MODELO PROPUESTO

The selection of a data-link layer protocol is the most important decision in the design of the network’s physical infrastructure. The data-link layer protocol is not only respon­ sible for strictly data-link layer functions, such as media access control, but also for the network’s physical layer implementation. Currently, the most commonly used data-link layer protocol on networks is Ethernet, with Token Ring running a distant second. However, there are several Ethernet variations that provide various levels of perfor­ mance, and selecting the correct one is crucial.

You need to consider a number of criteria when selecting the data-link layer protocol for use on a network. Because the data-link layer protocol dictates the nature of the network’s physical infrastructure, you must consider design elements such as the dis­ tance between workstations and the transmission speed you require. You must also consider the nature of the traffic the network will carry and its amount. Additionally, your budget is always an important consideration.

Selecting a Media Type

Although there have been other types of media used in the past, most LANs con­ structed today use either unshielded twisted pair (UTP) cable or fiber-optic cable. In most cases, UTP cable is sufficient and much less expensive, but fiber-optic cable pro­ vides a viable alternative when you have special performance requirements. Network media that consist of cables are called bounded media. Recent developments in wire- less LAN technologies have also made unbounded media (networks that don’t use cables) a viable and practical solution.

Unshielded Twisted Pair UTP is a type of copper cable that consists of four pairs of wires, each of which is twisted together and contained inside a protective sheath. The quality of a particular UTP cable is specified by its category rating. Category 5 (or CAT5) UTP is the most commonly used today, although there are higher grades avail- able for special applications (such as 1000Base-T Gigabit Ethernet networks). The con­ nectors on UTP cables are called RJ-45 and are similar in appearance to telephone cable connectors, except that they have eight pins instead of four.

UTP is one of the cable types supported by all forms of Ethernet and by Token Ring as well. As a network medium, it is the most cost-efficient selection because it is the same type of cable used by telephone networks. In new construction, it is common for the same contractor to install both the telephone and data network cables at the same time. You install UTP cable using a star topology, in which you connect each workstation on the network to a central hub (or repeater), as shown in Figure 1-2. You can then con­ nect hubs to create a larger and more complex network. On an Ethernet network, UTP cable supports distances of up to 100 meters between each workstation and the hub. For most LAN installations, this is more than enough. If greater distances are required, you can modify the location of the hub in your network design or consider using fiber- optic cable, which can span longer distances.

Hub

Figure 1-2 A star topology

Each form of Ethernet that supports UTP cable has its own limitations regarding the number of hubs you can connect. This is one of the main reasons why you must select the data-link layer protocol you intend to use before you start designing the layout of your network infrastructure. For example, if you plan to use standard Ethernet running at 10 megabits per second (Mbps), you can connect up to four hubs on a single LAN, as shown in Figure 1-3. If you use Fast Ethernet (running at 100 Mbps), the Ethernet guidelines dictate that you can use no more than two connected hubs.

F01pm03

Figure 1-3 A four-hub Ethernet network

Off the Record When designing an Ethernet network, most network engineers use a basic set of cabling guidelines specified by the Ethernet standards. For example, the 10 Mbps Ethernet standard uses the 5-4-3 rule, which says that a network can consist of no more than five network segments, connected by four repeaters, with no more than three of those seg- ments being mixing segments. There are, however, more exacting formulae in the Ethernet standards that add the lengths of the individual cable segments and a coefficient for each hub to arrive at a more precise configuration for the network. In other words, using the more complex formula, you might discover that you are able to exceed the number of hubs speci- fied in the basic Ethernet guidelines if the lengths of your cables are short enough. When designing a complex Ethernet network, you should consult the Ethernet standards and use the more precise formula to ensure that your design falls within the specified requirements.

Fiber Optic Although it can use the same topology and many of the same data-link layer protocols support it, fiber-optic cable operates on a different principle than UTP and all other copper-based cables. The actual network medium in a fiber-optic cable is a strand of plastic or glass that carries signals in the form of light pulses. Because the signals are not electric, they are immune to electromagnetic interference. In an envi- ronment where interference levels are high, such as a factory floor, fiber-optic cable can eliminate the performance degradation that the interference causes on copper cable. Even in a normal office environment, you can install fiber-optic cable near fluo- rescent light fixtures or electric motors without any difficulties. When using UTP, your network design should keep the copper cables a safe distance away from these possi- ble sources of interference.

