This document discusses centralized and distributed messaging systems. A centralized system hosts all server resources in a central data center, allowing for easier management and security. A distributed system places servers locally at branch offices due to unreliable network connections between sites. Consider centralizing for cost savings if network prerequisites are met, and plan contingencies for potential single points of failure. Distributed systems are suitable when network conditions cannot support traffic to a central hub.

1. Centralized vs. Distributed Messaging Systems
If your company is composed of offices all connected by high-bandwidth,
reliable network connections, regardless of the distance between offices, you can
implement a centralized messaging system. A centralized messaging system means
that all of your Exchange servers are located and managed in a central data center
and that you have a single routing group. When planning your messaging system, it
is best to start with this model in mind because it is the most cost-effective and
easily managed.
If your company contains remote offices with low-bandwidth, high-latency,
unreliable network connections, you can introduce routing groups to control how
messaging traffic is routed from one location to another. However, remote
locations and multiple routing groups do not prevent you from centralizing your
administrative model. In addition, with the features in Microsoft Windows
Server™ 2003, Exchange 2003, and Microsoft Office Outlook® 2003, you also
have the opportunity to consolidate your server hardware by removing Exchange
servers from remote sites. With these changes, users can log on remotely to
Microsoft Windows® services and Exchange 2003 and experience fewer problems
related to performance degradation or connectivity.
This topic discusses the characteristics of centralized and distributed
messaging systems and gives you some guidelines for planning each model.
Centralized computing is computing done at a central location, using terminals
that are attached to a central computer. The computer itself may control all the
peripherals directly (if they are physically connected to the central computer), or
they may be attached via a terminal server. Alternatively, if the terminals have the
capability, they may be able to connect to the central computer over the network.
The terminals may be text terminals or thin clients, for example.
It offers greater security over decentralized systems because all of the processing is
controlled in a central location. In addition, if one terminal breaks down, the user can
simply go to another terminal and log in again, and all of their files will still be
accessible. Depending on the system, they may even be able to resume their session
from the point they were at before, as if nothing had happened.
This type of arrangement does have some disadvantages. The central computer
performs the computing functions and controls the remote terminals. This type of
system relies totally on the central computer. Should the central computer crash, the
entire system will go down.
Characteristics of a Centralized Messaging System
A centralized messaging system consists of a large data center that hosts all
server resources, including the Active Directory® directory service global catalog
servers, domain controllers, and Exchange servers. The data center supports all
messaging system users, whether they connect locally or remotely. The following are
characteristics of a centralized messaging system:
• Data is hosted and managed in a centralized location regardless of whether
the users are connected remotely. This contrasts with the distributed model,
where users have local access to mailboxes but server administration is more
complex.
• Software upgrades can be rolled out from a centralized location.
• The data center incorporates power-insulating devices such as an
uninterruptible power supply (UPS) and "hot site" or "cold site" contingencies.
A hot site is a full-service commercial site that provides all of the equipment
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2. needed for a company to continue operations in the event of a disaster. A
cold site is a service that provides space but that the company must furnish
and set up. A hot site gets the company up and running faster, but a cold site
is a less expensive option.
Business requirements associated with reducing cost and security
requirements are usually the driving forces behind centralizing systems. The
requirements revolve around location centralization (reducing the number of sites
that provide server resources), physical consolidation (replacing smaller servers with
high-end servers), administrative consolidation, and data consolidation
(centralizing storage solutions that provide backup and disaster recovery
capabilities).
Important Considerations
Consider a centralized design only if prerequisites in the following areas are
already met or are included in the project plan:
• Data center hardware costs Weigh the cost of installing high-end servers
and clusters in the data center against the administrative cost savings of
centralizing the servers. It is recommended that you cluster the back-end
servers to build high availability and redundancy into the system, but this
involves greater costs up front. However, these costs may be more than
offset by reductions in operational costs, infrastructure costs, reduced
downtime, and greater scalability.
• Contingency planning When you centralize your server and data resources
across the organization, you increase the possible single points of failure. You
must formulate contingency plans in the event your data center is subjected
to a catastrophic event.
• Operational and administrative cost reductions Centralizing server
resources reduce operational costs because service capacity and growth are
achieved with better use of resources. It also reduces infrastructure costs
associated with storage and backup requirements.
• Data storage With larger centralized data volumes, you must use more
reliable storage systems to improve the integrity of your data. In addition, by
reducing the complexity of your server infrastructure, you can more readily
restore services and data when a failure occurs.
• Security A centralized model gives you easier security management, thus, a
greater degree of control. This control makes it easier for security staff to
maintain up-to-date virus signatures and take timely action in response to
security incidents. Another advantage of a centralized design is that it locates
your servers in a data center that you can physically secure.
Decentralized computing is a trend in modern day business environments. This is the
opposite of centralized computing, which was prevalent during the early days of
computers. Decentralized computing is the allocation of resources, both hardware and
software, to each individual workstation, or office location. In contrast, centralized
computing exists when the majority of functions are carried out, or obtained from a
remote centralized location.
A decentralized computer system has many benefits over a conventional centralized
network. Desktop computers have advanced so rapidly, that their potential performance
far exceeds the requirements of most business applications. This results in most desktop
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3. computers remaining idle (in relation to their full potential). A decentralized system can
utilize the potential of these systems to maximize efficiency. However, it is debatable
whether these networks increase overall effectiveness.
All computers have to be updated individually with new software, unlike a centralized
computer system. Decentralized systems still enable file sharing and all computers can
share peripherals such as printers and scanners as well as modems, allowing all the
computers in the network to connect to the internet.
A collection of decentralized computers systems are components of a larger computer
network, held together by local stations of equal importance and capability. These
systems are capable of running independently of each other.
Characteristics of a Distributed Messaging System
A branch office or distributed messaging deployment is one where numerous
branch offices or smaller distributed sites have slow connections to a
corporate hub or data center. The branches contain their own Exchange servers,
domain controllers, and global catalog servers. A distributed messaging system is
usually adopted when the network cannot handle traffic to a central hub for services,
so the operating system and messaging servers are placed locally. User
requirements may be another factor. If the requirements for user experience and
availability cannot be met by connecting to a data center, you may have no choice
but to place servers in the remote sites.
An Exchange branch office deployment has the following characteristics:
• The messaging system consists of a large number of locations (branches),
each containing an Exchange server, domain controllers, and at least one
global catalog server.
• The branch office locations usually contain a small or varying number of
users.
• The network is usually structured as a hub-and-spoke topology.
• The network connections between the branch office locations and the central
hub or data center are typically low-bandwidth, high-latency, or unreliable.
The main reasons behind deploying a distributed messaging system include
the following:
• The company's users are dispersed across sites.
• The company's network infrastructure cannot handle traffic to a central hub
for services.
• The user requirements dictate that a server be placed locally to provide
optimal user experience and availability.
Important Considerations
• Operational and administrative costs Distributed messaging systems
require more servers and result in higher operational and administrative
costs.
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4. • Data storage With distributed servers, the service infrastructure is more
complex, which makes it more difficult to restore services and data when a
failure occurs.
• Network connections For remote offices, it is recommended that the
network connection to the hub site or data center be no less than 56 Kbps
between servers. Between a hub and an office, however, a higher connection
speed is recommended.
• Security The physical security of servers in branch offices is a major
consideration. In a branch office design, you must take precautions to ensure
that servers are not located in open areas and that they are physically
secured.
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