Managing Information Technology

74 Part I • Information Technology

The token bus design is central to the
Manufacturing Automation Protocol (MAP), which was
developed by General Motors and adopted by many
manufacturers. MAP is a factory automation protocol (or
set of standards) designed to connect robots and other
machines on the assembly line by a LAN. In designing
MAP, General Motors did not believe it could rely on a
contention-based LAN with a probabilistic delay time
before a message could be sent. An automobile assembly
line moves at a fixed rate, and it cannot be held up because
a robot has not received the appropriate message from
the LAN. Therefore, General Motors and many other
manufacturers have opted for the deterministic token
bus LAN design.
The third LAN standard is the token ring, originally
developed by IBM, which combines a ring topology
(see Figure 3.4) with the use of a token as described for the
token bus. A device attached to the ring must seize the
token and remove it from the ring before transmitting a
message; when the device has completed transmitting, it
releases the token back into the ring. Thus, collisions can
never occur, and the maximum delay time before any
station can transmit is deterministic. The usual implemen-
tation of a token ring involves the use of a wire center into
which cables from individual devices are plugged, creating
a physical star but a logical ring.
All three types of wired LAN designs are in use
today. Token bus dominates the manufacturing scene, and
Ethernet leads token ring by a wide and growing margin in
office applications. But the hottest type of LAN in the
early twenty-first century is the wireless LAN, to which
we will now turn.

Wireless Local Area Networks. Mostwireless LANs
in use today are of the Wi-Fi(short for wireless fidelity)
variety, although WiMAX networks are beginning to

appear (more on WiMAX later). Wi-Fi LANs are rapidly

proliferating. Wi-Fi technology has obvious advantages for

people on the move who need access to the Internet in

airports, restaurants, and hotels and on university campus-

es. Wi-Fi is also gaining acceptance as a home or neigh-

borhood network, permitting an assortment of laptop and

desktop computers to share a single broadband access

point to the Internet. Numerous city-wide Wi-Fi networks

were developed in the early twenty-first century, but many

of these have been commercial failures; it now appears that

other technologies, such as WiMAX and other 4G

networks operated by the cellular carriers, will form the

basis of wide-area wireless networks (Martin, 2008;

Merron, 2007). Wireless LANs have also moved into the

corporate and commercial world, especially in older build-

ings and confined spaces where it would be difficult or

impossible to establish a wired LAN or where mobility is

paramount. Even in newer buildings, wireless LANs are

often being employed as overlay networks.In such cases

Wi-Fi is installed in addition to wired LANs so that

employees can easily move their laptops from office to

office and can connect to the network in places such as

lunchrooms, hallways, and patios (FitzGerald and Dennis,

2009, p. 244).

Today’s Wi-Fi LANs use one of the standards incor-

porated in the IEEE 802.11 family of specifications. All

these standards use the shared Ethernet design (logical bus,

physical star; see Figure 3.7) and the CSMA/CA Protocol,

which is an abbreviation for Carrier Sense Multiple Access

with Collision Avoidance. CSMA/CA is quite similar to

CSMA/CD used in traditional Ethernet, but it makes

greater efforts to avoid collisions. In one approach to

collision avoidance, any computer wishing to transmit a

message first sends a “request to transmit” to the wireless

access point (WAP). If no other computer is transmitting,

the WAP responds by sending a “clear to transmit” signal

Bus

Laptop

Laptop Laptop Laptop Laptop

Laptop

Switch to

rest of network

Wireless Access Point

FIGURE 3.7 Wi-Fi Local Area Network Topology