The backbone of any client/server system is the network on which it is installed.
It is the network that moves data from the server to the user.
The network might be a 10Mbps Ethernet or it might be the Internet itself.
Although network design is well beyond the scope of this book, the Visual Basic
client/server developer should have a grasp of how his or her data is moved in order to properly model the
application.
The expression network topology refers to how the network is physically laid out.
In contrast, the expression network architecture refers to how the network is
implemented on that topology. In the following sections, I briefly discuss the more common topologies and
architectures.
The star network topology implies a network where all computers are connected to a hub
(a hub is a box similar to a telephone switchboard containing minimal intelligence).
Multiple hubs are connected to one another. Star networks are simple to maintain but
offer minimal security constraints.
If a hub goes down, the entire network goes down.
In a ring network, all computers are connected via a continuous cable.
A multiplexing unit monitors the network, allowing only one packet to circle the
network at a time.
The packet is continually sent from computer to computer.
Each computer examines the packet to see if it is addressed to that workstation.
If a computer needs to send information, it waits for the packet and attaches a
"request" to transmit.
The multiplexing unit gives permission to each workstation in turn in a procedure that
is akin to a parliament (giving each "member" a turn to speak but also allowing for higher
priority speeches such as error messages).
If the packet is accidentally destroyed, the multiplexer recreates it.
IBM’s Token Ring architecture runs on a ring technology.
It has the advantage of being very reliable, but because of the extra overhead of
packet monitoring, it is somewhat expensive to maintain and does not scale to a large number of users.
The bus topology connects computers in a single line of cable.
Although each computer is typically connected to a hub, the internal wiring of the hub
still connects each computer serially.
When a workstation wants to send a packet of information, it "listens" for
the line to be clear through a process known as electronic sensing.
Only one packet can travel on the network at a time, and if two computers happen to
transmit at the same time, a packet collision occurs.
Each workstation listens for such collisions and, when they are detected, waits a
random amount of time and then resends the packet.
The most common implementation of bus technology is Ethernet.
Although fairly simple and inexpensive to install and maintain, the network is
vulnerable to any breaks in the cable.
Further, as more users are added to the network, performance can degrade drastically.
Still, Ethernet is by far the most common architecture.
To maintain satisfactory performance, network engineers break a large Ethernet network
into smaller LANs connected via bridges.