MHz vs. Mbits

The emergence of faster, higher throughput networking standards has highlighted the need to understand that the data rate expressed in Mbits/s and the bandwidth required to support a high data rate are different, yet related quantities. Many users have confused the terms Mbits/s and MHz. The former (Mbits/s) is a measure of the rate at which binary data can be transmitted; this rate is expressed in millions of bits per seconds, abbreviated as Mbps or Mbits/s. The latter (MHz) expresses a frequency of a pure sinusoidal signal.

A bandwidth of 100 MHz indicates that a cabling system in general, can transmit sinusoidal signals with a frequency of up to 100 MHz with an acceptable level of performance. The vague and general statement "acceptable level of performance" must be clearly defined. For this complicated task one will typically rely on an established standard or on a detailed transmission specification for each of the underlying transmission characteristics.

The relationship between Mbits/s and MHz for a network cabling system depends on the signal encoding used for the binary data as well as the desired data rate. The signal encoding for 10BASE-T Ethernet systems and the 16 Mbits/s Token Ring network imposes at least a one-to-one relationship between bandwidth and data rate. This means that the 10BASE-T Ethernet system requires a cabling system which supports a 10 MHz bandwidth (with transmission performance specified in the IEEE 802.3 standards document).

Similarly, 16 Mbits/s Token Ring requires a cabling plant with bandwidth performance of 16 MHz (although the IEEE 802.5 standard defines the desired performance for the underlying electrical parameters up to 25 MHz). This one-to-one relationship between Mbits/s and MHz is not a general requirement. It is imposed by the Manchester encoding techniques used by Ethernet and Token Ring. On the other hand, the fast Ethernet standard 100BASE-TX specifies a different signal encoding scheme (called MLT-3) which enables it to transmit 100 million bits per second with a bandwidth requirement of 80 MHz as specified in IEEE 802.3u (furthermore, the majority of the transmitted energy is below 32 MHz).

Most of the generic cabling standards, specify the transmission performance of cabling links in terms of the frequency range of sinusoidal signals that can be transmitted by the link with a specified level of quality. The parameters which are used to characterise this level of quality are attenuation, crosstalk, and/or the combined effect of these two parameters called attenuation-to-crosstalk ratio (ACR). 

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