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Cabling And Bandwidth Performance Requirements: Megabits vs. Megahertz

Infrastructure
Posted by James Donovan on 9 September 2015 Connect with James on LinkedIn Estimated reading time: 4 minutes

Specifying Cabling Bandwidth Performance:

Often, confusion can arise when assessing and specifying cabling system bandwidth performance and other performance requirements for current and future high speed data applications.

The confusion relates to the terms ‘Megabits per second’ (Mbps) and ‘MegaHertz’ (MHz), or ‘Gigabits per second’ (Gbps) and ‘GigaHertz’ (GHz). Mbps and MHz are NOT the same thing although their numerical values may in some instances coincide. Cabling transmission properties such as insertion loss and crosstalk are typically specified as a function of Hertz (Hz), or cycles per second. Digital data is transmitted as a series of ‘0’s and ‘1’s, called bits. The speed at which these digital symbols are transmitted is measured in bits per second.

Simply put, 1000 Mbps (Ethernet) does not mean the system is operating at 1000 MHz, and therefore the actual requirements for the cabling channel relies on an understanding of the relationship between the bit rate and the frequency. This relationship is founded on what is known as bandwidth-efficient digital communication that is based on technology and science that has been around for decades.

In simple terms, there are three ways of transmitting higher bit rates over cabling or any other transmission media for that matter, one is to improve the transmission channel bandwidth performance, the second is to improve the technology in the electronics, and thirdly and more commonly is a requirement for a mixture of both.

Bandwidth performance and coding / Modulation schemes

Bandwidth-efficient line codes are used to provide higher bit rates in a given bandwidth, or, alternatively, they are also used to reduce the required bandwidth for a fixed bit rate. Examples of the first type of uses of this technology were voiceband modems and digital radio. The first bandwidth efficient data transmission schemes were developed about fifty years ago for voiceband modem applications. Since then, the techniques have been used widely in Broadband, Cellular Wireless, Wi-Fi and other systems that rely on bandwidth efficiency.

Looking at applications such as Gigabit Ethernet and beyond, understanding the impact of coding and other techniques is vital to fully realize the potential of a particular grade of cabling system, whether it is copper cabling or fiber optic cabling.

Increasingly higher date rates require an increasing amount of bandwidth performance because of the band-limited nature of the available channels. Bandwidth-efficiency comprises a product of two components – two completely different and independent techniques, which are multilevel encoding and efficient spectral shaping.

With multilevel encoding, the pulses that are sent through the communication channel carry enough information to represent blocks of bits rather than one single bit. For a given bit rate, the usage of multilevel-encoded pulses results in a decrease of the rate (often referred to as the Baud rate) at which the pulses have to be sent through the channel, which, in turn, decreases the bandwidth requirement for the transmission channel.

However the more levels used the more susceptible the system is to noise and requires closer understanding and definition of the transmission channel (cabling) to cater for it and/or compensation techniques are employed to overcome the noise such as crosstalk cancellation etc.

With efficient spectral shaping, the square-shaped pulses that are used with line codes are smoothed out in time to eliminate any sharp voltage transitions. As a result, fewer high frequency components are generated by the pulses, which also decreases the bandwidth requirement for the channel. Designers have improved the bandwidth-efficiency of their transceivers, over the years, by making steady incremental progress on both the multilevel encoding and spectral shaping fronts.

The definition of Bit-rate

For definition: the bit rate is the number of bits transmitted in one second. The symbol or baud rate refers to the number of signal units, or symbols, per second that are required to represent those bits. Bit rate equals the symbol/baud rate times the number of bits represented by each signal unit. Bit rate is always greater than or equal to the symbol/baud rate.

Hopefully, this helps explain the difference between bit-rate and bandwidth, and also the potential relationship between the two. It also shows the relationship is quite complex and that the actual bandwidth requirement of a system is based on a multitude of factors.

Pulling this all together shows that the bandwidth, bit rate, signal strength, noise, channel impairments (and therefore cabling performance), and the bit error rate of the system are all interlinked. Improvements in one or more area affect and/or rely upon improvements in others, and specifically improvement in cabling performance can only result in a positive outcome for the overall system.