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is responsible for making connections This can be something as simple as software running on a PC or on a dedicated hardware appliance like an IP phone or voice conferencing station One requirement of all H323 terminals is that they must support voice communications other types of communications, like video or data, are optional An H323 gatekeeper is a central point for all multimedia calls and provides call control services to the terminals that register with it Its two main functions are to perform address translation (can be NAT as well as telephone number to IP address translation) and bandwidth management Note that the gatekeeper is not necessary to set up connections directly between two terminals if the two terminals wishing to communicate know each other s addressing information, they can set up the connection directly This is different from Skinny and SIP As I just mentioned, a gatekeeper is unnecessary However, a gatekeeper does make it easier to deploy multimedia services on a large scale A gatekeeper is the central repository for addressing information terminals register their addressing information with the gatekeeper, and the gatekeeper gives this information to querying terminals In this sense, it functions something like a hybrid PBX/DNS server H225 defines the RAS (Registration, Admission, and Status) protocol that the terminals and gatekeepers use to communicate with each other

are particularly useful because they permit operations on the database to be made available to external applications, without exposing any of the internal details of the database 9 covers procedural extensions of SQL as well as many other new features of SQL:1999

There are three basic ways that a connection can be made between two terminals: A terminal can contact a gatekeeper for address translation information and then set up the connection directly to the destination terminal this requires both terminals to be registered with the gatekeeper A terminal can contact a gatekeeper and have the gatekeeper handle the call signaling and control information between the two terminals this requires both terminals to be registered with the gatekeeper A terminal, knowing the destination terminal address, can set up the connection directly to the destination without the assistance of a gatekeeper

415 Summary

The next few sections will cover the interaction of the terminal with the gatekeeper as well as between the two terminals

.

Commercial database systems do not use the terse, formal query languages covered in 3 The widely used SQL language, which we studied in this chapter, is based on the formal relational algebra, but includes much syntactic sugar SQL includes a variety of language constructs for queries on the database All the relational-algebra operations, including the extended relational-algebra operations, can be expressed by SQL SQL also allows ordering of query results by sorting on speci ed attributes View relations can be de ned as relations containing the result of queries Views are useful for hiding unneeded information, and for collecting together information from more than one relation into a single view Temporary views de ned by using the with clause are also useful for breaking up complex queries into smaller and easier-to-understand parts SQL provides constructs for updating, inserting, and deleting information A transaction consists of a sequence of operations, which must appear to be atomic That is, all the operations are carried out successfully, or none is carried out In practice, if a transaction cannot complete successfully, any partial actions it carried out are undone Modi cations to the database may lead to the generation of null values in tuples We discussed how nulls can be introduced, and how the SQL query language handles queries on relations containing null values The SQL data de nition language is used to create relations with speci ed schemas The SQL DDL supports a number of types including date and time types Further details on the SQL DDL, in particular its support for integrity constraints, appear in 6 SQL queries can be invoked from host languages, via embedded and dynamic SQL The ODBC and JDBC standards de ne application program interfaces to access SQL databases from C and Java language programs Increasingly, programmers use these APIs to access databases We also saw a brief overview of some advanced features of SQL, such as procedural extensions, catalogs, schemas and stored procedures.