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Networking Fundamentals
Multiple Input
Multiple Output
(MIMO)
a wireless
networking tech-
nology that transmits
two or more streams
of data to increase
data throughput and
the range of the wire-
less network.
spatial multiplexing
a wireless
networking tech-
nology that transmits
two or more streams
of data in the same
frequency channel.
An 802.11n device is compatible with 802.11a, 802.11b, and 802.11g, but may
not support MIMO technology when paired with these devices. Multiple Input
Multiple Output (MIMO) is a wireless networking technology that uses two
or more streams of data transmission to increase data throughput and the range
of the wireless network. Transmitting two or more streams of data in the same
frequency channel is referred to as spatial multiplexing.
802.11n incorporates the Multiple Input Multiple Output (MIMO) technology
using 5-GHz and 2.4-GHz frequencies with an expected data rate of approximately
300 Mbps to 600 Mbps. The exact speed depends on the number of simultaneous
data streams transmitted. Some 802.11n devices are advertised with data rates
much higher than specifi ed in the standard.
MIMO was designed as an integral part of the 802.11n standard. Some
manufacturers have incorporated MIMO into the latest revisions of their 802.11g
devices to take advantage of producing higher data rates.
Prior to MIMO, Wireless Access Points and devices sent a single stream of
data between the transmitter and receiver. MIMO uses two or more antennae,
thus creating two or more streams of data simultaneously. For example, an
802.11n wireless device that transmits data at 150 Mbps can apply MIMO and
raise the data rate to 300 Mbps. The latest 802.11n standard is 144.4 Mbps or
rounded up to 150 Mbps. By using MIMO, 802.11n devices can raise the expected
data rate to nearly 300 Mbps when using a 20-MHz channel and 600 Mbps when
using a 40-MHz channel inside the assigned 2.4-GHz frequency range. The
40-MHz channel can carry twice the data as the 20-MHz channel.
The WI-FI Alliance uses a similar method in its standard and achieves a
maximum throughput of 450 Mbps as posted on their Web site at the time of this
writing. Visit the Wi-Fi Alliance Web site (http://wi-fi.org).
802.11 Access Method
802.11 networks rely on Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA) as a media access protocol. Do not confuse this with
CSMA/CD, which is used by IEEE 802.3 networks. The Carrier Sense Multiple
Access (CSMA) portion of the technology is the same for both the 802.11 and
802.3 networks. The difference is in the Collision Detection (CD) versus Collision
Avoidance (CA). Collision Detection (CD) detects a collision on the network after
it occurs, while Collision Avoidance (CA) attempts to avoid a collision.
To understand the CSMA/CA process, look at Figure 4-19. The laptop
equipped with a wireless card fi rst listens for network traffi c. If the airwaves are
clear, it signals the WAP with a request to send (RTS) message. The WAP returns
either a clear to send (CTS) or busy signal to the laptop. The process is repeated
until the laptop is cleared to send data. After the data has been sent to the
WAP, the WAP sends an acknowledge (ACK) to the laptop. Collision avoidance
technology solves the problem of broadcast storms, which are associated with
collision detection technology. The collision detection technology is designed to
accept network collisions, wait, and then resend the complete transmission.
The CSMA/CA access method was selected over CSMA/CD because of the
nature of wireless media. In a typical Ethernet environment that uses cable, a
collision can be detected anywhere on a segment by all nodes in that segment.
The same is not true of a wireless network. Look at Figure 4-20 and compare
the two networking technologies. In the cable-based network, all nodes are
connected to the same segment via a hub. All nodes can detect any broadcast that
takes place. There is no need to broadcast an intention to communicate.