Transmission Modes

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 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 transmit receive 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. RS232cConnectors

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 • DTR 20 Data Terminal Ready • DSR 6 Data Set Ready • RTS 4 Request to Send • CTS 5 Clear to Send • DCD 8 Data Carrier Detect DTE DCE DTE DCE DTE DCE DTE DCE DTE DCE

15. RS232c • Timing • TC 15 Transmit Clock • RC 17 Receive Clock DTE DCE DTE DCE Used in synchronous communications

16. PC Modem DTE DCE DTR DSR DCD RTS CTS TD RD RD TD SG SG Modem PC DCE DTE DTR DSR DCD RTS CTS RD TD TD RD SG SG Modem Connection

17. Null Modem PC PC TD TD RD RD RTS RTS CTS CTS DSR DSR DCD DCD DTR DTR SG SG

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 B Type A

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