Wireless Community Networks
A Guide for Library Boards, Educators, and Community Leaders
Chapter 7. WANs, MANs, and the Internet
If there is a rule about computer networks, it is this: networks grow. Once an organization chooses to embrace technological solutions in its day-to-day operations, more and more jobs present themselves as viable tasks for technological solutions. So more technology is acquired. With various people needing access to information in computerized—electronic—format, networks of computers are created so that information can be transferred quickly and easily between computers. As we'll see below, these networks grow.
This section presents the basic terminology you will be confronted with in dealing with sharing information resources through a multi-entity, or community, computer network. This chapter also builds on the previous one, describing the basic components you will need to expand several local area networks into something larger. It's called a wide area network. Let's start with some definitions.
Types of Networks
In the previous chapter we introduced the concept of a basic network of computers, called a local area network or LAN. Remember that LANs are formed to provide a means of sharing information departmentally, or among a subset of computers. A local area network of computers might be formed for many purposes:
- to share a specific printer or printers
- to share databases on CD-ROMs or hard drives
- to enable computer users in a group to send electronic messages to each other
- as a central repository to store and retrieve files for multiple users
- to provide shared access to the Internet
As technology is used more to store information electronically and to promote effective staff communication, there is a more frequent need to share information among many or all departments in an organization. For county governments and other organizations with dispersed departmental operations—including county libraries with branches—the LANs in each department are joined together to create a network of computer networks. This broader type of network is called a wide area network or WAN.
The LANs being networked into a WAN are not directly wired in many cases. Some departmental sites will be connected to other sites using a leased data circuit from the local telephone company. Some sites with minimal access needs can be connected with just regular voice telephone lines. We'll discuss these options under "Communications Links" below.
If all of the LANs being joined are located in the same community, the resulting large network is sometimes referred to as a metropolitan area network or MAN. Because there is no difference in how these networks are connected, many network specialists simply refer to LANs and WANs. In this manual we refer to all organization-wide networks as WANs.
Computer networks, both LANs and WANs, are created for various purposes. Some organizational networks are created only for use by organizational staff and managers. Depending on how they are used and who has access to them, these organizational networks have been given different designations. You will be familiar with some of these. Others may be new to you. Still others you may never see outside the context of this discussion. They are included just to provide completeness.
When an organization has information it shares only with staff and employees over an organizational network from a world wide web (or other traditional Internet) server using Internet protocols, the network is called an intranet. This means the network is only available inside the confines of the organization. This term has become popular with the advent of commercial access to the Internet. The web format has become so familiar that many companies are now formatting internal information the same way.
When two or more totally separate networks are joined so that information from each can be shared, the relationship is called an inter-network, or internet for short. A wide area network between a school and a public library, for instance, would be an internet.
When a company or organization has internal information to which it allows access by a partner or collaborating organization, the network connection between the two is called an extranet (as opposed to an intranet).
Obviously, the Internet (with a capital I) is the world's largest example of an internet. It spans the globe, linking computer networks of thousands of companies and organizations.
Creating a WAN
In order to create a wide area network linking two or more separate LANs, one needs three general components:
- a communications link
- a hardware device preparing data to be transmitted or received over the communications link
- a network device connecting a LAN to the communications link; these keep traffic destined for a local computer on the LAN and allow traffic destined for an external computer to pass through
While communication links "look" different, they all accomplish the same goal: they provide a "pipeline" allowing data signals to pass between networks. They look different because they use different media: copper wire, glass tubes, and airwaves. For our purposes, the only quantitative differences between the media will be the maximum amount of data that can travel through them in any one period of time and the cost—initial and ongoing—of using the media.
There are also qualitative differences between the various media. Chief among these is signal degradation. This is like sound waves moving through air. As sound travels through air, it spreads out with increasing distance so that it appears to diminish or grow fainter. Signal strength for all media diminishes, or attenuates, with distance, but the distance for each varies. However, when leasing data circuits or phone lines, attenuation is not a problem because the local telephone company is responsible for the quality of the signal, regenerating it when necessary.
The four most common methods of internetwork connectivity, listed in order of increasing data speeds, are analog (regular voice) telephone lines, digital telephone lines, leased data circuits, and private cable. Each of these is examined below.
