15.6 / IEEE
802.15
501
back-to-back packets equal
to
the size
of
the token bucket.
If
the
token
bucket size
is
large, this back-to-back run may
be
long enough to exceed
the
recipient's capacity.
To limit this effect, the maximum transmission
rate
bounds how fast successive
packets may be placed
on
the network.
The
latency
is
the maximum acceptable delay between transmission
of
a bit by
the
sender
and its initial transmission over the air, expressed in microseconds.
The
delay variation
is
the
difference, in microseconds,
between
the
maximum
and
minimum possible delay
that
a
packet
will experience. This value
is
used by
applications to
determine
the
amount
of
buffer space
needed
at
the
receiving side
in
order
to restore
the
original
data
transmission pattern.
If
a receiving application
requires
data
to
be
delivered in
the
same
pattern
that
the
data
were transmitted,
it
may
be
necessary for
the
receiving host briefly
to
buffer
data
as they are received
so
that
the receiver
can
restore
the
old transmission
pattern.
An
example
of
this is
a case
where
an
application wishes to send
and
transmit
data
such as voice sam-
ples, which
are
generated
and
played at regular intervals.
The
amount
of
buffer
space
that
the
receiving host is willing to provide determines
the
amount
of
varia-
tion in delay
permitted
for individual packets within a given flow.
The
IEEE
802.15 Working Group for Wireless Personal
Area
Networks (PANs) was
formed to develop standards for short range wireless PANs (WPANs).A PAN
is
com-
munications network within a small area in which all of the devices
on
the network
are typically owned by one person
or
perhaps a family. Devices
on
a PAN may include
portable and mobile devices, such
as
PCs, Personal Digital Assistants (PDAs), periph-
erals, cell phones, pagers, and consumer electronic devices. The first effort by the
working group was to develop 802.15.1, with the goal of creating a formal standard of
the Bluetooth specification; this standard was approved in 2002.
Because most or all
of
the planned 802.15 standards would operate in the same
frequency bands as used by 802.11 devices,
both
the 802.11 and 802.15 working
groups were concerned about the ability of these various devices to successfully
coexist. The 802.15.2 Task
Group
was formed
to
develop recommended practices for
coexistence.This work resulted in a recommended practices document in 2003.
Following the 802.15.1 standard, the 802.15 work went in two directions. The
802.15.3 task group
is
interested in developing standards for devices
that
are low
cost
and
low power compared
to
802.11 devices,
but
with significantly higher
data
rates
than
802.15.1.
An
initial standard for 802.15.3 was issued in 2003 and, as
of
this
writing, work continues
on
802.15.3a, which will provide higher
data
rates than
802.15.3, using the same
MAC
layer. Meanwhile,
the
802.15.4 task group developed
a
standard
for very low cost, very low power devices at data rates lower than
802.15.1, with a standard issued in 2003.
Figure 15.17 shows the current status
of
the 802.15 work.
Each
of
the
three
wireless PAN standards has
not
only different physical layer specifications
but
dif-
ferent requirements for the
MAC
layer. Accordingly, each has a unique
MAC
speci-
fication. Figure 15.18, based
on
one in [ZHEN04], gives
an
indication
of
the relative
scope
of
application
of
the wireless
LAN
and PAN standards. As can
be
seen,