fedorffixxzz Posted October 15, 2012 Report Posted October 15, 2012 Salutare, recent profesorul ne-a intrebat de ce cablul Ethernet Cat5e foloseste toate cele 8 fire pentru transmisia de date in comparatie cu Ethernet Cat5 unde sunt nevoite doar 2 perechi (4 fire) pentru 100mb si doar 2 perechi (4 fire) pentru 10mb? Stie cineva raspunsul?Va multumesc anticipat. Quote
scorpio2k2 Posted October 15, 2012 Report Posted October 15, 2012 pe 4 fire ai 100mbps iar pe 8 ai 1gbps. 10mbps ai pe 2 fire. Quote
fedorffixxzz Posted October 15, 2012 Author Report Posted October 15, 2012 pe 4 fire ai 100mbps iar pe 8 ai 1gbps. 10mbps ai pe 2 fire.Ok. Insa am intrebat, de ce gigabit foloseste toate cele opt fire pentru transmisie/receptie si nu doar patru ca in 100 mbps? Care este motivul? Cum functioneaza transmisia si receptia? De ce 8 fire? Quote
M2G Posted October 15, 2012 Report Posted October 15, 2012 Nu ti se pare logic ca sa folosesti mai multe fire cu cat ai banda mai mare? Pe unde sa circule bitii aia daca nu prin fire?Nu poti atinge viteze de 1 gbps cu 4 fire din cauza limitarilor fizice ale firelor. De asta e nevoie de mai multe. Quote
fedorffixxzz Posted October 15, 2012 Author Report Posted October 15, 2012 (edited) Nu ti se pare logic ca sa folosesti mai multe fire cu cat ai banda mai mare? Pe unde sa circule bitii aia daca nu prin fire?Nu poti atinge viteze de 1 gbps cu 4 fire din cauza limitarilor fizice ale firelor. De asta e nevoie de mai multe.Este logic, insa cum transmite bitii? Care este motivul si care este mecanismul in spatele motivului? Lasand la o parte capacitatea necesara pentru transmisia pe cablu.Spre exemplu la 100 mbps cand sunt folosite toate firele, restul de fire ramase sunt folosite pentru a transmite un semnal continuu, iar cand o interferenta intervine, este calculata interferenta cu semnalul continuu pentru corectare.De ce la Gigabit sunt folosite toate cele opt fire? Edited October 15, 2012 by fedorffixxzz Quote
vexyro Posted October 15, 2012 Report Posted October 15, 2012 (edited) Uite ?i standardul, c? tu asta vrei:100mpbsIn 100BASE-T2, the data is transmitted over two copper pairs, 4 bits per symbol. It uses these two pairs for simultaneously transmitting and receiving on both pairs thus allowing full-duplex operation. First, a 4 bit symbol is expanded into two 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; see the standard for details. This is needed to flatten the bandwidth and emission spectrum of the signal, as well as to match transmission line properties. The mapping of the original bits to the symbol codes is not constant in time and has a fairly large period (appearing as a pseudo-random sequence). The final mapping from symbols to PAM-5 line modulation levels obeys the table on the right. 100BASE-T2 was not widely adopted but the technology developed for it is used in 1000BASE-T.1000mpbs1000BASE-T (also known as IEEE 802.3ab) is a standard for gigabit Ethernet over copper wiring.Each 1000BASE-T network segment can be a maximum length of 100 meters (328 feet), and must use Category 5 cable or better (including Cat 5e and Cat 6).Autonegotiation is a requirement for using 1000BASE-T[7] according to Section 28D.5 Extensions required for Clause40 (1000BASE-T). At least the clock source has to be negotiated, as one has to be master and the other slave.In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of adaptive equalization and a 5-level pulse amplitude modulation (PAM-5) technique. The symbol rate is identical to that of 100BASE-TX (125 Mbaud) and the noise immunity of the 5-level signaling is also identical to that of the 3-level signaling in 100BASE-TX, since 1000BASE-T uses 4-dimensional trellis coded modulation (TCM) to achieve a 6 dB coding gain across the 4 pairs.Since negotiation takes place on only two pairs, if two gigabit devices are connected through a cable with only two pairs, the devices will successfully choose 'gigabit' as the highest common denominator (HCD), but the link will never come up. Most gigabit physical devices have a specific register to diagnose this behaviour. