General Media Considerations

In addition to identifying the characteristics of network media and their associated cabling, the Network+ exam requires knowledge of some general terms and concepts that are associated with network media. Before looking at the individual media types, it is a good idea to first have an understanding of some general media considerations.

Broadband Versus Baseband

Networks employ two types of signaling methods: baseband and broadband. Baseband transmissions use digital signaling over a single wire. Communication on baseband transmissions is bidirectional, allowing signals to be sent and received but not at the same time. To send multiple signals on a single cable, baseband uses something called Time Division Multiplexing (TDM). TDM divides a single channel into time slots.

In terms of LAN network standards, broadband transmissions, on the other hand, use analog transmissions. For broadband transmissions to be sent and received, the media has to be split into two channels. Multiple channels are created using Frequency Division-Multiplexing (FDM).

Simplex, Half Duplex, and Full Duplex

Simplex, half duplex, and full duplex are referred to as dialog modes, and they determine the direction in which data can flow through the network media.

Simplex allows for one-way communication of data through the network, with the full bandwidth of the cable being used for the transmitting signal. One-way communication is of little use on LANs, making it unusual at best for network implementations. Far more common is the half-duplex mode, which accommodates transmitting and receiving on the network but not at the same time. Many networks are configured for half-duplex communication.

The preferred dialog mode for network communication is the full-duplex mode. To use full duplex, both the network card and the hub or switch must support full duplexing. Devices configured for full duplexing are capable of transmitting and receiving simultaneously. This means that 100Mbps network cards are capable of transmitting at 200Mbps using full-duplex mode.

Media Interference

Depending on where network cabling (commonly referred to as media) is installed, interference can be a major consideration. Two types of media interference can adversely affect data transmissions over network media: electromagnetic interference (EMI) and crosstalk.

EMI is a problem when cables are installed near electrical devices, such as air conditioners or fluorescent light fixtures. If a network media is placed close enough to such a device, the signal within the cable might become corrupt. Network media vary in their resistance to the effects of EMI. Standard UTP cable is susceptible to EMI, whereas fiber cable with its light transmissions is resistant to EMI. When deciding on a particular media, consider where it will run and the impact EMI can have on the installation.

A second type of interference is crosstalk. Crosstalk refers to how the data signals on two separate media interfere with each other. The result is that the signal on both cables can become corrupt. As with EMI, media varies in its resistance to crosstalk, with fiber-optic cable being the most resistant.

For the Network+ exam, remember that fiber-optic cable offers the greatest resistance to EMI and crosstalk.

Attenuation

Attenuation refers to the weakening of data signals as they travel through a respective media. Network media varies in its resistance to attenuation. Coaxial cable is generally more resistant than UTP, STP is slightly more resistant than UTP, and fiber-optic cable does not suffer from attenuation at all. That's not to say that a signal does not weaken as it travels over fiber-optic cable, but the correct term for this weakening is 'chromatic dispersion,' rather than attenuation.

It's important to understand attenuation or chromatic dispersion and the maximum distances specified for network media. Exceeding a media's distance without using repeaters can cause hard-to-troubleshoot network problems. Most attenuation or chromatic dispersion related difficulties on a network require using a network analyzer to detect them.

Data Transmission Rates

One of the more important media considerations is the supported data transmission rate or speed. Different media types are rated to certain maximum speeds, but whether or not they are used to this maximum depends on the networking standard being used and the network devices connected to the network.

The transmission rate of media is sometimes referred to incorrectly as the bandwidth. In truth, the term bandwidth refers to the width of the range of electrical frequencies or amount of channels that the media can support.

Transmission rates are normally measured by the number of data bits that can traverse the media in a single second. In the early days of data communications, this measurement was expressed as bits per second (bps), but today's networks are measured in Mbps (megabits per second) and Gbps (gigabits per second).

The different network media vary greatly in the transmission speeds they support. Many of today's application-intensive networks require more than the 10Mbps offered by the older networking standards. In some cases, even 100Mbps, which is found in many modern LANs, is simply not enough to meet current network needs. For this reason, many organizations deploy 1Gbps networks, and some now even go for 10Gbps implementations.