Page 133 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes antenna, which is wirelessly connected to other hub antennas, computers, servers, peripherals
and hosts via cabled connections. They also connect multiple hub antennas for transmission
between rooms, floors and buildings. In order to serve multiple hosts, spread-spectrum radio
technology is used to make effective use of limited bandwidth. Spread spectrum involves
scattering of packets of a data stream across a range of frequencies, rather than using a single
transmission frequency. A side benefit of spread-spectrum is of increased security, as the signal
is virtually impossible to intercept. Some wireless LANs also use direct sequence transmission
which means that a signal is sent simultaneously over several frequencies and therefore increasing
its chances of getting through to the access hub. Figure 8.12 shows an example of wireless
network block diagram.

Figure 8.12: Wireless LAN Configurations

The wireless network is becoming popular due to easy to setup feature and no cabling is involved.
The computers can be connected anywhere in the home and office without the need for wires.
Some of the features of wireless networks are given below:
Wireless LANs are a relatively immature technology but becoming popular very fast.

Acquisition costs are not particularly low when compared to wired LANs, although
reconfiguration costs are virtually nonexistent.
WLAN is mostly a mix of wire and wireless media having an access point or wireless
router that is connected to a wired network via a coaxial cable, universal serial bus (USB)
or Ethernet connection.
Frequency range lies in 900 MHz, 2 GHz and 5 GHz bands.

A hub antenna is located at a central point from where line-of-sight can be established
with the various terminal antennae.
Bandwidth of a wireless radio LAN is approximately 4 Mbps.

The effective throughput is more in the range of 1 to 2 Mbps per hub.
The infrared transmission technique can also be used. PDA (Personal Digital Assistant)
make widespread use of infrared to establish links with hosts and other PDA for data
transfer. Enhanced infrared technology recently has been demonstrated at speeds of 1.5, 4,
and even 155 Mbps.

Error performance and security are issues of some significance.
IEEE 802.11a and IEEE 802.11b are wireless network standards with a data rate of only 2
Mbps and 11 Mbps respectively. They have a distance limitation up to 100 feet from the
access point router. This uses 2.4 GHz band.
IEEE 802.11g allows speeds up to 54 Mbps and continues to use the 2.4 GHz band.

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