What, exactly, does a router do? And how does it do it? Although the answer to the first question can be relatively simple—a router routes data packets along networks—all of the complexity lies in "how."
Routers come in many shapes and sizes: a room full of rack-mounted, dedicated systems for enterprise clouds; off-the-shelf commercial boxes from manufacturers like Linksys or D-Link; and DIY solutions the size of a deck of cards, built on hobbyist platforms like Raspberry Pi.
Leaving aside the overall meta architecture of packet exchange, the core hardware of a modern commercial router (aimed at the small business and home networking markets) distinguishes it from enterprise or application-specific solutions. Specifically, such a router emphasizes convenience. Single devices act as a DSL modem, router, wireless access point, media server and connection to the smart kitchen sink. Also, the processor and connectivity are geared toward a completely different profile (high-bandwidth use for gaming and streaming, along with simultaneous connections from multiple devices, including smartphones and fridges) than enterprise-oriented platforms.
This article focuses on the hardware that runs modern consumer routers. And in today's routers, SoC (system on a chip) solutions are universal—all of the hardware we cover comes in the form of integrated SoCs. These multi-function systems comprise a variety of configurations with different capabilities, making it more difficult to dig into their respective architectures; but they simplify the router design process. There are far fewer devices to consider when a single board comes with everything built onto it.
That doesn't mean completely integrated systems are the only off-the-shelf setups worth considering, though. Even in the world of SoCs, factors like cost, power consumption and OEM requirements do create a world of chimeric SoC solutions, with multiple high-power radios or transceivers pressed in to service a higher-level processor/memory board, or an xDSL modem connected to one of the ports of a more general-purpose processor. Solutions with discrete RAM or flash memory modules and transceivers with separate radio chips do exist, but they are few and far between. And so we focus on the two classes of SoCs that are most often seen in the wild: processor/all-in-one chips and transceivers.
A Note On Software/Firmware
Commercial routers overwhelmingly favor Linux as an operating system, and often employ a customized version. A lightweight Web server is almost always installed as well for user-controlled device configuration.
Another OS, VxWorks, is used only in enterprise-class systems, but it merits a mention if only because it is used on the Mars Reconnaissance Orbiter, and I would buy a commercial router running VxWorks regardless of how unwieldy the final system becomes.
Finally, an open, Linux-based standard, OpenWrt, is being embraced by more OEMs, even though it was previously mostly used by prosumer hobbyists.