Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 47

resulted in another MAN, which has been standardized as IEEE 802.16
and is popularly known as WiMAX.
2.2.2.4 Wide Area Networks
A WAN (Wide Area Network) spans a large geographical area, often a
country or continent. Each of these offices contains computers intended for
running user (i.e., application) programs. The rest of the network that con-
nects these hosts is then called the communication subnet, or just subnet for
short. The job of the subnet is to carry messages from host to host. In most
WANs, the subnet consists of two distinct components: transmission lines
and switching elements. Transmission lines move bits between machines.
They can be made of copper wire, optical fiber, or even radio links. Most
companies do not have transmission lines lying about, so instead, they lease
the lines from a telecommunications company. Switching elements, or just
switches or routers, are specialized computers that connect two or more
transmission lines. When data arrive on an incoming line, the switching ele-
ment must choose an outgoing line on which to forward them.
The WAN, as we have described it, looks similar to a large wired
LAN, but there are some important differences that go beyond
long wires. Usually in a WAN, the hosts and subnet are owned
and operated by different people. Second, the routers will usu-
ally connect different kinds of networking technology. Lastly, subnet(s)
can be entire LANs themselves. This means that many WANs will in
fact be Internetworks or composite networks that are made up of more
than one network.
Rather than lease dedicated transmission lines, a company might connect
its offices to the Internet. Virtual Private Network (VPN) allows connections
to be made between the offices as virtual links that use the underlying capac-
ity of the Internet. Compared to the dedicated arrangement, a VPN has the
usual advantage of virtualization, which is that it provides flexible reuse of
a resource (Internet connectivity), and the usual disadvantage of virtualiza-
tion, which is a lack of control over the underlying resources.
2.2.3 Network Models
Now that we have discussed layered networks in the abstract, it is time to
look at some examples. We will discuss two important network architectures:
the OSI reference model and the TCP/IP reference model. Although the pro-
tocols associated with the OSI model are not used any more, the model itself