Cisco ONS 15600 Reference Manual, Release 5.0

SONET Topologies and Upgrades

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Note The terms "Unidirectional Path Switched Ring" and "UPSR" may appear in Cisco literature. These terms do not refer to using Cisco ONS 15xxx products in a unidirectional path switched ring configuration. Rather, these terms, as well as "Protected Mesh Network" and "PPMN," refer generally to Cisco's path protection feature, which may be used in any topological network configuration. Cisco does not recommend using its path protection feature in any particular topological network configuration.

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This chapter explains Cisco ONS 15600 SONET topologies and upgrades. To provision topologies, refer to the Cisco ONS 15600 Procedure Guide.

Chapter topics include:

•Point-to-Point and Linear ADM Configurations

•Bidirectional Line Switched Rings

•Path Protection Configurations

•Dual-Ring Interconnect

•Subtending Rings

•Path-Protected Mesh Networks

•In-Service Topology Upgrades

The ONS 15600 usually operates as a hub node in networks that include ONS 15454s, ONS 15327s, and ONS 15310-CLs. Single nodes are installed at geographic locations where several ONS SONET topologies converge. A single ONS 15600 node might be a part of several ONS SONET rings/networks.

To avoid errors during network configuration, Cisco recommends that you draw the complete ONS SONET network topology on paper (or electronically) before you begin the physical implementation. A sketch ensures that you have adequate slots, cards, and fibers to complete the topology.

7.1  Point-to-Point and Linear ADM Configurations

You can configure ONS 15600s as a line of add/drop multiplexers (ADMs) by configuring one OC-N port as the working path and a second port as the protect path. Unlike rings, point-to-point (two node configurations) and linear (three node configurations) ADMs require that the OC-N ports at each node are in 1+1 protection to ensure that a break to the working path automatically routes traffic to the protect path.

Figure 7-1 shows two ONS 15600 nodes in a point-to-point ADM configuration. Working traffic flows from Slot 1/Port 1 at Node 1 to Slot 1/Port 1 at Node 2. You create the protect path by creating a 1+1 configuration with Slot 1/Port 1 and Slot 2/Port 1 at Nodes 1 and 2.

Figure 7-1 Point-to-Point ADM Configuration

7.2  Bidirectional Line Switched Rings

The ONS 15600 can support 32 concurrent two-fiber bidirectional line switched rings (BLSRs). Each BLSR can support up to 24 ONS 15600s. Because the working and protect bandwidths must be equal, you can create only OC-48 or OC-192 BLSRs. For information about BLSR protection channels, see the "6.7  Protection Channel Access Circuits" section on page 6-14.

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Note For best performance, BLSRs should have one LAN connection for every ten nodes in the BLSR.

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In two-fiber BLSRs, each fiber is divided into working and protect bandwidths. For example, in an OC-48 BLSR, STSs 1 to 24 carry the working traffic, and STSs 25 to 48 are reserved for protection (Figure 7-2). Working traffic (STSs 1 to 24) travels in one direction on one fiber and in the opposite direction on the second fiber. CTC circuit routing routines calculate the shortest path for circuits based on many factors, including user requirements, traffic patterns, and distance. For example, in Figure 7-2, circuits going from Node 0 to Node 1 will typically travel on Fiber 1, unless that fiber is full, in which case circuits will be routed to Fiber 2 through Node 3 and Node 2. Traffic from Node 0 to Node 2 (or Node 1 to Node 3) can be routed on either fiber, depending on circuit provisioning requirements and traffic loads.

Figure 7-2 Four-Node, Two-Fiber BLSR

The SONET K1, K2, and K3 bytes carry the information that governs BLSR protection switches. Each BLSR node monitors the K bytes to determine when to switch the SONET signal to an alternate physical path. The K bytes communicate failure conditions and actions taken between nodes in the ring.

If a break occurs on one fiber, working traffic targeted for a node beyond the break switches to the protect bandwidth on the second fiber. The traffic travels in a reverse direction on the protect bandwidth until it reaches its destination node. At that point, traffic is switched back to the working bandwidth.

Figure 7-3 shows a traffic pattern sample on a four-node, two-fiber BLSR.

Figure 7-3 Four-Node, Two-Fiber BLSR Traffic Pattern Sample

Figure 7-4 shows how traffic is rerouted following a line break between Node 0 and Node 3.

•All circuits originating on Node 0 that carried traffic to Node 2 on Fiber 2 are switched to the protect bandwidth of Fiber 1. For example, a circuit carrying traffic on STS-1 on Fiber 2 is switched to STS-25 on Fiber 1. A circuit carried on STS-2 on Fiber 2 is switched to STS-26 on Fiber 1. Fiber 1 carries the circuit to Node 3 (the original routing destination). Node 3 switches the circuit back to STS-1 on Fiber 2 where it is routed to Node 2 on STS-1.

