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Transmission Modes • Serial Transmission One bit is transmitted on a circuit at a time Usually there is one transmit circuit and one receive circuit transmit receive receive transmit Used with modem ports, early mouse ports, some printer ports Transmission Modes • Parallel Transmission Multiple bits transmitted simultaneously Multiple circuits (lines) must be used transmit receive One entire byte could be transmitted in one interval Printers have often been connected to PCs using this mode Transmission Techniques • How do we organize the bits for transmission? • How do we keep bits synchronized? • If we transmit bytes, what distinguishes the start of each byte? • How is the data rate determined? • We must define the rules, the standards, in order for different equipment to properly communicate Transmission Techniques • Asynchronous Transmission Also called Start-Stop Each character is framed by start and stop bits Hence, each character is individually synchronized Spacing between characters is undefined • May be short • May be long Asynchronous Transmission • Consider the character ‘Z’ ASCII code • In binary, this can be represented as 101 1010 • Often, an additional bit is added for parity 0101 1010 • This would be called even parity, the number of one bits is even • Some times, the parity bit might not be used Asynchronous Transmission • To the ASCII character, additional bits are added • By convention Start bit = positive value Stop bit = negative value • We can now represent this character as follows Asynchronous Transmission • The width of the pulse determines the speed of transmission • Width of a pulse must be set at both ends, that is, both end must agree on this ahead of time • Note that bits are sent one at a time, not a character at a time • This is a serial transmission. Interface Specification • Now that we know how we might encode data we need to define an interface • How many circuits? What will each do? • If voltages, what are the levels, what are the tolerances? • What types of connectors will we agree on? • Typically, interfaces have four major areas to define Interface Specification • Mechanical Physical connection Connector specifications • Electrical/Optical Voltage levels Meaning of each circuit Rate at which voltages change Determines data rates, distances • Functional Defines the meaning of each circuit Generally are broad categories of functional circuits • • • • Control Ground Data Timing • Procedural Defines sequences of events for establishing connections and data exchange RS232c (EIA) V.24 (ITU) • The is the most common interface • In the past, was used for everything, like USB interfaces are used now. • This is a serial (asynchronous) interface • Common past interface between modems and PCs, or Data Terminal Equipment (DTE) Data Communications Equipment (DCE) RS232c • Mechanical Various connectors have been defined • • • • DB25 DB15 DB9 RJ45 • Electrical Digital signal (2 states – voltage) Voltage measured with respect to a common ground Voltage: -15v to -3v (1) +15v to +3v (0) RS232c Connectors RS232c Functional Assignment of functions to specific circuits • Ground FG SG 1 7 Frame ground Signal ground • Data transmission TD 2 DTE RD DTE Transmit data DCE 3 Receive data DCE RS232c • Control DTR 20 DTE DSR DCE 6 DTE RTS 4 DTE Request to Send DCE 5 DTE DCD Data Set Ready DCE DTE CTS Data Terminal Ready Clear to Send DCE 8 Data Carrier Detect DCE RS232c • Timing • TC 15 DTE • RC DCE 17 DTE Transmit Clock Receive Clock DCE Used in synchronous communications Modem Connection PC DTE Modem DCE Modem DCE PC DTE DTR DTR DSR DCD DSR DCD RTS RTS CTS CTS TD RD RD TD RD TD TD RD SG SG SG SG Null Modem PC PC TD RD TD RD RTS CTS RTS CTS DSR DCD DTR DSR DCD DTR SG SG Synchronous Transmission • In asynchronous transmission, each character was ‘framed’ with start-stop bits • Synchronous transmission involves collecting characters into blocks and ‘framing’ each block. • Does not involve extra start-stop bits • But, may involve special characters or special strings to delimit each block • Hence, transmission are synchronized on blocks rather than characters Synchronous Transmission • Since synchronous transmission involves long string of bits, it is possible clocks at receivers to ‘slip’ • After some time the receiver might be sampling too close to the edge of a bit time rather than in the middle • For this reason, synchronous modems usually provide timing circuits to indicate when the transmit a bit and when to sample one • Synchronous communications can also use an RS232 interface Limitations of RS232c • Limited distances (originally limited to 50 ft.) • Limited speeds (originally limited to 20 Kbps, maximum 115 Kbps) • Communications is point-to-point • Some lack of functions Loopback • Does not lend itself to support modern devices such as scanners, cameras, etc • Other interfaces have been developed to address these issues (RS449) • RS232 has still remained the most commonly used interface of all time Serial Interfaces Universal Serial Bus (USB) Developed to overcome most RS232 limitations Has become the commodity serial interface Allows connection of up to 126 devices on single port (multipoint) using hubs Plug and Play support incorporated into Operating Systems Two versions • USB 1.1 (1996) up to 12 Mbps • USB 2.0 (2000) up to 480 Mbps USB Connectors Type A Type B IEEE-1394 • High speed multipoint serial interface like USB • Speeds up to 800 Mbps • Complements USB • Developed by Texas Instruments and implemented widely by Apple Computer • Also uses two types of connectors IEEE-1394 Connector