TCP/IP Tutorial and Technical Overview - IBM Redbooks

Fixed versus mobile wireless
There are two types of wireless devices: fixed and mobile. Fixed devices are
stationary and draw their power from a utility main. An example of such a device
is a wireless router plugged into a wall outlet. Conversely, mobile devices are
those that have the capability of movement. Naturally, these are powered from
batteries. An example of this is a mobile computer.
Effects of multipath
Similar to a wired IP network, it is possible for radio signals to traverse different
paths between a source and destination. This can occur when one signal
encounters an obstruction. This can introduce delays into the traversal of signals
and is called multipath distortion.
System operating margin
The system operating margin defines the range in which error free reception is
achieved. This is calculated in dB as the difference between the received signal
level and the receiver’s sensitivity. For example, if the received signal is -15 dB,
and the sensitivity of the receiver is -10 dB, the system operating margin is 5 dB.
Free space loss
Free space loss is similar to path loss, except that path loss is experienced
between any two radio points and thus incorporates signal loss through various
types of media. Conversely, free space loss is specific to the lessening of a
signal as it traverses free space.
Decibel over isotropic (dBi)
Before decibel isotropic (dBi) units can be understood, the concept of an
isotropic antenna must first be explained. An isotropic antenna is theoretical, and
produces uniform signal strength in every direction, called isotropic radiation.
This sphere can then be used as a point of reference when measuring an actual
antenna’s strength. This measurement is made in units of dBi, and compares the
antenna’s strength relative to the isotropic radiation that would be created by an
isotropic antenna of the same strength. This is illustrated in Figure 10-2 on
page 394.
Chapter 10. Wireless IP 393