High-Performance Networking Unleashed

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- 6 -

Hubs

by Frank C. Pappas and Emil Rensing

As you continue in your quest to build the perfect networking environment for your organization, you will be faced with many decisions. One of the most basic yet most important is the topology of your network. That decision sets the stage for everything that is yet to come: what levels of performance and reliability you can expect, what kind of administrative and support issues you will have to face, how you can scale or expand your network, and how large a dent the network will make in your operating budget.

In many cases, the thought of having to purchase hubs prevents most people from building 10base-T networks. While a fear of the unknown is natural, the fear of hubs is sometimes more like an uncontrollable phobia. If you are in the position of having to upgrade an existing 10base-2 LAN, you should not let the additional cost of hubs stop you from enjoying the increased performance and flexibility of your infrastructure that hubs can offer. In fact, the cost of a hub will probably be somewhere in the neighborhood of 20 to 150 dollars more per workstation, depending on the type of hub you decide to purchase. Later on, we will discuss why there is such a price difference--which we hope will help you to determine the right hub for your needs. In all probability, you will spend more on wiring than you will on any other component--especially if you have to wire an entire building and want things to look neat.

What Is a Hub?

As its name implies, a hub is a center of activity. In more specific network terms, a hub, or concentrator, is a common wiring point for networks that are based around a star topology. Arcnet, 10base-T, and 10base-F, as well as many other proprietary network topologies, all rely on the use of hubs to connect different cable runs and to distribute data across the various segments of a network. Hubs basically act as a signal splitter. They take all of the signals they receive in through one port and redistribute it out through all ports. Some hubs actually regenerate weak signals before re-transmitting them. Other hubs retime the signal to provide true synchronous data communication between all ports. Hubs with multiple 10base-F connectors actually use mirrors to split the beam of light among the various ports.

Figure 6.1 shows what a basic 10base-2 network might look like. Notice the way the machines are connected to each other. Remember that data being sent from the first system in the chain must be handled by each system between the originating and destination systems.

FIGURE 6.1. A basic diagram of a 10base-2 network. Notice how data must travel through each device between the source and target device.

Figure 6.2 shows the same network as Figure 6.1 if it were built using 10base-T. You can see how the topology is different and how a hub fits into the network topology.

FIGURE 6.2. A basic diagram of a 10base-T network. Notice the hub, which is the device to which all systems initially connect.

For example, on a 10base-T network, all of your devices will be physically wired to one or more hubs using unshielded, twisted-pair cabling. Your hub will have multiple ports and possible multiple types of ports so that you can connect many devices to it. You may need to connect multiple hubs together. If that is the case, you may want to use one of the other higher-speed ports on your hubs to build a backbone to your network. Each hub--and possibly your network servers--should connect directly to your high-speed backbone. Because the majority of communication on most LANs is between workstations and the primary servers, your backbone of network wire or segment will play a very important part in the overall performance of your network.

Notice in Figure 6.3 how the backbone connects the multiple hubs and servers directly over a dedicated connection. That dedicated connection ideally should be high-speed Ethernet or some other kind of high-speed connection, possibly fiber-optic.

Token-Ring networks also have devices on them that can be referred to as hubs. A Multi- Station Access Unit or MSAU can also be considered a type of hub, because it serves a similar purpose to an Ethernet hub. However, MSAUs use mechanical switches and relays to reroute packets to each active device in serially--not in parallel like Ethernet hubs. To reduce confusion between types of networks, Token-Ring MSAUs will not be discussed as hubs.

FIGURE 6.3. A diagram of a more complex 10base-T network. Notice the way that this 10base-T network connects each hub using a backbone. Typically, a backbone can be a higher-speed type of connection such as FDDI, and your server can be connected directly to the backbone.

Who Needs Hubs?

There is a simple way to determine whether you need a hub on your LAN. If you are building a network with a star topology and you have two or more machines, you need a hub. There is, however an exception to this rule. If you are building a 10base-T network and you have only two machines, you can connect them to each other without using a hub. You do, however, need to have a special jumper cable to match the transmitter leads of one system with the receiver leads of the other system and vice-versa. These kinds of cables are becoming more and more common. If you cannot find one, however, you can easily make one. It is also important to point out that the parts required to assemble them are relatively inexpensive. If you have a small length of twisted-pair cable, two RJ-45 connectors, and a crimping tool, you can build one yourself using Table 6.1 as a guide. It shows how the pins must be connected on a section of Ethernet cable to allow connections between two systems without the use of a hub. This type of Ethernet jumper is commonly referred to as a cross-over cable.

