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Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.

Megahertz, Megabits, MegaConfusion

Are you puzzled by transmission terminology?

The terms megahertz (MHz) and megabit (Mb) are two of the most misunderstood and incorrectly referenced terms in the category cabling1 arena today. Many end-users mistakenly believe that systems tested to frequencies over and above the minimum ANSI/TIA/EIA-568-A and ISO/IEC/11801 requirements will support higher data transmission rates. For example, we often hear that cables tested above 100 MHz are required to run ATM 155 Mb/s2 systems. These types of statements are generated based upon the false assumption that one bit of information is transmitted per one signal cycle. Because, there is no direct conversion between frequency and bit rate, it is best to rely upon the system specifications themselves to determine which category cabling systems are most suitable for your intended application.

Signals which oscillate (i.e. a sinusoid) are described by the number of times they cycle in a second. Signal frequency is measured in units of hertz. For example, figure 1 depicts a 5 hertz signal. The '568-A and '11801 requirements specify category 5 transmission performance for the signaling frequency ranges of 1 MHz (one million cycles per second) to 100 MHz (one hundred million cycles per second). These standards requirements are applications independent, meaning that the method used to transmit information over the available signaling frequency range is left unspecified.

Figure 1

Rate of information transfer is measured in units of bits per second. The term bit, an abbreviation for binary digit, represents an electrical state of either 'on' or 'off' and is used in various binary information encoding schemes. The bit rate for category 5 transmission systems, typically measured in megabits or millions of bits per second (Mb/s), is dependent upon the signaling scheme used by a specific application's protocol; it is not a frequency dependent parameter. Most applications designed for use with category 5 cabling deliver different bit rates for transmission up to 100 MHz. For example, 100BASE-TX3 transmits 100 Mb/s while ATM PMDI transmits 155 Mb/s. Additional protocols under evaluation for operation over category 5 cabling include Gigabit Ethernet and 622 Mb/s ATM.

A typical transceiver system operates by encoding and decoding bits of information for signal transmission over an available frequency bandwidth. Signal shaping techniques, which include modulation of amplitude, frequency, or phase and filtering are used to transform the desired bit rate into an acceptable signaling or baud rate. Baud describes the signaling speed over the transmission media and is not directly related to the number of bits to be transferred. For example, if a specific transmission scheme requires a timing bit to ensure error free transmission for every three data bits, the baud rate will be 25 percent higher than the bit rate. Applications designers are continually challenged to develop systems which transmit higher bit rates. By developing more efficient signaling schemes and migrating to full duplex transmission (transmitting and receiving at the same time), higher bit rates can be realized without changing the signal frequency bandwidth. For existing and emerging applications that are specified for operation up to over category 5 cabling, there is no advantage to be gained by evaluating transmission performance beyond 100 MHz.

Because the '568-A and '11801 transmission specifications are applications independent, these standards serve to create a level playing field for the evaluation of telecommunications components. Comparing NEXT, attenuation, and return loss characteristics versus frequency (as opposed to bit transmission rate) eliminates the need to know what encoding scheme is being used and how the schemes being compared are related.

If you need to know whether a cabling component or system is compatible with an application that operates at "X" Mb/s, consult the manufacturer or your system integrator. If you provide information on both the bit rate and application (i.e. the applicable LAN standard or encoding scheme used), the manufacturer or system integrator should be able to recommend a transmission performance category for your cabling that is suitable for that application.

Different Encoding Schemes Representing the same Number of Bits
Polar Non Return to Zero (NRZ) - Used for ATM 155 Mb/s

MLT-3 Used for 100BASE-TX

1 Category cabling specifications are provided in ANSI/TIA/EIA-568-A and ISO/IEC 11801.

2 ATM Forum Physical Medium Dependent Interface Specification for 155 Mb/s over Twisted-Pair Cable, AF-PHY-00015.000, September 994

3 IEEE Media Access Control (MAC) Parameters, Physical Layer, Medium Attachment Units, and Repeater for 100 Mb/s Operation, Type 100BASE-T, 802.3u-1995

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