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44 Chapter 2
while asynchronous transfers are delay-tolerant and are able to wait for available
bandwidth. USB control protocols are designed specifically to give a low protocol
overhead, resulting in highly effective utilization of the available bandwidth.
This available bandwidth is shared among all connected devices and is allocated using
“ pipes, ” with each pipe representing a connection between the host and a single device.
The bandwidth for a pipe is allocated when the pipe is established, and a wide range of
different device bit rates and device types can be supported concurrently. For example,
digital telephony devices can be concurrently accommodated ranging from 1 “ bearer ”
plus 1 “ demand ” channel (64 kbps—see ISDN above) up to T1 capacity (1.544 Mbps).
USB employs NRZI (non return to zero inverted) as a data encoding scheme. In NRZI
encoding, a 1-bit is represented by no change in output voltage level and a 0-bit is
represented by a change in voltage level ( Figure 2.12 ). A string of 0-bits therefore causes
the NRZI output to toggle between states on each bit cycle, while a string of 1-bits causes
a period with no transitions in the output.
NRZI has the advantage of a somewhat improved noise immunity compared with the
straight encoding of the input data stream as output voltages.
2.4.2 Physical Layer Technologies—Wireless Networks
The PHY layer technologies that provide the Layer 1 foundation for wireless networks
will be described further in Parts III, IV and V, where LAN, PAN and MAN technologies
and their implementations will be covered in detail.
Data bits
1 0 1 1 0 0 1 0 1 0
Non return
to zero (NRZ)
Non return
to zero
inverted (NRZI)
Bit
period
Figure 2.12 : USB NRZI data encoding scheme
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