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.The data is transmitted over four copper pairs, eight bits at a time. First, eight bits of data are expanded into four 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; this is similar to what is done in 100BASE-T2, but uses different parameters. The 3-bit symbols are then mapped to voltage levels which vary continuously during transmission.Sunt mai multe standarde de fiecare parte, dar principiul este similar.L.E.Spre exemplu la 100 mbps cand sunt folosite toate firele, restul de fire ramase sunt folosite pentru a transmite un semnal continuu, iar cand o interferenta intervine, este calculata interferenta cu semnalul continuu pentru corectare.Din câte ?tiu celelalte 4 fire pot fi folosite cu un adaptor s? mai transmi?i înc? 100mpbs. E posibil s? ai ?i tu dreptate. Edited October 15, 2012 by vexyro Quote
jojo123 Posted October 15, 2012 Report Posted October 15, 2012 (edited) 4 perechi X 250 Mbps = 1 GbpsAi o explicatie aici //vezi figura 7Spre exemplu la 100 mbps cand sunt folosite toate firele, restul de fire ramase sunt folosite pentru a transmite un semnal continuu, iar cand o interferenta intervine, este calculata interferenta cu semnalul continuu pentru corectare.Te inseli, 4 fire nu sunt folositePin Name Description1 TX+ Transmit Data+2 TX- Transmit Data-3 RX+ Receive Data+4 n/c Not connected5 n/c Not connected6 RX- Receive Data-7 n/c Not connected8 n/c Not connected Edited October 15, 2012 by jojo123 Quote
M2G Posted October 15, 2012 Report Posted October 15, 2012 Sa ajuns la concluzia ca nu e optim sa faci corectari in pachete daca acestea vin eronate. Daca exista zgomot si un anumit pachet este trimis cu erori, receptorul face un request pentru acel pachet si sursa il retransmite. Asta se intampla din cauza faptului ca e mai rapid sa retrasmiti pachetul respectiv decat sa il corectezi. Acum cativa ani se foloseau algoritmi de corectie deoarece viteza retelelor era mai mica dar acum nu se mai practica. Asta se intampla in cazul TCP/IP.Pentru UDP pachetele se pierd pur si simplu si poti sa observi asta in streamingul video. Cand iti mai apar din cand in cand "patratele pe ecran" pixeli care ar trebui sa fie acolo si nu sunt. Astea sunt defapt pachete pierdute.In legatura cu firele, materialele din care sunt construite au o limita de cate date poti trimite la un momentdat pe ele. Nu stiu exact sa iti raspund dar sunt multi factori care influenteaza rata maxima de trasmisie printr-un cablu.De exemplu un cablu cu o ecranare mai buna poate sa ajunga la performante mai mari dar din cauza stratului suplimentar de izolatie este mai greu de indoit. Lucru la care administratorii de sistem trebuie sa se gandeasca atunci cand proiecteaza o retea.Vitezele maxime mai sunt influentate de lungimea cablului, cantitatea de cupru din fire si multe alte asptecte. Quote
Xander Posted October 15, 2012 Report Posted October 15, 2012 Nu sunt neaparat mai putin flexibile am pe acasa un cord de extensie(3-4m) cat 6 de la zyxel ... si este la fel de flexibil ca un cat 3 ( doar putin mai elasitc - isi revine la forma initiala mai usor decat cat 3, nu se intinde:) Quote
jojo123 Posted October 15, 2012 Report Posted October 15, 2012 Nu sunt neaparat mai putin flexibile am pe acasa un cord de extensie(3-4m) cat 6 de la zyxel ... si este la fel de flexibil ca un cat 3 ( doar putin mai elasitc - isi revine la forma initiala mai usor decat cat 3, nu se intinde:)Pai nu conteaza daca e cat 3, 5 6 ai doua tipuri de cablu flexibil si rigid.De regula la o cablare faci ceva de genulswitch--patch cord flexibil---patch pannel--cablu rigid--priza rj45--patch cord flexibil --calculatorhttp://www.comtec-int.ro/upload/tinymce/File/Cablu%20Utp%20rigid%20cat%205e(1).jpghttp://www.comtec-int.ro/admin/upload/tinymce/File/UTP%20flexibil%20cat%205e.jpg Quote