To keep costs to a minimum the existing infrastructure will always be analysed with a view to re-use, where possible. There are certain considerations on older, multimode fibre installations relating to the distance and Ethernet bandwidth associated with 62.5/125 multimode fibre.
There are other considerations to make, often relating to the topology of the existing fibre. Many sites have the fibre installed in a fibre ring. Historically this would have given a resilience when connecting layer 2 Ethernet products which utilise healing protocols, such as RSTP (rapid spanning tree protocol).
RSTP operates by closing off ports using software to stop data travelling continuously around a fibre optic ring that could cause broadcast storms. The protocol will only open those closed ports should a fibre break, enabling all network traffic to travel on an alternate route.
This data routing uses the MAC addresses of all devices on the network; at the point of a fibre loss RSTP will instruct all the devices on the network to reconverge (flush the MAC address table) to allow communication to take place so the network devices can re-build a new MAC address table to cater for the change of topology caused by the fibre break.
Reconvergence time will differ dependent on the number of devices on the network. In a large prison with high camera numbers this time can very often exceed acceptable losses of network transmission.
On these occasions AMG would always advise the use of layer 3 network switches. At layer 3, ring healing protocols like OSPF (open shortest path first) can be applied to significantly reduce network downtime when re-routing traffic due to a broken fibre ring.
Moving to layer 3 will add complexity to the network design and very often lead to a low-level design document (LLD) being required. The LLD will map the data flow on the network and cater for all the tiers.