•Circuits originating on Node 2 that normally carry traffic to Node 0 on Fiber 1 switch to the protect bandwidth of Fiber 2 at Node 3. For example, a circuit carrying traffic on STS-2 on Fiber 1 switches to STS-26 on Fiber 2. Fiber 2 carries the circuit to Node 0 where the circuit switches back to STS-2 on Fiber 1 and is then dropped to its destination.

Figure 7-4 Four-Node, Two-Fiber BLSR Traffic Pattern Following Line Break

7.2.1  BLSR Bandwidth

BLSR nodes can terminate traffic coming from either side of the ring. Therefore, BLSRs are suited for distributed node-to-node traffic applications such as interoffice networks and access networks.

BLSRs allow bandwidth to be reused around the ring and can carry more traffic than a network with traffic flowing through one central hub. BLSRs can also carry more traffic than a path protection operating at the same OC-N rate. Table 7-1 shows the bidirectional bandwidth capacities of two-fiber BLSRs. The capacity is the OC-N rate divided by two, multiplied by the number of nodes in the ring minus the number of pass-through STS-1 circuits.

Table 7-1 Two-Fiber BLSR Capacity

OC Rate	Working Bandwidth	Protection Bandwidth	Ring Capacity
OC-48	STS 1-24	STS 25-48	24 x N - PT
OC-192	STS 1-96	STS 97-192	96 x N - PT

Figure 7-5 shows an example of BLSR bandwidth reuse. The same STS carries three different traffic sets simultaneously on different spans around the ring: one set from Node 3 to Node 1, another set from Node 1 to Node 2, and another set from Node 2 to Node 3.

Figure 7-5 BLSR Bandwidth Reuse

7.2.2  BLSR Fiber Connections

Plan your fiber connections and use the same plan for all BLSR nodes. For example, make the east port the farthest slot to the right and the west port the farthest slot to the left. Plug fiber connected to an east port at one node into the west port on an adjacent node. Figure 7-6 shows fiber connections for a two-fiber BLSR with trunk (span) cards in Slot 4 (west) and Slot 12 (east). Refer to the Cisco ONS 15600 Procedure Guide for fiber connection procedures.

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Note Always plug the transmit (Tx) connector of an OC-N card at one node into the receive (Rx) connector of an OC-N card at the adjacent node. Cards display an SF LED when Tx and Rx connections are mismatched.

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Figure 7-6 Connecting Fiber to a Four-Node, Two-Fiber BLSR

7.3  Path Protection Configurations

Path protection configurations provide duplicate fiber paths around the ring. Working traffic flows in one direction and protection traffic flows in the opposite direction. If a problem occurs in the working traffic path, the receiving node switches to the path coming from the opposite direction.

CTC automates ring configuration. Path protection traffic is defined within the ONS 15600 on a circuit-by-circuit basis. If a path-protected circuit is not defined within a 1+1 or BLSR line protection scheme and path protection is available and specified, CTC uses path protection as the default. You can set up a maximum of 64 OC-48 path protection configurations or 16 OC-192 path protection configurations for each ONS 15600 node.

A path protection circuit requires two DCC-provisioned optical spans per node. Path protection circuits can be created across these spans until their bandwidth is consumed.

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Note If a path protection circuit is created manually by TL1, data communications channels (DCCs) are not needed; therefore, path protection circuits are limited by the cross-connection bandwidth, or the span bandwidth, but not by the number of DCCs.

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The span bandwidth consumed by a path protection circuit is two times the circuit bandwidth, since the circuit is duplicated. The cross-connection bandwidth consumed by a path protection circuit is three times the circuit bandwidth at the source and destination nodes only. The cross-connection bandwidth consumed by an intermediate node has a factor of one.

Figure 7-7 shows a basic path protection configuration. If Node 0 sends a signal to Node 2, the working signal travels on the working traffic path through Node 1. The same signal is also sent on the protect traffic path through Node 3.

Figure 7-7 Basic Four-Node Path Protection

If a fiber break occurs, Node 2 switches its active receiver to the protect signal coming through Node 3 (Figure 7-8).

Figure 7-8 Path Protection with a Fiber Break

Because each traffic path is transported around the entire ring, path protection configurations are best suited for networks where traffic concentrates at one or two locations and is not widely distributed. path Protection capacity is equal to its bit rate. Services can originate and terminate on the same path protection, or they can be passed to an adjacent access or interoffice ring for transport to the service-terminating location.

