- The IEEE wireless LAN
is 802.11
- There are different versions
in existence:
- 802.11a: (1999)
operates in the 5 GHz band with a maximum net data rate of 54 Mbit/s
- 802.11b: (1999)
operates in the 2.4 GHz band with a maximum net data rate of 11 Mbit/s
This became very popular
(BTW, microwave ovens operates in the 2,4 GHz range !!!)
- 802.11g: (2003)
operates in the 2.4 GHz band with a maximum net data rate of 54 Mbit/s
It is fully backwards compatible with 802.11b.
- 802.11n: (2009)
amendmentment
to the
existing 802.11
standards....
Most important amendment: MIMO (sounds like Nemo the fish...)
MIMO: multiple input, multiple output
- MIMO
Signal Processing uses multiple antennas
that
send and receive data at the same time to improve signal coherence.
- The receiver listens to multiple signals on multiple antennas at the same time.
- MIMO
Signal Processing uses multiple antennas
that
send and receive data at the same time to improve signal coherence.
- 802.11a: (1999)
operates in the 5 GHz band with a maximum net data rate of 54 Mbit/s
- An 802.11 Lan is
subdivided into cells
- Each cell is called a
Basic Service Set (BSS)
- A BSS is controlled by
an Access Point (AP)
- Most 802.11 networks consist
of multiple APs
The APs are interconnected together by a backbone network.
Example:
The backbone network is called a distributed system in 802.11 (something that a computer scientist would not call....)
The whole interconnected wireless LAN is called a Extended Service Set (ESS)
- 802.11 Wireless router and network adapter
- Physics:
- The signal strength of wireless transmission attenuates very rapidly - at a rate of 1/r3 where r is the distance)
- This results in a
limited range
of reception.
Example:
Notes:
- The letters indicate the physical location of each node
- The circles
indicate the transmission range
of each node
- Node B is in A's range.
- Nodes A and C are in B's range.
- Nodes B and D are in C's range.
- Node C is in D's range.
- The letters indicate the physical location of each node
- Other facts:
- The range
is relatively short:
only
a few hundred of feet.
(E.g., wireless hotspots in a cafe does not go beyond a few hundred feet)
- Du of the short distance, transmissions of nodes will received by their neighbor nodes almost instantaneously - just as the case in a LAN
- The range
is relatively short:
only
a few hundred of feet.
- Question:
- Since the propagation delay is so short, can a wireless node use carrier sensing to avoid collisions ?
Answer:
- No and the reason is pretty length....
- Difference between
wireless LAN and
wired LAN:
- Wired LAN:
-
A transmission from any node
will be "heard"
by
all nodes
in the network
- Wireless LAN:
-
A transmission from a node
will only
be "heard"
by
nodes within the range
of the transmitting node
The limited range makes a significant difference in the ability to prevent collision by sensing the channel
- Wired LAN:
-
There are two
well-known problems
caused by the
limited range of
transmissions:
- The Hidden node problem
- The Exposed node problem
- Hidden node:
- A hidden node is a
node that is outside your range
I.e., you cannot be aware of its existence (because you cannot transmit to nor receive anything from that node)
- A hidden node is a
node that is outside your range
- Example: (hidden node)
- Node A is
outside the range
of node C
So Node A is a hidden node (with respect to Node C)
- Node A is
outside the range
of node C
- Hidden node problem:
- Due to limited range, your transmissions may collide with transmissions from hidden nodes without you being aware of them
Example: (hidden node problem)
- Node A is a
hidden node
(with respect to Node C)
Suppose: Node C wants to send a message to D...
- Node C senses that the channel and find that it is idle (no transmission) ....
- Node C can be
misled by the result
because the
hidden node A can
be transmitting:
The Hidden Node problem:
- C
cannot hear
A's transission
- But, if Node C does transmit, it will cause an collision at node B !!!
- C
cannot hear
A's transission
- Exposed node:
- A exposed node is a node that is within your range
- Example: (exposed node)
- Node B is
inside the range
of node C
So Node A is a hidden node (with respect to Node C)
- Node B is
inside the range
of node C
- Exposed node problem:
- Due to limited range, your transmissions may not interfere with transmissions from exposed nodes even when they are transmitting
Example: (exposed node problem)
- Suppose Node B is currently
transmitting
to Node A:
Suppose: Node C wants to send a message to D...
- Node C senses that the channel and find that it is busy ....
- Node C can be
misled by the result
because the
transmission
from B ⇒ A
will not interfere
with the transmission
from C ⇒ D:
The Exposed Node problem:
- C
hear
B's transission
- But, if Node C does transmit (to D), it will not cause a collision !!!
- C
hear
B's transission
- Summary of the above results:
- If a wireless node finds the
channel idle,
there is the
possibility
than its transmission will
collide with a
current transmission from a
hidden node...
- If a wireless node finds the channel busy, there is the possibility than the current transmission will not collide with its own transmission...
Conclusion:
- Carrier sensing is
not
very effective
in wireless networks...
(because the information that a node get from sensing the carrier does not tell him whether his transmission will or will not interfere with another transmission)
- But... it is not completely useless either :-)
- If a wireless node finds the
channel idle,
there is the
possibility
than its transmission will
collide with a
current transmission from a
hidden node...
- Normal cases:
- Usually, there are
some hidden nodes but
most of the
nodes are
not hidden
- So carrier sensing
still
makes sense...
- But:
carrier sensing alone
is
not
sufficient
In addition to carrier sensing, we need heavier duty techniques in wireless communication to avoid collision...
- Usually, there are
some hidden nodes but
most of the
nodes are
not hidden
- Measures to improve reliablity in wireless communication:
- Use spread frequency spectrum
(frequency hopping)
to allow signal to
overcome
multiple simultaneous transmissions
- Use a reservation technique to prevent neighboring nodes from interfering
- Use spread frequency spectrum
(frequency hopping)
to allow signal to
overcome
multiple simultaneous transmissions