1. 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 transmit Used with modem ports, early mouse ports, some printer ports

2. Transmission Modes Parallel Transmission  Multiple bits transmitted simultaneously  Multiple circuits (lines) must be used receivetransmit One entire byte could be transmitted in one interval Printers have often been connected to PCs using this mode

3. 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

4. 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

5. Asynchronous Transmission Consider the character ‘Z’ ASCII code In binary, this can be represented as 1 0 1 1 0 1 0 Often, an additional bit is added for parity 0 1 0 1 1 0 1 0 This would be called even parity, the number of one bits is even Some times, the parity bit might not be used

6. 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

7. 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.

8. 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

9. 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

10. 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)

11. 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)

12. RS232c Connectors

13. RS232c Functional  Assignment of functions to specific circuits Ground  FG 1 Frame ground  SG 7 Signal ground Data transmission  TD 2 Transmit data DTE DCE  RD 3 Receive data DTE DCE

14. RS232c Control  DTR20Data Terminal Ready  DSR 6Data Set Ready  RTS 4Request to Send  CTS 5 Clear to Send  DCD 8Data Carrier Detect DTE DCE

15. RS232c Timing TC15Transmit Clock RC17Receive Clock DTE DCE Used in synchronous communications

16. Modem Connection PC Modem DTEDCE DTR DSR DCD RTS CTS TDRD RDTDSG Modem PC DCEDTE DTR DSR DCD RTS CTS RDTD TDRDSG

17. Null Modem PCTDRDRTSCTSDSRDCDDTRSG

18. 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

19. 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

20. 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

21. 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

22. USB Connectors Type A Type B

23. 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

24. IEEE-1394 Connector