modem error and protocols

8/12/2019 Modem Error and Protocols

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M O D E M S

PROTOCOLS & ERRORS

ALVAREZ | CASCANO | HINAHON | ESPEDIDO | VALERIO


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What is a Protocol?

A uniform set of rules that enable two devices toconnect and transmit data to one another. Protocolsdetermine how data are transmitted betweencomputing devices and over networks. They defineissues such as error control and data compressionmethods. The protocol determines the following:type of error checking to be used, data compression

method (if any), how the sending device will indicatethat it has finished a message and how the receivingdevice will indicate that it has received the message.


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Modulation Protocols

Modulation protocols determine howmodems convert digital data into analog signals thatcan be sent over a phone line.


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The conversion between digital and analog data is

governed by proprietary and international standards,called modem protocols.


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The difference between Protocol and Modulation.

Modulationis the method that the signal is beingtransmitted by the transmitter so that the receiver

will recognize it as a valid signal.

Protocolis the method that the data to betransmitted are arranged and sent to the receiver.


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Modulation Speed

The speed at which modems transmit data is called

the throughput. This is measured in bits per second (bps).


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Modulation Speed

The bps rateof a modem is the measure of howmany bits per second the modem transmits.

10bps = 1cps (character per second)

1character = 8data bits plus 1start bit and 1stop

bit.

Current modems can sometimes transmit up to 115,200 bits per

second.


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Modulation Speed

The baud rateof a modem is the measure of

how many times per second a modem's signalchanges.

Because of limitations in standard phone lines, it is hard to get baud

rates higher than 2,400. Most high speed modems run at 2,400 baud.


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Protocol Working Overview

v.90

v.34

v.32biz

v.32

v.90

v.34

v.32biz


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History

1960

110bps modems

Teletype machines

No computers involved


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History

1960

AT&T300 bps modem Sold commercially in 1962.

Slow and expensive modems Used for transmitting data between mainframe computers or for

connecting a dumb terminal to a mainframe computer overphone lines.


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History

1980s

Bulletin Board Systems (BBS)


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History

1990s

Internet

BBS almost died (some got into the internet) and

Internet started gaining popularity (thoughexpensive)


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Bell Standard and its Implications

At 1200 bps and below there were twostandards CCITT (ITU at present)

Bell

A Bell modem cannot communicate with aCCITT modem

Bell standard at that time was usedpredominantly in the US

Today, all modems fall under the ITUspecifications


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Modulation Protocols Terms

CCITT. International Telegraph and TelephoneConsultative Committee (CCITT, from French: ComitConsultatif International Tlphonique etTlgraphique) (Note: CCITT changed its name to ITU-Ton March 1, 1993).

ITU-T. International Telecommunications Union,Telecommunication Standardization Sector.V.xxxITU-T(CCITT) series of "recommendations" (standards)relating to telecommunications.

bis"the second ter"the third PEP. Packet Ensemble Protocol (Telebit proprietary). HST. High Speed Technology (US Robotics proprietary).


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HISTORY

ITU modem standards.

The International Telegraphic Union (ITU) is an

international association that establishes worldwidecommunication standards. Its standards areprefaced by the letter -V- (called the V series) andinclude the following:


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A N h P l U d i h F 6K


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A Note on the Protocol Used in the Faster 56KModems

When the 56K modems were first introduced therewere two competing standards

One was the X2 standard proposed by US Robotics

that is now part of 3Com The competing protocol was knows as the Kflex56

standard A joint effort between Lucent and Rockwell


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ITU Standard for 56K Modems

Both standards have now been superceded by theITU V.90 standard

The vendors now produce modems that operate

under the ITU V.90 protocol The vendors also offer upgrades to the older X2 and

Kflex modems so that they could operate under V.90


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v.92 Features

V.92 is an ITU-T recommendation, titledEnhancements to Recommendation V.90, thatestablishes a modem standard allowing near 56 kb/sdownload and 48 kb/s upload rates. With V.92 PCM

is used for both the upstream and downstreamconnections; previously 56K modems only used PCMfor downstream data.

