Now that we have taken a look at the commonly used LAN network architectures, let's concentrate on the different connectivity strategies that can be used to link your computers and other devices together. As you can see from our discussion of the various network architectures, the standard connectivity medium is some sort of cabling. The most popular cabling type is still copper wire. However, other technologies that make use of wireless technologies and wiring systems that are already in place (such as the telephone or electrical wiring system in a building) are also available and are becoming increasingly popular.
Let's take a look at some of the standards for network cables. Then we can look at some of the other connectivity strategies and how they are being used in different LAN settings.
Network cabling consisting of copper wire has been the predominant network connectivity medium since the very beginning of the local area network. Copper-based cable transmits the data stream along the wire as an electrical signal. The discrete changes in the electrical signal on the cable distinguish the 1s and 0s in the bit stream. You already know that the network interface card (which we discussed in detail in the last chapter) has the job of taking the PC's information and getting it out onto the network medium in a format that is compatible with that particular medium (for example, electrical information is placed on copper wire, and light information is placed on fiber- optic cable).
Copper-based network cabling takes two major forms: coaxial cable and twisted-pair cable. Although the prices for fiber-optic cable (which does not experience interference because it uses light energy rather than electrical energy) are dropping and more fiber-optic LANs (particularly LAN backbones) are popping up all the time, copper wire is still the predominant wire type. Figure 4.3 provides a look at thinnet coaxial, thicknet coaxial, Category 5 twisted-pair cable, and fiber-optic cable, respectively.
Figure 4.3. Copper-based and fiber-optic cables are used as the physical media for LANs.
Let's take a closer look at copper-based possibilities. We can then take a look at fiber-optic cable.
Two types of coaxial cable are used for networking computers: thicknet and thinnet. Thicknet is a heavy-gauge coaxial cable that is fairly inflexible and requires special equipment (over and above a simple network card) to connect the computer to the network backbone.
Installations of thicknet (RG-8 and RG-11 coaxial cable) are practically nonexistent, but some can still be found in certain settings, such as in manufacturing companies. This is because this thicker version of coaxial cable is well shielded (encased in a coating of material that looks somewhat like aluminum foil) and therefore doesn't suffer as much from interference as thinnet cable (a manufacturing environment with a lot of machinery in use can certainly produce potential interference problems when the cabling runs close to large electrical devices).
Thicknet networks are characterized by a cable backbone that is tied to servers and workstations on the network by vampire taps (the taps actually pierce the cable). The transceiver is actually attached to the tap (rather than a network interface card), and then the computer is connected to the transceiver/tap by a drop cable (which connects to the NIC on the computer).
Thinnet (RG-58 coaxial cable) was the cable of choice at one time because of its relative ease of installation and its low cost. Thinnet LANs employ a bus topology, where a T-connector is attached to each computer's network card. The computers are then chained together using appropriate lengths of cable. Thinnet installations require that each end of the network be terminated , and terminators are placed on the downside T-connector of the computers that reside on either end of the network.
Although it is valid for us to discuss coaxial cabling, in reality you are going to find that twisted-pair cable is the network medium of choice for most LANs. Let's take a look at twisted-pair wiring and how it works.
Twisted-pair wire comes in a number of different grades. Everyone is familiar with the twisted-pair wire used for the connection between your telephone and the phone wall outlet. Twisted pair is so named because the cable is actually made up of a number of single copper wires that are twisted together.
Why are the pairs of wire twisted? You already know that copper wire is susceptible to interference (as discussed in an earlier Tip sidebar). Two copper wires running parallel to each other would actually interfere with each other quite a bit. Believe it or not, by twisting the wires together, the interference (or crosstalk ) between the wires is minimized.
Twisted-pair wire comes in two major flavors: unshielded twisted pair (UTP) and shielded twisted pair (STP). The big difference between UTP and STP is that the STP wires are encased in a foil wrap that protects them from interference.
UTP is the most commonly used network wiring. It is inexpensive, flexible, and light, thus making it very easy to work with. UTP cable is terminated with an RJ-45 connector, which is kind of the big brother to the RJ-11 male connector that you find on a telephone cord.
UTP cable has been placed in different categories based on data-transmission capabilities. You will often hear UTP referred to as CAT 3 or CAT 5 . Table 4.1 provides a listing of these various UTP categories.
Table 4.1. Twisted Pair Categories
Fiber-optic cable is a high-speed alternative to copper wire and is often employed as the backbone of larger corporate networks. However, the drop in the price of fiber-optic cable has started to make it a possibility for other LAN uses. Fiber-optic cable uses glass or plastic filaments to move data and provides greater bandwidth as well as longer cable runs (up to 2 kilometers, depending on the network architecture). With the need for network speed seemingly on the rise, fiber-optic installations are becoming commonplace.
Fiber-optic cable uses pulses of light as its data-transfer method. This means a light source is required, and lasers or light-emitting diodes are used. Fiber-optic cable is more expensive and more difficult to install than copper cable, but fiber-optic's ability to move data faster and farther makes it an excellent alternative to copper.
