Finally, immediately following an SCO transmission from the master.

Finally, a device in park mode not only stops listening,
but also gives up its active member address. It is only a member of the piconet
in that it remains synchronizing with the frequency-hopping pattern.

1.4.3    Baseband

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The Indoor Propagation base band provides transmission
channels for both data and voice communication. The basic indoor propagation
system with Bluetooth supports asynchronous data links with three synchronous
voice channels. Synchronous connection-oriented (SCO) links are typically used
for voice transmission. These are point-to-point symmetric connections that
reserve tome slots in order to guarantee timely transmission. The slave device
is always allowed to respond during the time slot immediately following an SCO
transmission from the master. A master link can communicate with single slave
link directly and also support two SCO links to different masters. SCO packets
are never retransmitted. 

Asynchronous connectionless (ACL) links are typically used
for data transmission. Transmissions on these links are established on a
per-slot basic (in slots not reserved for SCO links). ACL links support
point-to-multipoint transfers of either asynchronous or synchronous data. After
an ACL transmission from the master, only the addressed slave device may
respond during the next time slot, or if no device is addressed the packet is
considered a broadcast message. Most ACL links include packet retransmission.

1.4.4    Link

The base band state machine is controlled largely by the
link manager. This firmware, generally provided with the link control hardware,
handles link setup, security and control. Its capabilities include
authentication and security services, quality of service monitoring, and base
band state control. The link manager controls paging. Changing slave modes, and
handling required changes in master / slave roles. It also supervises the link
and controls handling of multi-slot packets.

Link managers communicate with each other using the Link
Management Protocol (LMP) which uses the underlying base band services
including the generation of management packets that communicated as payload.
Payloads are differentiated from logical link control and adaptation protocol
(L2CAP) packets by a bit in the ACL header. They are always sent as the
single-slot packet with higher priority than L2CP packets. This helps ensure
the integrity of the link under high traffic demand.

1.4.5    Transmission Signal

single carrier system modulates information onto one carrier using frequency,
phase, or amplitude adjustment of the carrier. For digital signals, the
information is in the form of bits, or collections of bits called symbols, that
are modulated onto the carrier. As higher bandwidths (data rates) are used, the
duration of one bit or symbol of information becomes smaller. The system
becomes more susceptible to loss of information from impulse noise, signal
reflections and other impairments. These impairments can impede the ability to
recover the information sent. In addition, as the bandwidth used by a single
carrier system increases, the susceptibility to interference from other
continuous signal sources becomes greater. This type of interference is
commonly labeled as carrier wave (CW) or frequency interference. Frequency
division multiplexing (FDM) extends the concept of single carrier modulation by
using multiple subcarriers within the same single channel. The total data rate
to be sent in the channel is divided between the various subcarriers. The data
do not have to be divided evenly nor do they have to originate from the same
information source. FDM offers an advantage over single-carrier modulation in
terms of narrowband frequency interference since this interference will only
affect one of the frequency sub-bands. The other subcarriers will not be
affected by the interference. Since each subcarrier has a lower information
rate, the data symbol periods in a digital system will be longer, adding some
additional immunity to impulse noise and reflections. FDM systems usually
require a guard band between modulated subcarriers to prevent the spectrum of one
subcarrier from interfering with another. 
These guard bands lower the system’s effective information rate when
compared to a single carrier system with similar modulation.

FDM system discussed above can use a set of subcarriers that were orthogonal to
each other to achieve a higher level of spectral efficiency as the guard bands
needed to allow individual demodulation of subcarriers in an FDM system would
no longer be necessary. The use of orthogonal subcarriers would allow the
subcarriers’ spectra to overlap, thus increasing the spectral efficiency. As
long as orthogonality is maintained, it is still possible to recover the
individual subcarriers’ signals despite their overlapping spectrums. If the dot
product of two deterministic signals is equal to zero, these signals are said
to be orthogonal to each other. Orthogonality can also be viewed from the
standpoint of stochastic processes. If two random processes are uncorrelated,
then they are orthogonal. Given the random nature of signals in a
communications system, this probabilistic view of orthogonality provides an
intuitive understanding of the implications of orthogonality in Communication.

signal in the vector space of the DFT can be represented as a linear
combination of the orthogonal sinusoids. One view of the DFT is that the
transform essentially correlates its input signal with each of the sinusoidal
basis functions. If the input signal has some energy at a certain frequency,
there will be a peak in the correlation of the input signal and the basis
sinusoid that is at that corresponding frequency. This transform is used at the
Communication transmitter to map an input signal onto a set of orthogonal
subcarriers, i.e., the orthogonal basis functions of the DFT. Similarly, the
transform is used again at the Communication receiver to process the received
subcarriers. The signals from the subcarriers are then combined to form an
estimate of the source signal from the transmitter. The orthogonal and
uncorrelated nature of the subcarriers is exploited in Communication with
powerful results. Since the basis functions of the DFT are uncorrelated, the
correlation performed in the DFT for a given subcarrier only sees energy for
that corresponding subcarrier. The energy from other subcarriers does not
contribute because it is uncorrelated. This separation of signal energy is the
reason that the Communication subcarriers’ spectrums can overlap without
causing interference 1. Communication was introduced in the 1950s but was
first implemented in the 1960s. It was originally developed from the
multi-carrier modulation techniques used in high frequency military radios. A
patent for Communication was granted in the 1970s. However, when Communication
was first introduced, it was not very popular because of the cost and
complexity of large arrays of sinusoidal generators and coherence demodulators
2. The actual widespread use of Communication started after the inverse
discrete Fourier transform (IDFT) and discrete Fourier transform (IDFT) made
the Communication implementation possible without the use of large number of
sinusoidal generators.