Technology overview-Noise

Written on 6:45 AM by ooe

Copper access transmission systems face a variety of impairments that present barriers to their operation. These can be broadly classified as intrinsic or extrinsic to the cable environment.

Examples of intrinsic noise impairments are thermal noise, echoes and reflections, attenuation and crosstalk. There are also other components that reside in the cable infrastructure that can impair the operation of DSL systems. These include surge protectors, RFI filters, and in some networks, bridged taps and loading coils.

Another intrinsic impairment is the condition of the cable infrastructure which may exhibit faults such as split pairs, leakage to ground, low insulation resistance, battery or earth contacts, and high-resistance joints. All these impairments reduce DSL performance.

Examples of extrinsic impairments are impulsive noise originating from lightning strikes, electric fences, power lines, rotating machinery, electric arc welders, switches, fluorescent lighting, etc. There is also radio interference from a variety of sources, such as AM broadcasting and SW/amateur radio transmitters.

The noise sources mentioned above can alternatively be classified as capacity limiting or performance limiting. Capacity-limiting noise is usually slow changing, such as thermal noise and crosstalk. These noise levels are often predictable and relatively easy to take into account when a Telco creates well-engineered deployment-planning rules.

Performance limiting noise, such as impulses and RFI, is intermittent in nature. It is geographically variable and unpredictable, and so is usually accounted for in planning rules by using a safety margin. DSL systems seek to use additional signal processing such as error correction with interleaving, and adaptive line codes, to mitigate such sources of noise.