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Physical Layer

* Digital transmission can be either parallel or serial in mode.
* In parallel transmission, a group of bits is sent simultaneously with each bit on a separate line.
* In serial transmission, there is only one line and the bits are sent sequentially.
* Serial transmission can be either synchronous or asynchronous.
* In asynchronous serial transmission, each byte is framed with a start bit and a stop bit. There may. be a variable-length gap between each byte.
* In synchronous serial transmission, bits are sent in a continuous stream without start and stop bits and without gaps between bytes. * Regrouping the bits into meaningful bytes is responsibility of the receiver.
* A DTE is.. source or destination for binary digital data.
* A DCE receives data from a DTE and changes it into a form appropriate for network transmission. It can also perform the reverse transformation.
* A modem is a DCE that modulates and demodulates signal.
* A modem changes digital signal to analog signals using ASK, FSK, PSK or QAM modulation.
* A regular telephone line uses frequencies between 600 Hz to 3000 Hz for data communication. This requires a bandwidth of 2400 Hz.
* ASK modulation is especially susceptible to noise.
* Signal travel from transmitter to receiver via a
* Multiplexing is the simultaneous transmission of multiple signals across a single data link.
* Two types of multiplexing are frequency-division multiplexing and time division multiplexing.
* In FDM, each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal.
* In TDM, digital signals from n devices are interleaved with one another, forming a frame of data.
* TDM can be classified as either synchronous or asynchronous.
* Wave division multiplexing (WDM) is similar to concept to FDM.
* The signal being multiplexed, however are light waves.
* Switching is a method in which multiple communication devices are connected to one another efficiency.
* A switch is intermediary hardware or software that links devices together temporarily.
* There are three fundamental switching methods circuit switching, packet switching and message switching.
* In circuit switching, packets from a single device travel or dedicated links to the destination.
* In space-division switch, the path from one device to another is spatially separate from other parts.
* Multistage switches can reduce the number of cross points needed, but blocking may result.
* In time-division switch, the inputs are divided in time, using TDM. * A control unit sends the input to the correct output device.
* Packet switching has two approaches datagram approach and virtual circuit approach.
* In message switching, a node receives a message stores it and then sends its.
* PSTN is an example of circuit-switched network.
* Space and time division switches may be combined.

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