Use of radio frequency wireless connectivity is increasing. As the main subject of this manual, it is described in great detail in the next chapter, "How RF Wireless Connections Work."
Analog Telephone Line
Unknown to many consultants and grant administrators, it is possible to use a plain old telephone line, also called a POTS line or an analog line, to connect an entity's local area network to some other external computer. This is true whether the connection is to the Internet or to another network in another building. In many TIF grants, libraries have installed multiple phone lines and Internet accounts, one for each microcomputer connected to the Internet. Unfortunately, this results either in unnecessary expense or under-utilized performance.
If the computers were connected in a local area network, the whole network could then be connected to the Internet over a single phone line and Internet account using a network device called an asynchronous router (see next paragraph for more details). This would reduce the library's ongoing costs by half or more. If the ongoing expense is manageable, a second phone line and Internet account could be attached to the router as well. In this two-line scenario, the phones appear to be "bonded" so that each computer is capable of receiving web files at up to double the performance of a single line. A single-line LAN connection can support from two to four computers at generally acceptable levels of performance. A double-line connection will support five to ten computers, depending on how often all computers are actively engaged in Internet use.
Analog phone lines require that a modem be connected to the phone line to change the data signal from a digital signal the computer understands to a range of sounds (analog) which can be transmitted over the phone line. The modem performs the opposite function on an inbound signal. Multiple computers can share this single phone line, by attaching an asynchronous router to the modem. Then the router is connected to a network hub, which in turn is connected to the computers. The router acts like a traffic cop on the network, examining each piece of data and sending it to over the phone line if it is bound for the external network, or on to the appropriate computer if it is bound for the internal network.
Analog phone lines provide at best a data transfer rate of 56Kbps. In Chapter 5 we determined that this is equivalent to sending about four double-spaced typed pages per second. In actuality, the effective data transfer rate is much less. Analog phone lines commonly suffer from noise. Noise includes such phenomena as line crossover—when you can hear someone else's phone conversation in the background—and the crackling of lightning or static somewhere along the connection. When noise occurs, data is corrupted and must be retransmitted. These retries lower the average data transfer of the line. In some communities, the greatest transfer rate possible over a regular phone line is 9600bps or less.
While very low, this level of bandwidth is acceptable for some applications, especially basic Internet connectivity. To learn more about how traffic travels over such a connection, and how you can connect more than one computer to the Internet using one phone line and Internet account, see the sidebar "Packets over POTS."
Packets Over POTS
Digital Telephone Line
In some areas, usually metropolitan, another type of phone service is available which uses only digital signals. The bandwidth available through these lines is higher and more effective than that offered by analog phone lines. The most common type of digital line is called an ISDN (Integrated Services Digital Network) line. An ISDN line provides up to two channels, or data paths, for connectivity, each providing a maximum 64Kbps data transfer rate. Using both channels at the same time provides up to 128Kbps of bandwidth.
Digital lines are much cleaner lines than regular analog lines. They are less affected by "noise," so their effective data transfer rates are much better than analog lines. While the maximum bandwidth of an ISDN line is almost two-and-a-half times as great as an analog line, its effective bandwidth is commonly three-and-a-half to four times as great.
Another type of digital line is called an ADSL (Asynchronous Digital Subscriber Line) line. ADSL technology provides various combinations of data transfer rates. When equalizing transmit/receive data rates, it is common to achieve bandwidth in excess of 600Kbps. Unfortunately, it is not yet available in most markets. If your local phone company is willing to provide a "dry line," or alarm line, to the two ends of a network connection, at least one vendor sells the equipment needed to create one's own ADSL connection. However, most phone companies are reluctant to do so for competitive purposes.
Leased Data Circuit
Data circuits provide a wall outlet that looks like a regular phone line connection. Nevertheless, they are not phone lines because they are not switched; that is, one cannot dial multiple places. In fact, one does not dial at all. A data circuit is a digital connection (normally) between two distinct entities, often called point-to-point service. The line becomes active when the network equipment on each end is connected and begins the communications process. Data circuits are "on" 24 hours a day. They are priced on a flat monthly rate. However, the rate is distance sensitive. The greater the distance between the two entities being connected, the higher the cost.
