN links). In SIMATIC PCS 7, the Terminal Bus connects OS Clients to OS Servers.

Attempting to extend the physical Terminal Bus across a WAN by placing native PCS 7 OS Client PCs at the remote site causes severe performance degradation. While available bandwidth (e.g., 10 Mbps) might appear adequate, native WinCC clients engage in high-frequency, synchronous SQL database transactions and RPC/DCOM handshakes with the OS Server. Over a WAN, physical transmission delay and router jitter introduce round-trip latencies ranging from 20 ms to 80 ms, causing picture switch times to balloon to 15-30 seconds or freezing the client completely.

Latency and Bandwidth Bottlenecks in WinCC Client-Server Architecture

The engineering limits governing PCS 7 Terminal Bus communication include:

Round-Trip Time (RTT / Latency): Siemens guidelines strictly prescribe that Terminal Bus network latency must not exceed 5 ms (absolute maximum tolerable: 20 ms under light load). WAN connections across 100 km routinely exceed 30 ms.
Synchronous Picture Loading: When an operator switches pictures on a 4-monitor client, the client must instantiate hundreds of OCX faceplates, load tag descriptors from server packages, and establish cyclical tag advises. Over high-latency WAN links, serialized packet acknowledgment creates severe visual lag.
The Virtualization Solution: By locating virtualized OS Clients (e.g., VMware vSphere or Microsoft Hyper-V) directly on the plant's local 1 Gbps Terminal Bus, latency between client and server remains < 1 ms. Only the compressed graphical display frames and operator keystrokes/mouse clicks are transmitted across the 10 Mbps WAN using Remote Desktop Protocol (RDP) or PCoIP.

Architecture Comparison Matrix: Native WAN vs RDP / PCoIP

The following table compares architectural topologies for remote multi-monitor operation:

Performance / Design MetricNative OS Client over WAN VPNVirtualized OS Client + RDP / PCoIPWinCC WebUX / Web NavigatorMax Tolerable Latency (Ping)< 5 ms (Strictly enforced)Up to 100 ms without degradationUp to 150 msRequired Bandwidth (4 Monitors)> 20 Mbps burst per client1.5 to 3.0 Mbps total2 to 4 Mbps per sessionPicture Switch Time10 to 30+ seconds (Unusable)1.5 to 2.5 seconds (Meets SLA)2 to 3 secondsFailure Mode on Packet LossWinCC freeze / Server disconnectMomentary video blur / auto-resumeWeb browser reconnect promptMulti-Monitor SupportNative hardware multi-headFull multi-monitor RDP spanBrowser window spanning

Engineering Architecture for Multi-Monitor Remote Control Rooms

To implement a robust, high-performance remote control center over a 10 Mbps WAN:

Deploy Virtualized OS Clients in the Plant Data Center:
- Install a certified industrial virtualization server (e.g., SIMATIC IPC1047 or enterprise hypervisor) on the local plant network.
- Create virtual machines for each OS Client running Windows 10 Enterprise LTSC.
- Connect the virtual network adapters directly to the local physical 1 Gbps Terminal Bus virtual switch.
Configure Multi-Monitor Remote Desktop Services:
- On the virtualized OS Clients, enable Remote Desktop and configure multi-monitor spanning support.
- On the remote control room thin clients (or PCs with 4 physical display outputs), launch the RDP client using the command: mstsc.exe /multimon /v:<VM_IP_Address>.
- Enable RDP 8.0/10 graphics compression and UDP transport for smooth dynamic process updates.
Establish Redundant Leased Line / VPN Routing:
- Ensure the 10 Mbps WAN link utilizes Quality of Service (QoS), prioritizing RDP traffic (TCP/UDP port 3389) over standard file transfers.
- Configure redundant firewall routers (e.g., SCALANCE S615 or SCALANCE SC600) to provide automatic WAN failover.

Verification

Verify remote control room performance:
1. Measure network latency from the remote control room to the plant gateway using ping -t <Gateway_IP>. Confirm average latency is stable (e.g., 15-25 ms) with zero packet drops.
2. Connect the 4-monitor remote session via RDP.
3. Perform picture changes across complex technological areas on all 4 screens simultaneously.
4. Using a stopwatch, verify that complete graphic rendering and live tag population completes within 2 to 3 seconds.
5. Trigger an operator command (e.g., motor start) and confirm immediate visual feedback on the remote display.

Limitations and Operational Precautions

Native Terminal Bus Prohibition: Never deploy native PCS 7 OS Clients over commercial WAN connections; doing so violates Siemens system specifications and voids performance warranties.
Emergency Local Override: Always maintain at least one physical OS client or engineering station locally within the plant to ensure operational capability if the WAN link is severed.
Cybersecurity Compliance: Remote access over WAN must comply with IEC 62443 process security standards, utilizing IPsec VPN encryption, multi-factor authentication (MFA), and DMZ boundary firewalls.

Engineer FAQ

Why does bandwidth not solve the native Terminal Bus WAN problem?

Bandwidth determines volume capacity, but latency determines transaction turnaround time. WinCC's database architecture executes sequential round-trip handshakes; higher bandwidth cannot overcome the physical latency of long-distance cabling.

Can Microsoft RDP accurately display 4 monitors simultaneously?

Yes. Modern Microsoft Remote Desktop Protocol natively supports multi-monitor configurations using the /multimon parameter, perfectly aligning with 4-screen operator consoles.

What is the bandwidth consumption of one RDP multi-monitor session?

Under typical process automation conditions with moderate dynamic tag animations, an RDP session spanning four 1080p monitors consumes between 1.5 and 3 Mbps sustained bandwidth.

Can WinCC Web Navigator be used instead of RDP?

Yes, WinCC Web Navigator is an approved remote access option, but multi-monitor spanning and full engineering client integration are generally more straightforward with virtualized OS Clients over RDP.