Characteristics Specified in the IEEE 802 Standards
The IEEE standards specify the characteristics of the networking systems, including speed, access methods, topologies, and media. Although you don't need detailed knowledge of all these IEEE standards in real-world applications, a general understanding of these standards will be an asset.
Many factors contribute to the speed of a network. The standard defines the maximum speed of a networking system. The speed normally is measured in megabits per second (Mbps), although some faster network systems use gigabits per second (that is, Gbps, where 1Gbps is equivalent to 1000Mbps).
Some networks are faster than others. For example, a token ring (802.5) network has a maximum speed of 16Mbps. Many ethernet networks (802.3 variants) now operate at 100Mbps and far beyond. However, the maximum speed attainable on a network can be affected by many factors. Networks that achieve 100% of their potential bandwidth are few and far between.
Access methods govern the way in which systems access the network media and send data. Access methods are necessary to ensure that systems on the network can communicate with each other. Without an access method, it would be possible for two systems to communicate at the exclusion of every other system. Access methods ensure that everyone gets an opportunity to use the network.
Several access methods are used in networks; the most popular are CSMA/CD, CSMA/CA, and a distant third would be token passing. Other methods, such as demand priority are sometimes found as well. We'll look at each of these access methods separately.
Carrier Sense Multiple Access/Collision Detection
Carrier Sense Multiple Access/Collision Detection (CSMA/CD) is defined in the IEEE 802.3 standard. CSMA/CD is the most common media access method because it is associated with 802.3 ethernet networking, which is by far the most popular networking system.
On a network that uses CSMA/CD, when a system wants to send data to another system, it first checks to see whether the network media is free. It must do this because each piece of network media used in a LAN can carry only one signal at a time. If the sending node detects that the media is free, it transmits, and the data is sent to the destination. It seems simple.
Now, if it always worked like this, you wouldn't need the CD part of CSMA/CD. Unfortunately, in networking, as in life, things do not always go as planned. The problem arises when two systems attempt to transmit at exactly the same time. It might seem like a long shot that two systems will pick the same moment to send data, but we are dealing with communications that occur many times in a single second—and most networks have more than two machines. Imagine that 200 people are in a room. The room is silent, but then two people decide to say something at exactly the same time. Before they start to speak, they check (listen) to see whether someone else is speaking; because no one else is speaking, they begin to talk. The result is two people speaking at the same time, which is similar to a network collision.
Collision detection works by detecting fragments of the transmission on the network media that result when two systems try to talk at the same time. The two systems wait for a randomly calculated amount of time before attempting to transmit again. This amount of time—a matter of milliseconds—is known as the backoff.
When the backoff period has elapsed, the system attempts to transmit again. If the system doesn't succeed on the second attempt, it keeps retrying until it gives up and reports an error.
The upside of CSMA/CD is that it has relatively low overhead, meaning that not much is involved in the workings of the system. The downside is that as more systems are added to the network, more collisions occur, and the network becomes slower. The performance of a network that uses CSMA/CD degrades exponentially as more systems are added. Its low overhead means that CSMA/CD systems theoretically can achieve greater speeds than high-overhead systems, such as token passing. However, because collisions take place, the chances of all that speed translating into usable bandwidth are relatively low.
Despite its problems, CSMA/CD is an efficient system. As a result, rather than replace it with some other technology, workarounds have been created that reduce the likelihood of collisions. One such strategy is the use of network switches that create multiple collision domains and therefore reduce the impact of collisions on performance. See Chapter 3, "Networking Components and Devices," for information about using switches.
Table 6.1 summarizes the advantages and disadvantages of the CSMA/CD access method.
Table 6.1. Advantages and Disadvantages of CSMA/CD
It has low overhead.
Collisions degrade network performance.
Utilizes all available bandwidth when possible.
Priorities cannot be assigned to certain nodes. Performance degrades exponentially as devices are added.
Instead of collision detection as with CSMA/CD, the Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) access method uses signal avoidance rather than detection. In a networked environment, CSMA/CA is the access mechanism used in Apple's LocalTalk network and with the 802.11 wireless standards.
On CSMA/CA networks, each computer signals its intent to transmit data signals before any data is actually sent. When a networked system detects a potential collision, it waits before sending out the transmission allowing systems to avoid transmission collisions. The CSMA/CA access method uses a random backoff time that determines how long to wait before trying to send data on the network. When the backoff time expires, the system will again "listen" to verify a clear channel on which to transmit. If the media is still busy, another backoff interval is initiated that is less than the first. The process continues until the wait time reaches zero, and the media is clear.
CSMA/CA uses a broadcast method to notify its intention to transmit data. Network broadcasts create a considerable amount of network traffic and can cause network congestion, which could slow down the entire network. Because CSMA/CD and CSMA/CA differ only in terms of detection and avoidance, they share similar advantages and disadvantages, as shown previously in Table 6.1.
Although token passing, defined in the IEEE 802.5 standard, was once a popular media access method, the domination of ethernet networking has pushed it far into the background. Although it might not be popular, it is clever.
On a token-passing network, a special data frame called a token is passed among the systems on the network. The network has only one token, and a system can send data only when it has possession of the token.
When the data arrives, the receiving computer sends a verification message to the sending computer. The sender then creates a new token, and the process begins again. Standards dictate how long a system can have control over the token.
One of the big advantages of the token-passing access method is the lack of collisions. Because a system can transmit only when it has the token, no contention exists. Even under heavy load conditions, the speed of a token-passing system does not degrade in the same way as a contention-based method such as CSMA/CD. In a practical scenario, this fact makes token passing more suitable than other access methods for applications such as videoconferencing.
However, token passing does have drawbacks. The creation and passing of the token generate overhead on the network, which reduces the maximum speed. In addition, the software and hardware requirements of token-passing network technologies are more complex—and therefore more costly—than those of other media access methods.
As discussed in Chapter 1, topologies dictate both the physical and logical layouts of the network. Remember that topologies include bus, star, ring, mesh, and wireless. Each of the IEEE LAN standards can be implemented by using the topology specified within the standard. Some standards, such as 802.3 (ethernet), have multiple physical topologies but always use the same logical topology.
Each IEEE specification defines what media are available to transport the signal around the network. The term media, which is the plural of medium, generically describes the methods by which data is transported from one point to another. Common network media types include twisted-pair cable, coaxial cable, infrared, radio frequency, and fiber-optic cable. See Chapter 2, "Media and Connectors," for a detailed discussion of media types.