The Cisco ONS is interoperable with our existing Cisco ONS UPSR architecture and it also gives us 16 rings of completely interoperable ONS. BLSR,Bi-directional Line Switched ring. There are two types of BLSR deployed in various networks. i. 2-fiber BLSR ii. 4-fiber BLSR. How is Bi-directional Line Switched Ring (SONET) abbreviated? BLSR stands for Bi-directional Line Switched Ring (SONET). BLSR is defined as Bi-directional.

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SONET systems can be configured as point to point terminals, linear add-drop chains and rings.

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To understand how the UPSR works refer to figure 1 which depicts a four node ring. For purposes of discussion the outer fiber loop is referred to as ring1 and the inner fiber loop is referred to as ring2. In this example a circuit is to be set up between nodes A and C. The input at node A travels clockwise on ring1 and egresses at node C. The signal egresses at node A completing the circuit, establishing communications between nodes A and B.


Notice that all that is needed for this circuit to be operational is the ring1 fiber unidirectional path. In this expamle ring1 is the working path. Ring2 will become the protection path as shown in figure 2.


Protection traffic now rides on the ring2 counter clockwise fiber path. At the output we implement a selector switch which chooses the signal that comes out of the network element.

Pathing switching is on an individual path basis. For example if there are 84 VT1. When you have a UPSR think head end bridge, tail end switch. Note that nodes B and D are pass throughs.

So if nodes A and C had 3 DS-s provisioned between them then rring entire ring bandwidth would be used and no traffic could be added at nodes B and D.

Once a node uses a time slot it is no longer available for use by any other node. A fiber cut between nodes B and C is shown in figure 3.

The blue lines show the traffic from A to C head end bridge. When a drop node in this case node C detects the AIS-P on the working path the selector performs a a path switch to ring2. Because node C is adjacent to the fiber cut all nlsr its selectors will make a path switch. This is shown in red. Node A will therefore receive its signal from B on ring2. Node A is not adjacent to the fiber so all of its path selectors will switch based upon path integrity of each individual path independent of the status of any other path.


This makes the UPSR particularly easy to provision and manage. When the fiber between B and C is restored the selector at node C will not switch back because the there is not any path degradation to cause the selector to switch.


If the selector were to switch back to ring1 the signal would take a hit during the switching interval. Since both ring1 and ring2 signals are good why switch? When a fault occurs the node is allowed 10mS to detect the failure and 50mS to make the switch. The total restoration time for each path, i. In the case of VT signals it is 3 VT superframes.

This can affect the restoration speed of a large UPSR.