The Bus Configuration

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The Bus Configuration

The bus configuration has its roots with coaxial cable in simple networks where desktop machines are simply connected together so that they can share information with each other. Traffic, here defined as voltage applied to the wire by any machine that needs to communicate, is applied to the bus, or the wire connecting the machines (see Figure 2.7).

Any time a machine needs to access information from another machine, it simply sends out a sequenced variation of voltage in a frame that the destination machine can understand, process, and respond to. Notice in this configuration (Figure 2.7) that other machines on the network are also listening for frames and will open the frame up long enough to determine whether it is destined for them as well.

In this configuration, clients and servers can be randomly placed on the network, because they are all capable to listening to frames sent by a machine. The main selling point behind this type of network is that it is somewhat simple to set up, and can scale fairly well with the addition of relatively inexpensive hardware, such as repeaters or bridges. The keyword here is relatively. Remember, adding more machines to a bus type network simply adds more machines that will be competing for the wire to transmit.

One problem with this type of network architecture occurs when two machines try to communicate and send their frames on the wire at the same time. This is the electrical equivalent of a train wreck for 1s and 0s, or what is commonly referred to as a collision on the network. Any machine listening on the network for frames has no idea what to make of the chaotic confusion that results from a collision. Imagine trying to listen to fifteen or twenty people trying to talk at the same time to different people, and even possibly in different languages.

Thankfully, network cards are designed with algorithms to alleviate some of the chaos surrounding collisions and ground rules for avoiding them in the future. One common design called Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) implements a standard set of rules for the transmission of frames on a network.

This simple concept (CSMA/CD) defines the relative politeness of machines on the network. When a network card wants to use the wire to transmit data, it listens first to determine whether another machine is already in the process of transmitting. If the network is idle (silent), the machine may transmit its own frames. If, in the course of transmitting, another network card also begins to transmit, a collision occurs. Each network card is instructed to stop transmitting, wait a random amount of time, and then listen again before trying to retransmit the data.

At the blazing speeds that data is transferred, it might seem that collisions are not a problem, and on small networks this is true; however, as networks grow in size and as the data being transferred between machines increases, the number of collisions also increases. It is possible to put so many machines on a network segment that the capability of machines to communicate is slowed down, if not stopped altogether. If too many machines try to communicate at the same time, it is nearly impossible for network cards to transmit data without collisions. This scenario is often referred to as saturating your bandwidth (the amount of sustainable data transfer rate) and should be avoided if at all possible.

To conceptualize this, just imagine the traffic on any rural road and how the traffic increases as the surrounding area becomes more developed. More and more people move into the area and use the roads until it becomes somewhat congested. A quick trip to the store may have taken five minutes initially, but now it takes fifteen minutes to run to the store, despite the fact that the distance hasn’t changed. Further development and growth of the area into, say, a metropolitan city, leads to more people and more traffic, until eventually the trip to the store takes two hours because of the constant traffic jams. The usual effect of this is frustration and a commitment not to go to the store during rush hours.

The scenario described above can happen with computer networks as well. The inability to access resources in a timely manner because of saturated bandwidth can lead to productivity losses and frustrated users. One method that has been used to help reduce collisions is specifying a smaller frame size for sending data. By specifying small frame sizes, network cards must stop more often to allow other network cards the opportunity to transmit. This means computers can only send a small amount of data at any one time.

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