Chapter 11, Circuits and Tunnels

This chapter explains Cisco ONS 15454 STS and VT circuits and VT and DCC tunnels. To provision circuits and tunnels, refer to the Cisco ONS 15454 Procedure Guide.

11.1 Circuit Types

For an explanation and examples of circuits and VT tunnels, see the "Cross-Connect Card Capacities" section. You can create unidirectional or bidirectional, revertive or non-revertive circuits. You can have circuits routed automatically or you can manually route them. The auto range feature eliminates the need to individually build circuits of the same type; CTC can create additional sequential circuits if you specify the number of circuits you need and build the first circuit.

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Before cards are installed. The ONS 15454 allows you to provision slots and circuits before installing the traffic cards. (To provision an empty slot, right-click it and select a card from the shortcut menu.) However, circuits will not carry traffic until you install the cards and place their ports in service.

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Cards are installed; ports are out of service. You must place the ports in service before circuits will carry traffic.

Note After cards are installed and their ports are placed in service, circuits will carry traffic as soon as the signal is received. In this chapter, "cross-connect" and "circuit" have the following meanings: Cross-connect refers to the connections that occur within a single ONS 15454 to allow a circuit to enter and exit an ONS 15454. Circuit refers to the series of connections from a traffic source (where traffic enters the ONS 15454 network) to the drop or destination (where traffic exits an ONS 15454 network).

11.2 Cross-Connect Card Capacities

The ONS 15454 XC, XCVT, and XC10G cards perform port-to-port time-division multiplexing (TDM).

XCs and XCVTs have capacity to terminate 288 STSs, or 144 STS cross-connections (each STS cross-connection uses two STS ports on the cross-connect card STS matrix). XC10Gs have capacity for 1152 STSs or 576 STS cross-connections. Table 11-1 shows STS capacities for the XC, XCVT, and XC10G cards.

11.2.1 VT1.5 Cross-Connects

XCVTs and XC10Gs can map up to 24 STSs for VT1.5 traffic. Because one STS can carry 28 VT1.5s, the XCVT and XC10G cards can terminate up to 672 VT1.5s or 336 VT1.5 cross-connects. However, to terminate 336 VT1.5 cross-connects:

Table 11-1 XC, XCVT, and XC10G Card STS Cross-Connect Capacities
Card	Total STSs	STS Cross-connects
XC	288	144
XCVT	288	144
XC10G	1152	576

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Each STS mapped for VT1.5 traffic must carry 28 VT1.5 circuits. If you assign each VT1.5 circuit to a different STS, the XCVT and XC10G VT1.5 cross-connect capacity will be reached after you create 12 VT1.5 circuits.

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ONS 15454s must be in a bidirectional line switched ring (BLSR). Source and drop nodes in UPSR or 1+1 (linear) protection have capacity for only 224 VT1.5 cross-connects because an additional STS is used for the protect path.

Table 11-2 shows the VT1.5 capacities for ONS 15454 cross-connect cards. All capacities assume each VT1.5-mapped STS carries 28 VT1.5 circuits.

Table 11-2 XC, XCVT, and XC10G VT1.5 Capacities
Card	Total VT1.5s (BLSR)	VT1.5 Cross-Connect Capacity (BLSR)	VT1.5 Cross-Connect Capacity (UPSR or 1+1)
XC	0	0	0
XCVT	672	336	224
XC10G	672	336	224

Figure 11-1 shows the logical flow of a VT1.5 circuit through the XCVT/XC10G STS and VT matrices at a BLSR node. The circuit source is an EC-1 card using STS-1. After the circuit is created:

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Two of the 24 XCVT or XC10G STSs available for VT1.5 traffic are used (one STS for VT1.5 input into the VT matrix; one STS for VT1.5 output).

In Figure 11-2, a second VT1.5 circuit is created from the EC-1 card. In this example, the circuit is assigned to STS-2:

If you create VT1.5 circuits on nodes in a UPSR or 1+1 protection, an additional STS is used for the protect path at the source and drop nodes. Figure 11-3 shows a VT1.5 circuit at a UPSR source node. When the circuit is completed:

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Three of the 24 STSs available for VT1.5 mapping on the XCVT or XC10G are used (one input and two outputs, one output for the working path and one output for the protect path).

