It is often helpful to connect different LANs. For example, two different
departments in a large business may each have its own LAN, but if there is
enough need for data communication between the departments, then it may be
necessary to create a link between the two LANs.
Each LAN is governed by a protocol, which is a set of rules and formats for
sending and receiving data. If two LANs are built around the same communication
rules, then they can be connected with a bridge or a router. A
bridge is a relatively simple device that looks at the information in each
packet header and rebroadcasts data that is traveling from one LAN to
another. A router is a more complicated device that stores the
addressing information of each computer on each LAN and uses this
information to act like an electronic post office, sorting data and sending it
along the most expedient route to its destination. Bridges forward data from one
network to another but are not suitable in many large organizations because of
the amount of packets bridges send to all networks connected to the bridge.
Routers, on the other hand, send packets only to the desired network, reducing
overall net-work traffic.
In some cases, a router can also be used to connect two different types of
LANs. However, a router only "routes" data: it knows how to send it to the
correct location, and that's all. If you need a more sophisticated connection
between networks, you need a gateway, a computer system that connects two
networks and translates information from one to the other. Packets from
different networks have different kinds of information in their headers, and the
information can be in different formats. The gateway can take a packet from one
type of network, read the header, and then encapsulate the entire packet into a
new one, adding a header that is understood by the second network, as shown in
Figure 7.8.
Wide Area Networks
Typically, a wide area network (WAN) is two or more LANs connected together,
generally across a wide geographical area using high speed or dedicated
telephone lines. For example, a company may have its corporate headquarters and
manufacturing facility in one city and its marketing office in another. Each
site needs resources, data, and programs locally, but it also needs to share
data with the other site. To accomplish this feat of data communication, the
company can attach a router to each LAN to create a WAN. Figure 7.9 shows a
typical WAN connecting two LANs.
The Internet is the ultimate WAN because it connects many thousands of
computers and LANs around the world. Most of the commercial online ser-vices and
large bulletin boards were not WANs when they started out because, typically,
users dialed in to a single computer or a group of computers housed at a single
site. (Commercial online services are businesses you connect to via modem that
offer their own internal products and services, one of which may be Internet
access.) However, today most of these systems provide connections to other
specialized services and to the Internet, so they are now more like WANs.
File Server Networks
Describing a network as a LAN or a WAN gives a sense of the physical area
the network covers. However, this classification does not tell you anything
about how individual computers on a network, called nodes, interact with other
computers on the network.
Many networks include not only nodes but also a central computer with a large
hard disk that is used for shared storage. This computer is known as the file
server, network server, or simply, server. Files and programs used by more than
one user (at different nodes) are generally kept on the server.
One relatively simple implementation of a network with nodes and a file
server is a file server network. This is a hierarchical arrangement in which
each node can have access to the files on the server but not necessarily to
files on other nodes. When a node needs information on the server, it requests
the entire file containing the information. In other words, the file server is
used simply to store and forward (send) files (see Figure 7.10).
Client/Server Networks
One popular type of server based network is client/server computing, a
hierarchical strategy in which individual computers share the processing and
storage workload with a central server. This type of arrangement requires
specialized software for both the individual node and the network server. It
does not, however, require any specific type of network. Client/ server software
can be used on LANs or WANs and a single client/server pro-gram can be used on a
LAN where all the other software is based on a simple file server relationship.
The most common example of client/server computing involves a database that
can be accessed by many different computers on the network. The data-base is
stored on the network server. Also stored on the server is the server portion of
the database management system (DBMS), the program that allows users to add
information to, or extract it from, the database. The user's computer (which can
be called the node, workstation, or client) stores and runs the client portion
of the DBMS.
Now, suppose that the user wants information from the database. For example,
suppose that the database is a list of customer purchases, and the user needs to
know the names of customers in the Wichita area who made purchases of more than
$500. The user uses the client software to describe the information that is
needed and sends the request to the server. The server soft-ware searches the
database, collects the relevant customer names, and sends them back to the
client. The client software then presents the information to the user in a way
that makes sense. This process is shown in Figure 7.11.
Client/server software is valuable to large, modern organizations because it
distributes processing and storage workloads among resources efficiently. This
means that users get the information they need faster.
Client/server computing is also a commonly used model on the Internet. Users
typically have client software that provides an easily used interface for
interacting with this giant WAN. Other types of processing, such as receiving,
storing, and sending e-mail messages, are carried out by remote computers
running the server part of the relevant software.
Peer-to-Peer Computing
A third arrangement is a peer-to-peer network, in which all nodes on the
network have equal relationships to all others, and all have similar types of
software. Typically, each node has access to at least some of the resources on
all other nodes, so the relationship is nonhierarchical. If they are set up
correctly, Windows 95 and its predecessor, Windows for Workgroups, give users
access to the hard disks and printers attached to other computers in the
net-work. A peer-to-peer network is shown in Figure 7.12.
In addition, some high-end peer-to-peer networks allow distributed computing,
which enables users to draw on the processing power of other computers in the
network. That means people can transfer tasks that take a lot of CPU power--such
as creating computer software--to available computers, leaving their own
machines free for other work.
Peer-to-peer LANs are commonly set up in small organizations (fewer than 50
employees) or in schools, where the primary benefit of a network is shared
storage, printers, and enhanced communication. Where large data-bases are used,
LANs are more likely to include client/server relationships.
A peer-to-peer network can also include a network server. In this case, a
peer-to-peer LAN is similar to a file server network. The only difference
between them is that the peer-to-peer network gives users greater access to the
other nodes than a file server network does (see Figure 7.12).
