This AD diagram example was redesigned from the picture "Site links" from the book "Active Directory for Dummies".
"Site links represent the Active Directory replication paths between sites.
These paths are manually defined so that the designer has control over which network links the replication traffic occurs on. These site links also control how clients are directed to domain controllers when there’s no DC in the client’s local site. Each site link has the following attributes:
(1) Connected sites: A site link is defined by the sites to which it connects. A site link can connect two or more sites together.
(2) Network transport: Site links support replication communication over IP-based RPCs or with the Simple Mail Transport Protocol (SMTP). You normally want to use RPC whenever possible, but you can use SMTP when the sites you’re linking don’t support RPC.
(3) Cost: Each site link has a cost associated with it. Costs are used to assign preferences to links that determine which link should be followed when multiple link paths are available between sites. The cost represents what it “costs” to use this site link relative to the other site links and affects replication traffic as well as how users are assigned a domain controller. Links with lower cost values have preference over links with higher cost values. Cost values range from 1–32,767; the default being 100.
(4) Frequency: The frequency value defines how often a replication occurs
when using this site link (the replication latency). You can configure the time between replications from a minimum of 15 minutes to a maximum of 10,080 minutes (one week). The default frequency is 180 minutes.
(5) Schedule: The schedule dictates when this link is active and available for replication between the sites. The schedule can also control which days of the week the link is available. Normally, the schedule is set so that the link is available 24 hours a day, but you can set up different schedules on a per-day-of-the-week basis.
By creating a site link, you enable two or more sites to be connected and to share the same site link attributes (transport, cost, frequency, and schedule). By default, site links create transitive connectivity between sites.
If you create a site link between sites A and B and another site link between
sites B and C, an automatic connection (known as a site link bridge) is created between sites A and C..." [Steve Clines and Marcia Loughry, Active Directory® For Dummies®, 2nd Edition. 2008]
The Active Directory diagram example "Site links" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Active Directory Diagrams solution from the Computer and Networks area of ConceptDraw Solution Park.