Figure 11-4 shows a second VT1.5 circuit that was created using STS-2. When the second VT1.5 circuit is created:

Unless you create VT tunnels (see the "VT Tunnels" section), VT1.5 circuits use STSs on the XCVT/XC10G VT matrix at each node that the circuit passes through.

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Two STSs are used at each node in the Figure 11-1 example, and three STSs are used at each node in the Figure 11-3 example.

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In the Figure 11-2 example, three STSs are used at the source and drop nodes and four STSs are used at pass-through nodes. In Figure 11-4, six STSs are used at the source and drop nodes and four
STSs at the pass-through nodes.

11.2.2 VT Tunnels

To maximize VT matrix resources, you can tunnel VT1.5 circuits through ONS 15454 pass-through nodes (nodes that are not a circuit source or drop). VT1.5 tunnels provide two benefits:

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They allow you to route VT1.5 circuits through ONS 15454s that have XC cards. (VT1.5 circuits require XCVT or XC10G cards at circuit source and drop nodes.)

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When tunneled through nodes with XCVT or XC10G cards, VT1.5 tunnels do not use VT matrix capacity, thereby freeing the VT matrix resources for other VT1.5 circuits.

Figure 11-5 shows a VT tunnel through the XCVT and XC10G matrices. No VT1.5-mapped STSs are used by the tunnel, which can carry 28 VT1.5s. However, the tunnel does use two STS matrix ports on each node that it passes through.

Figure 11-6 shows a six-node ONS 15454 ring with two VT tunnels. One tunnel carries VT1.5 circuits from Node 1 to Node 3. The second tunnel carries VT1.5 circuits from Node 1 to Node 4. Table 11-3 shows the VT1.5-mapped STS usage at each node in a ring based on protection scheme and use of VT tunnels. In the Figure 11-6 example, the circuit travels west through Nodes 2, 3, and 4. Subsequently, VT-mapped STS usage at these nodes is greater than at Nodes 5 and 6.

Table 11-3 VT1.5-Mapped STS Use in Figure 6-6
Node	VT Tunnel (BLSR)	VT Tunnel (UPSR, 1+1)	No VT Tunnel (BLSR)	No VT Tunnel (UPSR)	No VT Tunnel (1+1)
1	4	6	4	6	3
2	0	0	4	2	4
3	2	3	4	3	5
4	2	3	2	3	3
5	0	0	0	2	0
6	0	0	0	2	0

When planning VT1.5 circuits, weigh the benefits of using tunnels with the need to maximize STS capacity. For example, a VT1.5 tunnel between Node 1 and Node 4 passing (transparently) through Nodes 2 and Node 3 is advantageous if a full STS is used for Node 1 - Node 4 VT1.5 traffic (that is, the number of VT1.5 circuits between these nodes is close to 28). A VT tunnel is required if:

However, if the Node 1 - Node 4 tunnel will carry only a few VT1.5 circuits, creating a regular VT1.5 circuit between Nodes 1, 2, 3, and 4 might maximize STS capacity.

When you create a VT1.5 circuit, CTC determines whether a tunnel already exists between source and drop nodes. If a tunnel exists, CTC checks the tunnel capacity. If the capacity is sufficient, CTC routes the circuit on the existing tunnel. If a tunnel does not exist, or if an existing tunnel does not have sufficient capacity, CTC displays a dialog box asking whether you want to create a tunnel. Before you create the tunnel, review the existing tunnel availability, keeping in mind future bandwidth needs. In some cases, you may want to manually route a circuit rather than create a new tunnel.

11.3 DCC Tunnels

SONET provides four data communications channels (DCCs) for network element operations, administration, maintenance, and provisioning: one on the SONET Section layer and three on the SONET Line layer. The ONS 15454 uses the Section DCC (SDCC) for ONS 15454 management and provisioning.

