Electronic – What makes a high speed USB cable high speed

Please note that USB is 480M bits per second, while uPs have their data usually 8 or 32 bit wide. So the rate of system data transfer goes down to 60MHz or 15Mhz only. USB controllers use hardware access to uP memory (aka direct memory access, or "bus mastering") to transfer blocks of data between the system memory and USB PHY (Physical layer transceiver). The transfers are done in 8 or 16-bit parallel format, and the PHY performs parallel-to-serial conversion. The system usually has plenty of "elasticity" buffers (FIFOs) to provide coherent USB packets in full accords with specified data rates.

The rate of how fast a system can prepare the read/write memory buffers depends on overall processor throughput, so low-performing processors might not achieve maximum USB throughput.

The original 26ns limitation was imposed in the original USB1.1 FS (Full Speed) type of transmission (83ns UI timing). The spec cites two reasons for cable length limit, (a) Cable attenuation (sounds very funny today), and (b) "interference with transmitter". The 26ns corresponds to about 5m of cable. With source-termination there always will be a reflection, but I am not sure if the blown-up transmitter is the main reason for this limitation.

After introduction of USB2.0 and advent of HS termination scheme (on both ends), there is no reflections. The limitation of 5m now comes from:

(1) inability of industrial-quality cables (mass produced) to provide decent eye diagram at the end of 5m cable: the eye amplitude drops, and accumulated jitter becomes a problem. So the 5m limit was a good compromise between HS signal quality and FS signal legacy.

(2) The other limitation comes from the 5-tier hub architecture of USB, and includes hub repeater propagation time, device turnaround response, and cable delays across all five tiers. The USB2.0 protocol mandates a time-out for function response (ACK/NAK/...) of 1700ns worst case. The USB2.0 specifications align all these requirements into one coherent architecture.

Actually, it is not clear what comes first, cost of cables, or the protocol handshake time-out. But all these requirements are alinged, resulting in the standard cable prop delay of 26ns one-way.

If one can make a very good quality cable and use one-tier link, the cable can be of about 125m long, as this answer explains.

EDIT: If the actual question is "I connected my USB2.0 host port to some USB hub (with extra switches in data path) via a 20-m cable made of CAT7a, and it doesn't work. Why?", then the answer is:

The USB2.0 link has five elements in series: (1) Host port, (2) Device port, (3) cable, (4) host connector, and (5) device connector. All five components must perform above specifications for a 40-meter cable (100-ohms out of spec) to work.

(1) Host port must drive the HS signal at the high end of specs (440mV), and the port receiver sensitivity must be on the low end of squelch threshold (under 100mV);

(2) The device port must do the same as (1). Extra components in device path are usually not improving signal integrity, mostly due to substantial parasitic capacitance.

(3) The cable must be able to deliver at least 100mV eye opening across it, and edge jitter must keep the eye open.

(4) Interconnect on both ends must be perfect (board trace - connector - cable connector) on both ends.

If high-bandwidth test equipment (1-2 GHz scope and TDR time-domain reflectometer) with an expensive set of test fixtures is not available, it is impossible to determine which element of this link is at fault. It is likely that all five elements are contributing to inability of 20-meter link to deliver decodable signal. The specification call for 26ns prop limit across HS cable is not the limiting factor in this situation.