Mesh applications often require a string-of-pearls formation where the end-to-end path is non line-of-site (NLOS). Additionally, there may be points along the path where only backhaul is needed - no client Access Point. The baseline two radio MD4250-AAxx  supports this requirement.
 
If wireless client access is needed, the MD4350-AAIx has one 2.4GHz Access Point (AP) as the third radio in slot 2, in addition to the 5.8GHz wireless backhaul up link and downlink radios is slots 0,1 respectively (see above). The third radio is also used to bridge networks. In the figure below, a 5GHz based backhaul is bridged to a 2.4GHz based sub tree.
 

Higher client density or congested areas require more focused reception. MD4458-AAII houses two access points to provide twice the client access radio power. The two AP radios may be on the same channel and not interfere, using sectored antennas. The 4458-AIII may also serve as an edge node: with one uplink and three AP radios

The two radio uplink-downlink backhaul comes in two flavors. 4250-AAxx uses radios configured to operate in the 5GHz spectrum. 4220-IIxx uses radios configured to operate in the 2.4 GHz spectrum. When deciding whether to use 2.4GHz based products (4220) vs. the 5.8GHz (4250) product, RF pollution must be considered. Wi-Fi 802.11b/g 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 downlink will slow the performance of of the node-to-node link. 4220 mesh nodes thus should be limited in servicing few child nodes. On the positive side, 2.4 GHZ has about twice the range at 5GHz, so 4220 are preferable where range supersedes backhaul capacity.
   
In all other situations, 5GHz mesh nodes (4455-AAIA, 4458-AAII, 4452-AAIA, 4454-AAAA, 4250-AAxx) are suggested.