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AFB  ®
Technology News for People Who Are Blind or Visually Impaired
From the American Foundation for the Blind
 May 2002 Issue  Volume 3  Number 3

Access Issues

Networking at Lunch: Part 1

Lunches are a great time to network, but those who say so probably don't have home computer network design and implementation in mind. Nevertheless, I spent several lunches explaining the principles of building a home network to a lunch buddy. Each time we met for lunch, he would report on his progress and ask the next set of questions that launched our lunchtime discussions.

Since home computer networks have been a frequent topic on some of the blindness-related mailing lists I read, I decided to relate our network lunches for AccessWorld readers. Don't be put off by blindness. There's nothing here that can't be done without sight. Sometimes accessibility is little more than a good explanation and the self-confidence to proceed. So, come to lunch and listen in as we build a home computer network.

Lunch 1: Why Set Up a Home Network?

Computers have moved rapidly from the domain of corporation and university settings to a prominent place in the home. In fact, more and more homes now use more than one computer. In my case, there are often four computers in my home. I have two systems in my home office, my college-student daughter has a system in her room, and I frequently bring my laptop home from work. Operating multiple systems as unconnected islands limits what can be done, can increase overall computing costs, and prevents you from taking advantage of some real conveniences. Sharing a dial-up Internet account across multiple systems is worse than waiting your turn to use the one dormitory pay phone behind all your classmates. Moving a file by floppy disk to the one machine with the color printer attached can become tedious as you work through multiple drafts of a document. Do you really want to spend the additional money to buy duplicates of extra peripheral devices like CD burners or zip drives for all your systems?

These are reasons to install a home data network. A single cable or DSL Internet connection can be shared simultaneously from multiple networked computers. Printers can be used by any system on the network. And data can be moved to the particular system with those special devices. Beyond these applications, a network has other advantages, such as quickly making backup copies of important data from another hard drive and sharing a single copy of data from the source system with the other machines on the network. For example, I have extracted most of my audio CD music collection to MP3 files on my main system; through the network, I can play this music on any computer in the house.

Although networks are useful if you use a dial-up connection to your Internet service provider (ISP), they are far more important when a cable modem or DSL service is in place. As we discuss network components, hardware selection, and software setup, I will assume that you use a high-speed Internet connection.

Lunch 2: What Are the Basic Components and Concepts in a Home Network?

Many terms are thrown around when discussing a computer network. Among the more prevalent are NIC, hub, switch, router, DMZ host, firewall, IP address, DHCP, and wireless access point. Let's take a minute to sort out this technospeak and discuss what you need to know about network components and concepts.

To communicate via a network, a computer must have a hardware device that provides the connection to the network. This device is the network interface card, usually denoted by the acronym NIC. The NIC is a hardware circuit card that is installed in your system just like a modem or sound card. It has an RJ-45 jack on the back that is similar to a modular telephone jack, although somewhat larger. NICs are available from several manufacturers across a range of prices, depending on the number of features. But a basic NIC that will meet the needs of most users should cost about $20.

At this point, it's worth saying a few words about network speed. The information travels at a specific speed as it traverses the wires in your network. This speed is determined by the NIC and other network components and is typically either 10 Mbps or 100 Mbps—that is, 10 million or 100 million bits per second. Some older network devices were limited to 10 Mbps, but all newer equipment supports the faster 100 Mbps. As you shop for your network hardware, select components that support 100 Mbps. Some will be marked 10/100 Mbps, indicating that the device can run at either speed. These devices are fine to use in your home network.

Package Deal

In addition to a standard speed, the NIC places the data in a well-defined package as it is sent over the network. These packages are called packets, and are defined in the IP (Internet protocol) international standard. Each packet includes a source and destination address, as well as the actual data and a mathematical check sum that is used to determine if any of the information in the packet was corrupted in transit. Each computer must be assigned a unique IP address in the network. IP addresses consist of four sets of numbers separated by periods, each between 0 and 255. Thus, an IP address on a home network may be 192.168.1.101. All computers on the Internet have such an address, but we usually refer to them by a name, such as <www.afb.org. This URL (uniform resource locator) is actually translated to a numeric IP address by a network component called a domain name server (DNS). In the case of www.afb.org>, it gets translated to IP address 216.205.124.42. Besides this computer address, a complete IP address also includes a port number. The port number indicates the service that is being requested on the addressed computer, such as a web page, e-mail, or file transfer. For now, the important part of all this is to remember that each of your computers must have a unique IP address within your home network. Later, I'll tell you how to assign these addresses.

Getting Connected

Although two NICs can be connected directly to each other with a special cable called a "crossover" cable, networked computers are not usually connected in this manner. Rather, each NIC is connected to a network hub or switch. Hubs and switches are available that support a variety of ports, such as four ports, eight ports, or even more for large networks. Here the number of ports refers to the number of other devices that can be connected. Thus, a four-port hub can connect up to four computers or other network components. A hub is a passive device. That is, it simply takes any electrical signal entering via one of its ports and sends it out through all other ports. Essentially, it performs the function of tying all the computers together in a conference call. Each computer connected to the hub receives all data packets and is responsible for looking at the destination address and discarding any packets that are not addressed to it. So, if computer 1 is sending to computer 2 while computer 3 is sending to computer 4, computer 2 will accept the packets addressed to it from computer 1 and ignore the packets addressed to computer 4.

