[3 pages — 3070 words]

INTRODUCTION

In previous lessons, we discussed traditional Ethernet and its dependency on repeater sets to expand the length of these networks. As Ethernet evolved to incorporate twisted-pair cabling and star topology, a repeating hub was necessary in order to connect the various link segments together. With the introduction of switching hubs as a replacement for repeating hubs, network performance was enhanced by breaking up one collision domain into several collision domains. However, is it necessary for modern Ethernet networks to only incorporate switches? The answer is no. Repeating hubs have their place, but you must understand the tradeoffs when not selecting switch technology.



Repeating Hubs

Since modern Ethernet is wired in a star topology, a hub is required to expand a network beyond two stations. A repeating hub is intended for shared Ethernet or half-duplex Ethernet where only one station can transmit at a time otherwise collisions occur. Collisions must be sensed by all stations on the network in order to discard the transmitted frame which has been corrupted and to notify all stations that bus arbitration is occurring. Based upon a back-off algorithm, one station will win bus arbitration thus gaining sole access to the network and, therefore, allowing it the ability to transmit without error. A repeating hub must be transparent to the network and must, therefore, reinforce collisions so that stations connected to repeating hub ports can continue to participate in network arbitration as if the repeating hub was not present. Repeating hubs must perform in other ways in accordance to IEEE 802.3 requirements for repeater sets. They must:

  • Restore the amplitude of the signal
  • Restore the symmetry of the signal
  • Retime the signal
  • Rebuild the preamble
  • Enforce collisions on all segments
  • Extend fragments
Repeaters and repeating hubs are physical layer devices operating on symbols sent over the wire. Repeaters do not understand Ethernet frames or protocols. Their role is to simply extend distance and to facilitate star topology.

Switching Hubs

A much more sophisticated hub is the switching hub. A switching hub or "switch" is not a repeater but actually a bridge which acts upon the content of the Ethernet frame it receives and forwards the frame to the appropriate port on the switch. For example, if station A is connected to switch port 1 and station B and C are each connected respectively to ports 2 and 3, a message from A to B should only require the passing of the data from ports 1 to 2 but not to port 3. The switch could do this if it knew all the station locations. The switch learns these locations by observing the source addresses of Ethernet frames sent to its ports. It builds up a table of station addresses for each port and studies this table before forwarding the frame. If the switch does not know the location of a particular station address, it floods the frame to all ports except the port on which the frame was received. As soon as the unknown station announces its location by initiating a frame, the switch will update its table accordingly. Like a repeater, a switching hub is also effective in expanding a network beyond two stations. However, unlike a repeater, a port on a switch becomes an end point to the link segment attached to its port. Each switch port functions like any other Ethernet station except it passes along the data and station addresses generated by originating stations. Therefore, each switch port must abide by the same arbitration rules for collision detection and access resolution, but it need not pass collision information to other switch ports because these ports reside in separate collision domains. Switching hubs are fundamentally different from repeating hubs and, therefore, their performance can differ as well.

Increasing Distance

Twisted-pair segment lengths cannot exceed 100 m and, therefore, the need to extend network distances is frequently encountered. If there is a need to extend an Ethernet network, a switch can provide an advantage over a repeating hub. Repeating hubs are considered part of the collision domain and for reliable operation of a shared Ethernet network, the network diameter must not exceed that of the collision domain (see Figure 1). This distance limit is based upon the round-trip propagation delay between the two furthest stations on the network, and it cannot be such that collisions are not sensed by all stations within a prescribed time. Although repeating hubs can be cascaded for greater network distance, there are restrictions, that usually limit the number of repeating hubs to four. The rules for cascading repeating hubs were explained in previous lessons. The simplified rule is the 5-4-3 rule that states there cannot be more than five segments, four repeaters and three mixing segments. A mixing segment is a bus segment such as thick-wire and thin-wire coaxial cable. Since modern Ethernet only uses link segments consisting of either twisted-pair or fiber-optic cabling, the mixing rule can be dropped. Therefore, having five 100 m twisted-pair segments and four repeaters is fine. Since fiber optic segments can be up to 2 km in length, we need to impose limits to this rule. Instead of having four repeaters, we can have three with two interconnecting fiber optic segments of only 1 km in length. The expansion rules for repeating hubs are indeed confusing, but they are necessary in order that the network diameter not exceed the collision domain. What is even more confusing is that these rules only apply to 10 Mbps operation.

Figure 1 — With shared Ethernet, all devices and associated cabling must reside in a single collision domain.

One way of simplifying the expansion rules is by avoiding the collision domain restrictions. Switching hubs are not part of the collision domain since they are end devices on a network (see Figure 2). Therefore, adding one switch to a network without switches can effectively allow the network diameter to double without concern of exceeding the collision domain. Additional switches can be cascaded without the limit imposed on repeating hubs. In other words, if there is no collision domain issue to begin with, (the network diameter does not exceed the collision domain) then no collision domain issue will be introduced by adding a switch. In fact, switches can continue to be added beyond the four limit of repeating hubs regardless of data rate. This makes expansion rules much easier. However, the collision domain rules for each link segment on a particular port must be followed since it is possible that repeating hubs are attached to switch ports. The collision domain restrictions remain, but they are not aggravated by adding switches. It is also possible to regain the 2 km segment length in fiber optic ports if the fiber segment is between two switch ports. This is true at 10 Mbps and possible at 100 Mbps. To achieve large network diameters, especially at higher speeds, switches may be the only option.

Figure 2 — Because switches break the network into multiple collision domains, the physical size of the network is virtually unlimited.

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