If your company is composed of offices that are all connected by high-bandwidth and 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 servers that are running Exchange are located and managed in a central data center. When planning your messaging system, it is best to start by considering this model because it is the most cost-effective and easily managed.

If your company contains remote offices with low-bandwidth, high-latency, or unreliable network connections, you can introduce servers 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 2007, and Microsoft Office Outlook 2007, you can also consolidate your server hardware by removing servers that are running Exchange from remote sites. With these changes, users can log on remotely to Microsoft Windows services and Exchange 2007 and experience fewer problems related to a decrease in performance or connectivity.

Service Level Management

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, warm site, or cold site contingencies. A hot site is a full-service commercial site that is up and running continuously with data replicated to it, so that it can be used immediately. A warm site is a full-service site that provides all the equipment needed for a company to continue operations if a disaster were to occur. However, the equipment is not ready for immediate use, and some administrative tasks are required to make the site user-ready. A cold site is a service that provides space, but it is a site that the company must furnish and set up. A hot site gets the company operational 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).

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   Compare the cost of installing high-end servers and clusters in the data center to the administrative cost savings of centralizing the servers. We recommend that you cluster the back-end servers to build high availability and redundancy into the system, but this choice does involve greater initial costs. 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 server and data resources across the organization, you increase the number of possible single points of failure. You must make contingency plans in the event a catastrophic event affects your data center.
  
  
• Network outages   Consider the impact that a network outage will have on users in remote locations. If the users have Cached Exchange Mode enabled in Outlook, this consideration is less of an issue.
  
  
• Operational and administrative cost reductions   Centralizing server resources can reduce operational costs because service capacity and growth are achieved by having 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. Additionally, by reducing the complexity of the server infrastructure, you can more easily restore services and data when a failure occurs.
  
  
• LAN and WAN connectivity   If your current network does not provide the type of bandwidth and speed required for centralizing servers, you have to build a network upgrade into the project plan.
  
  
• Security   A centralized model gives you easier security management, and therefore, more 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.
  
  

A branch office or distributed messaging deployment is one where many branch offices or smaller distributed sites have slow connections to a corporate hub or data center. The branches contain their own servers that are running Exchange, 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. Therefore, 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 position servers in the remote sites.

An Exchange branch office deployment has the following characteristics:

• The messaging system consists of many locations (branches), and each contains a server that is running Exchange, 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 for 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.
  
  

Consider the following issues when you think about a distributed design:

• Software upgrades   Rolling out important updates can be much more challenging in a distributed messaging system.
  
  
• Using Outlook Anywhere   If you want to use Outlook Anywhere (formerly RPC over HTTP), all computers in your messaging environment that users will have to use with Outlook Anywhere communication must be running Windows Server 2003. This requirement extends to all global catalog servers and all servers that are running Exchange that your Outlook users will access.
  
  
• Operational and administrative costs   Distributed messaging systems require more servers and cause higher operational and administrative costs.
  
  
• 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. Features such as local continuous replication (LCR) and cluster continuous replication (CCR) are especially useful in a distributed messaging environment. For more information about LCR, see Local Continuous Replication. For more information about CCR, see Cluster Continuous Replication.
  
  
• Network connections   For remote offices, we recommend that the network connection to the hub site or data center be no less than 64 kilobits per second (Kbps) between servers. However, we recommend a higher connection speed between a hub and an office.
  
  
• Security   The physical security of servers in branch offices is a major consideration. In a branch office design, you must take precautions to make sure that servers are not located in open areas and that the servers are physically secured.