Learn about semijoin, relational calculus, SQL, Computer Networks, and Data Communication including types of networks and network topologies. Understand the basics of data transmission, modulation, multiplexing, and network performance. Explore various network topologies like star, ring, and bus networks.
01 / 89Comprehensive Study on Distributed Database Management Systems
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Comprehensive Study on Distributed Database Management Systems
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Tuple Relational Calculus • Based on first-order predicate logic • Expressed as { t | F(t)} • Where t is a tuple variable and F is well-formed formula BZUPAGES.COM
Atomic Formula • Tuple-variable membership expression: specified as R(t) or R.t • Conditions • s[A] Θ t[B] • S[A] Θ c BZUPAGES.COM
SQL • Language based on Tuple-oriented Calculs BZUPAGES.COM
Example • Select EMP.eName, DEP.dName from EMP, DEP where EMP.dNo = DEP.dNo BZUPAGES.COM
Domain Relational Calculus • Domain variable ranges over values in a domain and specifies a tuple BZUPAGES.COM
A query in DRC x1,x2,….xn|F(x1,….xn) where F is a wff and Xs are free variables • Implementation: QBE • Query by Example (QBE) is a method of creating database queries using examples based on a text string, the name of a document or a list of documents. The QBE system converts the user input into a formal database query. This approach allows the user to perform powerful searches without the need of having to learn a more formalized query mechanism such as Structured Query Language (SQL). BZUPAGES.COM
Computers are called Nodes, sites, hosts, in general node or host is meant the hardware and site means hw+sw • Other equipment at nodes is also possible like Printers, Disks etc. • Equipment connected via links and channels, link is a physical thing where as channel is logical one BZUPAGES.COM
Mode of Operation • Simplex: link operates in only one direction, like printers • Half Duplex: can transmit in both directions, but not simultaneously, link has to be “turned around” • Full Duplex: Simultaneously both ways BZUPAGES.COM
Performance of a Communication System • Bandwidth • Mode of Operation • Software employed • Redundancies within message • Headers and trailers with the message BZUPAGES.COM
Block Error Check Text Header • Source Address • Destination Address • Message Number • Packet Number • Acknowledgement • Control Information BZUPAGES.COM
Star • All communication via a central node • Excessive load on the central node • Disadvantage is that if the central node fails all network goes down BZUPAGES.COM
Ring Network Unidirectional Ring Ring Interface BZUPAGES.COM
Ring Network • Computers connected with transmission media in the form of loop • Each station also serves as a repeater, it repeats the signal that it receives • Control is generally managed via a Token BZUPAGES.COM
A token is circulated on the around the network, with certain bit pattern to indicate the network is free • Any site wanting to communicate grabs the token, sets it to busy and then sends the message • When communication is over, the site again sets token to free BP BZUPAGES.COM
Bus Topology • Common channel used to transmit and receive data • Link control is performed as • CSMA • CSMA/CD • In addition, token can also be used BZUPAGES.COM
CSMA/CD • Behave in CSMA, except that node keep listening to bus after even they have transmitted • The purpose is to detect if some collision has occurred • Collision occurs when multiple sites try to transmit at the same time BZUPAGES.COM
Meshed Network • Every computer connected with every other. • Gives maximum reliability, but is not practicable even for not a very large network BZUPAGES.COM
Point to Point • One or more links between sender and receiver • Link may be direct or via intermediate connections • Sender and receiver’s addresses are placed in the header of the message BZUPAGES.COM
Broadcast Networks • Common channel utilized by all nodes • Message received by all, ownership checked • Multicasting: message sent to a certain subset of nodes in nw • Generally Radio or Satellite based BZUPAGES.COM
In Satellite based, each site beams transmission to satellite • That beams it back at a different frequency • Broadcast Networks can also use Microwave that can be over Satellite or Terrestrial BZUPAGES.COM
Local Area NW • Metropolitan Area NW • Wide Area NW • Distinction between them is blurred, still they exist • Major categorization is probably Protocols, to be discussed next BZUPAGES.COM
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