Wednesday, 3 April 2013

Detailed operation of BLSR & Squelching.


Operation – Traffic flow :


Bi-directional traffic between two nodes is transported over a subset of the "ring sections" or "spans". In this configuration, Minimum capacity equals line rate. Capacity is in general expressed as number of AU4, or bandwidth. The bandwidth is provided by an integer number of AU4 payload.



Maximum bandwidth capacity :


 Here, each span has, in each direction, a capacity of up to half the number of AU4 in the STM-N (i.e. 8 AU4 for an STM-16 section). All traffic from a node goes to adjacent  nodes.

Max. capacity = 0.5 (line rate) x number of nodes.

Note: This Max is achieved only of the working traffic is transported only between two adjacent nodes.


Extra Traffic:


We can utilize shared protection bandwidth for Extra traffic. This extra traffic is not protected & it  could be lost when a failure of working traffic occurs

Operations – Fiber Cut :

Let us consider a scenario, where fiber cuts between A&B. We have a working traffic from A-C and C-A. This failure interrupts A-C and C-A traffic . Now Node A and Node B detect failure


Now node A and node B will switch the traffic to protection path. No dedicated protection bandwidth - only  used when protection required.  Only nodes next to the failure know about the protection switch.  No traffic lost.


Operations – Node Failure:


Let us consider that we have live traffic from D-F and F-D. If node B fails, Failure interrupts D-F and F-D traffic. Node A and C detect failure




Now Both  node A & C switch the traffic to protection channels. Only nodes next to the failure know about the protection switch. In this scenario, only   Traffic to/from  failed node lost.


Squelching Problem :


When a node fails, traffic terminating on those nodes cut off by failures could be misconnected to other nodes on the ring in case of using a local fail-over decision .

Consider a scenario, where we have active traffic from Node F-B , B-F and E-B, B-E. If Node B fails,

Squelching misconnection occur : Node F now talking to Node E instead of Node B






This can be avoided by path AIS Insertion. STM Path AIS is inserted instead of the looped STM-1#7. No mis-connections



Squelching Summary :

Squelching is in general used when extra traffic is used, it is used when normal traffic is switched to the protection entity and replaces the extra traffic. Squelching prevents that in case protection switch is active the normal traffic is output instead of the original extra traffic by outputting AU-AIS. You can also read clause 7.2.3.2 of ITU-T G.841

Squelching is required to assure that misconnections are not made. It is  required for bidirectional line switched rings only, since it is the only ring to provide a reuse capability of STM-1s around the ring. This is only required when nodes are cut off from the ring. Also this is only required for traffic terminating on the cut off nodes.

A ring map that includes all STM and VC Paths on the ring is available at every node on the ring. Squelching is also required for extra traffic since the extra traffic may be dropped when a protection switch is required.

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