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

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Chapter 4 making connections

Chapter 4:

Making Connections


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

  • List the alternatives to dial-up modems, including T-1 modems, cable modems, ISDN modems, and DSL modems


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

Dial-up Modems (continued)


Chapter 4 making connections

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


Chapter 4 making connections

Internal vs. External Models (continued)


Chapter 4 making connections

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


Chapter 4 making connections

Modems for Laptops (continued)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

The 56k Digital Modem (continued)


Chapter 4 making connections

The 56k Digital Modem (continued)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

Channel Service Unit/Data Service Unit

(CSU/DSU)


Chapter 4 making connections

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


Chapter 4 making connections

Cable Modems (continued)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

Modem Pools (continued)


Chapter 4 making connections

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


Chapter 4 making connections

Data Terminal Equipment and Data

Circuit-Terminating Equipment


Chapter 4 making connections

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)


Chapter 4 making connections

Interface Standards (continued)

  • Interface standards can consist of four components:

    • Electrical

    • Mechanical

    • Functional

    • Procedural


Chapter 4 making connections

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


Chapter 4 making connections

Interface Standards (continued)


Chapter 4 making connections

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


Chapter 4 making connections

EIA-232F and RS-232


Chapter 4 making connections

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


Eia 232f and rs 232 continued

EIA-232F and RS-232 (continued)


Chapter 4 making connections

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


Chapter 4 making connections

X.21 (continued)


Chapter 4 making connections

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)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

Asynchronous Connections


Chapter 4 making connections

Asynchronous Connections (continued)


Chapter 4 making connections

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


Chapter 4 making connections

Synchronous Connections (continued)


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

Terminal-to-Mainframe Computer

Connections (continued)


Chapter 4 making connections

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


Chapter 4 making connections

Terminal-to-Mainframe Computer

Connections (continued)


Chapter 4 making connections

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 4 making connections

Making Computer Connections in Action

(continued)


Chapter 4 making connections

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?


Chapter 4 making connections

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


Chapter 4 making connections

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


Chapter 4 making connections

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)


Chapter 4 making connections

Summary

  • Dial-up and digital modems

  • Alternatives to dial-up:

    • T-1

    • Cable

    • ISDN

    • DSL

  • Modem pools

  • Four components of interface standards


Chapter 4 making connections

Summary (continued)

  • Interface Standards:

    • EIA-232F

    • FireWire

    • Universal Serial Bus

    • SCSI

    • iSCSI

    • InfiniBand

    • Fibre Channel


Chapter 4 making connections

Summary (continued)

  • Asynchronous and synchronous data link interfaces

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

  • Terminal-to-mainframe connections


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