Kompresija pri downloadanju z Interneta

Kot sem napovedal v zadnji/prejšnji objavi, bom danes na kratko pisal o t.i. “kompresiranju” podatkov pri downloadanju iz Interneta na hard-disk na lokalnem računalniku; seveda, tu govorim izključno o tem, kako stvari jaz vidim oz. doživljam z mojim 56K dial-up modem adapterjem, pri čemer je t.i. “actual line transmission speed” (kar je tu ključno) pri meni celo samo 28.8 oziroma 31.2 kb/s, kolikor pač trenutno zmorejo telefonski kabli po Sloveniji. Naj pa povem še to glede same kompresije, da se ta dejansko odvija zelo zelo poredko in to izključno samo za posebne, ehm, recimo jim “oblike” podatkov. S tem pač mislim preprosto to, da ima kompresija najraje razne večkrat ponavljajoče se znake oz. sekvence znakov in podobno.

Torej, preden bom postal link do diskusije, ki sem jo o tem odprl že zelo zelo dolgo nazaj (če se ne motim, je bil to eden od mojih prvih postov na tem forumu) na “Ars OpenForum” forumu, bom postal tale dva linka: Data compression, Bandwidth compression do člankov na “Wikipediji” in pa dva screenshota GUI-a od programa DU Meter (ali alter. link ), s katerim tudi zaradi mojega logičnega “principa uporabe” WWW-ja oz. “principa brskanja po” WWW-ju (pri čemer skratka stalno pazim, da se ves čas, ko sem “online”, nekaj downloada; za več o tem glej razne moje prejšnje objave) redno spremljam pretok podatkov iz mojega PC-ja na druge računalnike in obratno.

Screenshota hitrosti no. 1, 2:

DU Meter - kompresija - DUMeter-speed-1.png DU Meter - kompresija - DUMeter-speed-2.png

(kot vedno, klikni slikico za originalno velikost)

Moram pa tudi omeniti, da kar “DU Meter”prikaže v screenshotih zgoraj je povprečna hitrost oz. pretok podatkov (angl. izraz “average”), torej ne trenutni/momentalni pretok, ampak pretok podatkov v večjem časovnem razponu/obdobju, kar je v programovem “Help-u” razloženo takole: “Display average values … Normally, DU Meter displays immediate readings of data throughput. However, when this feature is enabled, DU Meter will display an average over a range of several seconds

In pa kot rečeno še link do “56K dial-up modem adapter – speed rate mistery“: http://episteme.arstechnica.com/…/610007526631 diskusije, katero sem, kot rečeno, odprl na Ars forumu že dolgo nazaj (glejte tudi screenshote v njej) in v kateri mi je uporabnik z nickom Paladin napisal kot odgovor na moj post tole spodaj.

What you are seeing is compression. Not an increase in the actual line transmission speed.

Basically there are some types of data that can be compressed (most commonly using STAC compression) as the data is transferred across your dialup connection. When you see speeds like that you are getting lucky essentially. The data is hitting the dialup aggregation equipment at the ISP, that equipment recognizes that the data con be compressed and that your dialup software/modem will support that, and then it compresses it. Then your computer realizes that a lot more data is coming in in the same amount of time that it would normally take to send less data and surmises that the connection suddenly sped up. In reality the data is just squished more efficiently into the same size/speed connection as normal.

If you always get dialup connection speeds of 28.8 or 31.2 then yes, most likely your phone line quality/provisioning is the culprit. You could cajole the phone company to provision you a new line but there is no guarentee that the new line would be any better. The most common cause for limited dialup speeds like that is the instance of too many digital to analog conversion taking place on the line between the 2 modems involved in the internet connection. There is nothing you can do about that except to move next door to the CO.

Basically you are lucky that the compression is working right. It makes your online world a little better every now and then.

Kot vidite, mi je on v tem postu razložil, da ko se kompresija odvija/dogaja (jasno, če jo moj modem oz. njegov drajver sploh podpira), potem OS napačno “misli”, da se je povečal zgoraj omenjeni “line transmission speed” (ker pač pride “skozi” več podatkov v istem časovnem okviru) in zato zgoraj omenjeni program to tako tudi prikaže. V resnici pa so podatki samo bolj učinkovito stisnjeni skupaj in skratka ne gre za povečanje hitrosti same povezave na Internet!!

Tadej

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