7.4  Dual-Ring Interconnect

Dual-ring interconnect (DRI) topology provides an extra level of path protection for circuits on interconnected rings. DRI allows users to interconnect BLSRs, path protection configurations, or a path protection with a BLSR, with additional protection provided at the transition nodes. In a DRI topology, ring interconnections occur at two or four nodes.

The drop-and-continue DRI method is used for all ONS 15600 DRIs. In drop-and-continue DRI, a primary node drops the traffic to the connected ring and routes traffic to a secondary node within the same ring. The secondary node also routes the traffic to the connected ring; that is, the traffic is dropped at two different interconnection nodes to eliminate single points of failure. To route circuits on DRI, you must choose the Dual Ring Interconnect option during circuit provisioning. Dual transmit is not supported.

Two DRI topologies can be implemented on the ONS 15600:

•A traditional DRI requires two pairs of nodes to interconnect two networks. Each pair of user-defined primary and a secondary nodes drop traffic over a pair of interconnection links to the other network.

•An integrated DRI requires one pair of nodes to interconnect two networks. The two interconnected nodes replace the interconnection ring.

For DRI topologies, a hold-off timer sets the amount of time before a selector switch occurs. It reduces the likelihood of multiple switches, such as:

•Both a service selector and a path selector

•Both a line switch and a path switch of a service selector

For example, if a path protection DRI service selector switch does not restore traffic, then the path selector switches after the hold-off time. The path protection DRI hold-off timer default is 100 ms. You can change this setting in the UPSR Selectors tab of the Edit Circuits window. For BLSR DRI, if line switching does not restore traffic, then the service selector switches. The hold-off time delays the recovery provided by the service selector. The BLSR DRI default hold-off time is 100 ms and cannot be changed.

7.4.1  BLSR DRI

Unlike BLSR automatic protection switching (APS) protocol, BLSR DRI is a path-level protection protocol at the circuit level. Drop-and-continue BLSR DRI requires a service selector in the primary node for each circuit routing to the other ring. Service selectors monitor signal conditions from dual feed sources and select the one that has the best signal quality. Same-side routing drops the traffic at primary nodes set up on the same side of the connected rings, and opposite-side routing drops the traffic at primary nodes set up on the opposite sides of the connected rings. For BLSR DRI, primary and secondary nodes cannot be the circuit source or destination.

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Note A DRI circuit cannot be created if an intermediate node exists on the interconnecting link. However, an intermediate node can be added on the interconnecting link after the DRI circuit is created.

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Ring interworking on protection (RIP) circuits act as protection channel access circuits. RIP tables, which contain the primary and termination node IDs, can be accessed from secondary DRI nodes. In CTC, you set up RIP circuits by selecting the PCA option when setting up primary and secondary nodes during DRI circuit creation.

Figure 7-9 shows ONS 15600s in a traditional BLSR DRI topology with same-side routing. In Ring 1, Nodes 3 and 4 are the interconnect nodes, and in Ring 2, Nodes 8 and 9. Duplicate signals are sent from Node 4 (Ring 1) to Node 9 (Ring 2), and between Node 3 (Ring 1) and Node 8 (Ring 2). The primary nodes (Nodes 4 and 9) are on the same side, and the secondary nodes (Nodes 3 and 8) provide an alternative route. In Ring 1, traffic at Node 4 is dropped (to Node 9) and continued (to Node 10). Similarly, at Node 9, traffic is dropped (to Node 4) and continued (to Node 5).

Figure 7-9 ONS 15600 Traditional BLSR Dual-Ring Interconnect (Same-Side Routing)

Figure 7-10 shows ONS 15600s in a traditional BLSR DRI topology with opposite-side routing. In Ring 1, Nodes 3 and 4 are the interconnect nodes, and in Ring 2, Nodes 8 and 9. Duplicate signals are sent from Node 4 (Ring 1) to Node 8 (Ring 2), and between Node 3 (Ring 1) and Node 9 (Ring 2). In Ring 1, traffic at Node 4 is dropped (to Node 9) and continued (to Node 8). Similarly, at Node 8, traffic is dropped (to Node 3) and continued (to Node 4).

Figure 7-10 ONS 15600 Traditional BLSR Dual-Ring Interconnect (Opposite-Side Routing)

Figure 7-11 shows ONS 15600s in an integrated BLSR DRI topology. The same drop-and-continue traffic routing occurs at two nodes, rather than four. This is achieved by installing an additional OC-N trunk at the two interconnect nodes. Nodes 3 and 8 are the interconnect nodes.