Crash Course

Building a large network around a hub is inefficient. In addition to discarding packets addressed to other systems, collisions can occur. A collision takes place when two systems attempt to send a packet over the network at exactly the same time. The packets collide on the network, and the resulting electrical signal is an unintelligible combination of the two packets. Remember the mathematical check sum mentioned earlier? When a collision occurs, the recipient detects that the information is not correct and the sending system must retransmit the packet.

Using a switch, rather than a hub, as the center of the network is a great improvement. A network switch is an intelligent device that reads the "to" and "from" fields in each packet. It also keeps track of which IP addresses can be reached via each of its ports. Thus, in the earlier example, the packets from computer 1 that are addressed to computer 2 will be sent only to the port connected to computer 2. Computer 3's packets addressed to computer 4 are sent only out of the switch port to computer 4. Not only are collisions avoided, but each of these two data transfers can use the network's full 100 Mbps bandwidth. Furthermore, security has been improved, since packets that are not addressed to a given computer are not sent to its NIC for eventual discarding.

In reality, large networks are built around a combination of switches and hubs. However, a typical home network needs only one. Either a hub or a switch will be fine. Hubs are inexpensive (about $20), while a switch costs more (about $45).

A router is the most complex device on a network. It is used to transfer specific packets between two computer networks. For example, a router can be used to connect your home network to your Internet DSL or cable modem connection. It has a port attached to each of the networks being interconnected. The router, itself, has a unique IP address on each network, in this case, one on your private home network, called the LAN (local area network), and one on your ISP's network, called the WAN (wide area network). The router includes a microprocessor that performs network address translation (NAT) services. It's the magic of address translation that allows multiple systems on a home network to share a single IP address from the ISP to access the Internet. The router translates different computers' requests for web pages to different service ports so that it appears to the ISP to be multiple web browsers open on the same computer.

It's worth noting that Windows 98 Second Edition and later versions of Windows offer Internet connection sharing. Using this service allows one computer to send all its Internet traffic to a second computer, which, in turn, sends it out to the Internet. The advantages are that there are fewer network components and that even a dial-up connection can be shared. But the significant drawbacks are that the computer sharing the connection must always be on when the other computer needs to access the Internet, and part of its processing capacity and memory are tied up managing the connection sharing. Therefore, subsequent discussions will focus on using network components, rather than the Internet connection sharing service.

Private, Keep Out!

Since our second lunch is running out of time, let's quickly cover the basics of the other network concepts. A firewall is software or hardware that protects a computer from unauthorized outside requests for data or services. Basically, it allows you to reach out and bring information into your system while limiting anyone else's ability to reach into your system. Since one of the advantages of a home network is to be able to share information easily across your computers, a software firewall on each system isn't the best choice. But a router controlling the interface between the home and public Internet serves as an excellent hardware firewall. It gives the trusted users within your household easy access to your networked systems, but prevents unwanted access attempts from anyone outside your home.

A DMZ (demilitarized zone) host is a computer whose services are accessible from the outside public network. Corporations that want their customers to access some information via the Internet but protect the rest of their information place one or more computers in a DMZ. In the case of a home network, if you are hosting web pages for outside access, one of your computers can be defined in the router as a DMZ host for this purpose.

DHCP (dynamic host configuration protocol) service is used to assign a unique IP address to each computer automatically at boot up. This is often an easier task than manually assigning unique IP addresses when the computers are first connected to the network. Also, if a laptop is used in both a home and office network, it can be assigned a suitable IP address automatically in each network by that network's DHCP service.

The final concept before we head back to work is a wireless access point. So far, we've talked about networks that are connected to the hub or switch by wires. Wireless connections are also available. If a wireless access point is a part of your home network, any computer with a wireless NIC can be part of the network. In reality, a hybrid arrangement of both wired and wireless devices is usually the best approach. Wireless is a great solution for connecting a PC in a bedroom far away from the other systems, or for connecting a laptop for use on the porch or patio. One wireless access point can serve multiple wireless NICs on the network, essentially acting as a hub. The wireless portion of the network operates at a throughput speed of 11 Mbps, but is fully compatible with either a 10 or 100 Mbps network. The range of a wireless network is similar to that of a cordless telephone, and the data can be encrypted to prevent eavesdropping in a crowded neighborhood or apartment. A wireless access point for your network will cost about $150, and a wireless NIC can be found for just under $100. Although the hardware for the wireless access point is available as a stand-alone network component that can be connected to an existing hub, switch, or router, the more economical package is a combination device, such as the Linksys, Dlink, or Netgear router, switch, and access point combined in a single unit.

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