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To access the contents, click the chapter and section titles.
Oracle Performance Tuning and Optimization
Token Ring A Token Ring network differs from an Ethernet network in the basic way the NIC gets access to the network. In an Ethernet network, each NIC simply checks to see whether the network is available and starts transmitting. The Token Ring network uses a more orderly approach. In a Token Ring network, a token is sent around the ring, somewhat like a semaphore. A NIC can transfer data only when it has the token. The token is sent around the ring in a circular fashion (see Figure 37.3).
In the Token Ring, just as with Ethernet, the NIC sends the data in a structure called a packet. These packets contain not only the data network protocol information but also information concerning the NIC that sent the packet and the receiving NICs address. When the device driver sends a packet to the NIC to be sent across the network, the NIC waits until the token is available and then transmits the data. In a Token Ring network, the structure passed around the network is called a token. This token is continually passed around the ring in a circular fashion. If the NIC that gets the token does not need to transmit at that time, it quickly sends the token on to the next NIC in the ring. The token-passing approach is efficient because it prevents collisions. Token Ring networks are very popular and exhibit very good performance but they are sometimes difficult to manage. Because the token must be passed to each of the machines in the ring, it is necessary to keep the size of each ring as small as possible. Because it is very difficult to maintain a large number of machines on a single ring, there is a need for multiple rings. Token Ring networks have a bandwidth capability of 16 megabits/second. As you know, in Ethernet, it is very difficult to actually achieve a throughput near the limits of the medium; with Token Ring, however, it is possible to get very near that limit. The token-passing method eliminates the chance of collision and allows you to run at or near maximum throughput. Fiber Optics Fiber optics network controllers have been on the market for several years and have become quite popular when high-speed networking is a must. Fiber optics (also known by the mysterious acronym FDDI) controllers operate in a manner very similar to that of Ethernet controllers. Fiber optics networks have a bandwidth of 100 megabits/second. In a manner similar to Ethernet, the FDDI controller creates an FDDI packet that contains a network packet generated by the network protocol driver. Additional FDDI information is placed around the network packet to create the FDDI packet. The structure of the packet is determined by the network protocol being used. Just as with Ethernet, in an FDDI network, it is possible to get both deferred packets and collisions. You must take the same kind of precautions to avoid these conditions. Just as with Ethernet, it may be necessary to divide your FDDI network functions into separate segments. As the network is driven closer to its maximum bandwidth, the number of collisions increases exponentially. The more traffic there is on the network, the more likely your packet will collide with another packet. Even though your network may be able to theoretically handle 100 megabits/second, it is unlikely that you will ever achieve this. These things make collisions more likely:
With smaller packets, there is likely to be more wasted time between packets (that is, time during which there is no network activity). With larger packets, there is less inactivity on the network. Larger packets are more efficient and perform better. As you see in Chapter 38, Tuning the Network Components, there are ways to avoid collisions and increase the performance of an Ethernet network by segmenting the network and reducing overhead. Other Technologies As is every other aspect of the computer industry, networking is changing and improving rapidly. Other new technologies being introduced or on the horizon include such things as ATM, HPPI (High Performance Parallel Interface), and Fibre Channel networks. ATM has been very well received and promises high bandwidth rates in several modes. The theoretical bandwidths of ATM are approximately 600 megabits/second and approximately 2.5 gigabits/second in different modes. The ATM networks will gain in popularity in the near future. HPPI and Fibre Channel are both emerging standards that in the future may provide for even higher performance networks. These standards are currently under development and may take years to finally make it to the mainstream computer network.
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