A Service Delivery Location (SDL) refers to a physical location where Exchange and other servers reside. The network at and throughout an SDL must provide sufficient bandwidth and reliability to support Exchange, directory services, and other applications. An SDL must provide all dependent services that Exchange requires through resources deployed locally at and throughout the SDL. The minimum set of dependent services includes:

• Network availability, usually in the form of a local area network (LAN).
  
  
• Name resolution using Domain Name System (DNS).
  
  
• Directory services using Active Directory domain controllers and global catalog servers.
  
  

Other services that an SDL may provide include public external network connectivity (Internet) and perimeter network isolation, although these services are implementation specific and not a requirement of every SDL.

An SDL may be comprised of one or more subnets and may contain one or more Active Directory sites. SDLs correspond to a single physical building or campus environment with a common backbone network with LAN or greater speed and the appropriate level of redundancy. SDLs are always separated from one another by a wide area network (WAN) link.

A Client Service Location (CSL) refers to any location from which Exchange services may be accessed by a collective group of clients. A CSL may be co-located with an SDL on a common LAN, reside on a LAN physically separated from the SDL over a WAN, MAN, or Internet link; or refer to devices that use a common client access protocol over a public network. Examples of clients include Microsoft Office Outlook 2007, Outlook Web Access, Outlook Voice Access, Post Office Protocol 3 (POP3), and Internet Message Access Protocol 4 (IMAP4) clients, such as Outlook Express and Microsoft Exchange ActiveSync.

A logical topology for Exchange 2007 maps groupings of resources together to provide scoping for either features or security. Logical topologies help map resources closer to your business model.

With Exchange 2007, the logical topology refers to Active Directory forest design. Possible logical topologies for Exchange 2007 include single forest, multiple forest, and dedicated Exchange resource forest deployments.

For more information about Exchange 2007 logical topologies, see Logical Topologies.

A physical topology for Exchange 2007 maps physical elements to geographical locations. A physical topology is typically used to describe a network or the location of servers. Possible physical topologies for Exchange 2007 include single server, multiple server, and multiple site deployments. Physical topologies also frequently classify the distribution of servers and management roles into two primary categories: centralized servers and administration, and distributed servers and administration.

For more information about Exchange 2007 physical topologies, see Physical Topologies.

Exchange 2007 is an enterprise messaging system, and it requires three basic elements: directory services, transport mechanisms, and storage. Specifically, Exchange requires and uses Active Directory for directory services, various TCP/IP-based protocols for transport—such as the Simple Mail Transfer Protocol (SMTP) and the Hypertext Transfer Protocol (HTTP)—and the Extensible Storage Engine (ESE), which is built into Exchange, for storage.

Although Exchange has many components that make up an Exchange topology, all components that are required by Exchange can be categorized into three layers that combine to form an Exchange topology:

• Network layer
  
  
• Active Directory layer
  
  
• Exchange layer
  
  

From the perspective of an Exchange topology, each of these layers can be described in terms of both a logical topology and a physical topology. For more information about Exchange topologies, see Organization Topologies.