1. Data Communications and Computer Networks Chapter 3 Wireless Media In Action DataMining Corporation has one office in Chicago and one in Los Angeles. There is a need to transmit large amounts of data between the two sites. DataMining is considering using a Very Small Aperture Terminal satellite system.

2. Data Communications and Computer Networks Chapter 3 Wireless Media In Action Cost is proportional to high amount of traffic with very high reliability. Speed is high enough to support company’s needs. Distance can easily expand across the U.S. Satellite systems are robust in most environments. Security can be very good with encryption.

3. Data Communications and Computer Networks Chapter 3

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

5. 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.

6. 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.

7. 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 1-5 protocols. Most modern modems can support the fax standards.

8. Data Communications and Computer Networks Chapter 4 Modems Modems can support numerous security features including callback security, and password protection. Self-testing (loop-back) is the the ability of a modem to test itself and its connection. Local loop-back testing tests the local computer and modem connection while remote loop-back testing tests the connection between the local computer and the remote modem.

9. Data Communications and Computer Networks Chapter 4

10. 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. Either type of modem typically requires a memory buffer and an Interrupt line (IRQ) to be assigned in order to communicate with the computer processor.

11. Data Communications and Computer Networks Chapter 4

12. Data Communications and Computer Networks Chapter 4 Bandwidth Limitations If two modems are communicating data and both are attached to analog telephone lines (analog-to-analog), then the fastest possible data transfer speed is 33.6 Kbps. It can be proven that, due to telephone line bandwidth limitations and the noise introduced by analog-to-digital conversions done in the telephone network, these analog- to-analog transfers can never go much faster than this.

13. Data Communications and Computer Networks Chapter 4

14. Data Communications and Computer Networks Chapter 4 Breaking Bandwidth Limitations A 56K modem (56,000 bps) can achieve data transfer speeds greater than 33.6 Kbps when the data sender has a 64,000 bps digital network connection (standard digitized voice channel) rather than an analog connection. A 56Kbps modem cannot not actually achieve 64Kbps due to telephone network signaling constraints.

15. Data Communications and Computer Networks Chapter 4

16. Data Communications and Computer Networks Chapter 4

17. Data Communications and Computer Networks Chapter 4 Breaking Bandwidth Limitations A 56K modem also does not achieve exactly 56Kbps because the FCC will not let the modem transmit at the power level necessary to support 56K, so the best the modem can do under perfect conditions is approximately 53K A 56K modem will typically not even achieve 53K if there is substantial noise on the line. Typical users can get around 40Kbps – 45 Kbps.

18. Data Communications and Computer Networks Chapter 4 56K Modem Rates are Asymmetric Finally – remember that a 56K modem only achieves higher data rates in the download direction – from the digitally connected site to the 56K modem user. Data uploads (from the 56K modem up to the digitally connected site) still only go at a maximum of 33.6 Kbps because they require an analog-to-digital conversion.

19. Data Communications and Computer Networks Chapter 4 Alternatives to Traditional Modems A T1 line is a digital service offered by the telephone companies and can transfer data as fast as 1.536 Mbps (both voice and computer data). To support a T1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection. CSU/DSU is sometimes (incorrectly) called a digital modem. More will be said about T1 in Chapter 12.

20. Data Communications and Computer Networks Chapter 4

21. 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. Some cable modem services only provide downstream data transfer and require a telephone line for the upstream connection.

22. Data Communications and Computer Networks Chapter 4

23. Data Communications and Computer Networks Chapter 4

24. 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 IDSN and DSL in Chapter 12.

25. Data Communications and Computer Networks Chapter 4 Modem Pools A relatively inexpensive technique that allows multiple workstations to access a modem without placing a separate modem on each workstations. Modem pools can also be used to allow external users to dial into a business or corporate network via a modem in the modem pool.

26. Data Communications and Computer Networks Chapter 4

27. 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. These circuit cables are typically short (no more than a few dozen feet long).

28. Data Communications and Computer Networks Chapter 4

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

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

31. 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.

32. Data Communications and Computer Networks Chapter 4

33. 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.

34. Data Communications and Computer Networks Chapter 4

35. Data Communications and Computer Networks Chapter 4

36. Data Communications and Computer Networks Chapter 4 RS-232 and EIA-232E 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.

38. 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.

39. Data Communications and Computer Networks Chapter 4

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

41. Data Communications and Computer Networks Chapter 4 Interfacing a Computer and a Peripheral 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.

42. 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.

43. Data Communications and Computer Networks Chapter 4

44. Data Communications and Computer Networks Chapter 4

45. Asynchronous Transmission Asynchronous = “no synchronization” In Asynchronous transmission, transmitter and receiver data clocks do not need to be synchronized to each other –If sender clock is a little faster or slower, receiver will still know where start bit begins, and count out 8 bits from there. However, Asynchronous transmission is inefficient (about 20% of the bits are overhead – Start/Stop)

46. 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 transfer data in a more efficient manner.

47. Synchronous Transmission In Synchronous transmission, transmitter and receiver data clocks must be synchronized. This is accomplished by requiring transmitter to use a self-timing code to represent bits, such as Bipolar AMI Code. Receiver continuously watches voltage changes to determine precise bit transmission rate (bit timing synchronization)

48. Data Communications and Computer Networks Chapter 4

49. 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.

50. Simplex Data flows one way only

51. Half Duplex Data flows only one way at any particular time, but can flow either direction.

52. Full Duplex Data can flow both ways simultaneously

53. 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.

54. Data Communications and Computer Networks Chapter 4

55. 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.

56. Data Communications and Computer Networks Chapter 4

57. 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

58. Data Communications and Computer Networks Chapter 4