In the mid-1990s, network switches became prevalent, and networks began to flatten out (i.e., include more devices per broadcast domain). A switch differs from a shared hub in one significant way: A server or workstation connected to a switch port has available to it all bandwidth on that port. In other words, a 100Mbps Ethernet switch provides the full 100Mbps of bandwidth to each device connected to each port on the switch; media is no longer shared. Thus a switch allows for the flattening of a broadcast domain to include many more devices, which translates into fewer routers and more switches on a typical LAN.
Switches usually can move traffic around a network at much greater speeds than routers can because switches operate only at Layer 2—the data link layer of the OSI model. Because switches don't need to make higher-layer routing decisions and maintain complicated routing tables, they can move packets quickly.
If you implement a switched network in today's networking environment, you'll likely use a variety of bandwidths to accommodate your needs. Ethernet is the most common Layer 2 protocol, and it delivers several speeds, including 1Gbps, 100Mbps, and the venerable 10Mbps. (Some vendors are working on a 10Gbps Ethernet standard.) Each of these bandwidths is available on various physical media, including traditional copper cable and fiber optic cable. In general, fiber optic cable can carry higher bandwidths over greater distances than copper cable can, so that fact might drive some of your choices. You might ask, "Why wouldn't I implement 1Gbps Ethernet everywhere?" The most obvious answer is cost: The more bandwidth you deploy, the higher the cost. For that reason, my rule of thumb is to deploy only the bandwidth that I think I'll need today, while allowing for some growth for tomorrow. Most network hardware has a useful life of about 3 to 5 years, so you should plan for your needs for at least that long.
Servers typically need more bandwidth than individual workstations because servers must satisfy requests from hundreds, if not thousands, of workstations. Nowadays, it's not uncommon to find server segments using switched Gigabit Ethernet to each server, with switched 100Mbps probably the bare minimum you should consider.
You also need to consider how much bandwidth to provide to your desktop systems. Given that large organizations might have hundreds or thousands of desktops, providing Gigabit Ethernet to the desktop might be prohibitively expensive. A good idea is to keep an eye on network usage to determine who your biggest bandwidth consumers are. You might find that the graphics department needs 100Mbps for every desktop, whereas your call center users might be just fine with dedicated 10Mbps.
Whatever your choice of bandwidth, make sure that the hardware you choose—whether switch, router, or shared hub—provides you with opportunities for expansion without requiring you to throw out the device when you upgrade your bandwidth. Most medium- and high-end switches are organized with removable cards in an expandable chassis, so when you need to upgrade your server farm to Gigabit Ethernet, you can pop out that 100Mbps card and put in the faster version without a lot of expense and headaches.
Virtual LANs
Even though a broadcast domain on a typical switch can contain more than 500 devices, you might find it beneficial to segment switched traffic the same way you can segment routed traffic. Most intelligent switches support the concept of Virtual LANs (VLANs). A VLAN is simply a way of defining a routing boundary within a switch device. Typically, you specify a set of ports on one switch to be part of one VLAN and another set of ports on the same switch or on a different switch to be part of another VLAN. In effect, you're creating a routing boundary between these two groups of switch ports—a boundary that functions as if you had put a router between the two groups. In this case, however, the switch performs the routing between the two groups of devices and creates two separate broadcast domains. VLANs let you segment your network without having to deploy costly routers in addition to your switches.
WANs
Let's look now at using WANs for internal networks. As Figure 2 shows, you can deploy an internal WAN to connect disparate locations. Some organizations have offices spread over the country, if not the world. For example, many large banks have vast branch-office networks, with thousands of locations that contain servers and workstations that are part of the organization's internal networks. You typically have two ways to build such an internal network.
The first and most common way is to build a private WAN by using your own or a third-party carrier network. Large telcos such as AT&T, MCI, and Sprint provide private frame relay networks that let you efficiently and cost-effectively extend your private IP network to many locations. Frame relay is a common Layer 2 WAN protocol that provides a network cloud that lets you serve many locations at once, as Figure 4 shows. Deploying a frame-relay network or similar private WAN is like extending your internal network to all your organization's locations. The private WAN typically has no contact with the Internet, so if users in your branch offices need to get to the Internet, they must come through the frame relay cloud to use the Internet Point of Presence (POP) at your headquarters.
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nhit_whit July 22, 2004 (Article Rating: