"Wait I thought you said, 'digital signals were not subject to signal degradation'?" This is true as long as the medium is also digital, like a CD. However, notice that the digital signal is sent "at a fixed frequency." This indicates an analog medium. Think of it like this: One bit of data is either a one or a zero. To signal a one let's say that the pulse has to be between 3-5 kHz and anything lower is a zero. That one bit alone is an analog signal. However, all the bits as a whole is a digital signal. As this bit travels over the medium is is being subjected to degradation of the frequency. The further it travels, the more it degrades to the point where it will eventually be interpreted as a zero instead of a one. Each bit is subjected to the same degradation. Some might experience less, some might experience more, but all degrade which is why repeaters are used over longer lines. It is also worth mentioning that repeaters are also used for wireless signals as well and they work in much the same way. Before things start getting confused, let me point something out. Data that is sent out from your computer is always digital and the medium that it travels on is almost always analog.
Now, with broadband transmission, data is transmitted as a continuous electromagnetic or optical wave (see an analog wave instead of a digital pulse) [1]. This is a form of analog signaling and as mentioned in Part 3, can be divided into channels, over which different data can be transmitted. This is called multiplexing, or more specifically, frequency division multiplexing (FDM). Briefly, FDM divides the medium into two or more channels of different frequencies. These channels are combined into an aggregate frequency that can be sent over the medium. Each channel is separated from one another by an unused frequency so that data from one frequency does not bleed into the one next to it. This is called "crosstalk" and the unused frequencies are called "guard bands" [2]. Falling back to the busy street example you could think of the guard bands as concrete barriers between the lanes. They keep all cars (data) in their proper lane (channel).
Now, the receivers must have some means to dial into the right channel, like the cable box you have for your TV and the cable modem you use for your computer. Cable companies use broadband to deliver hundreds of channels and high-speed Internet to customers. Broadband transmissions are also half-duplex. However, two methods are used to create full-duplex broadband. One is called mid-split broadband which splits bandwidth into two groups of channels, one for sending and one for receiving. The other method is called dual-cable broadband, which just as its name implies, uses a transmission cable and receiver cable. Similarly to baseband, broadband has a way to improve transmission distances. Amplifiers detect and amplify weak signals then rebroadcast then to the next section of line [1].
Since, baseband signals are of very low frequencies, they are not good for transmitting over large distances because degradation is much more pronounced in low frequency waves. However, baseband is very efficient over short distances and is therefore commonly used in local area network (LAN) communication. In addition, baseband does not have to be modulated into an analog signal, so less equipment is needed to communicate between devices. The Ethernet cable that you have going from your modem to your computer or router is a baseband line. Most office networks are set up on a baseband network with a broadband connection to the Internet.
Broadband uses much higher frequencies and therefore is much better for transmitting over long distances. Wide area networks (WAN) use broadband to communicate. In fact, the whole Internet is really just one big WAN. Broadband is generally faster than baseband, but only because of the multiple signals factor. The data is really traveling in general at about the same rate. It's just that more signals can be passed through broadband and therefore, the resulting data transfer rate is higher. However, broadband is more expensive to implement due to the equipment involved and cost of service. Since broadband travels over cables that are buried underground and are usually owned by another entity, you have to pay to use them. Since baseband does not require much special equipment and because you can run the lines above ground, it is much less expensive to implement [3], not to mention that you own the lines.
This concludes the Part 5 of the the Data Transmission and Protocol series. In Part 6 I will go over the final set of transmission methods, serial and parallel. These transmission methods are generally used in peripheral communication. How does your computer communicate with your monitor? How about the printer? You will know after reading Part 6. Then in Part 7 I will begin contrasting and explaining protocols starting with Transmission Control Protocol and Internet Protocol.
[1] Tomsho, G., Tittel, E., & Johnson, G. (2004). Guide to networking essentials. Thomson Course Technology: Boston, MA. ISBN-13: 978-1-4188-3718-1
[2] General Telecom. (1998). Multiplexing Techniques. Retrieved from http://bit.ly/mTPngG
[3] James, E. (2010). Broadband. Retrieved from http://bit.ly/oZRc2t
[3] James, E. (2010). Broadband. Retrieved from http://bit.ly/oZRc2t
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