Four common terms are used to describe data circuits: 56K line, T-1 line, fractional T-1 line, and T-3 line. Due to recent state legislation lowering the cost of T-1 lines for public libraries, schools, and telemedicine centers in Southwestern Bell and many GTE-served communities, 56K lines are falling out of favor due to their limited bandwidth.
T-1 lines are data circuits with a maximum bandwidth of 1.544Mbps. These circuits are capable of being divided into 24 distinct channels, which can carry different streams of data, and, in fact, different types of data. Each channel, effectively represents 64Kbps of bandwidth.
Some T-1 circuits are ordered only using a portion of the channels available. These circuits are called fractional T-1 lines. Common increments of bandwidth in a fractional T-1 line are 384K, 512K, and 768K.
T-3 circuits are similar to T-1 lines in that they can be channelized or used "full pipe." Each of the 28 channels in a T-3 line equates to a T-1 line, or 1.544Mbps of bandwidth. When non-channelized, a T-3 circuit is capable of providing 45Mbps bandwidth. And, like T-1 circuits, fractional T-3 circuits can be ordered.
At this point in application development, T-1 circuits suffice for most community networks. However, when motion video transmission is more prevalent, perhaps in two-to-five years, the bandwidth limitation of a T-1 circuit will be a constraint on service.
Private Data Cable
The most common alternative to leased phone lines or data circuits is privately installed fiber optic cable. Especially for school districts where campuses are located "next door" to each other, laying fiber optic cable between campuses is the most cost-effective method of creating linking networks in separate buildings. But for those cases where a highway, private property, or physical barrier such as a lake must be crossed, the initial cost of laying fiber optic cable can be considerable. Costs can easily be in the tens of thousands of dollars in small communities and the hundreds of thousands of dollars in larger cities. Plus, right-of-way fees may be incurred for crossing private property or using utility poles to hang the cable.
The use of commercial wireless technologies to create municipal and wide area network links is a very recent development. It is ideal in situations where physical barriers or right-of-way fees prevent fiber optic installation. We cover wireless connectivity in greater detail in the next chapter.
Any discussion of network equipment can quickly become very technical. In this introduction to the concepts we purposely simplify the description of components. Our purpose is to acquaint you with the component parts and what they do, not necessarily teach you how they do it.
For each of the connectivity options listed above, certain hardware is required to connect the local area network to the transmission medium. In most cases, two separate pieces of hardware are required. The first translates network data into signals that can be carried over a particular medium. These include modems, channel service unit/ digital service units (CSU/DSUs), radio transceivers, and others. The second is a network device that determines whether data can come into the network or go out across the WAN connection. These include bridges, routers, and switches.
A third component, a combination of computer hardware and software, is used in many cases to protect an internal, private LAN from abuse by outside users. These devices are called firewalls. We discuss each of these in more detail below.
Modems and CSU/DSUs
Modems. In order to get a packet of data from one computer network to another over a phone line, a modem is used. (Modem is an acronym for modulator/demodulator.) A modem translates a data signal that works on a computer network into a signal that works over a phone line. Another modem performs the reverse function on the other end. It's all magic, of course!
Other types of modems are also used to connect networks to other types of phone lines, such as ISDN or ADSL lines. While they work a bit differently, these modems accomplish the same purpose. Note that in many cases modems for these digital lines are housed inside another network device called a router. We'll discuss routers in the next section.
CSU/DSUs. Once you move up the scale of connectivity options to a leased data circuit, another type of hardware is needed. A Channel Service Unit/Data Service Unit (CSU/DSU, also called a DSU/CSU) provides an interface between a network device like a bridge or router and the leased data circuit.
Not only does the CSU/DSU prepare network data packets for transport over a data circuit, it also maintains "channels" over the data circuit. When a data circuit is channelized, its bandwidth is segmented for different purposes. For example, one channel may carry voice traffic while other channels carry data traffic.
CSU/DSUs may be purchased as independent units. However, when acquiring a CSU/DSU and the associated router or bridge from the same manufacturer, it is usually less expensive to purchase the CSU/DSU as a module which slides into the router/bridge chassis.
Bridges and Routers
Whenever two networks are joined together, there be a traffic cop to determine where every packet goes. Any data broadcast over one of the networks might be destined for a computer on its local network or the remote network. Instead of having every packet traverse both networks, a network device is positioned at the connecting point and examines every data packet. Those addressed to a computer on the local network are ignored. Those addressed to a computer on the other network are sent over the connecting link. This maximizes the efficiency of the link and keeps unnecessary traffic off the LANs.
