Can someone explain the main differences between this and EdgeRouter? I'm looking to use this for school w 200 users

I use an Edgerouter (ER-POE5). I also use some products out of the Unifi line, like the Unifi Access Points with the Unifi Controller software installed on a Raspberry Pi. I don't specifically use the Unifi Security Gateway. I would say the main difference though is the way you manage the product. My EdgeRouter is more or less "standalone managed". It has a console port you can connect to, or you can manage it via the built in web interface, or you can SSH into it and use the CLI. Honestly, I really like the EdgeRouter because it does everything I need it to do very easily. It is capable of higher traffic volumes than this security gateway (my model does 2+ million packets per second), and it has great features like DHCP, a built in firewall, QOS, DNS, etc. It's relatively straightforward to figure it out. But it doesn't integrate into the "Unifi Controller" for centralized management. And that's really the big sell when it comes to the Unifi Security Gateway. You can install a single controller, and use that to manage all your Unifi devices. I have that installed to manage two access points. It may be a bit overkill for that. But, if you are at a school with a bunch of access points and switches (Unifi switches can also be managed through controller), then then Unifi Security Gateway might be nice if you want just a "single pane of glass". I would just look through the feature list carefully for the USG, and if it has the features you need, and if you are going to be buying all your switches and APs from the Unifi line also, then go for it as it may simplify your life. If this is going to be your only Ubiquiti product, then I probably recommend a standalone EdgeRouter that you manage directly.

Can this device be set up in a "transparent" mode? i already have a router/firewall but would like to be able to monitor more things on my unifi ap.

Yes, you can set it up in bridge mode.

Can the VoiP port be used as a second WAN port?

The ability to use the VoIP port as a second WAN has been added to the latest controller software. The feature itself is currently in beta but seems to work in limited testing

What can this do that an erx can't do? security gateway is $100 and erx is $60 and has more LAN ports. What am I missing?

The security gateway is intended to integrate with the UniFi platform and the integration is solid but still a little limited feature wise still. Administration of the gateway is literally done through UniFi; you don't really access the gateway directly. The EdgeRouters are their own standalone platform, development and web based options are farther along but no UniFi development. If you have a heavily Ubiquiti based network and want everything talking to each other, the security gateway is better. If you want to treat your router as a router and nothing more, the EdgeRouter line makes more sense.

ASIN	B00LV8YZLK
Batteries	Product Specific batteries required.
Best Sellers Rank	#8,299 in Computers & Accessories ( See Top 100 in Computers & Accessories ) #298 in Computer Routers
Brand	Ubiquiti Networks
Color	White
Date First Available	July 16, 2014
Hardware Platform	CONSUMER_ELECTRONICS
Is Discontinued By Manufacturer	No
Item Dimensions LxWxH	5.31 x 5.31 x 1.11 inches
Item Weight	12.3 ounces
Item model number	USG
Language	English, English, English, English, English
Manufacturer	Ubiquiti
Operating System	UniFi OS
Product Dimensions	5.31 x 5.31 x 1.11 inches
Series	USG
Voltage	240 Volts
Wireless Type	802.11b

Ubiquiti Unifi Security Appliance (USG), Single,White

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