Content

| 3.2 |  | Characteristics of Physical Layer Optimization Problems |
| 3.2.2 |  | Exceeding cable design limits, poor quality cabling and connections |

Attenuation
A common issue of exceeding the design limits of a media type is the attenuation of the bit-stream transmitted along the media. Attenuation depends on the media over which the traffic is being transmitted. Attenuation may occur to such an extent that the receiving device cannot always successfully distinguish the component bits of the stream from each other. This ends in a garbled transmission and results in a request from the receiving device for retransmission of the missed traffic by the sender.

Each network media is rated for a specific distance. Category 5 media has a maximum rated cabling distance of 100m. Beyond that distance, the signal must be regenerated or it may degenerate and be unreadable by the receiving end. To get an accurate measurement of the length of a cable, use a cable tester.

A far more common source of attenuation is a very poor connection resulting from a loose cable or dirty or oxidized contacts. One bad patch cable can easily cause an entire link to fail. Another source of this fault is the wrong Category cable used, such as Category 3 cable used for a link being tested to Category 5e limits.

Return Loss
Return loss is a measure of all reflections that are caused by the impedance mismatches at all locations along the link. It indicates how well the characteristic impedance of the cable matches its rated impedance over a range of frequencies. The characteristic impedance of links tends to vary from higher values at low frequencies to lower values at the higher frequencies. Return loss is expressed in decibels.

The termination resistance at both ends of the link must be equal to the characteristic impedance of the link to avoid reflections. A good match between characteristic impedance and termination resistance in the end equipment provides for a good transfer of power to and from the link and minimizes reflections. Return loss results vary significantly with frequency.

One small source of return loss is variations in the value of the characteristic impedance along the cable. This may be due to slight untwisting or separation of wires in the pairs, or due to variations in the metal of the wire and the uniformity of the insulation. Another source of return loss is reflections from inside the installed link, mainly from connectors. Mismatches predominantly occur at locations where connectors are present. The main impact of return loss is not on loss of signal strength but rather the introduction of signal jitter, the variation in delay over time from point-to-point. Jitter is of special concern for time sensitive traffic such as VoIP, excessive jitter seriously degrades call quality. Signal reflections truly cause loss of signal strength but generally this loss, due to return loss, does not create a significant problem.

Return loss is of particular concern in the implementation of Gigabit Ethernet.