Session 3

Objectives:By the end of this session, the student will 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•Discuss the basic operation of the EIA-232F interface standard•Cite the advantages of Firewire, Universal Serial Bus•Outline the characteristics of asynchronous and synchronous data link interfaces•Recognize the difference between half-duplex, full-duplex and simplex connections

Modem Terms

Modem TermsAutoanswer

Autodial

Autodisconnect

Autoredial

Fallback

Fall forward

3

Modem Terms

De facto StandardsMNP4

MNP5

FAX ModemsSpecial protocols are used, and are built into the modem.

SecurityCall back

Black list

Back door

4

Loop Back

5

Modems

6

56K Digital Modem

7

56K Modem is a hybrid of analog and digital signaling

56K Digital Modem

8

UpstreamPC – Modem – CO : 33.6Kbps (analog signaling) limited to 33.6Kbps due to analog signaling limitation on phone system

Downstream•CO – Modem – PC : near 56Kbps (digital signaling)•CO – CO at 64Kbps using PCM (8bits at 8KHz) – pure digital signaling•As local loop is analog – cannot achieve 64Kbps as quantizing noise is introduced

when converting from analog to digital•Noise is increased as power level must be lowered (FCC regulations)•Noise is increased the further you are from the CO •Downstream in high 40Kbps if near a CO

T1 Line

9

T1DSU – Data Service UnitCSU – Channel Service Unit

Carries 1.55Mbps over 56Kbps – 64Kbps leased lines

Cable Modems

10

Cable ModemsSpeed ranges 300Kbps – 2.5MbpsAsymmetric Upload / download speedsUses coaxial local loop with fibre between main distribution nodesData collisions only drawback

ISDN

11

ISDNIntegrated Services Digital Network

All digital

Modem required

More detailed examination when we discuss WAN technologies

DSL

12

DSLDigital Subscriber Loop

All digital phone service

Modem required

Speeds from 100sKbps - 1sMbps

More detailed examination when we discuss WAN technologies

Modem Pools

13

Interfaces

14

TerminologyDTE – Data Terminal Equipment (e.g. computer)

DCE – Data Communications Equipment (e.g. Modem)

Interchange circuits connect DTE to DCE

Components of an Interface

15

Electrical ComponentLine voltagesLine capacitanceOther electrical issues

Mechanical ComponentConnector / plug descriptionsPin configurations

Functional ComponentDescribes the function of each pin or circuit

Procedural ComponentDescribes how the particular circuits are used to perform an operation

EIA-232F

16

EIA-232F

17

EIA-449

EIA449Replaced EIA-232F with following improvements:

Allow for 2 possible standards (RS-422A and RS-423A) that can be used as the electrical component (both of these standards provide for faster transmission speeds between DTE / DCE)

Add 10 additional circuits, including loop-back testing (original RS-232 standard did not include loop-back)

Connector changed to 37 PIN connector from 25 PIN connector

Never caught on due to widespread use and acceptance of RS-232.

18

X.21

X.21Also intended to replace RS-232Uses 15 PIN connectorOn EIA-232 / RS-449 each signal is carried by its own dedicated lineOn X.21 4 lines are used to 'encode' various signals

19

Hayes Interface

Hayes InterfaceAT – attentionD - dialT - touch tone dialP - pulse dial+++ - go to command mode A - autoanswer modeE0 - do not locally echo charactersE1 - locally echo charactersS0=x - number of rings before answering (auto answer mode)H - hang up

ATDT9,4168684100

+++ATH

20

Firewire

IEEE1394Digital interfaceDeveloped by Apple.

400Mbps2 types of data connections - asynchronous

- iosynchronous (guaranteed data rate for multimedia)

21

USB (Universal Serial Bus)

USBDigital interface

Daisy chain devices (127 max)

USB 1.0 - 12MbpsUSB 2.0 - 480Mbps

22

Asynchronous Connections

23

Synchronous Connections

24

Connection Types

Connection TypesSimplex – one way communication. Similar to car radio.

Half-Duplex – two way communication, only one can talk at a time. Similar to a polite conversation or a CB radio

Full-Duplex – both sides can talk and listen simultaneously.

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