Table 6.1. Connecting the pins on a section of Ethernet cable to allow connections between two systems without the use of a hub.

RJ-45 Connector 1 RJ-45 Connector 2
Pin Number Function Pin Number Function
1 Transmit + <- -> 3 Receive +
2 Transmit - <- -> 6 Receive -
3 Receive + <- -> 1 Transmit +
6 Receive - <- -> 2 Transmit -

The preceding table shows how the pins must be connected on a section of Ethernet cable to allow it to be used to connect two systems together without the use of a hub. When you add a third machine to your mix, you will need a hub and two additional lengths of 10base-T. Remember to save your dual system 10base-T jumper. You never know when you may need it.

Types of Hubs

As you may have already guessed, hubs provide a crucial function on networks with a star top-ology. There are many different types of hubs, each offering specific features that allow you to provide varying levels of service. In the next section, we talk about some of the standard features of most hubs, the differences between passive, active, and intelligent hubs, as well as some of the additional features found in today's more high-performance hubs.

Basic Specifications

All hubs have a basic set of features that is determined in part by the types of cabling that run to the hub. In many respects, a hub is simply another network device that must perform within the standard parameters of the particular type of network to which the hub is connected. Despite the fact that hubs provide additional services to a network than simply an interface, they must still follow the restrictions placed on the medium by the IEEE.

The majority of connections to most hubs are through RJ-45 jacks. RJ-45 jacks are the standard connector type for many types of Ethernet that rely on twisted-pair cabling. From 10base-T to 100base-T, the cabling that runs from most workstations, printers, and other devices on your LAN to the hub is more than likely some type of twisted-pair cable, depending on the speed of the network. At either end of that cable is an RJ-45 connector.


NOTE: An RJ-45 connector looks similar to the connector that comes out of most North-American telephones except it is just a little bit wider. Although primarily used to connect devices that rely on twisted-pair Ethernet, RJ-45 connectors can also be used to connect Token Ring devices.

The length of each cable run to a hub is limited by the medium in use. (See Table 6.2.) For example, any length of 10base-T cabling cannot exceed 100 meters or roughly 330 feet in length. This is a limitation in the specification of 10base-T from the IEEE, not a limitation of any particular hub. For example, if your hub has a 10base-F connector to connect the hub to a high-speed backbone, the maximum run of that connection may be as far as 2 kilometers--as defined by the IEEE specification for 10base-F.

Table 6.2. The maximum distances of cable runs for different types of Ethernet, as determined by the IEEE.

Ethernet Type Distance
10base-2 185 meters/607 feet
10base-5 500 meters/1,640 feet
10base-T 100 meters/330 feet
10base-F 2 kilometers
10broad-36 3,600 meters/2.25 miles

Table 6.2 lists Ethernet specifications and the maximum length that a single run of each type can be. Remember that most of these runs can be extended with the use of Ethernet repeaters.

Of course, there are other standard requirements. Since hubs are electronic devices that take a single signal and broadcast it to multiple ports, hubs need a power source. Most hubs have LEDs that can be used to monitor various conditions. The two most common are LEDs to monitor power and active connections at particular ports. Other hubs have additional LEDs to monitor traffic on a particular port, as well as packet collisions on the LAN in general.


NOTE: One way to get in trouble with the FAA is to setup a LAN on an airplane using a battery-powered hub. While the promise of six-hours of multi-player Quake or Interstate '76 on a trip from New York to Los Angeles may seem like an acceptable risk, believe me when I say, "It is not!" Apparently, connecting electronic devices (such as computers and hubs) using external cables (such as unshielded twisted pair) is against FAA regulations.

Passive Hubs

Passive hubs, as the name suggests, are rather quiescent creatures. They do not do very much to enhance the performance of your LAN, nor do they do anything to assist you in troubleshooting faulty hardware or finding performance bottlenecks. They simply take all of the packets they receive on a single port and rebroadcast them across all ports--the simplest thing that a hub can do.

Passive hubs commonly have one 10base-2 port in addition to the RJ-45 connectors that connect each LAN device. As you have already read, 10base-5 is 10Mbps Ethernet that is run over thick-coax. This 10base-2 connector can be used as your network backbone. Other, more advanced passive hubs have AUI ports that can be connected to the transceiver of your choice to form a backbone that you may find more advantageous.

