Exchange Server 2010 implementations have three layers in their architecture: a network layer, directory layer, and messaging layer. The messaging layer is where you define and deploy the Exchange Server roles. The Exchange servers at the core of the messaging layer can operate in the following roles:
• Mailbox Server. A back-end server that hosts mailboxes, public folders, and related messaging data, such as address lists, resource scheduling, and meeting items. For high availability of mailbox databases, you can use database availability groups.
• Client access Server. A middle-tier server that accepts connections to Exchange Server from a variety of clients. This server hosts the protocols used by all clients when checking messages. On the local network, Outlook MAPI clients are connected directly to the Client Access server to check mail. Remote users can check their mail over the Internet by using Outlook Anywhere, Outlook Web App, Exchange ActiveSync, POP3, or IMAP4.
• Unified Messaging Server. A middle-tier server that integrates a private branch exchange (PBX) system with Exchange Server 2010, allowing voice messages and faxes to be stored with e-mail in a user’s mailbox. Unified messaging supports call answering with automated greetings and message recording, fax receiving, and dial-in access. With dial-in access, users can use Outlook Voice Access to check voice mail, e-mail, and calendar information; to review or dial contacts; and to configure preferences and personal options. To receive faxes, you need an integrated solution from a Microsoft partner.
• Hub transport Server. A mail routing server that handles mail flow, routing, and delivery within the Exchange organization. This server processes all mail that is sent inside the organization before it is delivered to a mailbox in the organization or routed to users outside the organization. Processing ensures that senders and recipients are resolved and filtered as appropriate, content is filtered and has its format converted if necessary, and attachments are screened. To meet any regulatory or organizational compliance requirements, the Hub Transport server can also record, or journal, messages and add disclaimers to them.
• Edge transport Server. An additional mail routing server that routes mail into and out of the Exchange organization. This server is designed to be deployed in an organization’s perimeter network and is used to establish a secure boundary between the organization and the Internet. This server accepts mail coming into the organization from the Internet and from trusted servers in external organizations, processes the mail to protect against some types of spam messages and viruses, and routes all accepted messages to a Hub Transport server inside the organization.
These five roles are the building blocks of Exchange organizations. Processors can be single core, dual core, or multiple core. A dedicated Mailbox server has a recommended maximum number of processor cores of 12, but a server with the Mailbox and other roles combined has a recommended maximum of 16. Note that although Exchange Server 2010 can support this number of processor cores, it might make more sense to scale out to multiple servers rather than to scale up the processor cores on a single server.
Because you can combine all of the roles except the Edge Transport server role on a single server, one of the most basic Exchange organizations you can create is one that includes a single Exchange server that provides the Mailbox server, Client Access server, and Hub Transport server roles. These three roles are the minimum required for routing and delivering messages to both local and remote messaging clients. For added security and protection, you can deploy the Edge Transport server role in a perimeter network on one or more separate servers.
Although a basic implementation of Exchange Server might include only one server, you’ll likely find investing in multiple servers is more effective in terms of time, money, and resources. Why? High availability is integrated into the core architecture of Exchange Server 2010.
With the Mailbox server role, you can configure automatic failover by making the Mailbox servers members of the same database availability group. Each Mailbox server in the group can then have a copy of the mailbox databases from the other Mailbox servers in the group. Each mailbox database can have up to 16 copies, and this means you can have up to 16 Mailbox servers in a group as well.
With the Client Access role, you can enable load balancing and failover support by making Client Access servers members of the same Client Access array. Each Client Access server in the array will then be able to support all client access features, including Outlook MAPI, POP3, IMAP4, Outlook Anywhere, Outlook Web App, and Exchange ActiveSync. You can use Client Access arrays to build groups of up to 32 load-balanced servers, starting with as few a two servers and incrementally scaling as demand increases. Servers that are members of an array cannot also have the Mailbox role. If you are using the Network Load Balancing service, Microsoft recommends no more than eight load-balanced servers.
Because of the built-in, high-availability features, the hardware you use with Exchange Server 2010 might be very different from the hardware you use with earlier releases of Exchange Server.
Source of Information : Microsoft Press - Exchange Server 2010 Administrators Pocket Consultant
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