Figure 7-11 ONS 15600 Integrated BLSR Dual-Ring Interconnect

7.4.2  Path Protection DRI

Figure 7-12 shows ONS 15600s in a traditional drop-and-continue path protection DRI topology. In Ring 1, Nodes 4 and 5 are the interconnect nodes, and in Ring 2, Nodes 6 and 7. Duplicate signals are sent from Node 4 (Ring 1) to Node 6 (Ring 2), and between Node 5 (Ring 1) and Node 7 (Ring 2). In Ring 1, traffic at Node 4 is dropped (to Node 6) and continued (to Node 5). Similarly, at Node 5, traffic is dropped (to Node 7) and continued (to Node 4).

Figure 7-12 ONS 15600 Traditional Path Protection Dual-Ring Interconnect

Figure 7-13 shows ONS 15600s in an integrated DRI topology. The same drop-and-continue traffic routing occurs at two nodes, rather than four. This is achieved by installing an addition OC-N trunk at the two interconnect nodes.

Figure 7-13 ONS 15600 Integrated Path Protection Dual-Ring Interconnect

7.4.3  Path Protection/BLSR DRI Handoff Configurations

Path protection configurations and BLSRs can also be interconnected. In BLSR/path protection DRI handoff configurations, primary and secondary nodes can be the circuit source or destination, which is useful when non-DCC optical interconnecting links are present. Figure 7-14 shows an example of a path protection to BLSR traditional DRI handoff.

Figure 7-14 ONS 15600 Path Protection to BLSR Traditional DRI Handoff

Figure 7-15 shows an example of a path protection to BLSR integrated DRI handoff.

Figure 7-15 ONS 15600 Path Protection to BLSR Integrated DRI Handoff

7.5  Subtending Rings

Subtending rings reduce the number of nodes and cards required and reduce external shelf-to-shelf cabling. The ONS 15600 supports ten concurrent rings. Figure 7-16 shows an ONS 15600 with multiple subtending rings.

Figure 7-16 ONS 15600 with Multiple Subtending Rings

Figure 7-17 shows a path protection subtending from a BLSR. In this example, Node 3 is the only node serving both the BLSR and path protection. OC-N cards in Slots 4 and 12 serve the BLSR, and OC-N cards in Slots 3 and 13 serve the path protection.

Figure 7-17 Path Protection Subtending from a BLSR

The ONS 15600 can support 32 BLSRs on the same node. This capability allows you to deploy an ONS 15600 in applications requiring SONET digital cross connect systems (DCSs) or multiple SONET ADMs.

Figure 7-18 shows two BLSRs shared by one ONS 15600. Ring 1 runs on Nodes 1, 2, 3, and 4. Ring 2 runs on Nodes 4, 5, 6, and 7. Two BLSRs, Ring 1 and Ring 2, are provisioned on Node 4. Ring 1 uses cards in Slots 4 and 12, and Ring 2 uses cards in Slots 3 and 13.

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Note Nodes in different BLSRs can have the same or different node IDs.

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Figure 7-18 BLSR Subtending from a BLSR

After subtending two BLSRs, you can route circuits from nodes in one ring to nodes in the second ring. For example, in Figure 7-18 you can route a circuit from Node 1 to Node 7. The circuit would normally travel from Node 1 to Node 4 to Node 7. If fiber breaks occur, for example between Nodes 1 and 4 and Nodes 4 and 7, traffic is rerouted around each ring: in this example, Nodes 2 and 3 in Ring 1 and Nodes 5 and 6 in Ring 2.

7.6  Path-Protected Mesh Networks

In addition to single BLSRs, path protection configurations, and ADMs, you can extend ONS 15600 traffic protection by creating path-protected mesh networks (PPMNs). PPMNs include multiple ONS 15600 SONET topologies and extend the protection provided by a single path protection to the meshed architecture of several interconnecting rings. In a PPMN, circuits travel diverse paths through a network of single or multiple meshed rings. When you create circuits, CTC automatically routes circuits across the PPMN, or you can manually route them. You can also choose levels of circuit protection. For example, if you choose full protection, CTC creates an alternate route for the circuit in addition to the main route. The second route follows a unique path through the network between the source and destination and sets up a second set of cross-connections.

For example, in Figure 7-19, a circuit is created from Node 3 to Node 9. CTC determines that the shortest route between the two nodes passes through Node 8 and Node 7, shown by the dotted line, and automatically creates cross-connections at Nodes 3, 8, 7, and 9 to provide the primary circuit path.

