Page 187 - Handout Computer Network.

Computer Network 2026

Since the AP knows from the set power-transmission bit that the node is going to sleep, it (the
AP) knows that it should not send any frames to that node, and will buffer any frames destined
for the sleeping host for later transmission.

A node will wake up just before the AP sends a beacon frame, and quickly enter the fully active
state (unlike the sleepy student, this wakeup requires only 250 micro seconds [Kamerman
1997]!).
The beacon frames sent out by the AP contain a list of nodes whose frames have been buffered
at the AP.
If there are no buffered frames for the node, it can go back to sleep. Otherwise, the node can
explicitly request that the buffered frames be sent by sending a polling message to the AP.

With an inter beacon time of 100 msec, a wakeup time of 250 microseconds, and a similarly small
time to receive a beacon frame and check to ensure that there are no buffered frames, a node
that has no frames to send or receive can be asleep 99% of the time, resulting in a significant
energy savings.

Personal Area Networks: Bluetooth networks seem to have quickly become part of everyday life.
Perhaps you’ve used a Bluetooth network as a “cable replacement” technology to interconnect
your computer with a wireless keyboard, mouse, or other peripheral device.

Or perhaps you’ve used a Bluetooth network to connect your wireless earbuds, speaker, watch,
or health monitoring band to your smartphone or to connect your smartphone to a car’s audio
system.
In all of these cases, Bluetooth operates over short ranges (tens of meters or less), at low power,
and at low cost. For this reason, Bluetooth networks are sometimes referred to as wireless
personal area networks (WPANs) or piconets.
Although Bluetooth networks are small and relatively simple by design, they’re packed with many
of the link-level networking techniques that we’ve studied earlier including time division
multiplexing (TDM) and frequency division, randomized backoff, polling, error detection and
correction, reliable data transfer via ACKs and NAKS.
And that’s just considering Bluetooth’s link layer! Bluetooth networks operate in the unlicensed
2.4 GHz Industrial, Scientific and Medical (ISM) radio band along with other home appliances
such as micro waves, garage door openers, and cordless phones.

As a result, Bluetooth networks are designed explicitly with noise and interference in mind. The
Bluetooth wireless channel is operated in a TDM manner, with time slots of 625 microseconds.
During each time slot, a sender transmits on one of 79 channels, with the channel (frequency)
changing in a known but pseudo-random manner from slot to slot.

This form of channel hopping, known as frequency-hopping spread spectrum (FHSS), is used so
that interference from another device or appliance operating in the ISM band will only interfere
with Bluetooth communications in at most a subset of the slots. Bluetooth data rates can reach
up to 3 Mbps.

207