A train, a bus or a crane moves past a series of fixed radios. Each time it switches from one to the next, the network performs a handover. For email that hardly matters. For live video, a slow handover shows up as frozen pictures, lost packets and gaps in the recording.
Why conventional roaming drops the connection
A standard Wi-Fi client has one radio. To move to the next access point, it has to let go of the current one, find the next, and connect again, which is called break-before-make. During that gap no traffic flows. Many systems also rely on a central controller to manage roaming, which adds a dependency and another point of failure.
Make-before-break with two radios
Anywhere nodes on a moving vehicle use two radios. One stays connected to the current fixed node while the other connects to the next. The node switches paths based on real-time link metrics only once the new link is ready, so there is no gap to wait through. Path switching typically takes place within 5 ms, and the nodes make their own decisions, so no central controller is needed.
What 5 ms means for a video stream
At 25 frames per second, a camera produces a new frame every 40 ms. A path switch that completes within 5 ms is shorter than the interval between two frames, which is why video from a moving train or crane can stay smooth through each handover.
Where it matters
- Trains and metros: CCTV, passenger Wi-Fi and diagnostics, with up to 700 Mbps per train
- Monorails and people movers: live video from moving cabins, as on the Danyang Monorail in South Korea
- Port cranes: video and data from cranes that travel along the quay, with immediate wireless backup if a fibre link breaks
- Tower cranes and heavy industry: reliable data and video from equipment that moves all day
- Vehicle depots: buses and fleet vehicles stay connected as they move between parking, cleaning and refuelling
Planning a mobile network
Handover only works if the next fixed node is reachable before the current link fades. Planning covers the spacing of trackside or quayside nodes, antenna choice and direction, and the throughput each vehicle needs. On trains, front and rear mobile nodes can be combined for 1 Gbps or more of aggregate throughput. The same AX52e node is used on the vehicle and at the trackside, running in mobile or static mode.
Equipment that rides on the vehicle
Onboard equipment faces vibration, shock, temperature swings and strict fire rules. The AX52e meets rolling-stock standards including EN 50121-3-2, EN 50155, EN 61373 and EN 45545-2, is rated IP67 and operates from -35°C to 65°C.
Questions to ask a supplier
- Is the handover make-before-break, or does the radio disconnect first?
- Does each mobile node have one radio or two?
- Does roaming depend on a central controller?
- What handover time has been measured in service, not only in the lab?
- Which rolling-stock certifications does the onboard unit hold?