In the same context, the designer should focus on the specific problem at hand and work to resolve it within any existing constraints. With new designs, it becomes more important to anticipate potential problems, which is the mark of an excellent designer. Cisco categorizes network problems into three specific areas: media, protocols, and transport. While these parameters may be oversimplified, they should help novice designers identify and resolve the issues that will confront them.
- Media The media category relates to problems with available bandwidth. Typically, this refers to too high a demand on the network as opposed to a problem with the media itself. Designers would likely use switches and segmentation to address this category of problem, although links of greater bandwidth would also be practical.
- Protocols Protocol issues include scalability problems. Many of the chapters in this text will discuss the problems with certain protocols due to their use of broadcasts. This usage may lead to congestion and performance problems, which would not be resolved with media modifications per se. Protocol issues are typically resolved with migrations to the Internet Protocol (IP), although some tuning within the original protocol can provide relief as well. IP is suggested primarily because of current trends in the market and advances that have increased its scalability.
- Transport Transport problems typically involve the introduction of voice and video services in the network. These services require more consistent latency than traditional data services. As a result, transport problems are typically resolved with recent Ethernet QoS (quality of service) enhancements or ATM switches. Transport issues may seem similar to media problems, but there is a difference. The transport category incorporates new time-sensitive services, whereas the media category is targeted more toward increased demand.
In modern network design, there are five common technologies, as enumerated below. Each provides unique benefits and shortcomings in terms of scalability and cost. However, many corporations also consider user familiarity and supportability along with these factors.
- Ethernet Includes FastEthernet, GigabitEthernet, and enhancements still under development to increase theoretical bandwidth. Ethernet is the most frequently deployed networking technology. Many network designs have included switched-to-the-desktop Ethernet, which increases available bandwidth without requiring a change at the workstation.
- Token Ring Token Ring is a very powerful networking technology that was frequently deployed in large financial institutions that started with mainframe systems. However, it never met with the success of Ethernet primarily because of the expense involved. Token Ring adapters were always significantly more expensive than Ethernet NICs (network interface cards), and many firms based their decisions on financial considerations. In later years, Ethernet was enhanced to FastEthernet and switching was added. This overcame many of Token Rings positive attributes and placed it at a significant disadvantage in terms of performance (16MB early-release Token Ring versus 100MB full-duplex Ethernet).
- FDDI Fiber Distributed Data Interface and its copper equivalent, CDDI, were very popular for campus backbones and high-speed server connections. Cost has prevented FDDI from migrating to the desktop, and advances in Ethernet technology have eroded a significant portion of the FDDI market share.
- ATM Asynchronous Transfer Mode was the technology to kill all other technologies. It is listed here as ATM, as opposed to ATM LANE, discussed below. In this context it is not considered a LAN technology, but ATM is frequently considered along with ATM LANE in LAN designs. There is no question that ATM will expand as a powerful tool in wide area network design and that many companies will first accomplish the integration of voice, video, and data using this technology. However, vendors are beginning to map IP and other transports directly onto fiber especially using the dense wavelength division multiplexing (DWDM) that has matured in the past few years. This technology may ultimately remove ATM from the landscape. Note that some large campus installations use ATM to replace FDDI ringsa design that does not include LANE.
- ATM LANE LAN Emulation on ATM is listed separately from ATM because the two serve different functions. ATM LANE was designed to work with legacy LAN technologies while providing a migration path to desktop ATM. Thus far, most companies have used the technology in small deployments. These organizations have selected Ethernet-based technologies for enhanced servicesa move that ultimately saves money. ATM LANE requires new equipment, training, support tools, and still-emerging standards that may not be sufficient to offset the benefits that are included with the technology. Quality of service and integration with video and voice were powerful motivators for companies to install ATM and ATM LANE, but the market has since moved many of these services to Ethernet.
Local Area Networks
Local area networks are found in the access layer of the hierarchical model. This coincides with their role of servicing user populations. Figure 2.1 illustrates the hierarchical models relationship to the local area network. Note that this design is not redundant.
FIGURE 2.1 The hierarchical model and local area networks
Designers require a number of components in the design and administration of the LAN. These include cabling, routers, and concentrators (hubs or switches).
|Within this text, routers are considered to be the only Layer 3 devices, while switches operate at Layer 2. This is consistent with the current exam objectives; however, modern switching products now address Layers 3 and 4, while development is in progress to expand awareness to Layer 5. This will improve caching and QoS functionality. Some consider these new switches to be little more than marketing hype, but there is little doubt that increased knowledge regarding the content of data will augment security and prioritization of flows. This text will not enter the debate of switch versus routerit will simply define switching as a Layer 2 function and routing as a Layer 3 function. Note that some hierarchical models use Layer 2 switches as the access layer, with the first router at the distribution layer.
Designers often ignore cabling in the network design process, although up to 70 percent of network problems can be attributed to cabling issues. Responsibility for infrastructure is left to facilities staff or other organizations, especially within large corporations. This is certainly not the best methodology for effective network deployments. The cable plant is the single most important factor in the proper maintenance of the network and, as noted in Chapter 1, the cable plant has the longest life cycle of any network component.