Service survivability has become more important than ever. This is because telecommunication is used increasingly for vital transactions such as electronic fund transfer, order processing, inventory control & many other business activities ( e.g : e-mail, internet access). Users are willing to pay more to get guaranteed service.
In SDH transmission system, Automatic Protection Switching ( APS) algorithms and performance/alarm monitoring are built in. This system allows the construction of linear point-to-point networks and synchronous ring topology networks which are self- healing in the event of failure. Also, to minimize the disruption of traffic, the protection switching must be completed within the specified time limit ( sub 50ms) recommended by ITU-T G.783 (linear networks) and ITU-T G.841 (ring networks).
Upon detection of a failure (dLOS, dLOF, high BER), the network must reroute traffic (protection switching) from working channel to protection channel. The Network Element that detects the failure (tail-end NE) initiates the protection switching. The head-end NE must change forwarding or to send duplicate traffic. Protection switching may be revertive (automatically revert to working channel)
Key ITU-T recommendations :
ITU-T recommendations define methods of protecting service traffic in SDH networks. Two important recommendations are :
1.Recommendation G.783 covers linear point to point networks.
2.Recommendation G.841 covers various configurations of multiplex section rings.
Linear ( point to point) protection :
In a linear network, protection is achieved through an extra protection fibre. It can protect the network from fiber or NE card failure. Different variants of linear protection are 1+1, 1:1 and 1:N.
How it works ?
Head-end and tail-end NEs have bridges (muxes). Head-end and tail-end NEs maintain bidirectional signaling channel. Signaling is contained in K1 and K2 bytes of protection channel. K1 – tail-end status and requests. K2 – head-end status .
Linear 1+1 protection :
This is simplest form of protection. Can be at OC-n level (different physical fibers) or at STM/VC level (called SubNetwork Connection Protection) or end-to-end path (called trail protection) Head-end bridge always sends data on both channels. Tail-end chooses channel to use based on BER, dLOS, etc. No need for signaling. For non-revertive cases, there is no distinction between. working and protection channels. BW utilization is 50%.
Linear 1:1 protection :
In this case, Head-end bridge usually sends data on working channel. When tail-end detects failure it signals (using K1) to head-end. Head-end then starts sending data over protection channel. When not in use, protection channel can be used for (discounted) extra traffic (pre-emptible unprotected traffic).
Linear 1:N protection:
This is verymuch similar to 1:1 protection with a small difference. Here, in order to save BW we allocate 1 protection channel for every N working channels. Here, N limited to 14.
Let us read about ring networks in next post.