CIS 304
LAN Hardware/Wiring & Installation

Lesson 8 - Exploring Hubs, Bridges, Routers, & Switches

Instructor: Michael P. Harris

  Key Terms  
Structured Wiring
intelligent hubs
physical network
managed hub
standalone hub
stackable hub
linking hubs
modular hub
dedicated LANs
network center

[Top] Structured Wiring Architecture

Management of larger networks and internetworking begins with "structured wiring". With structured wiring, all of the network stations are physically star wired to intelligent hubs. Intelligent hubs are hubs that can be monitored and managed by network operators. This combination of a star topology and intelligent hubs make troubleshooting and fault isolation easier and faster because each endstation is attached to the network on its own individual port, which means it can be monitored easily and, if necessary, can be easily turned off. In addition, structured wiring makes adding users to the network, moving them, or making other physical changes on the network very simple. Since both Ethernet and Token Ring networks can use twisted pair cable and can be configured in a physical star topology, a structured wiring architecture will support either network technology.

[Top] Hubs: The Central Connection Point

The hub is one of the most important elements of a LAN. It is a central connection point for wiring the network, and all stations on the LAN are linked to each other through the hub.

The cornerstone of the network is the intelligent hub, or concentrator, which serves as the control point for systems activity, management, and growth. By integrating any combination of connectivity, internetworking, and management capabilities into intelligent hubs, network managers can create the perfect physical network infrastructure for their environment.

The term concentrator is generally associated with 10BASE-T/100Base-TX Ethernet networks, while the term multistation access unit (MAU) is used to refer to the Token Ring wiring hub. Just as these two LAN technologies use different media access methods, concentrators and MAUs perform different media access functions internally, but at one level they perform the same function: They are both network wiring hubs.

A typical hub has multiple user ports to which computers and peripheral devices such as servers are attached. Each port supports a single 10BASE-T/100Base-TX twisted pair connection from a network station. When an Ethernet packet is transmitted to the hub by one station, it is repeated, or copied, over onto all of the other ports of the hub. In this way, all of the stations "see" every packet just as they do on a bus network, so even though each station is connected to the hub with its own dedicated twisted pair cable, a hub-based network is still a shared media LAN -- picture it as a LAN in a box.

[Top] Managed Hubs

Intelligent hubs have been defined as manageable hubs, meaning that each of the ports on the hub can be configured, monitored, enabled, or disabled by a network operator from a hub management console. Hub management can also include gathering information on a variety of network parameters, such as the numbers of packets that pass through the hub and each of its ports, what types of packets they are, whether the packets contain errors, and how many collisions have occurred. Each hub vendor has some type of management package it sells with its products. These applications vary in how much information they can gather, what commands can be issued, and how the information is presented to the network operator.

[Top] Standalone Hubs

Both hubs and MAUs come in three configurations: standalone hubs, stackable hubs, and modular hubs. Some products are combinations of the best configurations. Standalone hubs are -- as the term implies -- single box-level products with a number of ports. Standalone hubs usually include some method of linking them to other standalone hubs -- either by connecting them together with a length of 10BASE5 coaxial cable or cascading them using twisted pair between individual ports on each hub. Standalone hubs are usually the least expensive type of hub and are often not managed. They are best suited for small, independent workgroups, departments, or offices typically with fewer than 12 users per LAN.

Network A   illustrates four 100BASE-TX hubs connected together by a single cable. This cable could be a coaxial or an optical fiber cable. All of the hubs are part of a single LAN. Network B   illustrates two 100BASE-TX hubs cascaded. Here the cable connecting the two ports is unshielded twisted pair (CAT5) wire. All of the hubs that are cascaded in this fashion are part of a single LAN.

[Top] Stackable Hubs

Stackable hubs look and act like standalone hubs except that several of them can be stacked or connected together, usually by short lengths of cable. When they are linked together, they act like a modular hub in that they can be managed as a single unit. One manageable hub, used within a stack, can typically provide the management for all other hubs in the stack. These hubs are ideal when an organization wants to start with a minimal investment but knows that its LAN will grow. By utilizing stackable hubs, an organization doesn't need to invest in a large chassis, which may only have one or two cards in it for a considerable length of time until more are needed.

[Top] Linking Hubs

Each hub usually represents a single LAN. In most organizations it is desirable to interconnect all of the LANs, which means linking hubs in some way. One way to link hubs is to use an interrepeater link or cascaded segment. This type of connection simply repeats all of the packets from one hub to the other hub it is linked to, so that in effect the two hubs are part of the same LAN.