Hub

Hub Hub

Fiber-optic cables are also much less susceptible to attenuation than copper cables. Attenuation is the tendency of a signal to weaken as it travels along a medium. This signal weakening is one of the main reasons for the 100-meter UTP-cable length limit. All fiber-optic cables can exceed UTP cables in length, but there are several different types of fiber-optic cable, each with different length limitations. The multimode fiber- optic cable typically used on LANs can span distances ranging from 400 to 2,000 meters, while single-mode fiber can support cable runs as long as 100 kilometers. In performance and flexibility, fiber-optic cable is superior to UTP in almost every way. Fiber-optic cable is inherently more secure than copper cables because unauthorized people cannot easily tap into the cable and intercept its signals as they can with cop- per. The drawback of the medium is its additional cost and the special skills required to install and maintain it. Virtually all the tools used to install and test fiber-optic cable are different from those for copper-based cables. Installers must glue the connectors onto a fiber-optic cable—typically using a heat-cured glue and a small oven—while UTP connectors are crimped onto the cable. Also, because of the way light pulses are transferred through the cable, fiber-optic installers must be careful not to bend the cable too sharply.

The raw materials for a fiber-optic installation are far more expensive than their UTP counterparts. There are also far fewer fiber-optic installation contractors than UTP experts, and their skills come at a premium. In addition, many network administrators who have the ability to make minor repairs on UTP cables would be lost when faced with a malfunctioning fiber-optic connection.

Planning Generally speaking, network designs call for fiber-optic cable only when there are specific reasons for it, such as the need for extra-long cable runs or an environment with a lot of interference.

Wireless Networking Although wireless network technologies of various types have been around for many years, only recently have they become a viable medium for the average network installation. The older wireless LAN technologies were notoriously slow and unreliable, but a new standard published by the Institute of Electrical and Elec­ tronic Engineers (IEEE), called 802.11b, has boosted wireless LAN transmission speeds to 11 Mbps (faster than standard cabled Ethernet) and greatly increased their reliability. IEEE 802.11b networks can function using two different topologies. The ad hoc topol­ ogy, illustrated in Figure 1-4, consists of two or more computers equipped with wire- less network-interface adapters that can communicate with each other interchangeably.

Figure 1-4 A wireless network using the ad hoc topology

The infrastructure topology design, as shown in Figure 1-5, enables wireless computers to interact with a standard cabled network. In this topology, you connect a wireless transceiver called an access point to the cabled network, and the wireless computers can then interact with the rest of the network through the access point.

Access Point

Hub

Figure 1-5 A wireless network using the infrastructure topology

Wireless local area networking is becoming increasingly popular, now that the prod­ ucts are less expensive and more plentiful, but it is still a technology that is recom­ mended only for installations where a cabled topology is impractical. If you plan on

having computers on your network that are frequently moved to different locations, such as laptops or mobile kiosks, wireless can be an excellent solution. Wireless is also a good alternative in locations where running cables would be impractical, such as a lobby or plaza where cable would clash with the décor.

Before incorporating any wireless networking technology into your network infrastruc­ ture plan, however, it is strongly recommended that you test the proposed technology thoroughly in the actual locations where it will be used. Wireless transmissions are sub­ ject to interference from a wide range of environmental factors, including the number and composition of walls between the transceivers, proximity to machinery and other electrical equipment, and even climatic conditions. The effective transmission range of a wireless device can vary from location to location and even from minute to minute. You should always be sure that the medium you propose to use for your network be viable in the intended location before you invest time and money in the network infrastructure.

Security Alert Because of recent developments in the wireless networking standards, the popularity of wireless LAN technologies is growing quickly. However, the development of a security infrastructure for these networks has not been growing quite as fast. By definition, any wireless transceiver that comes into the effective transmission range of another trans­ ceiver of the same type has the potential to join that network. This creates a serious security hazard because unauthorized users with wireless terminals can conceivably access network resources from outside the organization. For more information on securing a wireless net- work, see Chapter 13, “Designing a Security Infrastructure.”

Selecting a Transmission Speed

Another important factor in the selection of a data-link layer protocol for your network infrastructure is the speed at which the network can transmit data. As with most data- processing technologies, Ethernet networks have gotten faster over the years, and the cutting edge of the technology is a trade-off between high transmission speeds and high equipment prices. Selecting a transmission speed for your network is a matter of determining your users’ current and future requirements and balancing them against your budget.

Ethernet offers the most flexibility in terms of transmission speed. The original Ethernet standard called for a 10 Mbps transmission speed. Fast Ethernet, introduced in the mid- 1990s, increased the maximum transmission speed to 100 Mbps, and Gigabit Ethernet— the current state of the art—runs at 1,000 Mbps, or 1 gigabit per second (Gbps). Work is also proceeding on a 10-Gbps Ethernet standard, products for which will surely appear on the market within a few years.