You can use the Line DCCs (LDCCs) and the SDCC (when the SDCC is not used for ONS 15454 DCC terminations) to tunnel third-party SONET equipment across ONS 15454 networks. A DCC tunnel end-point is defined by Slot, Port, and DCC, where DCC can be either the SDCC, Tunnel 1, Tunnel 2, or Tunnel 3 (LDCCs). You can link an SDCC to an LDCC (Tunnel 1, Tunnel 2, or Tunnel 3), and an LDCC to an SDCC. You can also link LDCCs to LDCCs and link SDCCs to SDCCs. To create a DCC tunnel, you connect the tunnel end points from one ONS 15454 optical port to another.

Each ONS 15454 can support up to 32 DCC tunnel connections. Table 11-4 shows the DCC tunnels that you can create.

Table 11-4 DCC Tunnels
DCC	SONET Layer	SONET Bytes	OC-3 (all ports)	OC-12, OC-48
SDCC	Section	D1 - D3	Yes	Yes
Tunnel 1	Line	D4 - D6	No	Yes
Tunnel 2	Line	D7 - D9	No	Yes
Tunnel 3	Line	D10 - D12	No	Yes

Figure 11-7 shows a DCC tunnel example. Third-party equipment is connected to OC-3 cards at Node 1/Slot 3/Port 1 and Node 3/Slot 3/Port 1. Each ONS 15454 node is connected by OC-48 trunk cards. In the example, three tunnel connections are created, one at Node 1 (OC-3 to OC-48), one at Node 2 (OC-48 to OC-48), and one at Node 3 (OC-48 to OC-3).

11.4 Multiple Drops for Unidirectional Circuits

Unidirectional circuits can have multiple drops for use in broadcast circuit schemes. In broadcast scenarios, one source transmits traffic to multiple destinations, but traffic is not returned back to the source.

When you create a unidirectional circuit, the card that does not have its backplane Rx input terminated with a valid input signal generates a loss of service (LOS) alarm. To mask the alarm, create an alarm profile suppressing the LOS alarm and apply it to the port that does not have its Rx input terminated.

11.5 Monitor Circuits

You can set up secondary circuits to monitor traffic on primary bidirectional circuits. Figure 11-8 shows an example of a monitor circuit. At Node 1, a VT1.5 is dropped from Port 1 of an EC1-12 card. To monitor the VT1.5 traffic, test equipment is plugged into Port 2 of the EC1-12 card and a monitor circuit to Port 2 is provisioned in CTC. Circuit monitors are one-way. The monitor circuit in Figure 11-8 is used to monitor VT1.5 traffic received by Port 1 of the EC1-12 card.

11.6 UPSR Circuits

Use the Edit Circuits window to change UPSR selectors and switch protection paths ( Figure 11-9). In this window, you can:

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Change PDI-P settings, perform maintenance switches on the circuit selector, and view switch counts for the selectors

11.7 Path Trace

The SONET J1 Path Trace is a repeated, fixed-length string comprised of 64 consecutive J1 bytes. You can use the string to monitor interruptions or changes to circuit traffic. Table 11-5 shows the ONS 15454 cards that support path trace. DS-1 and DS-3 cards can transmit and receive the J1 field, while the EC-1, OC-3, OC-48AS, and OC-192 can only receive it. Cards not listed in the table do not support the J1 byte.

Table 11-5 ONS 15454 Cards Supporting J1 Path Trace
Card	Receive	Transmit
DS1-14	X	X
DS1N-14	X	X
DS3-12E	X	X
DS3N-12E	X	X
DS3XM-6X	X	X
EC1-12	X
OC3 IR 4 1310	X
OC48 IR/STM16 SH AS 1310	X
OC48 LR/STM16 LH AS 1550	X
OC192 LR/STM64 LH 1550	X

The J1 path trace transmits a repeated, fixed-length string. If the string received at a circuit drop port does not match the string the port expects to receive, an alarm is raised. Two path trace modes are available:

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Automatic—The receiving port assumes the first J1 string it receives is the baseline J1 string.

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Manual—The receiving port uses a string that you manually enter as the baseline J1 string.

