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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x Pub Date: 06/17/98
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Network adapters range in price from less than $100 to much more than $1,000. What do you get for your money? Primarily, speed. The faster adapters can push data faster onto the cable, which means that the file server receives a request more quickly and sends back a response more quickly.
Data-Transfer Speeds on a LAN
Electrical engineers and technical people measure the speed of a network in megabits per second (Mbps). Because a byte of information consists of 8 bits, you can divide the Mbps rating by 8 to find out how many millions of characters (bytes) per second the network can handle theoretically.
Suppose that you want to transfer an entire 3 1/2-inch 720K floppy disks worth of information across a LAN. The rated speed of the LAN is 4Mbit/sec. Dividing 4Mbps by 8 tells you that the LAN theoretically can transmit 500K of data per second. This is equivalent to an average hard disks transfer rate. The data transfer rate for a floppy drive is 500Kbit/sec. The data from the 720K floppy disk takes at least a few seconds to transfer, as you can see from these rough calculations.
In practice, a LAN is slower than its rated speed. In fact, a LAN is no faster than its slowest component. If you were to transfer 720K of data from one workstations hard disk to the file server, the elapsed time would include not only the transmission time but also the workstation hard disk retrieval time, the workstation processing time, and the file servers hard disk and server CPU processing times. The transfer rate of your hard disk, which in this case is probably the slowest component involved in the copying of the data to the server, governs the rate at which data flows to the file server. Other peoples requests interleave with your requests on the LAN, and the total transfer time may be longer because the other people are using the LAN at the same time you are.
If you transfer the data from a 720K floppy disk to the file server, you see that it takes even longer. Floppy disk drives, as you know, are slower than hard disks. Your workstation uses the network in small bursts as it reads the data from the floppy disk. The workstation cannot send data across the LAN in this case any faster than it can read the data from the disk.
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ARCnet Adapters
ARCnet is one of the oldest types of LAN hardware. It originally was a proprietary scheme of the Datapoint Corporation, but today many companies make ARCnet-compatible cards. By modern standards, ARCnet is very slow, but it is forgiving of minor errors in installation. It is known for solid reliability, and ARCnet cable/adapter problems are easy to diagnose. ARCnet generally costs less than Ethernet, but hardware prices for Ethernet adapters have plummeted so much in recent years that the difference in price between the two is no longer that great an issue. ARCnet operates something like Token Ring, but at the slower rate of 2.5Mbps. The section Token Ring Adapters later in this chapter explains the basic principles on which ARCnet and Token Ring work.
Ethernet Adapters
The most widely used type of network adapter is Ethernet. Ethernet-based LANs allow you to interconnect a wide variety of equipment, including UNIX workstations, Apple computers, IBM PCs, and IBM clones. You can buy Ethernet cards from dozens of competing manufacturers. Ethernet comes in three varieties (Thinnet, UTP, and Thicknet), depending on the type of cabling you use. Thicknet cables can span a greater distance, but they are much more expensive. Ethernet traditionally operates at a rate of 10Mbps, but there are now Ethernet adapters available that operate at a rate of 100Mbps. These Fast Ethernet adapters are manufactured by Intel, Thomas-Conrad, and others.
There are even models that run at both 10 and 100Mbps speeds, allowing you to gradually upgrade your network by installing new NICs and hubs over an extended period of time. 100Mbps adapters only function at that speed when communicating through a high-speed hub to another 100 Mbps adapter.
Between data transfers (requests and responses to and from the file server), Ethernet LANs remain quiet. After a workstation sends a request across the LAN cable, the cable falls silent again. What happens when two or more workstations (and/or file servers) attempt to use the LAN at the same time?
Suppose that one of the workstations wants to request something from the file server, just as the server is sending a response to another workstation. A collision occurs. (Remember that only two computers can communicate through the cable at a given moment.) Both computersthe file server and the workstationback off and try again. Ethernet network adapters use an algorithm called Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) to deal with collisions, causing each computer to back off for a random amount of time. This method effectively enables one computer to go first. A certain number of collisions are therefore normal and expected on an Ethernet network, but with higher amounts of traffic, the frequency of collisions rises higher and higher, and response times become worse and worse. A saturated Ethernet network actually can spend more time recovering from collisions than it does sending data. IBM and Texas Instruments, recognizing Ethernets traffic limitations, designed the Token-Ring network to solve the problem.
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