Chapter 3 • Telecommunications and Networking 75to all computers on the wireless LAN, specifying the
amount of time for which the network is reserved for the
requesting computer.
In order to establish a wireless LAN, a wireless
network interface card(NIC) must be installed in each
computer. The wireless NIC is a short-range radio trans-
ceiver that can send and receive radio signals. At the heart
of a wireless LAN is the wireless access point (WAP),
which is a radio transceiver that plays the same role as a
hub in a wired Ethernet LAN. The WAP receives the
signals of all computers within its range and repeats them
to ensure that all other computers within the range can hear
them; it also forwards all messages for recipients not on
this wireless LAN via the wired network.
At the present time, there are four Wi-Fi LAN stan-
dards in use, with the newest standard (802.11n) approved
in 2009. The 802.11a Wi-Fi standard operates in the 5 GHz
(gigaHertz) band at data rates up to 54 mbps. The problem
with 802.11a is that the range is only about 150 feet; in
fact, the 54 mbps data rate can be sustained reliably only
within about 50 feet of the WAP. The 802.11b standard
operates in the 2.4 GHz band at data rates of 5.5 to 11 mbps.
The range of 802.11b LANs is typically 300 to 500 feet,
which is greater than that of 802.11a. The most widely
used Wi-Fi LAN standard today is 802.11g, which uses a
different form of multiplexing in the 2.4 GHz band to
achieve the same range as 802.11b (300 to 500 feet) with
data rates up to the 54 mbps of 802.11a. The recently
approved 802.11n standard uses both the 2.4 GHz and
5 GHz frequency ranges simultaneously (by using multiple
sets of antennas) in order to increase its data rate and its
range. The 802.11n standard provides five timesthe
throughput of earlier Wi-Fi standards—up to 300 mbps—
andtwice the range—up to 1,000 feet. Happily, the
802.11n standard is backward compatible with the earlier
Wi-Fi standards, so that it will co-exist with—and eventu-
ally replace—all of the earlier standards. Many commenta-
tors believe that the 802.11n standard, with its increased
speed, coverage, and reliability, will hasten the movement
to replace wired LANs with a wireless office environment
(Bulk, 2008; Moerschel, 2010).
The newest type of wireless network is WiMAX
(short for worldwide interoperability for microwave
access), which is based on the IEEE 802.16 family of
specifications. At the beginning of 2010 there were about
half a million WiMAX users in the United States, but
Clearwire—the leading vendor of WiMAX service—
expects that number to grow substantially over the next
few years. In early 2010, the Clearwire WiMAX service,
named Clear, was available in 27 markets across the
United States, covering 34 million people; Clearwire
expects those numbers to grow to 42 markets covering
120 million people by the end of 2010 (Clearwire, 2010).
In practical terms, WiMAX will operate very much like
Wi-Fi but over greater distances and for a greater number
of users.
There are actually two types of WiMAX. The IEEE
802.16d standard covers fixed-point wireless access, and it
is used to connect a central access point to a set of fixed
networks, such as from a branch office to a central office a
few miles away. Under ideal conditions, 802.16d provides
a data rate of 40 mbps for up to 20 miles, but actual data
rates and distances are much less. An important use of
802.16d is to connect multiple Wi-Fi public access points
(e.g., in a city-wide network) to a central switch so that
users can connect to the Internet.
By providing access to mobile users, the 802.16e
standard is designed to be direct competition for outdoor
Wi-Fi networks. The network is expected to provide up to
15 mbps of capacity and to have an effective range of up to
6 miles with a line of sight to the access point or 2 miles
without a line of sight; in practice, an 802.16e network
operates at about 4 to 8 mbps.Higher-Speed Wired Local Area Networks. LAN tech-
nology continues to advance as we move further into the
twenty-first century. The top speed of a traditional Ethernet
LAN is 10 mbps, but Fast Ethernet, operating at 100 mbps,
is now the most common form of Ethernet in new LANs.
Fast Ethernet uses the same CSMA/CD architecture and the
same wiring as traditional Ethernet. The most popular imple-
mentations of Fast Ethernet are 100 Base-T,which runs at
100 mbps over Category 5 twisted-pair cabling (four pairs of
wires in each cable), and 100 Base-F, which runs at
100 mbps over multimode fiber-optic cable (usually two
strands of fiber joined in one cable). Although the wiring for
Fast Ethernet could handle full-duplex communication, in
most cases only half-duplex is used.
Even newer and faster than Fast Ethernet is Gigabit
Ethernet, with speeds of 1 billion bps and higher. Gigabit
Ethernet is often used in backbone networks, to be
discussed in the next section. There are two varieties of
Gigabit Ethernet in use today: 1-gbps Ethernet, commonly
called1 GbE; and 10-gbps Ethernet, or 10 GbE. A 1 GbE
running over twisted-pair cables is called 1000 Base-T,
and it operates over one Category 5e cable (four pairs of
wires) by using an ingenious procedure to send streams of
bits in parallel. There are two versions of 1 GbE when
running over fiber-optic cabling: 1000 Base-SXuses multi-
mode fiber and 1000 Base-LXuses either multimode fiber
or single-mode fiber depending on the distances involved
(up to 1,800 feet with multimode fiber or over 16,000 feet
with single-mode fiber). Currently, 10 GbE is being
deployed in some backbone networks or in circumstances