Without routers, packets of data would wander aimlessly around the Internet. These stripped-down, purpose-built computers vary in size, power and number of port connections. Their sole task: read the address-headers of endless bundles of bits every millisecond, then forward them to their correct destinations. Like knots, routers hold together the individual strands of the World Wide Web.
Outwardly, a router looks a lot like a cable modem. Flashing indicator lights adorn its front; a series of cables and a power cord extend out from its back. Wireless routers also sport a vertical antenna. Open the casing and you will find a set of printed circuit boards. There is one for each active cable port. Closer in lies a motherboard replete with internal clock, central processing unit (CPU), strips of dynamic random access (DRAM) memory, Read Only Memory (ROM) and Flash Memory.
The printed circuit boards repackage outgoing data streams to fit the transmission requirements of the network to which its adjoining cable port connects. Conversely, they rid incoming data streams of these extraneous protocols. So for example, your router may have one board supporting Ethernet, another token ring. In edge routers at network gateways, one or more provisions the Internet's Transmission Control Protocol. In core routers interconnecting backhaul networks, others layer in Signal System 7, a telecommunications protocol.
The CPU at the heart of the motherboard reads the incoming message header and determines its destination. CPUs vary in size and speeds depending on the router's workload. Wireless home and broadband office devices clock in up at 11 Megagbits per second (Mbps) or 54 Mbps. 248 Mbps routers are under development. Much busier Edge Routers generally clock in anywhere between 10 and 100 Gigabits per second.
DRAM stores the operating system software, the running configuration file, and the routing tables. It is in constant use. As is a specialized type of DRAM used as a packet buffer feeding the CPU. Routers contain anywhere from 16 Megabytes to 4 Gigabytes of DRAM. Because it retains data when power is off, Non-Volatile Random Access acts as a backstop by storing the router's configuration file. Flash Memory also holds on to data, but can be easily changed. An updatable version of the Router's operating system is thus stored there. Permanent ROM contains code that will reboot the operating system in the event of a catastrophic failure.
The router contains the IP number of a message's recipient. Every computer on the net has its own unique number. To know where to send message next, the CPU consults a Routing Table stored in its main memory. This software tells the CPU the next "hop" or destination, the most cost-effective route, and the direct and indirect subnets the data packet must still navigate to get there. It then passes the message on to the appropriate circuit board to send.
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