Router Overview - Packets and Paths

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Whatever information is sent across the Internet (e-mail, Web page, and so on) is first broken into 1,500-byte packets. The packets are transmitted across a number of routers, each one sending the packet to the destination device. The packets will be transmitted via the best available route. This type of network is called a packet-switched network. Each packet could take the same route, or none of the packets could take the same route. Once the packets show up at the destination computer, they are reassembled. This process goes so quickly that you wouldn’t even know that the file was chopped into 1,500-byte packets and then reassembled.

Figure 4-3 illustrates how a packet-switched network operates. The routers in the Internet are linked together in a web. The packets follow the path of least resistance to ensure they arrive at their destination in a reasonable amount of time. It seems logical that the packets would go through the least number of routers to get to its destination. However, sometimes that isn’t feasible, because there may be congestion clogging the ideal path. Routers send the traffic around the congested portions of the Internet for increased speed and efficiency.

 

This may seem like a very complicated system—as compared to the process followed when placing a telephone call—but the system works for two important reasons:

The network can balance the load across different pieces of equipment on amillisecond-by-millisecond basis.

If there is a problem with one piece of equipment in the network while amessage is being transmitted, packets can be routed around the problem to ensure that the entire message is received.

The routers that make up the main back bone of the Internet can reconfigure the paths that packets take because they look at all the information surroundng the data packet, and they tell each other about line conditions, like problems sending and receiving data on various parts of the Internet.

All Shapes and Sizes

Not every router is responsible for the fate of packets whizzing across the Internet. Routers come in different sizes and do more or less, depending on how big and sophisticated they are. For instance:

▼ If you have enabled Internet Connection Sharing between two Windows XP-based computers, the computer that is connected to the Internet is acting as a simple router. This router does very little—it just looks at data to see which computer it’s meant for.

■ Routers that are used to connect small offices to the Internet do more. They enforce rules about security for the office LAN, and they generally handle enough traffic that they tend to be stand-alone devices.

▲ The biggest routers (the ones used to handle data at the major traffic points on the Internet) handle a lot of information—millions of packets each second. These are stand-alone devices that look more like Maytag made them than a computer company.

Let’s consider the medium-sized router—it’s probably something humming away in a small room at your business. This router only has two networks to deal with—your LAN and the Internet. The office LAN connects to the router via an Ethernet connection. The router might also have two connections to your company’s ISP—a T3 and an ISDN connection. For the most part, your traffic comes and goes via the T3 line. However, the ISDN line is used in the event something goes awry with the T3 line. In fact, the router is configured to send data across the ISDN line, because the configuration table has been programmed to switch over in case of an emergency.

This router is also tasked with another function—it’s a layer of security against outside attacks. Although firewalls are routinely used to prevent attacks, the router is also configured to keep the bad people out.

However, the backbone of the Internet uses the third kind of router we listed. Cisco’s Gigabit Switch Router 12000 Series of routers is the kind of equipment used to run the Internet. These routers are designed and built like supercomputers. For instance, the 12000 Series uses 200 MHz MIPS R5000 processors, which are the same kind of processors used in the computers that make special effects for the movies. Cisco’s largest router—the 12816—can handle up to 1.28 trillion bits of information per second.

This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today.  Buy this book now.

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