Most passive hubs are excellent entry-level devices that you can use as your starting points in the world of star topology Ethernet. Most eight-port passive hubs cost less than $200, and if you are upgrading from 10base-2, even the most inexpensive 10base-T setup will deliver a whole new world of performance.

Active Hubs

Active hubs actually do something other than simply rebroadcasting data. Generally, they have all of the features of passive hubs, with the added bonus of actually watching the data being sent out. Active hubs take a larger role in Ethernet communications by implementing a technology called store & forward where the hubs actually look at the data they are transmitting before sending it. This is not to say that the hub prioritizes certain packets of data; it does, however, repair certain "damaged" packets and will retime the distribution of other packets.

If a signal received by an active hub is weak but still readable, the active hub restores the signal to a stronger state before rebroadcasting it. This feature allows certain devices that are not operating within optimal parameters to still be used on your network. If a device is not broadcasting a signal strong enough to be seen by other devices on a network that uses passive hubs, the signal amplification provided by an active hub may allow that device to continue to function on your LAN. Additionally, some active hubs will report devices on your network that are not fully functional. In this way, active hubs also provide certain diagnostic capabilities for your network.

Active hubs will also retime and resynchronize certain packets when they are being transmitted. Certain cable runs may experience electromagnetic (EM) disturbances that prevent packets from reaching the hub or the device at the end of the cable run in timely fashion. In other situations, the packets may not reach the destination at all. Active hubs can compensate for packet loss by retransmitting packets on individual ports as they are called for and retiming packet delivery for slower, more error-prone connections. Of course, retiming packet delivery slows down overall network performance for all devices connected to that particular hub, but sometimes that is preferable to data loss--especially since the retiming can actually lower the number of collisions seen on your LAN. If data does not have to be broadcast over and over again, the LAN is available for use for new requests more frequently. Again, it is important to point out that active hubs can help you diagnose bad cable runs by showing which port on your hub warrants the retransmission or retiming.

Active hubs provide certain performance benefits and, sometimes, additional diagnostic capabilities. Active hubs are more expensive than simple, passive hubs and can be purchased in many configurations with various numbers and types of ports.

Intelligent Hubs

Intelligent hubs offer many advantages over passive and active hubs. Organizations looking to expand their networking capabilities so users can share resources more efficiently and function more quickly can benefit greatly from intelligent hubs. The technology behind intelligent hubs has only become available in recent years and many organizations may not have had the chance to benefit from them; nevertheless intelligent hubs are a proven technology that can deliver unparalleled performance for your LAN.

In addition to all of the features found in active hubs, incorporating intelligent hubs into your network infrastructure gives you the ability to manage your network from one central location. If a problem develops with any device on a network that is connected to an intelligent hub, you can easily identify, diagnose, and remedy the problem using the management information provided by each intelligent hub--that is, in the event it is a problem that cannot be remedied by the hub itself. This is a significant improvement over standard active hubs. Troubleshooting a large enterprise-scale network without a centralized management tool that can help you visualize your network infrastructure usually leaves you running from wiring closet to wiring closet trying to find poorly functioning devices.

Another significant and often overlooked feature of intelligent hubs is their ability to offer flexible transmission rates to various devices. Of course, intelligent hubs have additional ports for connecting high-speed backbones--just like other types of hubs. However, the intelligent hubs support standard transmission rates of 10, 16 and 100Mbps to desktop systems using standard topologies such as Ethernet, Token Ring or FDDI. That means that you can gradually upgrade your systems from 10Mbps connections to 100Mbps connections, or simply deliver faster transmission speeds to devices that need faster services.

In addition, to boost the flexibility in configuration and management of networks of mixed media and mixed levels of technology, intelligent hubs have incorporated support for other technologies such as terminal servers, bridges, routers, and switches. Additionally, modern intelligent hubs provide more comprehensive and easier-to-use network management software, which make them a crucial component of most comprehensive network management systems.

Advanced Features

There are many additional features supported by some of the more high-end hubs from various manufacturers. Some hubs feature redundant AC power supplies. If one should fail, the other takes over--and is fully capable of powering the entire unit. Other hubs have built-in DC power supplies to function in the event of a power outage. Redundant fans in some hubs provide cooling of the hub in the event that either fan fails. Other more commonly encountered advanced features in some intelligent hubs include automatic termination for coaxial connections, full hot-swap capabilities for connector modules, as well as the ability to automatically reverse the polarity of improperly wired 10base-T connections. More advanced intelligent hubs have features such as redundant configuration storage and redundant clocks. The redundant clocks allow any hub with an onboard clock on the network to act as a master to help in timing packet delivery. The redundant configuration storage feature available on some intelligent hubs is used when assigning various properties to various ports. Each similar hub on the network stores the configuration of one other hub, allowing configuration information to be restored by way of the intelligent link between hubs. Additionally, some manufacturers deliver modules for routing and bridging services that can live inside of the same chassis as your larger enterprise-level hubs.

