Client/Server Network Model
Peer−to−peer model networks evolved into the client/server model, in which the server shares applications
and data storage with the clients in a somewhat more centralized network. This setup includes a little more
security, provided by the operating system, and ease of administration for the multiple users trying to access
data.
A LAN in this environment consists of a physical wire connecting the devices. In this model, LANs enable
multiple users in a relatively small geographical area to exchange files and messages, as well as to access
shared resources such as file servers and printers. The isolation of these LANs makes communication between
different offices or departments difficult, if not impossible. Duplication of resources means that the same
hardware and software have to be supplied to each office or department, along with separate support staff for
each individual LAN.
WANs soon developed to overcome the limitations of LANs. WANs can connect LANs across normal
telephone lines or other digital media (including satellites), thereby ignoring geographical limitations in
dispersing resources to network clients.
In a traditional LAN, many limitations directly impact network users. Almost anyone who has ever used a
shared network has had to contend with the other users of that network and experienced the impacts. These
effects include such things as slow network response times, making for poor network performance. They are
due to the nature of shared environments.
When collision rates increase, the usefulness of the bandwidth decreases. As applications begin having to
resend data due to excessive collisions, the amount of bandwidth used increases and the response time for
users increases. As the number of users increases, the number of requests for network resources rises, as well.
This increase boosts the amount of traffic on the physical network media and raises the number of data
collisions in the network. This is when you begin to receive more complaints from the network’s users
regarding response times and timeouts. These are all telltale signs that you need a switched Ethernet network.
Later in this chapter, we will talk more about monitoring networks and solutions to these problems. But before
we cover how to monitor, design, and upgrade your network, let’s look at the devices you will find in the
network.
The Pieces of Technology
In 1980, a group of vendors consisting of Digital Equipment Corporation (DEC), Intel, and Xerox created
what was known as the DIX standard. Ultimately, after a few modifications, it became the IEEE 802.3
standard. It is the 802.3 standard that most people associate with the term Ethernet.
The Ethernet networking technology was invented by Robert M. Metcalfe while he was working at the Xerox
Palo Alto Research Center in the early 1970s. It was originally designed to help support research on the
“office of the future.” At first, the network’s speed was limited to 3Mbps.
Ethernet is a multiaccess, packet−switched system with very democratic principles. The stations themselves
provide access to the network, and all devices on an Ethernet LAN can access the LAN at any time. Ethernet
signals are transmitted serially, one bit at a time, over a shared channel available to every attached station.
To reduce the likelihood of multiple stations transmitting at the same time, Ethernet LANs use a mechanism
known as Carrier Sense Multiple Access Collision Detection (CSMA/CD) to listen to the network and see if it
is in use. If a station has data to transmit, and the network is not in use, the station sends the data. If two
stations transmit at the same time, a collision occurs. The stations are notified of this event, and they instantly
reschedule their transmissions using a specially designed back−off algorithm. As part of this algorithm, each
station involved chooses a random time interval to schedule the retransmission of the frame. In effect, this
process keeps the stations from making transmission attempts at the same time and prevents a collision.
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