11.8 Automatic Circuit Routing

If you select automatic routing during circuit creation, Cisco Transport Controller (CTC) routes the circuit by dividing the entire circuit route into segments based on protection domains. For unprotected segments of protected circuits, CTC finds an alternate route to protect the segment in a virtual UPSR fashion. Each path segment is a separate protection domain, and each protection domain is protected in a specific fashion (virtual UPSR, BLSR, or 1+1).

The following list provides principles and charactistics of automatic circuit routing:

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Circuit routing tries to use the shortest path within the user-specified or network-specified constraints. VT tunnels are preferable for VT circuits because VT tunnels are considered shortcuts when CTC calculates a circuit path in path-protected mesh networks.

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If you do not choose Fully Path Protected during circuit creation, circuits may still contain protected segments. Because circuit routing always selects the shortest path, one or more links and/or segments may have some protection. CTC does not look at link protection while computing a path for unprotected circuits.

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Circuit routing will not use links that are down. If you want all links to be considered for routing, do not create circuits when a link is down.

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Circuit routing computes the shortest path when you add a new drop to an existing circuit. It tries to find a shortest path from the new drop to any nodes on the existing circuit.

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If the network has a mixture of VT-capable nodes and nodes that are not VT capable, depending on the route found, CTC will automatically force creation of a VT tunnel. Otherwise, CTC asks you whether a VT tunnel is needed.

11.8.1 Bandwidth Allocation and Routing

Within a given network, CTC will route circuits on the shortest possible path between source and destination based on the circuit attributes, such as protection and type. CTC will consider using a link for the circuit only if the link meets the following requirements:

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The link has the required time slots to enforce the same time slot restrictions for BLSR

The same logic applies to VT circuits on VT tunnels. Circuit routing typically favors VT tunnels because, based on topology maintained by circuit routing, VT tunnels are shortcuts between a given source and destination. If the VT tunnel in the route is full (no more bandwidth), CTC asks whether you want to create an additional VT tunnel.

11.8.2 Secondary Sources and Drops

CTC supports secondary sources and drops. Secondary sources and drops typically interconnect two "foreign" networks, as shown in Figure 11-10. Traffic is protected while it goes through a network of ONS 15454s.

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CTC does not allow a secondary destination for unidirectional circuits because you can always specify additional destinations (drops) after you create the circuit

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Secondary sources and destinations are permitted only for regular STS/VT connections (not for VT tunnels and multicard EtherSwitch circuits)

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For point-to-point (straight) Ethernet circuits, only SONET STS endpoints can be specified as multiple sources or drops

For bidirectional circuits, CTC creates a UPSR connection at the source node that allows traffic to be selected from one of the two sources on the ONS 15454 network. If you check the Fully Path Protected option during circuit creation, traffic is protected within the ONS 15454 network. At the destination, another UPSR connection is created to bridge traffic from the ONS 15454 network to the two destinations. A similar but opposite path exists for the reverse traffic flowing from the destinations to the sources.

For unidirectional circuits, a UPSR drop-and-continue connection is created at the source node.

11.9 Manual Circuit Routing

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Choose a specific path, not just the shortest path chosen by automatic routing

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Choose a protected path for Multicard EtherSwitch circuits, allowing virtual UPSR segments

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All circuits, except Multicard EtherSwitch circuits in a shared packet ring, should have links with a direction that flows from source to destination. This is true for Multicard EtherSwitch circuits that are not in a shared packet ring.

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If you enabled Fully Path Protected, choose a diverse protect (alternate) path for every unprotected segment ( Figure 11-11).

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For Multicard EtherSwitch circuits, the Fully Path Protected option is ignored.

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For a node that has a UPSR selector based on the links chosen, the input links to the UPSR selectors cannot be 1+1 or BLSR protected (see Figure 11-12). The same rule applies at the UPSR bridge.

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Choose the links of Multicard EtherSwitch circuits in a shared packet ring to route from source to destination back to source (see Figure 11-13). Otherwise, a route (set of links) chosen with loops is invalid.

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Multicard EtherSwitch circuits can have virtual UPSR segments if the source or destination is not in the UPSR domain. This restriction also applies after circuit creation; therefore, if you create a circuit with UPSR segments, Ethernet node drops cannot exist anywhere on the UPSR segment (see Figure 11-14).

