1.2 • THE NETWORK EDGE 21
home in a residential access network. The Optical Carrier (OC) standard link speeds
range from 51.8 Mbps to 39.8 Gbps; these specifications are often referred to as OC-
n, where the link speed equals n × 51.8 Mbps. Standards in use today include OC-1,
OC-3, OC-12, OC-24, OC-48, OC-96, OC-192, OC-768. [Mukherjee 2006,
Ramaswamy 2010] provide coverage of various aspects of optical networking.
Terrestrial Radio Channels
Radio channels carry signals in the electromagnetic spectrum. They are an attractive
medium because they require no physical wire to be installed, can penetrate walls,
provide connectivity to a mobile user, and can potentially carry a signal for long dis-
tances. The characteristics of a radio channel depend significantly on the propagation
environment and the distance over which a signal is to be carried. Environmental con-
siderations determine path loss and shadow fading (which decrease the signal strength
as the signal travels over a distance and around/through obstructing objects), multi-
path fading (due to signal reflection off of interfering objects), and interference (due
to other transmissions and electromagnetic signals).
Terrestrial radio channels can be broadly classified into three groups: those that
operate over very short distance (e.g., with one or two meters); those that operate in
local areas, typically spanning from ten to a few hundred meters; and those that
operate in the wide area, spanning tens of kilometers. Personal devices such as wire-
less headsets, keyboards, and medical devices operate over short distances; the
wireless LAN technologies described in Section 1.2.1 use local-area radio channels;
the cellular access technologies use wide-area radio channels. We’ll discuss radio
channels in detail in Chapter 6.
Satellite Radio Channels
A communication satellite links two or more Earth-based microwave transmitter/
receivers, known as ground stations. The satellite receives transmissions on one fre-
quency band, regenerates the signal using a repeater (discussed below), and transmits
the signal on another frequency. Two types of satellites are used in communications:
geostationary satellites and low-earth orbiting (LEO) satellites.
Geostationary satellites permanently remain above the same spot on Earth. This
stationary presence is achieved by placing the satellite in orbit at 36,000 kilometers
above Earth’s surface. This huge distance from ground station through satellite back
to ground station introduces a substantial signal propagation delay of 280 millisec-
onds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps,
are often used in areas without access to DSL or cable-based Internet access.
LEO satellites are placed much closer to Earth and do not remain permanently
above one spot on Earth. They rotate around Earth (just as the Moon does) and may
communicate with each other, as well as with ground stations. To provide continuous
home in a residential access network. The Optical Carrier (OC) standard link speeds
range from 51.8 Mbps to 39.8 Gbps; these specifications are often referred to as OC-
n, where the link speed equals n × 51.8 Mbps. Standards in use today include OC-1,
OC-3, OC-12, OC-24, OC-48, OC-96, OC-192, OC-768. [Mukherjee 2006,
Ramaswamy 2010] provide coverage of various aspects of optical networking.
Terrestrial Radio Channels
Radio channels carry signals in the electromagnetic spectrum. They are an attractive
medium because they require no physical wire to be installed, can penetrate walls,
provide connectivity to a mobile user, and can potentially carry a signal for long dis-
tances. The characteristics of a radio channel depend significantly on the propagation
environment and the distance over which a signal is to be carried. Environmental con-
siderations determine path loss and shadow fading (which decrease the signal strength
as the signal travels over a distance and around/through obstructing objects), multi-
path fading (due to signal reflection off of interfering objects), and interference (due
to other transmissions and electromagnetic signals).
Terrestrial radio channels can be broadly classified into three groups: those that
operate over very short distance (e.g., with one or two meters); those that operate in
local areas, typically spanning from ten to a few hundred meters; and those that
operate in the wide area, spanning tens of kilometers. Personal devices such as wire-
less headsets, keyboards, and medical devices operate over short distances; the
wireless LAN technologies described in Section 1.2.1 use local-area radio channels;
the cellular access technologies use wide-area radio channels. We’ll discuss radio
channels in detail in Chapter 6.
Satellite Radio Channels
A communication satellite links two or more Earth-based microwave transmitter/
receivers, known as ground stations. The satellite receives transmissions on one fre-
quency band, regenerates the signal using a repeater (discussed below), and transmits
the signal on another frequency. Two types of satellites are used in communications:
geostationary satellites and low-earth orbiting (LEO) satellites.
Geostationary satellites permanently remain above the same spot on Earth. This
stationary presence is achieved by placing the satellite in orbit at 36,000 kilometers
above Earth’s surface. This huge distance from ground station through satellite back
to ground station introduces a substantial signal propagation delay of 280 millisec-
onds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps,
are often used in areas without access to DSL or cable-based Internet access.
LEO satellites are placed much closer to Earth and do not remain permanently
above one spot on Earth. They rotate around Earth (just as the Moon does) and may
communicate with each other, as well as with ground stations. To provide continuous
