Cabling bandwidth: What do you need to know?

Cabling bandwidth: What do you need to know?

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When determining the right cabling solutions for an installation, there are a variety of factors that need to be considered. Deployers need to balance the issue of cost versus performance, consider the distance they will need to cover and take into account any specific environmental factors.

These will all affect the type of cable that is best-suited for a deployment, and there are several key specifications deployers should look for when buying cable. And one of the most important will be the maximum bandwidth offered by the potential solution.

Bandwidth vs data rates

Bandwidth is often used interchangeably with data transfer rate, when in fact, from a cabling installer's perspective, they are two different properties, and this may often cause confusion among those who aren't familiar with the terms, or who think in terms of consumer-grade definitions.

For instance, when an internet connection is sold to a consumer, it will often be advertised as having a 'bandwidth' of a certain Mbps, when this is actually the data bandwidth rate. But there is also the signal bandwidth of the cable, which describes the frequency of the signal and determines how capable it is of transmitting a signal from one end to the other with minimal signal degradation.

This is a concern for both copper and fibre optic cables. As well as simple loss, copper cables can be affected by issues such as crosstalk, while fibre performance may be impacted by return loss.

By increasing the bandwidth of these signals, cable vendors are able to deliver the data at higher rates. While there are many complex factors related to bandwidth and frequency, in simple terms, the higher the bandwidth of a cable, the more data it can reliably carry.

For copper cabling, bandwidth is measured in megahertz (MHz), and each category of cable has a maximum bandwidth. These are:

Cat5 and Cat5e - 100MHz
Cat6 - 250MHz
Cat6a - 500MHz
Cat7 - 600MHz

Fluke Networks notes that, in days gone by, bandwidth frequency was closely related to data transfer rates. Cat5 cables, for example, have a maximum bandwidth of 100MHz and can deliver speeds of 100Mbps. However, technologies such as pulse amplitude modulation have allowed cables to go far beyond this rate, enabling Cat6 and newer cables to offer gigabit speeds and beyond with bandwidths of 250MHz.

Understanding fibre bandwidth

When it comes to multimode fibre optic solutions, there are five grades of cabling that are defined by their bandwidth, from OM1 to OM5. OM1 and OM2 are today usually restricted to legacy applications, with the majority of modern cabling infrastructures using OM3, OM4 or OM5 fibre.

Unlike with copper, where a higher category of bandwidth performance translates into support for higher data transmission rates, improvements in the grade of fibre offer increased transmission distance for the applications designed to run over it.

For example, a 10GBase-SR application with a data transfer speed of up to 10Gbps is only supported for distances of up to 33 metres on the oldest OM1 fibres. An OM3 cable, on the other hand, will deliver this for distances of up to 300 metres, while for OM5, the maximum distance is 500 metres.

However, multimode fibre also has a specification called effective modal bandwidth (EMB), which determines how much data a specific fibre is able to transmit at a given wavelength. This is dependent on the length of the cable, so for example, a fibre optic cable with an EMB of 200MHz/km can move 200MHz of data up to one kilometre, while a higher EMB can move more data the same distance, or it can send the same amount of data over a longer distance.

The EMB of a multimode cable is affected by the differential mode delay (DMD) of the cable. This is the difference in travel time between the fastest mode and the slowest within the cable, as when multiple modes of light travel through a multimode fibre, some travel faster down the centre, while others travel slower along pathways closer to the core-cladding interface. Fibre manufacturers design their fibre to limit DMD and allow for higher bandwidth.

Choosing the right type of cabling

When looking for cabling, there are a range of factors that must be considered, of which the data transfer rate is just one consideration. 

Installers will have to consider factors including:

  • What applications will need to be supported
  • The distance over which these applications must be supported
  • The anticipated lifespan of the infrastructure
  • How much budget is available

The amount of bandwidth available on a cable type will be important in meeting many of these needs, so it pays to take the time to understand what the various options are and what they may translate to in terms of real-world performance.

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