WI-FI (802.11)
This
section takes a closer look at a specific technology centered on the emerging
IEEE 802.11 standard, also known as Wi-Fi. Wi-Fi is technically a trademark,
owned by a trade group called the Wi-Fi alliance that certifies product
compliance with 802.11. 802.11 is designed for use in a limited geographical
area (homes, office buildings, campuses) and its primarily challenge is to
mediate access to a shared communication medium in this case, signals
propagating through space.
PHYSICAL PROPERTIES:
802.11
run over six different physical layer protocols. Five are based on spread
spectrum radio, and one on diffused infrared (and is of historical interest
only at this point). The fastest runs at a maximum of 54 Mbps.
The
original 802.11 standard defined two radio based physical layers standards, one
using frequency hopping and the other using direct sequence. Both provide up to
2 Mbps. Then physical layer standard 802.11 b was added. Using a variant of
direct exempt 2.4GHz frequency band of the electromagnetic spectrum. Then came
802.11a, which delivers up to 54 Mbps using a variant of FDM called orthogonal
frequency division multiplexing (OFDM). 802.11 a runs in the license-exempt
5GHz band. The most recent standard is 802.11g, which is backward compatible
with 802.11b.
COLLISION AVOIDANCE:
A wireless
protocol wait until the link becomes idle before transmitting and back off
should a collision occur. Consider the situation where A and C are both within
range of B but not each other. Suppose both A and C want to communicate with B
and so they each send it a frame. A and C are unaware of each other since their
signals do not carry that far. These two frames collide with each other at B,
but unlike an Ethernet, neither A or C is aware of this collision. A and C are
said to be hidden nodes with respect to each other.
A related
problem called the exposed node problem where each of the four nodes is able to
send and receive signals that reach just the nodes to its immediate left and
right. For EX: a B can exchange frames with A and C but it cannot reach D ,
while C can reach B and D but not A. Suppose B is sending to A. Node C is aware
of this communication because it hear B‟s
transmission. It would be a mistake, however, for C to conclude that it cannot
transmit to anyone just because it can hear B‟s
transmission. For example, suppose C wants to transmit to node D. This is not a
problem since C‟s
transmission to D will not interfere with A‟s ability
to receive from B.
802.11
addresses these two problems with an algorithm called multiple access with
collision avoidance (MACA). The idea is for the sender and receiver ot exchange
control frames with each other before the sender actually transmits any data.
This exchange informs all nearby nodes that a transmission is about to begin.
Specifically, the
sender
transmits a Request to send (RTS) frame to the receiver; the RTS frame
includes a field that indicates how long the sender wants to hold the medium.
The receiver then replies with a clear to send (CTS) frame. This frame
echoes this length field back to the sender. Any node that sees the RTS frame will
collide with each other.
802.11
does not support collision detection, but instead the senders realize the in
which case they each wait a random amount of time before trying again. The
amount of time a given node delay is defined by the same exponential backoff
algorithm used on the Ethernet.
DISTRIBUTION SYSTEM
Instead
of all nodes created equal, some nodes are allowed to roam and some are
connected to a wired network infrastructure. 802.11 calls these base stations access
points (AP), and they are
connected to each other by a so-called distribution system. A distribution system that connects three access
points, each of which services the nodes in some region. Although two nodes can
communicate directly with each other if they are within reach of each other,
the idea behind this configuration is that each node associates itself with one
access point. For node A to communicate with node E, for example, A first sends
a frame to its access point (AP-1), which forwards the frame across the
distribution system to AP-3 , which finally transmits the frame to E.
The
technique for selecting an AP is called scanning and involves the following
four steps:
1. The node
sends a probe frame;
2. All APs
within reach reply with a probe Response frames;
3. The node
selects one of the access points, and sends that AP an Association Request
frames;
4. The AP
replies with an Association Response frame.
Because
the signal from its current AP has weakened due to the node moving away from
it. Whenever a node acquires a new AP, the new AP notifies the old AP of the
change via the distribution system.
Here in
this fig., where node C moves from the cell serviced by AP-1 to the cell
serviced by AP-2.At some point, C prefers AP-2 over AP-1,and so it associates
itself with that access point.
The
mechanism just described is called active scanning since the node is actively
searching for an access point. APs also periodically send a BEACON frame that
the capabilities of the access point; these include the transmission rates
supported by the AP.
This is
called passive scanning, and a node can change to this AP based on the BEACON
frame simply by sending an ASSOCIATION REQUEST frame back to the access point.
FRAME FORMAT:
The frame
contains the source and destination node address, each of which is 48 bits
long, up to 2,312 bytes of data, and a 32-bit CRC. The Control field contains
three subfields of interest : a 6-bit Type field that indicates whether the
frame carries data, is an RTS or CTS frame, or is being used by the scanning
algorithm; and a pair of 1-bit fields-called ToDS and .
The
802.11 frame format is that it contains four, rather than two, address. how
these address are interpreted depends on the settings of the ToDS and FromDS
bits in the frame‟s Control
field. This is to account for the possibility that the frame had to be
forwarded across the distribution systems, which would mean that the original
sender is not necessarily the same as the most recent transmitting node.
Similar
reasoning applies to the destination address. In the simplest case, when one
node is sending directly to another, the DS bits are 0, Addr1 identifies the
target node, and Addr2 identifies the source node.
In the
most complex case, both DS bits are set to 1, indicating that the message went
from a wireless node onto the distribution system and then from the
distribution system to another wireless node. With both bits set, Addr1
identifies the ultimate destination, Addr2 identifies the immediate sender (the
one that forwarded the frame from the distribution system to the ultimate destination),
Addr3 identifies the intermediate destination (the one that accepted the frame
from a wireless node and forwarded it across the distribution system), and
Addr4 identifies the original source. In terms of the example given in fig.,
Addr1 corresponds to E, Addr2 identifies AP-3, Addr3 corresponds to AP-1, and
Addr4 identifies A.
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