Monday, July 16, 2007

Network Switching (Part I)

Physical Media and Switching Types

The following are the most popular types of physical media in use today:
Ethernet—Based on the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard. However, it doesn’t rely on the Carrier Sense Multiple Access Collision Detection (CSMA/CD) technology. It includes 10Mbps LANs, as well as Fast Ethernet and Gigabit Ethernet.

  • Token−Ring—Not as popular as Ethernet switching. Token−Ring switching can also be used to improve LAN performance.

  • FDDI—Rarely used, chiefly due to the high expense of Fiber Distributed Data Interface (FDDI) equipment and cabling.

The following are some of the protocol and physical interface switching types in use today:

  • Port switching—Takes place in the backplane of a shared hub. For instance, ports 1, 2, and 3 could be connected to backplane 1, whereas ports 4, 5, and 6 could be connected to backplane 2. This method is typically used to form a collapsed backbone and to provide some improvements in the network.

  • Cell switching—Uses Asynchronous Transfer Mode (ATM) as the underlying technology. Switch paths can be either permanent virtual circuits (PVCs) that never go away, or switched virtual circuits (SVCs) that are built up, used, and torn down when you’re finished.

Networking Architectures

Network designers from the beginnings of networking were faced with the limitations of the LAN topologies.In modern corporate networks, LAN topologies such as Ethernet, Token Ring, and FDDI are used to provide network connectivity. Network designers often try to deploy a design that uses the fastest functionality that can be applied to the physical cabling. Many different types of physical cable media have been introduced over the years, such as Token Ring, FDDI, and Ethernet. At one time, Token Ring was seen as a technically superior product and a viable alternative to Ethernet. Many networks still contain Token Ring, but very few new Token Ring installations are being implemented. One reason is that Token Ring is an IBM product with very little support from other vendors. Also, the prices of Token Ring networks are substantially higher than those of Ethernet networks. FDDI networks share some of the limitations of Token Ring. Like Token Ring, FDDI offers excellent benefits in the area of high−speed performance and redundancy. Unfortunately, however, it has the same high equipment and installation costs. More vendors are beginning to recognize FDDI and are offering support, services, and installation for it—especially for network backbones. Network backbones are generally high−speed links running between segments of the network. Normally,backbone cable links run between two routers; but they can also be found between two switches or a switch and a router. Ethernet has by far overwhelmed the market and obtained the highest market share. Ethernet networks are open−standards based, more cost−effective than other types of physical media, and have a large base of vendors that supply the different Ethernet products. The biggest benefit that makes Ethernet so popular is the large number of technical professionals who understand how to implement and support it.Early networks were modeled on the peer−to−peer networking model. These worked well for the small number of nodes, but as networks grew they evolved into the client/server network model of today. Let’s take a look at these two models in more depth.

Peer−to−Peer Networking Model

A small, flat network or LAN often contains multiple segments connected with hubs, bridges, and repeaters. This is an Open Systems Interconnection (OSI) Reference Model Layer 2 network that can actually be connected to a router for access to a WAN connection. In this topology, every network node sees the conversations of every other network node.In terms of scalability, the peer−to−peer networking model has some major limitations—especially with the technologies that companies must utilize to stay ahead in their particular fields. No quality of service, prioritizing of data, redundant links, or data security can be implemented here, other than encryption. Every node sees every packet on the network. The hub merely forwards the data it receives out of every port, asshown in Figure 1.1.

Figure 1.1: A flat network topology.

Early networks consisted of a single LAN with a number of workstations running peer−to−peer networks and sharing files, printers, and other resources. Peer−to−peer networks share data with one another in a non−centralized fashion and can span only a very limited area, such as a room or building.

Client/Server Network Model

Peer−to−peer model networks evolved into the client/server model, in which the server shares applicationsand data storage with the clients in a somewhat more centralized network. This setup includes a little more security, provided by the operating system, and ease of administration for the multiple users trying to access data.A LAN in this environment consists of a physical wire connecting the devices. In this model, LANs enable multiple users in a relatively small geographical area to exchange files and messages, as well as to access shared resources such as file servers and printers. The isolation of these LANs makes communication between different offices or departments difficult, if not impossible. Duplication of resources means that the same hardware and software have to be supplied to each office or department, along with separate support staff for each individual LAN.WANs soon developed to overcome the limitations of LANs. WANs can connect LANs across normal telephone lines or other digital media (including satellites), thereby ignoring geographical limitations in dispersing resources to network clients.In a traditional LAN, many limitations directly impact network users. Almost anyone who has ever used a shared network has had to contend with the other users of that network and experienced the impacts. These effects include such things as slow network response times, making for poor network performance. They are due to the nature of shared environments.When collision rates increase, the usefulness of the bandwidth decreases. As applications begin having to resend data due to excessive collisions, the amount of bandwidth used increases and the response time for users increases. As the number of users increases, the number of requests for network resources rises, as well.This increase boosts the amount of traffic on the physical network media and raises the number of data collisions in the network. This is when you begin to receive more complaints from the network’s users regarding response times and timeouts. These are all telltale signs that you need a switched Ethernet network. Later in this chapter, we will talk more about monitoring networks and solutions to these problems. But before we cover how to monitor, design, and upgrade your network, let’s look at the devices you will find in the network.


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