1. 1 Chapter 4 Making Connections Data Communications and Computer Networks: A Business User’s Approach

2. 2 Data Communications and Computer Networks Chapter 4 Introduction Connecting peripheral devices to a computer is normally not a simple task. Let’s examine the interface between a computer and a device. This interface occurs at the physical layer. We will start with the interface of a modem, one of the more common devices.

3. 3 Data Communications and Computer Networks Chapter 4 Modems Modern modems use combinations of amplitude, frequency, and phase modulation to achieve high data rates. The fastest dial-up modem at the moment is 56 kbps. Modems can support auto answer, auto dial, auto disconnect, and auto redial.

4. 4 Data Communications and Computer Networks Chapter 4 Modems Connection negotiation is the ability of a modem to automatically fall forward or fallback to faster or slower speeds, respectively. Modems can perform data compression and error correction and support the MNP 1-10 protocols. Most modern modems can support the fax standards.

5. 5 Data Communications and Computer Networks Chapter 4 Modems Modems can be internal, in which they plug into a slot inside a computer, or external, in which they are separate from the computer and require a serial cable and their own power supply. Internal modems do not require a serial cable but instead require an IRQ assigned.

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7. 7 Data Communications and Computer Networks Chapter 4 Modems Modems for laptop computers are typically of two forms. The first type of laptop modem is completely inside the laptop. A user plugs a modular phone cord (RJ-11) into a slot which is attached to the modem. The second type of laptop modem is about the size of a credit card and plugs into a special connector. The modular phone cord then plugs into this card.

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9. 9 Data Communications and Computer Networks Chapter 4 The 56k Digital Modem A 56k modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems. A 56k modem would actually achieve 64k except: 1. the local loop is still analog, thus analog signaling 2. the analog to digital conversion at the local modem introduces noise/error Combined, these shortcomings drop the speed to at best 56k.

10. 10 Data Communications and Computer Networks Chapter 4 The 56k Digital Modem A 56k modem does not achieve 56k either because the FCC will not let the modem transmit at the power level necessary to support 56k, so the best the modem can do is approximately 53k A 56k modem will not even achieve 53k if the connection between your modem and the remote computer contains an additional analog to digital conversion, or if there is significant noise on the line.

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13. 13 Data Communications and Computer Networks Chapter 4 The 56k Digital Modem A 56k modem is based upon one of two standards: V.90 - Upstream speed is maximum 33,600 bps V.92 - Newer standard which allows maximum upstream speed of 48 kbps (under ideal conditions) and can place a data connection on hold if the telephone service accepts call waiting and a voice telephone call arrives

14. 14 Data Communications and Computer Networks Chapter 4 Alternatives to Traditional Modems A T-1 line is a digital service offered by the telephone companies and can transfer data as fast as 1.544 Mbps (both voice and computer data). To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection. More will be said about T-1 in Chapter 12.

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16. 16 Data Communications and Computer Networks Chapter 4 Alternatives to Traditional Modems Cable modems allow high speed access to wide area networks such as the Internet. Most cable modems are external devices that connect to the personal computer through a common Ethernet card. Cable modems can provide data transfer speeds between 500 kbps and 2.5 Mbps

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18. 18 Data Communications and Computer Networks Chapter 4 Alternatives to Traditional Modems ISDN modems support ISDN connections. ISDN is an all-digital service capable of supporting data and voice, with data speeds up to 128 kbps. DSL modems support digital subscriber line service. DSL is quickly growing in popularity and provides a high- speed service between homes and Internet service providers. More on ISDN and DSL in Chapter 12.

19. 19 Data Communications and Computer Networks Chapter 4 Interfacing Connecting a device such as a modem (or DCE - data circuit-terminating equipment or data communicating equipment) to a computer (or DTE - data terminal equipment). The connections between the DTE and DCE are the interchange circuits.

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21. 21 Data Communications and Computer Networks Chapter 4 Interface Standards Many different groups contribute to interface standards: International Telecommunications Union (ITU) (formerly CCITT) Electronics Industries Association (EIA) Institute for Electrical and Electronics Engineers (IEEE) International Organization for Standards (ISO) American National Standards Institute (ANSI)

22. 22 Data Communications and Computer Networks Chapter 4 Interface Standards Interface standards can consist of four components: 1. The electrical component 2. The mechanical component 3. The functional component 4. The procedural component

23. 23 Data Communications and Computer Networks Chapter 4 Interface Standards 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. A standard connector is the ISO 2110 connector, also known as DB-25. The DB-9 connector has grown in popularity due to its smaller size.

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25. 25 Data Communications and Computer Networks Chapter 4 Interface Standards 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. For example, the functional component may describe two circuits, Request to Send and Clear to Send. The procedural component describes how those two circuits are used so that the DTE can transfer data to the DCE.

