CPSC 3600 - DAY 11 FEBRUARY 20, 2017 ================================================================================ TRANSMISSION MODES ------------------ Serial - One bit is sent at one time Parallel - Multiple bits are sent at the same time. Parallel Transmission The term parallel transmission refers to a transmission mechanism that transfers multiple data bits at the same time over seperate media. Typically used with wired medium over multiple independent wires. Signals on all wires are synchronized so that bit travels across each wire at precisely the same time. Typically the set of wires is located in a single cable. High speed - since it can send N bits at the same time, a parallel interface can uperate N times faster than the equiavalent serial interface. Match to underlying hardware - computer hardware uses parallel circuits Serial transmission advantages sends one bit at a time most communication systems use serial mode Reasons: They can be extended for longer distances due to less wires and costs. One wire shouldn't cause timing problems The sender and reciever must contain hardware that converts data from parallel to serial form. TRANSMISSION ORDER ------------------ Most Significant bit (MSB) Least Significant bit (LSB) A little-endian machine will send the LSB first. A bit-endian machine will send the MSB first. Sender and reciever must agree which order is transmitted first. The internet sends big-endian TIMING OF SERIAL TRANSMISSION ----------------------------- Asynchronous transmission can occur at any time, with an arbitrary delay between the transmission of the two data items. Synchronous transmission occurs continuously with no gap between the transmission of the two data items. Isochronous transmission occurs at a regular intervals with a fixed gap between the transmission of the two data items. Asynchronous Transmission Good for applications that generate data at random Disadvantages: There is a lack of coordination between the sender and reciever While a medium is idle, the receiver will not know how long it will wait until data arrives and might not be ready to recieve the data when it is sent. A preamble or a set of start bits are sent before the data is sent so the reviever hardware has time to synchronize with the incoming signal. RS 232-C - Main standard for character communication Specifies the details of physical communication Negative voltage = logical 1, positive voltage = logical 0 Each data item represents one character, either seven or eight bits. Sender can delay sending character as long as it wants, but once transmission begins, it must transmit all bits sequentially with no delay. Start bit- an extra 0 bit is sent before each character. Tells the reciever where a new character starts. Stop bit - a pause at the end of the character for at least the time required to send one bit. A stop bit is appended to every character. Synchronous Transmission Requires framing to allow for periods of idleness. Advantages: The receiver and sender constantly remain synchronized. Each character is sent without start or stop bits, eliminating the overhead. Frame an interface added to a synchronous mechanism that accepts and delivers a block of bytes each frame starts with a special bit sequence. An idle sequence tells the reciever that the sender has no data to send at the moment Framing allows for idle gaps while still maintaining a connection. Isochronous Transmission Designed to provide steady bit flow for multimedia applications that contain voice or video. Prevents delay known as jitters that can disrupt reception (pops in audio or video freeze) Designed to accept and send data at a fixed rate Sender must generate digitized audio or video continuously and the reciever must be able to accept and play the stream. In order for the transmission to be Isochronous, as system must be designed so the sender and reciever see a continuous stream of data SIMPLEX, HALF-DUPLEX, AND FULL-DUPLEX TRANSMISSION -------------------------------------------------- Simplex: Can only transfer data in a single direction. Examples: broadcast radio or television Full-Duplex: System allows transmission in two directions simultaneously Consist of 2 simplex mechanisms Example: voice telephone conversations Half-Duplex Shared transmission medium Can be used for communication in each direction, but cannot proceed simultaneously Example: walkie-talkie MULTIPLEXING AND DEMULTIPLEXING ------------------------------- Multiplexing - refers to the combination of information streams from multiple sources for transmission over a shared medium Multiplexor - a mechanism that implements the combination Demultiplexing - seperation of combination back into seperate info streams Demultiplexor - a mechanism that implements seperation Concept of Multiplexing: Each sender communicates with a single receiver. Although they carry independent communication, all pairs share a single TX line. The multiplexor combines information in such a way that the demultiplexor can seperate the information. Frequency Division Multiplexing (FDM) Underlying principle used for broadcast radio Splits data streams into different channels or frequencies Each sender-reciever pair is assigned a spaced frequency band. Gives a "private" communication. Limitations: If the frequencies are too close, combined signal may occur. Demultiplexing hardware would have to be able to divide these To prevent this, guard bands are implemented: Mandatory gap between sets of carrier frequencies FDM Variations Subchannel Allocation Intended to increase data rate. Splits the channel into 1/K carriers Sends 1/K of the data through each carrier