V.92 was first presented in August 1999. It was

intended to succeed the V.90 standards; however,with the spread of broadband Internet access, uptakewas minimal.


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v.92 Features

Quick Connect

This reduces negotiation times to around 10 secondsinstead of over 20 seconds. Quick connect works by

training the client modem on the first call; analogand digital characteristics are stored in a local profileand then retrieved for future connections.


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v.92 Features

"Modem on Hold" (MOH)[

This allows the connection to be temporarily severed

and then reconnected, reducing the possibility ofdropped connections. This is particularly useful forlines that have call waiting.


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v.92 Features

PCM upstream

Pulse-code modulation (or PCM) allows higher ratedigital transmissions over the analog phone lines.

PCM upstream provides a digital connection forupstream data, reducing latency and allowing for amaximum upload speed of 48 kbit/s. Previously thespeed was limited to a 33.6 kbit/s analog signal

under the previous V.90.


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V.44 compression

V.44 compression replaces the existing V.42biscompression standards. It generally allows for

between 10% and 120% better compression. In mostsituations the improvement is around 25%.


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Error Correction Protocols

Error Correction Protocols Line noise can cause significant errors in modem

connections. While a moment of line noise at 300bps can create several junk characters, that same

moment of noise at 28,800 bps can fill the screenwith garbage.

Error correction protocols combat the line noiseproblem by packetizing the data and providing

checksums to help determine if the data has beencorrupted in transmission. Error correction and datacompression ensure accurate, swift data transfers.


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Data Compression Protocols

Data Compression Protocols In the never ending quest to increase transmission rates,

several data compression protocols have been developed forserial transmissions. Data compression protocols shrink thedata that must be transmitted over a connection, thereby

increasing the transmission rate. The two most popular datacompression protocols for modems are MNP-5, with amaximum compression ratio of 2 to 1, and CCITT V.42bis,

with a maximum compression rate of 4 to 1. Note, however, that these data compression protocols only

help with compressible data. It is possible to decrease thetransmission rate using a data compression protocol withalready compressed data. In general, though, a datacompression protocol certainly helps more than it hinders.


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MNP Levels 1-4

Microcom Networking Protocol- (MNP-), developed

by Microcom Systems, Inc., enables error-free asyncdata transmission. Although MNP is proprietary, it

became an industry standard in the 1980s becauseusers demanded it from manufacturers. Bothmodems in a connection must use the same MNPprotocols.


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MNP Level 5

Incorporates the first four levels and also appliesa data-compression algorithm. It -compresses-data 2:1, so you can double the amount of data

you send at the modem's top transmission speed.

For example, with MNP Level 5, modems thattransmit data at 4800 bps send an amount ofdata equal to an uncompressed 9600 bps.


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MNP Level 6,7,9,10


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V.42, V.42 bis

These -V Series- protocols are internationally recognizedstandards for error control and data compression.

V.42 is the recommendation of the ITU for error control; itcontains two algorithms (LAPM, or Link Access Protocol,and MNP 1-4). When two V.42-compliant modemsestablish a connection, they use LAPM to control dataerrors and retransmit -bad- data blocks. If one modemsupports V.42 and the other supports only MNP, then the

two negotiate to use MNP protocol. V.42 bis corresponds roughly to MNP Level 5. V.42 can usually generate a 4:1 ratio of data compression,

depending on the type of file transmitted.


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The other benefit of V.42 (or MNP 4) is that it canimprove throughput. Before sending the data to aremote system, a modem with V.42 (or MNP 4)

assembles the data into packets and during thatprocess it is able to reduce the size of the data bystripping out the start and stop bits.


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Error correction basics

In general, these protocols break down a file into a series ofpackets containing a number of bytes from the original file.

Some sort of additional data, normally a checksum or CRC, isadded to each packet to indicate whether the packetencountered an error while being received .

The packet is then sent to the remote system, whichrecomputes the checksum or CRC of the data and compares itto the received checksum or CRC to determine if it wasreceived properly.

