0 Connection Maintenance Phase: The con-
`nection maintenance mechanism ensures that a
`satisfactory performance is delivered to the user.
`This process requires the use of flow control mech-
`anisms to ensure continuous playout at the client
`station. It ensures timely delivery of video frames
`to the client by monitoring a control signal re-
`turned via the TCP connection. The system thus
`adapts itself to load changes that can affect con-
`tinuous media playout.
`
`After connection establishment, video data is de-
`livered to the client via a dedicated UDP connection.
`
`The choice of UDP was based on the need to sup-
`port data delivery in real-time. Even though TCP is
`a more reliable protocol, it can introduce delays due
`to retransmission of lost packets, which is not accept-
`able for real-time video communication. In addition,
`video data are tolerant to losses and we exploit this
`characteristic in our design.
`
`5 Summary and Conclusion
`
`In this paper we have presented a distributed
`database organization for multimedia information sys-
`tems based on the use of metadata and resource
`servers. We have identified and illustrated the service
`
`primitives required for the management of continuous-
`media sessions in a distributed system. The con-
`cepts presented are illustrated by example of a proto-
`type distributed application called the Virtual Video
`Browser. The VVB employs a client-server architec-
`ture to support browsing of a metadatabase of video
`databases and to set up video sessions to remote video
`servers.
`
`The VVB employs a two phase process to serve its
`users. These are (1) a query phase during which user
`queries are sent to the metadata server and (2) a con-
`nection establishment and maintenance phase, during
`which connections are set up between the video server
`and the client machine. The VVB is currently in use
`on a testbed of Unix workstations interconnected via
`
`Ethernet. To be used as a viable alternative to ap-
`plications such as cou1'ses—on-demand, the VVB needs
`to support a large number of users and a rich data
`archive. We are currently considering the effort re-
`quired for such a scale—up.
`
`Acknowledgments
`
`0 Query Phase: The query phase involves the
`querying of the QDB to get all information re-
`quired by tl1e client to obtain a selected movie or
`scene. The query mechanism is implemented us-
`ing a remote procedure call (RPC) between the
`video-playout client and the QDB.6 The user be-
`gins the query by selecting a category from the
`Category Screen. The client-system then queries
`the remote database (the QDB) for movies be-
`longing to that particular category. The user can
`get more detailed information of the movies in-
`cluding a summary, poster, or textual description
`of a scene by further querying the QDB for movie-
`specific data,
`
`'”
`The Virtual Video Browser (VVB) Interface
`
`"I
`
`t
`__i__
`
`Query for Movie
`Playoui Mechanis
`Playout Controller
`
`
`
`
`RPC
`
`
`
`Video DB Server ._,
`
`Connection Setup
`& Flow Control.
`
`HR
`
`FC
`
`LO
`
`u ery DB Sewer
`
`iQ
`
`uery Database
`
`Figure 6: Mechanics of the VVB Operation
`
`After the query phase, a movie can be selected for
`playout. The video playout phase is divided into two
`parts;
`the connection setup phase, and the connec-
`tion management phase. During connection setup, the
`client checks with the VDB to see if it can support
`a new connection.
`If a connection can be set up, a
`negotiation phase is initiated during which QOS pa-
`rameters are negotiated.
`If the negotiation proceeds
`to a successful conclusion, the connection setup and
`maintenance phases are initiated.
`
`0 Connection Setup Phase: The connection-
`setup phase establishes a dedicated client-server
`UDP connection to facilitate continuous media
`
`data transfer. Synchronized flow control is satis-
`fied by a separate dedicated TCP control channel.
`
`GPOSTGRES is the DBMS used in the VVB implementation
`of the QDB.
`
`The development of the VVB prototype, including
`data collection, was made possible by the significant
`
`17
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`

`
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`
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