Making Connections

1
Data Communications and Computer Networks A
Business Users Approach Third Edition

• Chapter 4
• Making Connections

2
Objectives

• After reading this chapter, you should be able
  to
• Identify a dial-up modem and cite its basic
  operating characteristics
• Discuss the advantages of digital modems and
  recognize why they do not achieve the high
  transfer speeds advertised
• List the alternatives to dial-up modems,
  including T-1 modems, cable modems, ISDN modems,
  and DSL modems

3
Objectives (continued)

• Recognize the uses of a modem pool and its
  advantages and disadvantages
• List the four components of all interface
  standards
• Discuss the basic operations of the EIA-232F
  interface standard
• Cite the advantages of FireWire, Universal Serial
  Bus, SCSI, iSCSI, InfiniBand, and Fibre Channel
  interface standards

4
Objectives (continued)

• Outline the characteristics of asynchronous and
  synchronous data link interfaces
• Recognize the difference between half-duplex,
  full-duplex, and simplex connections
• Identify the operating characteristics of
  terminal-to-mainframe connections and why they
  are unique compared to other types of computer
  connection

5
Introduction

• Connecting peripheral devices to a computer is
  normally not a simple task
• Lets 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
Dial-up Modems

• Modern modems use combinations of amplitude,
  frequency, and phase modulation to achieve high
  data rates
• Fastest dial-up modem at the moment is 56 Kbps
• Modems can support
• Auto answer
• Auto dial
• Auto disconnect
• Auto redial

7
Dial-up Modems (continued)

• Connection negotiation - ability of a modem to
  automatically fall forward or fall back to faster
  or slower speeds, respectively
• Modems can
• Perform data compression and error correction
• Support the MNP 1-10 protocols
• Most modern modems can support fax standards

8
Dial-up Modems (continued)

• Modems can support numerous security features
  including blacklisting, callback security, and
  backdoor entry with password protection
• Self-testing (loop-back) - the ability of a modem
  to test itself and its connection
• Local loop-back testing - tests local computer
  and modem connection
• Remote loop-back testing - tests connection
  between local computer and remote modem

9
Dial-up Modems (continued)

10
Internal vs. External Models

• Internal - plug into a slot inside a computer
• Require an assigned IRQ
• External - separate from computer
• Require serial cable and their own power supply

11
Internal vs. External Models (continued)

12
Modems for Laptops

• Typically of two forms
• Completely inside the laptop
• User plugs modular phone cord (RJ-11) into a slot
  attached to modem
• Second type of is about the size of a credit card
  
• Plugs into a special connector
• Modular phone cord then plugs into this card

13
Modems for Laptops (continued)

14
The 56k Digital Modem

• A 56k modem (56,000 bps) achieves this speed
  through digital signaling as opposed to analog
  signaling used on all other modems
• A 56k modem would actually achieve 64k except
• Local loop is still analog, thus analog
  signaling
• Analog to digital conversion at local modem
  introduces noise/error
• Combined, these shortcomings drop the speed to at
  best 56k

15
The 56k Digital Modem (continued)

• A 56k modem does not achieve 56k either
• FCC will not let the modem transmit at power
  level necessary to support 56k
• Best is approximately 53k
• Will not even achieve 53k if connection between
  your modem and the remote computer contains
  additional analog to digital conversion, or if
  there is significant noise on the line

16
The 56k Digital Modem (continued)

17
The 56k Digital Modem (continued)

18
The 56k Digital Modem (continued)

• Based upon one of two standards
• V.90 - Upstream speed is maximum 33,600 bps
• V.92 - Newer standard with maximum upstream speed
  of 48 kbps (under ideal conditions)
• Can place data connection on hold if the
  telephone service accepts call waiting and a
  voice telephone call arrives

19
Alternatives to Dial-Up Modems

• T-1 line - digital service offered by telephone
  companies
• 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

20
Channel Service Unit/Data Service Unit
(CSU/DSU)

21
Cable Modems

• Allow high speed access to wide area networks
  such as the Internet
• Most are external devices that connect to the
  personal computer through a common Ethernet card
• Can provide data transfer speeds between 500 kbps
  and 2.5 Mbps
• A few services require a telephone line for the
  upstream connection

22
Cable Modems (continued)

23
ISDN and DSL Modems

• ISDN modems support ISDN connections
• ISDN - all-digital service capable of supporting
  data and voice, with data speeds up to 128 kbps
• DSL modems support digital subscriber line
  service
• Quickly growing in popularity
• Provides high-speed service between homes and
  Internet service providers
• More on IDSN and DSL in Chapter 12

24
Modem Pools

• A relatively inexpensive technique that allows
  multiple workstations to access a modem without
  placing a separate modem on each workstation
• 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