A very interesting aspect of fiber-optic cable is that because it uses light to transfer network data, it is not susceptible to tapping (that is, the stealing of data off the line itself). Copper cabling, on the other hand, can be tapped because it uses an electrical signal to send data. On networks where security is an issue, fiber-optic cable provides a more secure environment.
Phone and Electrical Wire Networks
One option for connecting PCs in a small network environment, such as a home peer-to-peer network or a small business network with a limited number of computers, is to use existing phone lines. Because the data communications on the network operate at a different frequency than the phone, phone communication is not disrupted by using the phone lines for a dual purpose (phone calls and the movement of data).
There is actually an organization called the Home Phoneline Networking Alliance (check out www.homepna.org for more information), which is attempting to create a standard for home networking products that use existing phone lines. Although many of the products available provide limited bandwidth, some supply 10Mbps throughput, which is as fast as the vanilla flavor of Ethernet. Phone line connectivity cards are available from a number of vendors . There is the Diamond PCI Phoneline card (it looks a lot like an Ethernet PCI NIC), called the Diamond Homefree. It can be used to connect PCs at 10Mbps by attaching them to any phone jack using a cable with RJ-11 ends (the same ends you find on any phone cord). Linksys also makes a phone line networking card. You will find that most of the phone-line products are compatible with the various flavors of Windows such as Windows 9x, 2000, and XP. Again, simple peer-to-peer networking is the aim of these products, as well as sharing a single Internet connection.
Other phone-line products take advantage of USB technology and use a small device that looks like a hub. The computers are attached to the device by a cable that runs from each computer's USB port. Your existing electrical lines also provide a source of wiring for connecting computers into a network. Again, a number of different companies make electrical or power-line networking products. This technology is considered to be another possibility for the home networking arena (as is phone-line technology) but could be used in some small business networking situations.
As with phone-line networking, using electrical lines as a connectivity medium allows computers to share files and printers and also share a single Internet connection. Power-line networks currently only offer bandwidth of around 350Kbps. This means that network communication is slower than some of the other alternatives (it is certainly not as fast as the phone-line alternatives).
The options for creating wireless networks have seemingly exploded over the last couple years . Wireless network connections take advantage of radio signals, infrared light, or lasers. For longer distances, wireless communications can also take place through cellular telephone technology (as seen with any number of cellular devices, such as Web-ready cellular phones), through microwave transmission, or via satellite. Because the microwave and satellite technologies fall more into the realm of wide area networking, let's take a closer look at radio LAN technology and infrared technology. We will discuss cellular technology in Chapter 17, "Networking on the Run."
Radio connectivity is used to extend the range of a LAN or make it easy for users with laptops to connect to the LAN without the use of cabling. Wireless radio LANs require that an access point be placed on the network for the computers that wish to connect to the LAN. For example, 3COM makes a wireless access point that attaches to an existing Ethernet network (Cisco also makes similar devices). It looks like a hub with an antenna on it. Computers outfitted with a wireless Ethernet card (these cards typically have a small antenna on them and otherwise operate like any other NIC using a transceiver) can connect to the access point. Figure 4.4 provides a diagram of a LAN that uses wireless technology for network access.
Figure 4.4. PCs outfitted with wireless network adapters connect to the LAN using a wireless access point.
The distance that a computer can be from the access point and still connect to the LAN varies from wireless product to wireless product. The 3COM specs for its wireless access point device claim a throughput of up to 11Mbps and a maximum range of 300 feet.
Wireless radio LANs are becoming increasingly popular in hospital settings (where nurses and doctors need to be mobile) and in business environments where a conference room must be quickly converted into a training room hosting multiple PCs connected to a LAN.
One issue related to wireless radio LANs is that it is fairly easy to intercept data. Although most radio LANs use one particular frequency, there are strategies for minimizing the possibility of data being intercepted (known as eavesdropping ).
Some wireless communications will actually break up the data and broadcast portions on different frequencies, thus making it harder to eavesdrop. Another method of diminishing the possibility of eavesdropping is known as frequency hopping . This is where the transmitter and the receiver switch (or hop ) frequencies periodically.
An important new set of specifications for wireless communication via radio signals is Bluetooth. A number of companies, including Microsoft, IBM, 3COM, and Nokia are creating Bluetooth devices such as keyboards, mice, personal digital assistants (PDAs), and cellular phones. Bluetooth is also being used to create networking devices.
Intel has certainly not been idle in relation to mobile computing and wireless communication. Intel has introduced the Centrino mobile computing technology, which integrates a wireless network connection into laptop computers that use the Intel Centrino processor.
Another possibility for wireless networking is infrared. Infrared systems use very high frequencies, just below visible light in the electromagnetic spectrum, to carry data. Like visible light, infrared signals can be blocked by opaque objects. Therefore, this technology is not suitable in all situations.
Because infrared technology is fairly limited in respect to the distance that two communicating devices can be apart, it is primarily used as a way for a laptop user to connect to a printer (both devices are outfitted with an infrared eye). Infrared is also becoming a popular method of syncing up personal data assistants, such as two Palm Pilots (we discuss handheld computing devices in Chapter 17, "Networking on the Run").