If it was, the receiver sends back an ACK (acknowledgement)message, prompting the sender to send the next packet. Ifthere was any problem, it instead sends a NAK (not-acknowledged) message, and the sender resends the damagedpacket.


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Error correction basics

In general, these protocols break down a file into a series ofpackets containing a number of bytes from the original file.

Some sort of additional data, normally a checksum or CRC, isadded to each packet to indicate whether the packetencountered an error while being received .

The packet is then sent to the remote system, whichrecomputes the checksum or CRC of the data and compares itto the received checksum or CRC to determine if it wasreceived properly.

If it was, the receiver sends back an ACK (acknowledgement)message, prompting the sender to send the next packet. Ifthere was any problem, it instead sends a NAK (not-acknowledged) message, and the sender resends the damagedpacket.


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Cable Internet access

Cable Internet access, shortened to cable Internet isa form of broadband Internet access that uses thecable television infrastructure. Like digital subscriberline and fiber to the premises services, cable Internetaccess provides network edge connectivity (last mileaccess) from the Internet service provider to an enduser. It is integrated into the cable televisioninfrastructure analogously to DSL which uses theexisting telephone network.


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DSL

A digital subscriber line (DSL) modem is a deviceused to connect a computer or router to a telephoneline which provides the digital subscriber line servicefor connectivity to the Internet, which is often calledDSL broadband.


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ADSL

More of the bandwidth of the line is dedicated todownstream (download) data than upstream

(upload) data, so, download rates are faster thanupload rates, because most users download muchlarger quantities of data than they upload.

The most widely available ADSL service today has

rates of 1.5 Mbit/s download and 64 to 640 kbit/supload. The distance limit for ADSL is 18 000 feet(5.5 km or 3.4 miles).


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Mobile Broadband Modem

A mobile broadband modem, also known as aconnect card or data card, is a type of modem thatallows a laptop, a personal computer or a router toreceive Internet access via a mobile broadbandconnection instead of using telephone or cabletelevision lines. A mobile Internet user can connectusing a wireless modem to a wireless InternetService Provider (ISP) to get Internet access.


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General packet radio service (GPRS) is a packetoriented mobile data service on the 2G and 3Gcellular communication system's global system formobile communications (GSM). GPRS was originallystandardized by European TelecommunicationsStandards Institute (ETSI) in response to the earlierCDPD and i-mode packet-switched cellulartechnologies. It is now maintained by the 3rdGeneration Partnership Project (3GPP).


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1G-2.4 kilobit per second (kbit/s) or less.

2G- digital, and offered faster dial-up speeds of9.6kbit/s or 14.4kbit/s without the need for a

separate modem. 3G networks have taken this approach to a higher

level, using different underlying technology but thesame principles. They routinely provide speeds over

300kbit/s


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HSDPA

High-Speed Downlink Packet Access (HSDPA) is anenhanced 3G (third-generation) mobile-telephonycommunications protocol in the High-Speed Packet

Access (HSPA) family, also dubbed 3.5G, 3G+ or

turbo 3G, which allows networks based on UniversalMobile Telecommunications System (UMTS) to havehigher data-transfer speeds and capacity. As of 2013

HSDPA deployments can support down-link speeds

of up to 42.3 Mbit/s. HSPA+ offers further speedincreases, providing speeds of up to 337.5 Mbit/swith Release 11 of the 3GPP standards.


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LTE (4G)

LTE, an acronym for Long Term Evolution,commonly marketed as 4G LTE, is a standard for

wireless communication of high-speed data formobile phones and data terminals. It is based on theGSM/EDGE and UMTS/HSPA networktechnologies, increasing the capacity and speed usinga different radio interface together with core networkimprovements.

Up to 100 Mbits/s

LTE Advanced-

Data speeds of LTE Advanced

LTE Advanced

Peak download 1 Gbit/s

Peak upload 500 Mbit/s


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In Summary

ITU specified protocols with respect to modems existfor the following.

Modulation

Error correction

Data compression

Different protocols apply to different speeds ofcommunication

A high speed modem can communicate with a lowspeed modem

modem error and protocols

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