Factors that influence Data Transfer Rates

The amount of data that can be transferred over even simple unshielded twisted-pair cables has increased dramatically over the last few years.

Many experts in the late 1980s believed that UTP cables would not support data rates in excess of 10Mbps.

Today, cables of 100 meters typically support data rates of 10Gbps. It is even possible that UTP cables could achieve greater data rates in the future.

When looking at a typical cabling catalog, there are more than 2,000 different cable types listed. The modern cabling world nowadays uses a huge range of engineering terms and communication buzzwords.

Cable design complexity has increased dramatically with the need for faster data rates. With an increase in data rates, what happens inside the cables becomes increasingly complex, while the probability that data signals will be corrupted also increases.

The cable’s electrical properties change when the data rates increases, signal travel distance decreases and signal distortion becomes more prevalent. Manufacturers of cables supporting frequencies bigger than 100MHz and 1000Base-T (Gigabit Ethernet) are faced with electrical problems that are not encountered at lower data rates and frequencies. These problems include arrival delays of the signal on different pairs of wires and different varieties of crosstalk.

High-Speed Data Transfer Obstacles

Electricity consists of electrons flowing inside a cable and colliding with each other. For a signal to be transmitted correctly, enough electrons must flow through the cable from the sender to the receiver. When the frequency, and as a result the potential data rate, increases in a cable, a number of factors obstruct the signal’s travel. These factors are important to both the person responsible for testing and certifying the cable and the person responsible for authorizing cable purchases.

Category 5e, 6, 6A, 7, and 7A cabling specifications list a number of these factors and the maximum (or minimum) acceptable values that cables can have to still be compliant.

Both 1000Base-T Ethernet and 10GBase-T uses complex modulation technology, resulting in the TIA specifying additional cabling performance characteristics which were not included in the original specifications.

These include pair-to-pair crosstalk and power-sum measurements, return loss, delay skew, ACR-N and ACR-F.

Some of the new performance characteristics relate to crosstalk, for example AN’EXT (near-end alien crosstalk) and AXT: AACRF (attenuation to alien crosstalk ratio, far-end), both used to express interaction between cables in a bundle.

Although crosstalk should be considered in all technologies, fast technologies such as 1000Base-T and 10GBase-T are more sensitive as they use four pairs in parallel for transmissions.

These new requirements are included in the current versions of the ANSI/TIA-568-C and ISO/IEC 11801 Ed. 2.2 standards.

Many transmission requirements are based on intricate mathematical formulas. For ease of use, values are calculated and listed in the specification for selected frequencies.

The actual requirement is however that the parameter must pass the test across the full bandwidth applicable to the cable category. This means that performance must be in accordance with the formula and consistent at any given frequency, from the lowest to the highest frequency specified.

The major 17 test parameters for communication cables are listed below:

Attenuation (signal-loss)

• Mutual capacitance (1)
• Conductor resistance (2)
• Insertion loss (3)
• Return loss (4)
• Impedance (5)

Noise

• Capacitance unbalance (6)
• Resistance unbalance (7)
• Far-end crosstalk (FEXT) (8)
• Near-end crosstalk (NEXT) (9)
• Power-sum FEXT (10)
• Power-sum NEXT (11)
• Power-sum ACR-F, ACR-N (12)
• Power-sum AACRF, ANEXT (13)
• Alien crosstalk (AACRF, ANEXT) (14)
• Attenuation-to-crosstalk ratio (ACR-F, ACR-N) (15)

Other

• Delay skew (16)
• Propagation delay (17)

In our next blog posts we will be looking more deeper at any of these factors, but for now, you can take a glance at actually how many the factors there really are!