+

MODEMS

+ CONTENTS

INTRODUCTION .

PROCEDURE. CLASSIFICATION OF MODEMS.

TYPES OF MODEM.

STANDARDS and PROTOCOLS.

MODEM FEATURES.

CRITERIA FOR BUYING.

+ BASIC PROCEDURE

+Classification of modemsRange

Short Haul Voice Grade (VG) Wideband

Operation Mode Half Duplex Full Duplex Simplex

Synchronization Asynchronous Synchronous

Modulation AM FM/FSK PM TCM

RANGE

Short Haul :

Work in short ranges - up to 15 km (called ‘local loops’)

Short haul modems tend to be cheaper than other modems.

Transmission rate lower to ensure consistent , error-free transmission.

+ Voice Grade :

Used for unlimited destination, using high data rate

Are expensive and maintenance and tuning are sophisticated

Wide Band: Used in large volume telephone-line multiplexing, dedicated

links.

Exceed high data rates

OPERATION MODE

Simplex :

Communication is Unidirectional. One station can transmit and other can receive.

Examples of simplex are keyboard , monitor, printer etc.

+

Half Duplex :Each station can both transmit and receive, but not at same time

At a time, if one device is sending, other can receive and vice versa

Example of half duplex are walkie-talkie

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Full Duplex :

Also called DUPLEXCan transmit and receive simultaneously at full speed

Full duplex modems will not work on half-duplex channels

SYNCHRONIZATION

Asynchronous : Can only handle data bytes with ‘start’ and ‘stop’ bits

No separate timing signal or clock b/w modem and DTE

Can be connected in different options to the communication media

Using 2-Wire or 4-Wire interface. Using interface to call unit/automatic answer, when dialing-

up

Synchronous :

Requires a clock signal Can handle continuous stream of data bits

Data bits are synchronized to clock signal

Separate clocks for transmitted and received bits

+TYPES OF MODEMS

INTERNAL MODEM

EXTERNAL MODEM

CABLE MODEM

FAX MODEM

WIRELESS MODEM

POKET MODEM

ISDN MODEM

DSL MODEM

56K MODEM

INTERNAL MODEM

EXTERNAL MODEM

TYPICAL INDICATERS ON EXTERNAL MODEMS FRONT PANAL

HS = HIGH SPEED

AA = AUTO ANSWER

CD =CARRIER DETECT

OH =OFF HOOK

RD =RECIVED DATA

SD =SEND DATA

TR =TERMINAL READY

MR = MODEM READY

+PINS OF EIA-232

1=DETECT CARRIER SERIAL

2=TRANSMIT CARRIER

3=RECEIVE DATA

4=DTE READY

5=SIGNAL GROUND

6=DCE READY

7=REQUEST OT SEND

8=CLEAR TO SEND

9=RING INDICATOR

PARALLEL

+CABLE MODEM

+

+FAX MODEM

A fax modem in PC can send data to a standard fax machine, which converts data into hard copy form.

Incoming faxes received as image files are saved

to the PC’s harddisk.

Fax modems provide certain advantages over stand

alone fax machines.

WIRELESS MODEMConvert digital data into  radio signals and back. 

Support a maximum data transmission rate of 19.2Kbps (kilobits per second).

Wireless modems can be used up to 20-30 miles (30-50 KM).

Operation is in the 900 MHz and 2.4 GHz. 

Modem names include –

BreeseLINK, LATNET-Radio Data Links, AirLink Wireless Modems, and Open Minds.

Pocket Modems :

Very compact external modems. Designed for Laptops. Directly plug into a port connector. No external power supply is needed.

+ISDN MODEMISDN is an abbreviation for Integrated Services Digital Network.

There is no such thing as a noisy or fuzzy line. 

always runs at 64kbps.

Call Setup is very fast (dialing, handshake and authentication), usually under 2 seconds (compared with 15 seconds at least for most analog modems).

Ability to get 2 calls at the same phone number (one line is for voice, and the other line for your internet connection).

+ ISDN SYSTEM

+DSL MODEM

DSL Modems: DSL (Digital Subscriber Line)  service is usually offered through  the phone the company/ISP.

You can leave your Internet connection open and still use the phone line for voice calls.

The speed is much higher than a regular modem (up to 1.5 Mbps )

DSL doesn't necessarily require new wiring; it can use the phone line you already have

+56K MODEMS The latest modems are equipped with many

facilities they have a speed which can even reach a data transfer speed of 56.6 kbps . They have all facilities like fax, auto-answering, auto-dialing etc.

They are asymmetrical in that the downloading

is a maximum of 56kbps.

The uploading can be maximum of 33.6 kbps.

+SPEED OF MODEM

Modem speed is rated in bps (bits per second).  So a 56k modem transmits at 56,000 bits per second in theory. While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second.   A modem will never connect higher than what it is rated for.

+STANDARDS

ITU :-- International Telecommunication. Union.

ISO :-- International Standard Organization.

CCITT :-- International Consultative Committee For Telephone And Telegraphy.

+STANDARDS

V.34 -  for 28.8k and 33.6k communications. 

K56Flex - for 56k communication,  created by Lucent and Rockwell. 

X2 - for 56k communication,  created by US Robotics (3com). 

V.90 - This is the International Standard for 56k modems.  

V.92 - This is an  ITU (international Telecommunication Union) standard for 56K modems, but three new features have been added -- faster connections (on 50%), better uploading, and data and voice support:

+DIFFERENT CCITT STANDARDS

+PROTOCOLS

Protocols are used by modems to transfer files .

Protocols are those rules & regulation that are followed by any company.

The widespread protocols for transferring files are:

X-modem Y-modem Z-modem

+ERROR CORRECTION PROTOCOLThere are 2 protocols that are used to correct errors on

a connection:

V.42 (LAPM) – This is the International Standard for Error Correction .

MNP 4–  an old protocol that most modems support. 

+DATA COMPRESSION

Compression helps to speed up transmission of information.  There are 2 protocols for compression:

V.42bis – This is the International Standard for Compression. It only works with V.42 Error Correction 

MNP5 – This is an older protocol that most modems support. This only works with MNP Error Correction.

+LATEST TRENDS OF MODEMS

VOICE MAIL

AUTO DETECTION/AUTO SWITCHING

CALLER ID

DSVD

FAX

QUICK CONNECT

+CRITERIA FOR BUYING Transmission Rate –

The transmission rate of a modem must not be below 14,400 bps. It must lie between 28,800 bps and 33,600 bps. They may exceed this speed.

Data / voice Transmission –

A modem must have capability of transmission of data as well as voice.

Auto dialing and answering –

Today many modems are available which have auto-answering and auto-dialing feature.

Fax capability –

A modem must have ability to fax if it is required. Cost -

According to the utility and speed cost of the modem must also be taken into consideration.

+Bibliography

Hardware Bible Winn.L.Rosch

TROUBLESHOOTING,MAINTAINING Stephen J.Bigelow

& REPAIRING

Data Comm. & Networking Behrouz .A. Forouzan

Internet

+

QUERIES

+

THANKYOU