[Top] Modular Hubs

Modular hubs are popular in networks because they are easily expanded and always have a management option. A modular hub starts with a chassis, or card cage, with multiple card slots, each of which accepts a communications card, or module. Each module acts like a standalone hub; when the communications modules are placed in the card slots in the chassis, they connect to a communications backplane that links them together so that a station connected to a port on one module can easily communicate with a station on another module.

Modular hubs provide a central point where multiple concentrators located in different wiring closets can be united into a LAN or WAN. The modular hub can be equipped with a wide variety of connectivity and network management modules designed to provide a customized solution for the creation of enterprise-wide LANs and WANs.

Modular hubs typically range in size from four to 14 slots, so the network can be easily expanded. Typically, several slots in a modular hub will be filled with 10BASE-T Ethernet modules. For instance, with 10 modules, each supporting 12 users, a single hub could support 120 users over 10BASE-T. The modules are linked by the high-speed backplane, which can also be used to connect the communications modules to a management module that manages all of the cards in the chassis. In addition to using one management module for a large number of ports, all of the modules share a common power supply. Another advantage of some modular hubs is that Ethernet, Token Ring, and even FDDI communications modules can be placed in the same chassis, using the same common power supplies.

[Top] Internetworking

The term internetworking refers to linking individual LANs together to form a single internetwork. This internetwork is sometimes called an enterprise network because it interconnects all of the computer networks throughout the entire enterprise. Workgroup LANs on different floors of a building or in separate buildings on a business campus can be linked together so that all of the computing systems at that site are interconnected. Geographically distant company sites can also be tied together in the enterprise-wide internetwork.

An individual LAN is subject to limits on such things as how far it can extend, how many stations can be connected to it, how fast data can be transmitted between stations, and how much traffic it can support. If a company wants to go beyond those limits -- link more stations than that LAN can support, for example -- it must install another LAN and connect the two together in an internetwork.

There are two main reasons for implementing multiple LANs and internetworking them. One is to extend the geographic coverage of the network beyond what a single LAN can support -- to multiple floors in a building, to nearby buildings, and to remote sites. The other key reason for creating internetworks is to share traffic loads between more than one LAN. A single LAN can only support so much traffic. If the load increases beyond its carrying capacity, users will suffer reduced throughput and much of the productivity achieved by installing the LAN in the first place will be lost. One way to handle heavy network traffic is to divide it between multiple internetworked LANs.

There are three major types of devices used for internetworking: bridges, routers, and switches. Today the most commonly used internetworking devices are high-speed routers, especially in wide area internetworks linking geographically remote sites. But routers are also heavily used in building and campus internetworks. Bridges have also been popular, even though they offer less functionality than routers, because they are less expensive to purchase, implement, and maintain.

LAN switches are a new class of internetworking device, and many people believe that switched internetworks will become the most common design for linking building and campus LANs in the future. Today's LAN switches and switching hubs are the first steps on a migration path to something called asynchronous transfer mode (ATM) switching, an emerging technology that will be widely implemented in both LANs and wide area networks in the coming years.

[Top] Bridges and Routers

Bridges and routers are both special kinds of devices used for internetworking LANs -- that is, linking different LANs or LAN segments together. Many organizations have LANs located at sites that are geographically distant from each other. Routers were originally designed to allow users to connect these remote LANs across a wide area network, but bridges can also be used for this purpose. By placing routers or bridges on LANs at two distant sites and connecting them with a telecommunications link, a user on one of the LANs can access resources on the other LAN as if those resources were local.

Bridges and routers link adjacent LANs. Local bridges and routers were first used to extend the area a network could cover by allowing users to connect two adjacent LANs to maintain performance by reducing the number of users per segment. Both Ethernet and Token Ring specify limits on maximum distances between workstations and hubs, hubs and hubs, and a maximum number of stations that can be connected to a single LAN. To provide network connectivity for more people, or extend it to cover a larger area, it is sometimes necessary to link two different LANs or LAN segments. Bridges and routers can both provide this function.

Today, however, these internetworking devices are also increasingly used to segment LANs to maintain performance by reducing the number of users per segment. When users on a single LAN begin to experience slower response times, the culprit is often congestion: too much traffic on the LAN. One method users are employing to deal with this is to break large LANs with many users into smaller LANs, each with fewer users. Adding new network users may require the organization to create new LANs to accommodate them. Implementing new applications on an existing LAN can create so much incremental traffic that the organization may need to break the LAN into smaller LANs segments to maintain acceptable performance levels.

