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There are important differences between a railroad system and a computer network, however. On a network, all traffic appears to move simultaneously, without interruption. Actually, this appearance of simultaneity is an illusion; in reality, the computers take turns accessing the network for brief periods of time. The more significant difference arises from the higher speed at which network traffic moves.
Multiple computers must share access to the cable that connects them. However, if two computers were to put data onto the cable at the same time, the data packets from one computer would collide with the packets from the other computer, and both sets of data packets would be destroyed. Figure 3.1 shows what happens when two computers try to access the network at the same time.
Figure 3.1 Collision occurs if two computers put data on the cable at the same time
If data is to be sent over the network from one user to another, or accessed from a server, there must be some way for the data to access the cable without running into other data. And the receiving computer must have reasonable assurance that the data has not been destroyed in a data collision during transmission.
Access methods need to be consistent in the way they handle data. If different computers were to use different access methods, the network would fail because some methods would dominate the cable.
Access methods prevent computers from gaining simultaneous access to the cable. By making sure that only one computer at a time can put data on the network cable, access methods ensure that the sending and receiving of network data is an orderly process.
Run the c03dem01, c03dem02, and c03dem03 videos
located in the Demos folder on the CD accompanying this book to
view a demonstration of how access methods help ensure orderly transmission
of data on a network.
Figure 3.2 Computers can transmit data only if the cable is free
Only when a computer "senses" that the cable is free and that there is no traffic on the cable can it send data. Once the computer has transmitted data on the cable, no other computer can transmit data until the original data has reached its destination and the cable is free again. Remember, if two or more computers happen to send data at exactly the same time, there will be a data collision. When that happens, the two computers involved stop transmitting for a random period of time and then attempt to retransmit. Each computer determines its own waiting period; this reduces the chance that the computers will once again transmit simultaneously.
With these points in mind, the name of the access method—carrier-sense multiple access with collision detection (CSMA/CD)—makes sense. Computers listen to or "sense" the cable (carrier-sense). Commonly, many computers on the network attempt to transmit data (multiple access); each one first listens to detect any possible collisions. If a computer detects a possible collision, it waits for a random period of time before retransmitting (collision detection).
The collision-detection capability is the parameter that imposes a distance limitation on CSMA/CD. Due to attenuation—the weakening of a transmitted signal as it travels farther from its source, discussed in Chapter 2, "Basic Network Media"—the collision detection mechanism is not effective beyond 2500 meters (1.5 miles). Segments cannot sense signals beyond that distance and, therefore, might not be aware that a computer at the far end of a large network is transmitting. If more than one computer transmits data on the network at the same time, a data collision will take place that will corrupt the data.
Run the c03dem04 and c03dem05 videos located in the Demos folder on the CD accompanying this book to view a demonstration of the CSMA/CD access method.
CSMA/CD is known as a contention method because computers on the network contend, or compete, for an opportunity to send data.
This might seem like a cumbersome way to put data on the cable, but current implementations of CSMA/CD are so fast that users are not even aware they are using a contention access method.
Run the c03dem06 video located in the Demos folder on the CD accompanying this book to view a demonstration of why CSMA/CD is considered a contention method.
The more computers there are on the network, the more network traffic there will be. With more traffic, collision avoidance and collisions tend to increase, which slows the network down, so CSMA/CD can be a slow-access method.
After each collision, both computers will have to try to retransmit their data. If the network is very busy, there is a chance that the attempts by both computers will result in collisions with packets from other computers on the network. If this happens, four computers (the two original computers and the two computers whose transmitted packets collided with the original computer's retransmitted packets) will have to attempt to retransmit. These proliferating retransmissions can slow the network to a near standstill.
The occurrence of this problem depends on the number of users attempting to use the network and which applications they are using. Database applications tend to put more traffic on the network than word-processing applications do.
Depending on the hardware components, the cabling, and the networking
software, using a CSMA/CD network with many users running several database
applications can be very frustrating because of heavy network traffic.
Run the c03dem07 video located in the Demos folder on
the CD accompanying this book to view a demonstration of the CSMA/CA access
The computer can now transmit data. Data is transmitted in frames, and additional information, such as addressing, is attached to the frame in the form of headers and trailers, discussed later in this chapter.
In Figure 3.3, the server is shown transmitting data. It takes control of the free token on the ring and sends data to the computer with the address 400080865402.
Figure 3.3 Token-passing access method
While the token is in use by one computer, other computers cannot transmit data. Because only one computer at a time can use the token, no contention and no collision take place, and no time is spent waiting for computers to resend tokens due to network traffic on the cable.
Run the c03dem08, c03dem09, c03dem10, c03dem11,
and c03dem12 videos located in the Demos folder on the CD
accompanying this book to view demonstrations of the token-passing access
This access method is based on the fact that repeaters and end nodes are the two components that make up all 100VG-AnyLAN networks. Figure 3.4 shows a demand-priority network. The repeaters manage network access by doing round-robin searches for requests to send from all nodes on the network. The repeater, or hub, is responsible for noting all addresses, links, and end nodes and verifying that they are all functioning. According to the 100VG-AnyLAN definition, an end node can be a computer, bridge, router, or switch.
Figure 3.4 Star-bus network access method for 100VG-AnyLAN is demand priority
Run the c03dem13 and c03dem14 videos located in the Demos folder on the CD accompanying this book to view a demonstration of the demand-priority access method.
As in CSMA/CD, two computers using the demand-priority access method can cause contention by transmitting at exactly the same time. However, with demand priority, it is possible to implement a scheme in which certain types of data will be given priority if there is contention. If the hub or repeater receives two requests at the same time, the highest priority request is serviced first. If the two requests are of the same priority, both requests are serviced by alternating between the two.
In a demand-priority network, computers can receive and transmit at the same time because of the cabling scheme defined for this access method. In this method, four pairs of wires are used, which enables quartet signaling, transmitting 25 MHz signals on each of the pairs of wire in the cable.
In a demand-priority network, there is communication only between the sending computer, the hub, and the destination computer. This is more efficient than CSMA/CD, which broadcasts transmissions to the entire network. In demand priority, each hub knows only about the end nodes and repeaters directly connected to it, whereas in a CSMA/CD environment, each hub knows the address of every node in the network.
Demand priority offers several advantages over CSMA/CD including:
By using four pairs of wires, computers can transmit and receive at the same time.
Transmissions are not broadcast to all the other computers on the network. The computers do not contend on their own for access to the cable, but operate under the centralized control of the hub.
Table 3.1 Features of Different Access Methods
|Feature or function||CSMA/CD||CSMA/CA||Token passing||Demand priority|
|Type of Communication||Broadcast-
|Type of access method||Contention||Contention||Noncontention||Contention|
|Type of network||Ethernet||LocalTalk||Token Ring