Lunghezza cavi...

[arturo83]

Ipotizzando di voler collegare un hard disk esterno al PC, mi servirebbe sapere se esiste un limite massimo alla lunghezza dei cavi utilizzati per la trasmissione dati tramite i seguenti protocollli:
- USB
- Firewire
- Ethernet (NAS)

In particolare, ho sentito dire che un cavo USB troppo lungo (oltre i 10 mt. ad es.) potrebbe non garantire una corretta trasmissione di dati...

[Glide]

per l'usb è sconsigliabile l'uso di un cavo oltre i 3 metri
per l'ethernet va bene anche più di 10mt, basta che sia di buona qualità e non abbia cavi elettrici troppo vicini
il firewire non lo so

[lukeskywalker]

il migliore è ethernet, ma alla fine conviene firewire per la velocità di trasferimento..

quoto

[teeodor]

il migliore è ethernet, ma alla fine conviene firewire per la velocità di trasferimento..

Puoi fare tutti i metri che vuoi e l'interferenza con la luce non è un grosso problema..
Esistono pure i cavi schermati!

[arturo83]

Intanto ringrazio tutti per le risposte :)
per l'usb è sconsigliabile l'uso di un cavo oltre i 3 metriTecnicamente quel'è il motivo?
il migliore è ethernet, ma alla fine conviene firewire per la velocità di trasferimento..il nuovo protocollo Firewire IEEE 1394b che arriva a 800Mbps batte tutti!
mentre l'IEEE 1394a con i suoi 400Mbps è un pò più lento dell'USB 2.0 (480Mbps)
è anche vero che a seconda della dimensione dei files da trasferire, in certi casi il 1394a può essere più conveniente dell'USB 2.0...
Rimane da capire se si può utilizzare un cavo Firewire lungo più di 5 mt...

[ilinux]

Il limite teorico massimo di un cavo Ethernet e' di 100 mt , purtroppo per i cavi Usb e Firewire esistono limiti ben piu' bassi di pochi metri , come ti e' gia' stato detto.
Inoltre i cavi piu' adatti a entrare in canaline sono gli Ethernet facili da essere "crimpati" cioe' inseriti i connettori , per le altre 2 tecnogie non e' per nulla facile se non impossibile crimparli.

[arturo83]

Ok, ho fatto una ricerca un pò più approfondita... :)

- USB: max 5 mt.Cables and Long-Haul Solutions


1. Why are there cable length limits, and what are they?
A: The cable length was limited by a cable delay spec of 26ns to allow for reflections to settle at the transmitter before the next bit was sent. Since USB uses source termination and voltage-mode drivers, this has to be the case, otherwise reflections can pile up and blow the driver. This does not mean the line voltage has fully settled by the end of the bit; with worst-case undertermination. However, there's been enough damping by the end of the bit that the reflection amplitude has been reduced to manageable levels. The low speed cable length was limited to 18ns to keep transmission line effects from impacting low speed signals.


2. I want to build a cable longer than 5 meters, why won't this work?
A: Even if you violated the spec, it literally wouldn't get you very far. Assuming worst-case delay times, a full speed device at the bottom of 5 hubs and cables has a timeout margin of 280ps. Reducing this margin to 0ps would only give you an extra 5cm, which is hardly worth the trouble.


3. What about using USB signal repeaters to make a cable longer than 5 meters?
A: Don't bother. The best solution is self-powered hub with a fixed 10m cable that had a one-port bus powered hub in the middle. The maximum range will still have to deal with the timeout, so any out of spec tweaking of the terminations between the two hubs and the timing budget still won't yield more than 5cm of extra distance. A better solution is described in the following question.


4. I really need to put a USB device more than 30 meters away from my PC. What should I do?
A: Build a USB bridge that acts as a USB device on one side and has a USB host controller at the other end. Use a long-haul signaling protocol like Ethernet or RS-485 in the middle. Using cables or short-haul fiber, you can get ranges upwards of a kilometer, though there's no reason why the long-haul link in the middle of the bridge couldn't be a pair of radio transceivers or satellite modems.
Embedded host solutions capable of doing this already exist. Also, two PCs connected via USB Ethernet adapters are essentially a slave/slave version of this master/slave bridge.

- IEEE 1394a: max 4.5 mt.
- IEEE 1394b: max 100 mt.
The Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) Standards Board has approved IEEE Standard 1394b, "High-Performance Serial Bus," which amends the IEEE 1394-1995 and IEEE 1394a-2000 standards. IEEE 1394b upgrades the prior standards by allowing for gigabit signaling and by extending signaling distance to 100 meters (vs. 4.5 meters in IEEE 1394-1995) in data storage, home network backbones and other systems.

The IEEE 1394-1995 standard was widely deployed and many digital consumer and non-consumer products based their primary external interface on it. IEEE 1394b expands the number and type of devices that can use this standard. The amendment also supports a broader range of interconnect media, from CAT5 unshielded twisted pairs and UTP5 to glass and plastic optical fiber. It allows for cable lengths of 50 meters for plastic optical fiber cables and 100 meters for glass optical fiber cables. It also supports SIN S100 (100Mb/s) operation over CAT-5 at lengths to 100 meters.

Under the new amendment, high-speed serial buses integrate with most IEEE standard 34-bit and 64-bit parallel buses, which enables low-cost interconnection among external peripherals. The new amendment is fully interoperable with 1394a-2000 and 1394-1995 for 6-pin and 4-pin connectors. It extends bus speeds to S800 and S1600, and has architectural support for S3200.

IEEE 1394b supports data/strobe signaling and the speeds inherent in IEEE 1394a-2000 and 1394-1995. It also adds beta-mode signaling for much higher data rates between beta-mode ports. For copper-cable connections shorter than 5 meters, ports on the PHY developed for IEEE 1394b can signal by either data-strobe or beta mode. These ports select the optimum connection method.

The new signaling system also provides for scalability as signaling rate increases and allows data transmission to overlap the transmission of arbitration signals in the reverse direction, which eliminates arbitration gaps in 1394b buses. In addition, a bus with all connections operating in beta mode is completely self-timed and does not need a setting for gap count.

The IEEE 1394b standard covers such elements as: cables and connectors for gigabit signaling; detection and resolution of physical loops in bus topology; circuit design for transmitting 8b/10b encoded signals; extension of the PHY/link interface for higher data rates over either an 8-bit parallel or bit-serial bus; protocols to encode bus arbitration signals as symbols; protocols for signal speed negotiation between peer devices; and testing and compliance procedures for gigabit connections.