Before discussing sites and replication, you need to understand some basic physical and network concepts.
1. The Three Types of Network
When designing networks, systems and network
administrators use the following terms to define the types of
connectivity between locations and servers:
Local area networks (LANs)
A local area network (LAN)
is usually characterized as a high-bandwidth network. Generally, an
organization owns all of its LAN network hardware and software.
Ethernet is by far the most common networking standard. Ethernet speeds
are generally at least 10Mbps and can scale to multiple gigabits per
second. Currently, the standard for Ethernet is the 10 Gigabit
Ethernet, which runs at 10 times the speed of Gigabit Ethernet (1Gbps).
Several LAN technologies, including routing and switching, are
available to segment LANs and to reduce contention for network
resources.
Wide area networks (WANs)
The purpose of a wide area network (WAN)
is similar to that of a LAN—to connect network devices together. Unlike
LANs, however, WANs are usually leased from third-party
telecommunications carriers and Internet service providers (ISPs).
Although extremely high-speed WAN connections are available, they are
generally costly for organizations to implement through a distributed
environment. Therefore, WAN connections are characterized by
lower-speed connections and, sometimes, nonpersistent connections.
The Internet
If you have not heard of the Internet, you must
have been locked away in a server room (without network access) for a
long time. The Internet is a worldwide public network infrastructure
based on the Internet Protocol (IP).
Access to the Internet is available through organizations known as
ISPs. Because it is a public network, there is no single "owner" of the
Internet. Instead, large network and telecommunications providers
constantly upgrade the infrastructure of this network to meet growing
demands.
Organizations now use the Internet regularly. For
example, it's rare nowadays to see advertisements that don't direct you
to one website or another. Through the use of technologies such as
Virtual Private Networks (VPNs), organizations can use encryption and
authentication technology to enable secure communications across the
Internet.
2. Exploring Network Constraints
In an ideal situation, a high-speed network would
connect all computers and networking devices. In such a situation, you
would be able to ensure that any user of your network, regardless of
location, would be able to quickly and easily access resources. When
you are working in the real world, however, you have many other
constraints to keep in mind, including network bandwidth and network
cost.
2.1. Network Bandwidth
Network bandwidth generally refers to the amount of
data that can pass through a specific connection in a given amount of
time. For example, in a WAN situation, a T1 may have 1.544Mbps
(megabits per second), or a standard analog modem may have a bandwidth
of 56 or 57.6Kbps (kilobits per second) or less. However, your LAN's
Ethernet connection may have a bandwidth of 100Mbps. Different types of
networks work at different speeds; therefore, it's imperative that you
always consider network bandwidth when you're thinking of how to deploy
domain controllers in your environment.
2.2. Network Cost
Cost is perhaps the single biggest factor in
determining a network design. If cost were not a constraint,
organizations would clearly choose to use high-bandwidth connections
for all of their sites. Realistically, trade-offs in performance must
be made for the sake of affordability. Some of the factors that can
affect the cost of networking include the distance between networks and
the types of technology available at locations throughout the world. In
remote or less-developed locations, you may not even be able to get
access through an ISP or Telecom beyond a satellite connection or
dial-up, and what is available can be quite costly. Network designers
must keep these factors in mind, and often they must settle for
less-than-ideal connectivity.
Before we considered the monetary value of doing
business, let's consider another definition of cost. When designing and
configuring networks, you can require certain devices to automatically
make data transport decisions based on an assigned network cost. These
devices are commonly known as routers, and they use routing protocols
to make routing decisions. One of the elements a router uses to
configure a routing protocol is its ability to adjust the cost of a
route. For example, a router might have multiple ways to connect to a
remote site, and it may have multiple interfaces connected to it, each
with different paths out of the network to which it is connected
locally. When two or more routes are available, you can set up a
routing protocol that states that the route with the lower cost is
automatically used first.
Another cost is personnel. Do you have the personnel
to do the job or do you need to hire a consultant? Remember that even
if you use individuals already on staff, they will be spending time on
these projects. When your IT team is working on a project, that is a
cost because they cannot also be working on day-to-day tasks.
All of these factors play an important role when you make your Active Directory implementation decisions.
