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272 CHAPTER NINE
It should theoretically be possible to buy any DOCSIS modem and use it on the cable
system, but this may not always be the case. DOCSIS provides data rates downstream
up to 56 Mbps and upstream rates up to 3 Mbps. However, all the data bandwidth must
be shared among all the users (and the sideband control information). More informa-
tion is provided at www.cable-modems.org/tutorial/01.htm (follow the page links) and
at www.iec.org/online/tutorials/cable_mod/ (follow the page links).

Local Area Networks (LANs)
One of the most popular wired communication systems for computers is the LAN and
the Internet. A LAN is a method of connecting computers together in a building or small
campus. The Internet is the network connecting computers together worldwide. We’ll
take a look at the physical layer first and then discuss some of the basics needed to plan
a LAN connection for the robot.

Physical Layer The most popular method of connecting to a LAN is the Ethernet.
Most computers have NIC cards or connections that can accept Ethernet connectors at
10BT/100BT data rates. 1000BT (and variants thereof) provides 10 times the band-
width, but we’ll ignore it for now. The top end of the commonly available data band-
width is 100 Mbps with 100BT. But as we discussed earlier, the Ethernet LAN is a
slotted Aloha system without reservations. As such, uncoordinated traffic tends to top
out at 18 percent of the raw capacity of the network. The transmission method is base-
band NRZ data, as discussed earlier, but the tough part is not the physical layer.
The toughest part of using a LAN is the configuration. It takes an expert to tame a
LAN system, so plan on consulting heavily with information technology (IT) person-
nel before deciding on a LAN for the robot. They’ll ask tons of questions before setting
up the LAN so the robot can use it. They’ll provide the IP address the robot needs to
function as well as the connectivity to the other computers the robot will need to
address. In addition, they can set up the robot with other services it may require, like
email and Internet access.
We must cover a couple of basics before turning the robot loose on the LAN. Books
that explain how to effectively use LANs are often a thousand pages long. The follow-
ing are the basic facts most often used in designing a LAN communication link for a
robot. These facts do not sufficiently explain all the details of how to finish the engi-
neering on the LAN link, but they outline the capabilities of a LAN so the robot’s com-
munication link can be planned.