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27. 27 Data Communications and Computer Networks Chapter 4 RS-232 and EIA-232F An older interface standard designed to connect a device such as a modem to a computer or terminal. Originally RS-232 but has gone through many revisions. The electrical component is defined by V.28, the mechanical component is defined by ISO 2110, and the functional and procedural components are defined by V.24.

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29. 29 Data Communications and Computer Networks Chapter 4 X.21 Another interface standard that was designed to replace the aging RS-232. Currently popular in Europe and with ISDN connections. Each circuit in the X.21 standard can contain many different signals. Since each circuit can transmit different signals, the combination of signals on the four circuits is much larger than if each circuit performed only a single function.

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31. 31 Data Communications and Computer Networks Chapter 4 High-Speed Interface Protocols Firewire - A bus that connects peripheral devices such as wireless modems and high speed digital video cameras to microcomputers, at 400 Mbps. Designated as IEEE 1394. Firewire supports asynchronous connections and isochronous connections (provides a guaranteed data transport at a pre-determined rate).

32. 32 Data Communications and Computer Networks Chapter 4 High-Speed Interface Protocols Universal Serial Bus (USB) - Modern standard for interconnecting modems and other peripheral devices to microcomputers. Support plug and play. USB can daisychain multiple devices. Like Firewire, USB is a high speed connection (USB 1.0 is only 12 Mbps, but USB 2.0 is 480 Mbps)

33. 33 Data Communications and Computer Networks Chapter 4 USB The USB standard defines all four components – electrical, mechanical, functional, and procedural (see www.usb.org) There are only 4 wires inside a USB connector (and the shell counts as one more)

34. 34 Data Communications and Computer Networks Chapter 4 High-Speed Interface Protocols SCSI - small computer system interface - technique for interfacing a computer to high-speed devices such as disk drives, CDs, and DVDs Need the appropriate SCSI adapters, interconnecting cables, and software iSCSI - Internet SCSI - technique for interfacing disk storage to a computer via the Internet; it looks like the disk storage is down the hall, but it could be anywhere on the Internet

35. 35 Data Communications and Computer Networks Chapter 4 High-Speed Interface Protocols InfiniBand - a serial connection or bus that can carry multiple channels of data at the same time with speeds up to billions of bits per second; more than just a single bus, InfiniBand is a network of high-speed links and switches Fibre Channel - similar to InfiniBand, but limited to the interconnection of 126 devices

36. 36 Data Communications and Computer Networks Chapter 4 RAID What about interfacing a computer to multiple disk drives? Redundant array of independent disks - a collection of techniques for interfacing multiple hard disk drives to a computer RAID-0 - data is broken into pieces and each piece is stored on a different disk drive (striping) RAID-1 - data is stored on at least two disk drives in duplicate (disk mirroring)

37. 37 Data Communications and Computer Networks Chapter 4 RAID RAID-3 - data is redundantly stored across multiple disk drives and error-checking information is kept on a separate disk RAID-5 - data is broken in pieces (stripes) and stored across three or more disks; error-checking information is stored along with the striped data

38. 38 Data Communications and Computer Networks Chapter 4 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 to the frame which can be used to detect errors.

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41. 41 Data Communications and Computer Networks Chapter 4 Synchronous Connections A second type of connection defined at the data link layer. A synchronous connection creates a large package (frame) that consists of header and trailer flags, control information, optional address information, error detection code (checksum), and the data. A synchronous connection is more elaborate but transfers data in a more efficient manner.

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43. 43 Data Communications and Computer Networks Chapter 4 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.

44. 44 Data Communications and Computer Networks Chapter 4 Terminal-to-Mainframe Computer 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 called the primary, and the terminals are called the secondaries.

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46. 46 Data Communications and Computer Networks Chapter 4 Terminal-to-Mainframe Computer Connections To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal. Two basic forms of polling include 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.

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48. 48 Data Communications and Computer Networks Chapter 4 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

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50. 50 Data Communications and Computer Networks Chapter 4 Making Computer Connections In Action 1 and 2 - DIN connectors for keyboard and mouse (what are the mechanical, electrical, and functional specs?) 3 - USB connectors 4 and 6 - DB-9 connectors 5 - Parallel port connector (Centronics) (USB someday?) 7, 8 and 9 - audio connectors Will Bluetooth or ??? replace all these someday?

51. 51 Data Communications and Computer Networks Chapter 4 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 a parity bit. 700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits.

52. 52 Data Communications and Computer Networks Chapter 4 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 of overhead. 1500 bytes payload + 5 byte overhead = 1505 byte frames.

53. 53 Data Communications and Computer Networks Chapter 4 Making Computer Connections In Action 467 frames * 1505 bytes/frame = 716,380 bytes (5,731,040 bits). Significantly less data than asynchronous (7,700,000 bits).