MD4455/MD4325 Mobile Node with Mobility Scanner
      The MD4455-AAIA is essentially the "mobile" version of the three radio MD4350 - there are two 5GHz 802.11a backhaul radios, and a 2.4GHz 802.11b/g AP radio.  The fourth radio slot on the MD4455-AAIA motherboard houses a scanning radio. The scanning radio listens to the RF environment and helps the node make judicious parent node selections in a high speed mobility environment. 
MD4355 Mobile Mesh Node The MD4355-AAxA is essentially the "mobile" version of the two radio MD4250 - there are two 5GHz 802.11a backhaul radios, slots 0,1, and no radio in slot 2.. The fourth radio slot on the MD4355-AAxA motherboard houses a scanning radio. The scanning radio listens to the RF environment and helps the node make judicious parent node selections in a high speed mobility environment. 
        The 4325-IIxI is a 2.4GHz version of the 4355, with three radios: one backhaul uplink, one backhaul downlink and one 2.4GHz scanner. The downlink radio doubles as both a backhaul and 2.4Ghz service radio. The 4325 is intended as a edge edge node. It will interoperate with the 4455: its 2.4GHz uplink will connect to the most suitable 4455 in its vicinity as a parent, using 2.4GHz radio on the 4455 as a parent downlink. [Illustration

Meshdynamics 4455-AAIA and 4355-AAxA mobility nodes are in deployment by US military and its allies. They validated that the MD4000 provides superior connectivity even at high speeds. Applications include video surveillance with cameras mounted on vehicles and connecting with nodes along the road. Both static and mobile units have been in operation along the Arizona Mexico border since 2005. Mobile units have also been employed in armored vehicle convoys in Iraq.

 Click to enlarge

  Left: Hybrid Mesh supports both multi-radio module and single radio edge units. 
  Right: Video Clip on How P3M Mesh Networks disperse and coalesce dynamically.

P3M Mobility A cluster of mobile MD4455 will automatically form a network with each other. If the cluster physically separates into two smaller clusters, each will then become its own independent network.  If two separate clusters/networks physically come together, one single network will form and all nodes + associated clients will then be able to share data. 

In a deployed combat environment in Iraq, MD4355 and MD4455 and vehicle-mounted nodes demonstrated reliable connectivity between convoy vehicles in simultaneous motion. Networks split and reformed dynamically and in real time. Applications supported by Meshdynamics systems operated flawlessly as vehicles moved through rugged terrain. P3M Meshdynamics nodes currently deployed in multiple military programs involving mesh mobility  in US, UK, Canada and Israel. [Customers]

Also see: P3M Press Release and  Illustration
Bridging Networks.
Meshdynamics Structured Mesh networks are inherently tree based. Radios in a Meshdynamics Mesh networks  may operate in one of three modes: Station, Access Point or (passive Scanner). A 2.4GHz downlink or parent Access Point provides services to 2.4GHz Child Stations as part of standard (and scalable) 802.11 infrastructure mode. These 2.4GHz child stations may act as pseudo roots for  2.4 GHz backhaul trees. [Illustration]
Choice of Backhaul: When deciding whether to use 2.4GHz based products (4220,4325) RF pollution must be considered. The clients are 2.4GHz and will interfere with a 2.4GHz node-to-node (backhaul) link. In addition, 802.11b clients - if attached to the 2.4GHz backhaul (e.g. 4220, 4325) slows the performance of of the node-to-node link.4220 and 4325 nodes should be limited in servicing few child nodes. On the positive side, 2.4 GHZ has about twice the range at 5GHz, so 4325, 4220 are preferable where range supersedes backhaul capacity.

In most other cases,5GHz nodes (4455-AAIA, 4458-AAII, 4452-AAIA, 4454-AAAA, 4250-AAxx) are suggested.  
Related Links

1. Military Mesh Networks  Overview on why Military Mesh Networks require third generation mesh. 
2. Meshdynamics Delivers Persistent Wi-Fi Network for Mobile Environments Performance and Mobility
3. Border Security Applications and Solutions: Network deployments along Mexico-Arizona border
4. High Speed Mobility Test -1  Glitch free video transmission over 4 hops, traveling at 50+ mph.
5. High Speed Mobility Test -2  Video feed from one vehicle to another.