Definition of Announcement in Network Encyclopedia.
What is Announcement
Announcement enables Microsoft NetShow clients to receive streaming multimedia information. Announcements, which are used only with Advanced Streaming Format (ASF) transmissions, are text files that have the extension .asx.

How it works
The function of an announcement depends on whether the transmission method is multicasting or unicasting. If the transmission method is multicasting, announcements enable NetShow clients to retrieve the channel file containing channel information.
If the transmission method is unicasting, announcements supply the client with information on how to connect to the NetShow server.
Once created, an announcement can be distributed to clients by several means: over the Web, through e-mail, or on a network share.