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I am planning a wireless network that will involve a wireless radio modem that must be mounted pretty far from my house on a post (Needs line of sight to work) and then getting the signal back over to the house using an outdoor wireless access point(something like this, for example).

Both the modem and the WAP can be powered by Power over Ethernet and I was wondering if I can chain them.

For example, can I replace this configuration...
Cat5 -> PoE Injector -> Cat5 -> WAP -> Cat5 --> PoE Injector-> Cat5 -> Modem

With this configuration...
Cat5 -> PoE Injector -> Cat5 -> WAP -> Cat5 -> Modem

Is this possible? Also, Is one of these two configurations a bad idea for some reason?

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what's a wireless radio modem? do you mean what i've heard is called a range extender that extends the range of your WAP. – barlop Aug 13 '11 at 1:15
the configuration you describe has cables on both sides of your WAP so not making use of wireless and who would an ethernet over mains plug, have cat5 cables on both sides, it can only have one side with cat5.. you have me baffled. This "Cat5 --> PoE Injector-> Cat5 -" is something you'd add if your WAP cannot reach your modem. – barlop Aug 13 '11 at 1:23
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@barlop It's not Ethernet over mains, it's power over Ethernet. The WAP is used for wireless - it links with the remote wireless device. – sblair Aug 13 '11 at 2:16
@barlop - It is used by WISP providers. In my example it is the subscriber module for the Motorola Canopy System (canopy-wireless-solutions.com/Products/PMP/Specifications/…) – JohnFx Aug 13 '11 at 15:16
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Generally no, because in switched Ethernet, all links are point-to-point. There should be one power sourcing equipment (PSE) device (e.g., an injector) per powered device (PD). It would be up to the switching fabric in the switch (in your case the Ethernet switch in the WAP) to support this.

An exception is the relatively new Cisco PoE passthrough [PDF] range of devices which:

can draw power from an upstream (PoE+/PoE-capable) switch or a router in the wiring closet, to power itself and to drive power downstream to the IP devices connected to it.

Another option is a HP IntelliJack Gigabit switch, which can power up to two devices, but only if supplied by a 802.3at-2009 (PoE+) source (or a local DC supply, as described in the manual PDF).

HP IntelliJack Gigabit switch

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