In all of these cases, it is still critical that users on one LAN be able to reach resources on other LANs within the organization. But the LANs must be connected in such a way that packets are filtered, so that only those packets that need to pass from one LAN to another are forwarded across the link. This keeps the packets sent between two stations on any one LAN from crossing over onto the other LANs and thereby congesting them. A general rule of thumb suggests that 80 percent of the packets transmitted on a typical workgroup or department LAN are destined for stations on that LAN. Both bridges and routers can be used to segment LANs.

[Top] Bridges

Bridges are the simpler, and often less expensive, type of device. Bridges filter packets between LANs by making a simple forward/don't forward decision on each packet they receive from any of the networks they are connected to. Filtering is done based on the destination address of the packet. If a packet's destination is a station on the same segment where it originated, it is not forwarded. If it is destined for a station on another LAN, it is connected to a different bridge port and forwarded to that port. Many bridges today filter and forward packets with very little delay, making them good for large traffic volumes.

[Top] Routers

Routers are more complex internetworking devices and are also typically more expensive than bridges. They use Network Layer Protocol Information within each packet to route it from one LAN to another. This means that a router must be able to recognize all of the different Network Layer Protocols that may be used on the networks it is linking together. This is where the term multiprotocol router comes from -- a device that can route using many different protocols. Routers communicate with each other and share information that allows them to determine the best route through a complex internetwork that links many LANs.

[Top] Switches

Switches are another type of device used to link several separate LANs and provide packet filtering between them. A LAN switch is a device with multiple ports, each of which can support a single endstation or an entire Ethernet or Token Ring LAN. With a different LAN connected to each of the switch's ports, it can switch packets between LANs as needed. In effect, it acts like a very fast multiport bridge -- packets are filtered by the switch based on the destination address.

Switches are used to increase performance on an organization's network by segmenting large networks into many smaller, less congested LANs, while still providing necessary interconnectivity between them. Switches increase network performance by providing each port with dedicated bandwidth, without requiring users to change any existing equipment, such as NICs, hubs, wiring, or any routers or bridges that are currently in place. Switches can also support numerous transmissions simultaneously.

Deploying technology called dedicated LANs is another advantage of using switches. Each port on an Fast Ethernet switch supports a dedicated 100 Mbps Ethernet LAN. Usually, these LANs comprise multiple stations linked to a 100BASE-TX hub, but it is also possible to connect a single high-performance station, such as a server, to a switch port.

Using LAN switches allows a network designer to create several small network segments. These smaller segments mean that fewer stations are competing for bandwidth, thereby diminishing network congestion.

In this case, that one station has an uncontested 100 Mbps Fast Ethernet LAN all to itself. Packets forwarded over it from other ports on the switch will never produce any collisions because there are no other stations on the LAN at that port.

As was noted earlier, LAN switching is a relatively new technology. Today's switching devices switch relatively large, variable-length LAN packets between different local area networks. ATM is another type of switching technology that switches small, fixed-length cells containing data. ATM networks can be run at much higher data rates than today's LANs. Eventually, they will be used to carry voice, video, and multimedia traffic, as well as computer-generated data over both short and long distances. ATM will be one of the dominant enterprise networking technologies of the future, and many companies are beginning to develop strategies to incorporate ATM in their existing LANs and LAN internetworks.

[Top] Networking Today

LAN technology is evolving. In the early 1980s LANs were strictly local area networks, linking small groups of computers in company departments. As workgroup LANs proliferated over the past 10 years, users began connecting them to form internetworks, first with bridges and later with routers. Today's networks typically comprise a combination of workgroup and campus hubs, bridges, and routers. Switches are also beginning to become more prevalent.

The next few years will see networks evolve to include more sophisticated LAN switches and switching hubs. They will be designed using several different types of components, both old and new. Ethernet and Token Ring LANs will be built with stackable workgroup hubs, which, in turn, will be interconnected by larger modular hubs that may incorporate LAN switching functionality. Large networks will include another layer of consolidation with network center hubs linking workgroup hubs and switches. Routers will continue to be used as gateways to the wide area network linking other buildings and remote sites.

For networks to deliver the performance today's users require, their many components must work together to deliver seamless connectivity between all of the users and computing systems throughout the enterprise. Flexibility to grow, power to support applications, and seamless connectivity are what users expect in the products they choose to build LANs and enterprise networks.

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