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Data & Network
Communications/Connections

Devices, Transmission Modes, Circuits & Software

Communications processors=minicomputers

. perform message switching
. located between computer and modem
. nonprogrammable

Multiplexors=switching devices

. route/channel flow of data
. intermix two way flow so can flow over one line

Concentrator=programmable; more expensive than multiplexor

Modem (modulator demodulator)=hardware device

. converts digital signals from terminals and CPU into analog signals for transmission
. receiving modem converts back to digital
. baud rates=speed data transmitted; one baud=one bit per second)

Communications channels=communications media for transmitting data

. narrowband=telegraph
. voiceband=telephone
. broadband=microwave circuits, satellite channels
. baseband network=type of LAN used for data communications

Transmission modes - asynchronous/start-stop transmission

. used for slow transmissions - keyboard terminal
. each character marked by start and stop code

Transmission modes - synchronous transmission

. transfers blocks of characters
. no start and stop codes
. modems must be synchronized

Transmission circuits

. simplex=one direction only - ex, display
. half duplex=both directions but not at same time
. full duplex=both directions at once; real time processing

Communications software

. complex
. receives input,
. locates program & loads into memory,
. transmits input to program,
. passes output to user,
. identifies and corrects errors
. logs activity
. manages buffers
. manages sequencing and proper routing of messages
. protocol=set of rules for transmitting messages

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