Bridges. One common type of network device used in connecting multiple networks is a bridge. A bridge is used for two different purposes: to break a large LAN into smaller segments so that traffic is more efficiently distributed, and to create a WAN link to another computer network. When used to segment a LAN into smaller components, only one bridge is required. The separate segments are connected to separate ports on the bridge. When used to connect two LANs into a WAN, two bridges are required, one for each side of the WAN link. Each monitors the traffic on its own LAN.
Bridges are the simplest, quickest traffic cops. But they're not as intelligent as other types. Bridges examine the packets on one network and then perform one of three actions:
- dropping, or ignoring, the packet because it is destined to another computer on the same network
- forwarding the packet to the appropriate external network link (there can be more than one other link) because it recognizes the destination computer address
- broadcasting the packet to all other links because it does not recognize the destination computer address
While quick, this does not provide the most efficient means of linking the networks. The broadcasting of packets can clog the networks involved or saturate the communications link. For better efficiency, a router is required.
Routers. Routers are configured into a network of networks the same way bridges are. However, they operate slightly differently. They actually "open" packets to read more information. Then they send the packet to appropriate network segment. Broadcasts to other segments are eliminated, increasing the effectiveness of all connections.
In an environment where there are multiple routers, the router can determine if there are multiple "paths" to the destination computer. They can also determine which of the possible paths is the most efficient. In this way, severe congestion over one popular path is avoided, increasing apparent throughput in the network connections. Almost all connections involving disparate networks now employ routers.
Wireless Bridges. In wireless network connections, a network device called a wireless bridge is used. Wireless bridges have two components included: a radio transceiver which enables the wireless connection and a network bridge which routes network traffic across the radio connection as needed.
The radio transceiver and associated circuitry performs a function similar to a CSU/DSU or modem. It prepares data packets for transmission across a medium with a different signaling structure than a network cable. It also receives radio signals and turns them back into data packets the bridge can forward to the network. See the section "Radio Transceivers" for more information about how data is transmitted over radio waves.
The bridge portion performs standard bridging functions. However, because of the improved routing available in a router, some bridges now have software performing routing functions. Some of them are referred to as wireless bridge/routers, or brouters.
Because of their specialized nature—that is, preparing network data to be transmitted over radio waves—wireless bridges must operate together seamlessly. This requires bridges on both sides of a radio connection to be made by the same company. When a large radio WAN is created, all of the bridges connecting the WAN must be made by the same company. (A new networking standard for wireless bridge and router products has emerged over the last year, so interoperability is a future possibility.) Because of this, product quality and manufacturer support are very important.
The last component we'll mention in relation to WANs is the firewall. Like the firewall between a car's engine and the passenger compartment—literally designed to keep fire in a car's engine compartment away from the passengers—network firewalls keep outside threats away from sensitive data available inside the network.
Whenever the networks from two different organizations are joined together, there is always a threat that someone from one organization (or someone breaking into that organization's network) will break into the other organization's network. Such break-ins may result in private data being stolen and distributed, valuable data being altered or destroyed, or entire hard drives being erased. In network terms these risks are called security risks. Add an Internet connection to this equation, and a whole class of dark threats, real or imagined, can be foreseen.
Firewalls are used as a means of preventing or minimizing the security risks inherent in connecting to other networks. Some routers have firewall capabilities and are called firewall routers. When they exist outside the router, firewalls normally take the form of a computer dedicated for this one purpose, with appropriate security software installed.
It is important that any organization connecting to others or to the Internet take security precautions. The topic of security is addressed in more detail in Chapter 9.
Linking to the Internet
The primary purpose of creating many community networks is to share a high-speed Internet connection among two or more organizations, rather than having these organizations pay higher total costs for separate, and sometimes lower-speed, service. With more commercial services being offered over the Internet, such connectivity enables the cooperating group of entities to negotiate lower fees for the service than the aggregate cost of having each entity contract for the same service separately.
We know it's an economically viable thing to do. The question is, how does this Internet connection work? The following two sections discuss briefly the two cost components of an Internet connection: a telecommunications link for the Internet connection and access charges for getting data traffic to and from the Internet.
