Local Area Network Technology:

Part 8 - 10BaseT and 100Base-TX: Hubs and Switches

by Jeffrey L. Carrell
Issue v2n1 - January 1998

For a number of years, we've built 10BaseT networks. We have been using hubs to connect the devices together, some are stand-alone and/or stackable designs, while others are the modular chassis design. With the advent of the 100BaseTX price point being driven down so quickly, many network designers are implementing 100BaseTX networks at a faster rate. Therefore, how do you connect all of those 10BaseT devices with 100BaseTX devices into a commonly accessible network?

Prevalent network design practices are recommending to connect workgroup departments, around 20 to 50 devices, together on 10BaseT hubs, when their overall use is considered light to medium duty. Then you connect these workgroup hubs either to 10BaseT switches or to 100BaseTX switches that have 10BaseT link ports. For those workgroups who need higher bandwidth, usually due to application demands, connect them together using 100BaseTX hubs, then connect those workgroup hubs to 100BaseTX switches. (see figure 2)

Now for some basics:

What is a hub?

Hubs, also known as concentrators, are the center of a star-topology network. As defined by the IEEE 802.3 specification, they are multiport repeaters. A repeater circuit regenerates the signal as it passes it on to the device, as opposed to merely just splitting the signal. When connecting hubs together in a series, the network is considered to be on the same segment or collision domain, meaning all traffic is "heard" by all devices. Therefore, all data is available to all of the devices connected to this network segment, without any other device intercepting and/or directing traffic.

What is a switch?

A switch is similar to a hub in the fact that each port is a repeater port, but it isolates the data traffic between its ports. The data packets are received by a switch port and stored temporarily until the switch knows which data port to send that data packet out to. This function is relying on the OSI Data Link Layer (see figure 1) address information, known as a MAC address. The MAC address is the unique physical hardware address of the device's Network Interface Card (NIC). All devices communicate at various levels within the OSI Model, but this is the lowest common denominator. So when designing or connecting many network segments together, a switch can be used to break up the collision domain to reduce the overall traffic that all the devices "here". A switch is always "listening" on all of its ports and building a table of known addresses. If data needs to be sent to another network segment in the switch, the data is "forwarded" to the appropriate port based on the table lookup.

There are many different vendors offering 10BaseT and 100BaseTX hub and switch products. Most of the time when you see stackable 10BaseT and 100BaseTX hubs, there is no connection between those network segments. Usually a stand-alone switch is used to physically connect those different network segments together. (see figure 2) Likewise there are many options when selecting switch products. Many switches are predominately multi-port 10BaseT or 100BaseTX, with a 100BaseTX "uplink" port. There are however some products available that are configurable 10BaseT or 100BaseTX by port, but these are usually much more expensive than fixed configuration ports.

Although it is always advantageous to purchase networking products from a single vendor, it is not always possible. There may be legacy equipment in your network, or the networking products were purchased based on lowest price. In addition, many networks have grown far beyond everyone's expectation in the company. So when making decisions on which hubs and/or switches to now add or build your network with, look for those vendors who can offer 10BaseT and 100BaseTX hubs, and also have switch technology that will be suited for your network design now, and possible changes and/or growth for the future.

The OSI Model

Figure 1

Multi Hub/Switch Design

Figure 2

Copyright © 1997 Jeffrey L. Carrell. All Rights Reserved.

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