Figure 7-19 Path-Protected Mesh Network

If full protection is selected, CTC creates a second unique route between Nodes 3 and 9 that passes through Nodes 2, 1, and 11. Cross-connections are automatically created at Nodes, 3, 2, 1, 11, and 9, shown by the dashed line. If a failure occurs on the primary path, traffic switches to the second circuit path. In this example, Node 9 switches from the fiber from Node 7 to the fiber from Node 11 and service resumes. The switch occurs within 50 ms.

PPMN also allows spans of different SONET line rates to be mixed together in virtual rings. Figure 7-20 shows Nodes 1, 2, 3, and 4 in an OC-192 ring.

Figure 7-20 PPMN Virtual Ring

7.7  In-Service Topology Upgrades

Topology upgrades can be performed in-service to convert a live network to a different topology. An in-service topology upgrade is potentially service-affecting, and generally allows a traffic hit of 50 ms or less. Traffic might not be protected during the upgrade. The following in-service topology upgrades are supported:

•Unprotected point-to-point or linear ADM to path protection

•Point-to-point or linear ADM to two-fiber BLSR

•Path protection to two-fiber BLSR

•Node addition or removal from an existing topology

You can perform in-service topology upgrades irrespective of the service state of the involved cross-connects or circuits, however all circuits must have a DISCOVERED status.

ONS 15600 circuit types supported for in-service topology upgrades are:

•Synchronous transport signal (STS)

•Unidirectional and bidirectional

•Automatically routed and manually routed

•CTC-created and TL1-created

•Ethernet (unstitched)

•Multiple source and destination (both sources should be on one node and both drops on one node)

You cannot upgrade stitched Ethernet circuits during topology conversions. For in-service topology upgrade procedures, refer to the "Convert Network Configurations" chapter in the Cisco ONS 15600 Procedure Guide. For procedures to add or remove a node, refer to the "Add and Remove Nodes" chapter of the Cisco ONS 15600 Procedure Guide.

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Note A database restore on all nodes in a topology returns converted circuits to their original topology.

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Note Open-ended path protection and DRI configurations do not support in-service topology upgrades.

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7.7.1  Unprotected Point-to-Point or Linear ADM to Path Protection

CTC provides a topology conversion wizard for converting an unprotected point-to-point or linear ADM topology to path protection. This conversion occurs at the circuit level. CTC calculates the additional path protection circuit route automatically or you can do it manually. When routing the path protection circuit, you can provision the USPR as go-and-return or unidirectional.

To convert from point-to-point or linear ADM to a path protection, the topology requires an additional circuit route to complete the ring. When the route is established, CTC creates circuit connections on any intermediate nodes and modifies existing circuit connections on the original circuit path. The number and position of network spans in the topology remains unchanged during and after the conversion.

Figure 7-21 shows an unprotected point-to-point ADM configuration converted to a path protection. An additional circuit routes through Node 3 to complete the path protection.

Figure 7-21 Unprotected Point-to-Point ADM to path protection Conversion

7.7.2  Point-to-Point or Linear ADM to Two-Fiber BLSR

A 1+1 point-to-point or linear ADM to a two-fiber BLSR conversion is manual. You must remove the protect fibers from all nodes in the linear ADM and route them from the end node to the protect port on the other end node. In addition, you must delete the circuit paths that are located in the bandwidth that will become the protection portion of the two-fiber BLSR (for example, circuits in STS 25 or higher on an OC-48 BLSR) and recreate them in the appropriate bandwidth. Finally, you must provision the nodes as BLSR nodes.

To complete a conversion from an unprotected point-to-point or linear ADM to a two-fiber BLSR, use the CTC Convert Unprotected/path protection to BLSR wizard in the Tools > Topology Upgrade menu.

7.7.3  Path Protection to Two-Fiber BLSR

CTC provides a topology conversion wizard to convert a path protection to a two-fiber BLSR. An upgrade from a path protection to a two-fiber BLSR changes path protection to line protection. A path protection can have a maximum of 16 nodes before conversion. Circuits paths must occupy the same time slots around the ring. Only the primary path through the path protection is needed; the topology conversion wizard removes the alternate path protection path during the conversion. Because circuit paths can begin and end outside of the topology, the conversion might create line-protected segments within path protection paths of circuits outside the scope of the ring. The physical arrangement of the ring nodes and spans remains the same after the conversion.

7.7.4  Add or Remove a Node from a Topology

You can add or remove a node from a linear ADM, BLSR, or path protection configuration. Adding or removing nodes from BLSRs is potentially service affecting, however adding and removing nodes from an existing 1+1 linear ADM or path protection configuration does not disrupt traffic. CTC provides a wizard for adding a node to a point-to-point or 1+1 linear ADM. This wizard is used when adding a node between two other nodes.