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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