Hi: Let's assume USB 1.1, full speed in the following and transfer rate = 1.5 Mbit/s (Low-Bandwidth). The signal is NZRI and embedded in it is the clock. Each bit is 1 / 1.5 us = 0.667us= 667ns. Well, this is rather the question I make. That is, has the bit time to be 667ns or may it be any T, T >= 667ns. Put other way: has the data rate to be 1.5Mbit/s or, maybe, 1.5Mbit/s is just an upper bound for the data rate and any data rate below 1.5Mbit/s is acceptable? And hence T, the bit time, can be longer than 667ns? This question could have been done assuming 2.0, as it also admits 1.5Mbit/s.

You can retrieve the documentation from http://www.usb.org/developers/docs/
From USB Specification rev. 2.0, p. 159:
The low-speed data rate is nominally 1.50 Mb/s. For low-speed functions, the required data-rate when
transmitting (TLDRATE) is 1.50 Mb/s ±1.5% (15,000 ppm). This allows the use of resonators in low cost, lowspeed
devices.

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Thank you very much, elucches. That is exactly what I wanted to know.