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VT tunnels cannot be an endpoint of a UPSR segment. A UPSR segment endpoint is where the UPSR selector resides.

If Fully Path Protected is chosen, CTC verifies that the route selection is protected at all segments. A route can have multiple protection domains with each domain protected by a different mechanism.

The following tables summarize the available node connections. Any other combination is invalid and will generate an error.

Table 11-6 Bidirectional STS/VT/Regular Multicard EtherSwitch/Point-to-Point (straight) Ethernet Circuits
# of Inbound Links	# of Outbound Links	# of Sources	# of Drops	Connection Type
-	2	1	-	UPSR
2	-	-	1	UPSR
2	1	-	-	UPSR
1	2	-	-	UPSR
1	-	-	2	UPSR
-	1	2	-	UPSR
2	2	-	-	Double UPSR
2	-	-	2	Double UPSR
-	2	2	-	Double UPSR
1	1	-	-	Two Way
0 or 1	0 or 1	Ethernet Node Source	-	Ethernet
0 or 1	0 or 1	-	Ethernet Node Drop	Ethernet

Table 11-7 Unidirectional STS/VT Circuit
# of Inbound Links	# of Outbound Links	# of Sources	# of Drops	Connection Type
1	1	-	-	One way
1	2	-	-	UPSR Head End
-	2	1	-	UPSR Head End
2	-	-	1+	UPSR drop and continue

Table 11-8 Multicard Group Ethernet Shared Packet Ring Circuit
# of Inbound Links	# of Outbound Links	# of Sources	# of Drops	Connection Type
At intermediate nodes only
2	1	-	-	UPSR
1	2	-	-	UPSR
2	2	-	-	Double UPSR
1	1	-	-	Two way
At source or destination nodes only
1	1	-	-	Ethernet

Table 11-9 Bidirectional VT Tunnels
# of Inbound Links	# of Outbound Links	# of Sources	# of Drops	Connection Type
At intermediate nodes only
2	1	-	-	UPSR
1	2	-	-	UPSR
2	2	-	-	Double UPSR
1	1	-	-	Two way
At source nodes only
-	1	-	-	VT tunnel end point
At destination nodes only
1	-	-	-	VT tunnel end point

Although virtual UPSR segments are possible in VT tunnels, VT tunnels are still considered unprotected. If you need to protect VT circuits either use two independent VT tunnels that are diversely routed or use a VT tunnel that is routed over only 1+1 or BLSR (or a mix) links.

11.10 Constraint-Based Circuit Routing

When you create circuits, you can choose Fully Protected Path to protect the circuit from source to destination. The protection mechanism used depends on the path CTC calculates for the circuit. If the network is comprised entirely of BLSR and/or 1+1 links, or the path between source and destination can be entirely protected using 1+1 and/or BLSR links, no PPMN (virtual UPSR) protection is used.

If virtual UPSR (PPMN) protection is needed to protect the path, set the level of node diversity for the PPMN portions of the complete path on the Circuit Creation dialog box:

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Required—Ensures that the primary and alternate paths of each PPMN domain in the complete path have a diverse set of nodes.

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Desired—CTC looks for a node diverse path; if a node diverse path is not available, CTC finds a link diverse path for each PPMN domain in the complete path.

When you choose automatic circuit routing during circuit creation, you have the option to require and/or exclude nodes and links in the calculated route. You can use this option to:

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Simplify manual routing, especially if the network is large and selecting every span is tedious. You can select a general route from source to destination and allow CTC to fill in the route details.

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Balance network traffic; by default CTC chooses the shortest path, which can load traffic on certain links while other links are either free or less used. By selecting a required node and/or a link, you force the CTC to use (or not use) an element, resulting in more efficent use of network resources.

CTC considers required nodes and links to be an ordered set of elements. CTC treats the source nodes of every required link as required nodes. When CTC calculates the path, it makes sure the computed path traverses the required set of nodes and links and does not traverse excluded nodes and links.

The required nodes and links constraint is only used during the primary path computation and only for PPMN domains/segments. The alternate path is computed normally; CTC uses excluded nodes/links when finding all primary and alternate paths on PPMNs.

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Circuits and Tunnels