Monday, 1 April 2013

NBN: Impact of different topologies: FTTN vs FTTP

Customer Access Networks (CAN) of Copper and Fibre are fundamentally different, a layout that right for FTTN Copper is exactly wrong for FTTP Fibre.

The "Transit Network", the equivalent for Fibre of Local and Central Telephone Exchanges with their interconnections, is worth ~$2 billion and set for completion in 2014. It won't be worth anything for a Fibre-to-the-Node install: the way the cables are routed, the topologies, are just too different:

  • Copper is bulky and very heavy in large cables, while Fibre is small and light, requiring different techniques and routing.
    • Copper is easily & quickly jointed, even with simple tools. Fibre needs precision equipment with highly trained operators for "fusion splicing".
    • Copper Network tends to be "star of stars", vs Fibre long loops.
    • Copper is expensive as well as heavy, so every effort is made to reduce the amount used. The thinest possible conductors (which have poor high-frequency response) and shortest, simplest runs.
    • Fibre cables are relatively light and thin compared to copper. The majority of the weight is in the buffering, packaging and strength-members.
      • Fibre cables "scale up" more slowly than copper.
    • Laying Fibre cables is dominated by trenching & pulling costs, not the internal conductor.
  • Copper joints are simple but require higher maintenance, as do the conductors: they must be kept dry. Older cables are insulated with thin paper and are kept pressurised with dry air. Alarms detect pressure drops, alerting crews to go look for the damage.
  • Fibre has much lower maintenance costs and can survive water immersion, but they are still sealed and packed to stop water-ingress. Being glass, it is fragile and must be carefully handled. Classic mis-handling faults are bending too tightly and
  • For xDSL use, Copper networks are tightly constrained on length
    • the 400m/800m/1500m 'rules' cannot be exceeded or access rates fall dramatically
    • Fibre loops are much longer: 10+km
  • Fibre networks often have high level redundancy, where Copper CAN's have none, it's too hard and very expensive.
    • Dual rings are the usual configuration for the active parts of fibre networks.
    • They are self-healing and protect against single failures or faults.
  • The GPON technology used by NBN Co allows multiple premises to share the same fibre core using "splitters".
    • The head-end transmits at 2.5Gbps, this is shared between customers.
      • presumably as 2*1Gbps services down to 100-150*12Mbps services.
      • I've only seen mentioned 20 services per fibre. 
        • There will be sharing limits due to the losses per splitter.
      • With Wave Division Multiplexing, using different laser colours, it is possible to send multiple separate signals down a single fibre.
        • This technology might lend itself to GPON networks, allowing much higher capacities over the same fibre/splitter infrastructure.
    • There are already higher speed GPON systems designed or available. We can assume from the current commercial availability of 40Gbps and 100Gbps active optical ethernet, that GPON speeds will increase 40-fold at least, with WDM available as the next evolution.
    • Customer Premises (SDU: Single Dwelling Units) are connected by single-fibre "drop cables". From the videos and images, only a single fibre is used carrying signals in both directions, unlike the normal computing arrangement for active optical networks where fibres are single direction.
    • Fibres are grouped into ribbons of 12 (coloured) fibres that can be jointed in a single operation.
    • Ribbons are grouped into bundles of 12 (144 fibres) per tube, around 1cm in diameter.
    • Tubes are grouped into cables, currently 4 or 6 tubes (576 and 864 fibres).
    • Underground cables are filled with a petroleum jelly to prevent water travelling along the cable if it is damaged. [Backhoes are great for this.]
What I can deduce from snippets of layout maps and private correspondence:
  • All networks must be built with significant unused capacity / connections. This allows:
    • maintenance as cables, joints or conductors/fibres fail.
    • additional services to be added, either by building or subscriber demand.
  • Copper networks seem to avoid back-tracking and are Star-networks.
    • An Exchange Area covering up to 20,000 services  is divided into a small number, eight (?) segments, fed from the exchange with large
    • These large cables are then jointed to successively smaller distribution cables and finally to premises drop-cables.
    • Large feeder cables eventually arrive at pits in "distribution areas" of 100-200 premises.
    • The physical marker of these is the kerbside "pillar" where technicians patch the small local distribution cables (10-20 pairs?) back to the exchange Distribution Frame.
    • The topology of Distribution Areas seems to be best modelled as:
      • a rectangular area, with a series of parallel "local loop" cables.
      • The exchange feed arrives at one side (the pillar) and local-loop cables exit to the other.
      • From the housing density in Australia, we can infer that local loops for 200 premise DA's are 800m - 1500m, with a fan-out of 10-20 pair cables down local streets.
  • Fibre networks, from the snippet of a map in an NBN Co media release, are organised as a single large loop, with some spurs, running around the roads of a Fibre Distribution Area.
    • Every house gets passed by a common high-capacity cable.
    • The one cable, around 4-5km, passes all 200 houses in a single area.

No comments:

Post a Comment

Note: only a member of this blog may post a comment.