Traffic Protection on SDH Optical Networks, Interview notes for SDH protection

        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.