Choosing a Hub

One of the most fascinating aspects of the computer industry is the rapid pace at which available products manage to evolve, consistently beating IT professionals in their attempt to maintain cutting-edge networks. Unlike most other industries, where core product lines experience revolutionary growth every five to fifteen (or potentially more) years, even the most conservative projections show networking technology undergoing logarithmic growth every year and a half--sometimes even more frequently. Part of the reason for this explosive growth, other than the obvious demand factors, is the rather large number of software publishers and hardware manufacturers dedicated to improving the speed and performance of computing. Because network computing is such a large component of computing in the business world, it is quite easy to see why there are so many vendors who specialize in networking hardware and software--all dedicated to making your LAN and WAN perform as well as they possibly can.

When it comes time for you to choose a vendor for your networking hardware, not necessarily just your hubs, there are three things you want to consider before making any purchases:

Breadth of Products Offered

As you begin your search for the perfect hub, you will undoubtedly encounter more vendors than you know what to do with, many of whom will be offering almost identical product lines. If your intent is to network only a few machines together, perhaps fewer than 12, you will more than likely be satisfied by selecting any of the many passive hubs offered on the market. If you have a long-term goal for your workgroup that includes a larger, more high-end network infrastructure, you may wish to make a more educated purchasing decision. Finding a vendor who makes a highly scaleable workgroup- to enterprise-level intelligent hub system and additional products such as routers, bridges, and network interface adapters that all function under the same network management system, might prove to be a bit of a challenge. While there are many vendors that make hubs for specific audiences, and it may be really easy to purchase any old product from your local computer store to get you started, it is important to look at the big picture. This step is critically important if you are on the verge of expansion or are just now implementing a star topology on your LAN as part of your overall upgrade path. Besides, many vendors will argue that their products work best in a brand-homogeneous environment.

Depth of Services Offered

Many larger vendors also provide networking consulting and installation services through local distributors and certified resellers. If you are in the position of having to build a network or upgrade an existing one, and you really do not want to get involved much further than paying the bills, you may find these services quite beneficial. If you feel that your LAN implementation is going to be a one-time-only type of project and have no immediate plans to upgrade until the technology significantly improves, you really have nothing to lose by picking any product that will fulfill your needs from any vendor. At the same time, there is no telling how any level of assistance from a qualified professional can benefit your implementation. Having someone to bounce an idea off of may change your whole outlook on what you are trying to accomplish with your network. Remember, you know what you need your network to do so that you can get your job done; however, only an experienced network engineer can help you build a network computing environment that will fulfill your needs in the best possible way.

Price of Product Versus Operational Savings

Many of the more high-end manufacturers of intelligent hubs and comprehensive network hardware solutions are significantly more expensive than their nearest competitors with seemingly similar product lines. While entry-level passive hubs obviously will be less than entry-level intelligent hubs, the three-digit price difference for the same number of ports and the same overall specified speed may seem like wasted money. However, depending on the scope of your project or the performance that you need from your LAN, the additional expense in hardware may save you lots of time, money, and aggravation when it comes down to adding additional nodes, adding routing capabilities, trying to troubleshoot problem connections, increasing operating speed on certain systems, or simply using your network. You should be aware that certain increased costs in hardware can offer substantial savings in operation later, when you need to be more flexible and open.

Summary

You can see how crucial hubs are to even the most basic LAN. All of the communicating you do over a star topology has to be routed through one or more hubs and it is crucial that your center of activity performs in a manner that is most beneficial for you. Basic, passive hubs are great for small networks--even in mixed protocol and/or multiple-operating system environments. 10base-T networks based upon the simplest and least expensive hubs can easily deliver performance and services that can rival even the most advanced 10base-2 networks. As your network grows and your needs increase, you will clearly see how better, more advanced systems will enhance your network performance. If you add active hubs, collisions and retransmissions decrease substantially. You may even see a performance increase in systems that were more problematic under basic, passive hubs. As your organization expands, and your network infrastructure grows, you will most likely take advantage of certain features of active hubs, and want to move as quickly as possible to more intelligent hubs that will become the nucleus of your overall network management system. It is for that very reason that the hub can make or break your network computing environment.


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