25
Modem Pools (continued)

26
Interfacing a Computer to Modems and
Other Devices

• 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

27
Data Terminal Equipment and Data
Circuit-Terminating Equipment

28
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)

29
Interface Standards (continued)

• Interface standards can consist of four
  components
• Electrical
• Mechanical
• Functional
• Procedural

30
Interface Standards (continued)

• Electrical component - deals with voltages, line
  capacitance, and other electrical characteristics
• Mechanical component - deals with items such as
  the connector or plug description
• Standard connector is the ISO 2110 connector,
  also known as DB-25
• DB-9 connector has grown in popularity due to its
  smaller size

31
Interface Standards (continued)

32
Interface Standards (continued)

• Functional component - describes the function of
  each pin or circuit used in a particular
  interface
• Procedural component - describes how 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

33
EIA-232F and RS-232

34
EIA-232F and RS-232 (continued)

• Older interface standard designed to connect a
  device such as a modem to a computer or terminal
• Originally RS-232
• Has gone through many revisions
• Electrical component is defined by V.28
• Mechanical component is defined by ISO 2110
• Functional and procedural components are defined
  by V.24

35
EIA-232F and RS-232 (continued)

36
X.21

• Another interface standard designed to replace
  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
• Combination of signals on the four circuits is
  much larger than if each circuit performed only a
  single function

37
X.21 (continued)

38
RAID

• Redundant array of independent disks - 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)

39
RAID (continued)

• RAID-3 - data is redundantly stored across
  multiple disk drives
• 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

40
FireWire

• Bus that connects peripheral devices such as
  wireless modems and high speed digital video
  cameras to microcomputers at 400 Mbps
• Designated as IEEE 1394
• Supports asynchronous connections and isochronous
  connections
• Provides guaranteed data transport at a
  pre-determined rate

41
Universal Serial Bus (USB)

• Modern standard for interconnecting modems and
  other peripheral devices to microcomputers
• Supports plug and play
• 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)

42
SCSI and iSCSI

• SCSI (small computer system interface) -
  technique for interfacing a computer to
  high-speed devices such as disk drives, CDs, and
  DVDs
• Need 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

43
InfiniBand and Fibre Channel

• 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

44
Data-Link Connections

• Asynchronous Connection
• 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
• A Start bit added to the front of the frame
• A Stop bit added to the end of the frame
• An optional parity bit can be added to the frame
  which can be used to detect errors

45
Asynchronous Connections

46
Asynchronous Connections (continued)

47
Synchronous Connections

• Type of connection defined at the data link layer
• Creates large package (frame) that consists of
  header and trailer flags, control information,
  optional address information, error detection
  code (checksum), and the data
• More elaborate but transfers data in a more
  efficient manner

48
Synchronous Connections (continued)

49
Half Duplex, Full Duplex, and Simplex
Connections

• Half duplex connection - transmits data in both
  directions but in only one direction at a time
• Full duplex connection - transmits data in both
  directions at same time
• Simplex connection - can transmit data in only
  one direction

50
Terminal-to-Mainframe Computer
Connections

• Point-to-point connection - direct, unshared
  connection between a terminal and a mainframe
  computer
• Multipoint connection - shared connection between
  multiple terminals and a mainframe computer
• Mainframe is called the primary
• Terminals are called the secondaries

51
Terminal-to-Mainframe Computer
Connections (continued)

52
Terminal-to-Mainframe Computer
Connections (continued)

• 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 first
  terminal, and this terminal passes the poll onto
  next terminal

53
Terminal-to-Mainframe Computer
Connections (continued)
54
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

55
Making Computer Connections in
Action (continued)

56
Making Computer Connections in
Action (continued)

• 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?

57
Making Computer Connections in
Action (continued)

• 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

58
Making Computer Connections in
Action (continued)

• If a synchronous connection is used, assume
  maximum payload size 1500 bytes
• Transferring 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

59
Making Computer Connections in
Action (continued)

• 467 frames 1505 bytes/frame 716,380 bytes
  (5,731,040 bits)
• Significantly less data than asynchronous
  (7,700,000 bits)

60
Summary

• Dial-up and digital modems
• Alternatives to dial-up
• T-1
• Cable
• ISDN
• DSL
• Modem pools
• Four components of interface standards

61
Summary (continued)

• Interface Standards
• EIA-232F
• FireWire
• Universal Serial Bus
• SCSI
• iSCSI
• InfiniBand
• Fibre Channel

62
Summary (continued)

• Asynchronous and synchronous data link interfaces
• Half-duplex, full-duplex, and simplex connections
• Terminal-to-mainframe connections