Making the Link
We've discussed the components required to create a wide-area network: communication links and bridges or routers to route data traffic over the links. Sometimes showing is better than telling. See Figure 1 for a diagram of a very simple WAN (actually a MAN) connecting the LANs of a school and public library and providing a link to the Internet.
The communication link between two or more LANs has a defining characteristic: you either pay a monthly fee or you pay to have a private connection implemented so that there is no ongoing cost. (Both require maintenance of the equipment, however.) Regular telephones, ISDN lines, and 56K and T-1 lines all require regular monthly payments to your local phone company. In some cases when entities install a private fiber optic cable, there may be recurring monthly or yearly fees, such as a payment to the electrical utility for the right to hang the fiber on utility poles. In other cases, the fiber link will be free of recurring charges. Wireless links involve no recurring fee.
The cost of voice-grade lines will range from $25-$40 per month. ISDN lines in most locations in Texas will cost $50-$60 per month with no per-minute charges. In some locations which are provided an extended ISDN link from a metropolitan area, the monthly cost will be about $90-$140. Data circuit costs rise sharply from there. Regular business costs of a T-1 line, which are related to the distance between the two points being connected, range from $200-$1,000 per month or more. Public schools and libraries qualify for reductions in these rates. But there is some question whether municipal and county data traffic can then be carried over them.
While the link between the two LANs can be created with wireless technology, to incorporate Internet access a separate communication link to the Internet is required. The Internet link will almost always take the form of a leased data circuit between the WAN's central site and an Internet Service Provider (ISP)—a company acting as a conduit between your network and the Internet. Few ISPs currently offer wireless connections to their networks. The leased circuit incurs the costs mentioned above. But communicating over the Internet involves a second cost as well: the cost of Internet access.
Paying for the Traffic
In addition to the communications link costs quoted in the previous section, a link to the Internet also requires payment to the ISP. High-speed access to the Internet is expensive. While dial-up accounts to the Internet cost around $20 per month, full-time, high-speed access to the Internet is much more expensive.
Commercial ISPs charge as much as $2,500 to $3,500 per month for guaranteed T-1 access to the Internet. Guaranteed access means the full bandwidth of a T-1 line is available to the Internet whether the customer uses it or not. ISPs also provide shared T-1 access to the Internet. Shared T-1 access means two or more T-1 lines are connected to a router which provides just T-1 access to the Internet. At any moment, only T-1 access is available. If one entity is using 25% of the total capacity, another entity will have only 75% of the capacity available.
Normally, shared access comes with some guaranteed level of service, such as 128Kbps or 384Kbps (1/12th and 1/4th the full capacity of a T-1 line). Since a high-speed connection to the Internet is seldom fully loaded, paying for shared access is more cost effective. Such connections range in price from $1,000 to $2,500 per month through a commercial ISP. Municipal and county governmental entities in Texas can get shared access to the Internet from $300 to $1,000 per month.
With the cost of T-1 Internet connectivity ranging from about $300 on the low end to $3,500 on the high end, sharing the cost of the Internet link is a primary benefit to creating a wide area network for public entities. Given the fact that one entity very rarely uses the entire capacity of its link, this sharing seldom results in degradation of service. These concepts are described in more detail in Chapter 1, "The Need for Community Networks."
In this section we looked at the basic components used in a wide area network. We covered the following concepts:
- Communication Links—the various types of telephone lines and data circuits used to transport data from one network to another. These include:
- Voice-grade telephone lines
- Digital telephone lines
- Point-to-point leased data circuits
- Private fiber optic cables
- Airwaves (wireless connections)
- Network Equipment
- "Modem"—any of a variety of devices that allow a computer or network to transmit data over telephone lines and other links
- CSU/DSU—hardware unit used to terminate and channelize a leased data circuit
- Bridge or Router—hardware used to control the flow of data traffic over two or more linked networks
- Wireless Bridge—a combination of a radio transceiver and a network bridge, linking two or more networks
- Firewall—hardware and software unit securing a network against unwanted external threats
- Linking to the Internet
- Network link—usually a leased high-speed data circuit, connecting a LAN or WAN to the Internet
- Access costs—the charges payed to an Internet Service Provider for tranferring and receiving data over the Internet
Written by Robert L. Williams.