Chapter 4 making connections
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Chapter 4: Making Connections. 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

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Chapter 4 making connections

Chapter 4:

Making Connections


  • 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

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

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


  • 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

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

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

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

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

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

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

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

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

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

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

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

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

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

Data Terminal Equipment and Data

Circuit-Terminating Equipment

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)

Interface Standards (continued)

  • Interface standards can consist of four components:

    • Electrical

    • Mechanical

    • Functional

    • Procedural

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

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

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


  • 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


  • 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)

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


  • 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

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)


  • 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

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

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

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

Half Duplex, Full Duplex, and Simplex


  • 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

Terminal-to-Mainframe Computer


  • 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

Terminal-to-Mainframe Computer

Connections (continued)

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

Terminal-to-Mainframe Computer

Connections (continued)

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

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?

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

Making Computer Connections in Action


  • 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

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)


  • Dial-up and digital modems

  • Alternatives to dial-up:

    • T-1

    • Cable

    • ISDN

    • DSL

  • Modem pools

  • Four components of interface standards

Summary (continued)

  • Interface Standards:

    • EIA-232F

    • FireWire

    • Universal Serial Bus

    • SCSI

    • iSCSI

    • InfiniBand

    • Fibre Channel

Summary (continued)

  • Asynchronous and synchronous data link interfaces

  • Half-duplex, full-duplex, and simplex connections

  • Terminal-to-mainframe connections