Restrictions on Legacy Ethernet Topologies

The topology of a local area network (LAN) can be described in two ways:

• The first is the physical topology, which describes the physical layout of the network media and the devices that connect to it.

• The second is the logical topology, which is not concerned with the actual physical connections, but with the logical path through the network that data can take from one place to another.

Several topologies are used with Ethernet, each with its own distance and other specifications. During the first few years of its development, Ethernet was run using a bus topology. When PCs caused corporate LANs to proliferate, new structured wiring standards led to the use of a star topology.

Limiting Factors of Ethernet Technologies

The two basic topologies that can be used to form an Ethernet local area network are the bus and the star. By using interconnecting devices, such as routers and switches, a larger network can be constructed, building on the bus and star to create a more complex network topology.

The restrictions that are imposed by a particular topology generally have to do with several factors:

• The network transmission media Imposes length and speed restrictions.

• Interconnecting devices Used to join different physical segments.

• The number of devices on the network Because Ethernet uses a broadcast method for data exchange, too many devices on the same network broadcast segment can cause congestion problems that can degrade performance.

• Media access mechanisms How the individual devices compete for or obtain access to the network media. In standard Ethernet networks, each workstation contends for access to the local media equally.

Interconnecting Devices and Cable Segment Length

Interconnecting devices and cable segment length are the most basic limiting factors for a local area network. As cable lengths get longer, the signal degrades (attenuation) until eventually it cannot be understood by another device attached to the same media. Even if you were to insert devices to strengthen or regenerate the signal at regular intervals, as is done with the public switched telephone network (PSTN), the length of the cable would be a problem because Ethernet networks rely on roundtrip timing to determine whether a packet has been properly sent. The sending station can't wait forever to determine whether a collision has occurred or whether its data was successfully transmitted on the wire with no interference.

The length of a cable segment depends on the type of cable:

• A segment of 10BASE-2, using coaxial cable (commonly called thinnet), can be as many as 185 meters, or 607 feet. With repeaters, the total diameter of the thinnet network is limited to 925 meters, or about 3,035 feet.

• For 10BASE-T Ethernet, using twisted-pair wiring, the workstation must be within 100 meters (328 feet) of the hub or switch.

• For Fast Ethernet environments, you can use different types of cable, from twisted-pair to fiber optic, and each of the Fast Ethernet specifications has different cable length limitations. For example, the 100-meter limit for any segment still applies for 100BASE-TX and 100BASE-T4 segments.

• 100BASE-FX (fiber-optic cable) has a maximum segment distance of about 2 kilometers. The distance advantage the 100BASE-FX has over the other cabling methods makes it more suitable for use as a network backbone medium at these speeds. However, there are network cards available that allow you to go ahead and bring fiber to the desktop now, if you can afford itand if you need the bandwidth. Fiber to the desktop today might be extreme unless you are supporting a high-end workstation, such as in a graphics development environment.

The 5-4-3 Rule

There is an easy way to remember what you can place between any two nodes on a legacy Ethernet LAN. The 5-4-3 rule means that there can be

• A maximum of five cable segments on the LAN

• A maximum of four repeaters or concentrators

• Only three segments containing cable with nodes attached

This is a general rule you should stick to when planning the network topology. Note, however, that the last part of the rule applies only to coaxial cable, such as 10BASE-2 or 10BASE-5. When nodes are connected using a hub or switch and twisted-pair wiring, each node has its own cable and can vary from a small workgroup of just a few computers to a much larger one supported by stacked hubs/switches.