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1. What is a data dictionary and what entries does it have?
The data dictionary refers to the dataData items, data structures, data flows, data storage, processing logic, external entities, etc.Define and describe, the purpose of which is to make a detailed description of each element in the data flow diagram.
A data dictionary is a user-accessible directory that records database and application source data.
Active data dictionary refers to the data dictionary whose content can be automatically updated by the DBMS when the database or application structure is modified.
Passive data dictionary refers to the data dictionary whose content must be manually updated when it is modified.
The composition of the data dictionary:
1. Data item
2. Data structure
3. Data flow
4. Data storage
5. Processing process
The content of the data dictionary includes:
1. Information about all schema objects in the database, such as tables, views, clusters, and indexes.
2. How much space is allocated, how much space is currently used, etc.
3. The default value of the column.
4. Constrain the integrity of information.
5. The user's name.
6. Permissions granted to users and roles.
7. Audit information on user access or use.
8. Other generated database information.
2. What is a channel multiplexing system, and what are its advantages and disadvantages?
Frequency reuse means that the same frequency is used simultaneously by several cells that are far enough apart.
The advantage is that it can increase the system capacity;
the disadvantage is that it will bring frequency interference.