1. 1 Chapter Four Making Connections

2. 2 Introduction Connecting peripheral devices to a computer has, in the past, been a fairly challenging task Newer interfaces have made this task much easier Let’s examine the interface between a computer and a device. This interface occurs primarily at the physical layer Chapter Four - Making Connections

3. 3 Interface Standards There are essentially two types of standards –Official standards, created by standards making organizations such as ITU (International Telecommunications Union), IEEE (Institute for Electrical and Electronics Engineers), EIA (Electronics Industries Association), ISO (International Organization for Standards), and ANSI (American National Standards Institute) –De-facto standards – protocols created by other groups that are not official standards but because of their widespread use, become “almost” standards Chapter Four - Making Connections

4. 4 Interface Standards There are four possible components to an interface standard: –Electrical component –Mechanical component –Functional component –Procedural component Chapter Four - Making Connections

5. 5 The Four Components The electrical component deals with voltages, line capacitance, and other electrical characteristics The mechanical component deals with items such as the connector or plug description. Chapter Four - Making Connections

6. 6 The Four Components The functional component describes the function of each pin or circuit that is used in a particular interface The procedural component describes how the particular circuits are used to perform an operation Chapter Four - Making Connections

7. 7 Two Example Interfaces In order to better understand the four components of an interface, let’s examine two popular interface standards –EIA-232F – an older standard originally designed to connect a modem to a computer –USB (Universal Serial Bus) – a newer standard that is much more powerful than EIA-232F Chapter Four - Making Connections

8. 8 RS-232 and EIA-232F Originally named RS-232 but has gone through many revisions The electrical component is defined by another standard: V.28 The mechanical component is often defined by ISO 2110, the DB-25 connector. The DB-9 connector is now more common than the DB-25. Chapter Four - Making Connections

9. 9

10. 10 RS-232 and EIA-232F The functional and procedural components are defined by the V.24 standard For example, V.24 defines the function of each of the pins on the DB-9 connector, as shown on the next slide Chapter Four - Making Connections

11. 11 RS-232 and EIA-232F Chapter Four - Making Connections

12. 12 RS-232 and EIA-232F The next slide shows an example of the procedural dialog that can be used to create a connection between two endpoints Note the level of complexity needed to establish a full-duplex connection Chapter Four - Making Connections

13. 13 Chapter Four - Making Connections

14. 14 Universal Serial Bus (USB) The USB interface is a modern standard for interconnecting a wide range of peripheral devices to computers Supports plug and play Can daisychain multiple devices USB 2.0 can support 480 Mbps (USB 1.0 is only 12 Mbps) Chapter Four - Making Connections

15. 15 Universal Serial Bus (USB) The USB interface defines all four components The electrical component defines two wires VBUS and Ground to carry a 5 volt signal, while the D+ and D- wires carry the data and signaling information The mechanical component precisely defines the size of four different connectors and uses only four wires (the metal shell counts as one more connector) Chapter Four - Making Connections

16. 16 Universal Serial Bus (USB) Chapter Four - Making Connections

17. 17 Universal Serial Bus (USB) The functional and procedural components are fairly complex but are based on the polled bus The computer takes turns asking each peripheral if it has anything to send More on polling near the end of this chapter Chapter Four - Making Connections

18. 18 Other Types of Connections The physical layer is not the only layer involved in a connection The data link layer is also involved Let’s look at the first of these data link layer connections – the asynchronous connection Chapter Four - Making Connections

19. 19 Asynchronous Connections A type of connection defined at the data link layer To transmit data from sender to receiver, an asynchronous connection creates a one-character package called a frame Added to the front of the frame is a Start bit, while a Stop bit is added to the end of the frame An optional parity bit can be added which can be used to detect errors Chapter Four - Making Connections

20. 20 Asynchronous Connections Chapter Four - Making Connections

21. 21 Asynchronous Connections Chapter Four - Making Connections

22. 22 Asynchronous Connections The term asynchronous is misleading here because you must always maintain synchronization between the incoming data stream and the receiver Asynchronous connections maintain synchronization by using small frames with a leading start bit Chapter Four - Making Connections

23. 23 Synchronous Connections A second type of connection defined at the data link layer A synchronous connection creates a large frame that consists of header and trailer flags, control information, optional address information, error detection code, and data A synchronous connection is more elaborate but transfers data in a more efficient manner Chapter Four - Making Connections

24. 24 Synchronous Connections Chapter Four - Making Connections

25. 25 Isochronous Connections A third type of connection defined at the data link layer used to support real-time applications The data must be delivered at just the right speed (real-time) – not too fast and not too slow Typically an isochronous connection must allocate resources on both ends to maintain real-time USB (and Firewire) can both support isochronous connections Chapter Four - Making Connections

26. 26 Half Duplex, Full Duplex, and Simplex Connections A half duplex connection transmits data in both directions but in only one direction at a time A full duplex connection transmits data in both directions and at the same time A simplex connection can transmit data in only one direction Chapter Four - Making Connections

27. 27 Terminal-to-Mainframe Connections A point-to-point connection is a direct, unshared connection between a terminal and a mainframe computer A multipoint connection is a shared connection between multiple terminals and a mainframe computer The mainframe is the primary and the terminals are the secondaries Chapter Four - Making Connections

28. 28 Terminal-to-Mainframe Connections Chapter Four - Making Connections

29. 29 Terminal-to-Mainframe Connections To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal Two basic forms of polling: roll-call polling and hub polling In roll-call polling, the mainframe polls each terminal in a round-robin fashion In hub polling, the mainframe polls the first terminal, and this terminal passes the poll onto the next terminal Chapter Four - Making Connections

30. 30 Terminal-to-Mainframe Connections Chapter Four - Making Connections Roll call polling

31. 31 Making Computer Connections In Action The back panel of a personal computer has many different types of connectors, or connections: –RS-232 connectors –USB connectors –Parallel printer connectors –Serial port connectors Chapter Four - Making Connections

32. 32 Making Computer Connections In Action Chapter Four - Making Connections

33. 33 Making Computer Connections In Action 1 and 2 – DIN connectors for keyboard and mouse 3 – USB connectors 4 and 6 – DB-9 connectors 5 – Parallel port connector (Centronics) 7, 8, and 9 – audio connectors Will Bluetooth or ??? replace these someday? Chapter Four - Making Connections

34. 34 Making Computer Connections In Action A company wants to transfer files that are typically 700K chars in size If an asynchronous connection is used, each character will have a start bit, a stop bit, and maybe a parity bit 700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits Chapter Four - Making Connections

35. 35 Making Computer Connections In Action If a synchronous connection is used, assume maximum payload size – 1500 bytes To transfer a 700K char file requires 467 1500- character (byte) frames Each frame will also contain 1-byte header, 1-byte address, 1-byte control, and 2-byte checksum, thus 5 bytes overhead Chapter Four - Making Connections

36. 36 Making Computer Connections In Action 1500 bytes payload + 5 byte overhead = 1505 byte frames 467 frames * 1505 bytes/frame = 716,380 bytes, or 5,731,040 bits Significantly less data using synchronous connection Chapter Four - Making Connections