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Table Of Contents
10.4.1 APC at the Amplifier Card Level
10.4.2 APC at the Shelf Controller Layer
10.5 ROADM Power Equalization Monitoring
10.7.1 Automatic Laser Shutdown
10.7.2 Automatic Power Reduction
10.8 Network-Level Gain—Tilt Management of Optical Amplifiers
10.8.1 Gain Tilt Control at the Card Level
10.8.2 System Level Gain Tilt Control
10.9 Optical Data Rate Derivations
10.9.1 OC-192/STM-64 Data Rate (9.95328 Gbps)
10.9.2 10GE Data Rate (10.3125 Gbps)
10.9.3 10G FC Data Rate (10.51875 Gbps)
10.9.4 ITU-T G.709 Optical Data Rates
Network Reference
This chapter explains the ONS 15454 dense wavelength division multiplexing (DWDM) network applications and topologies. The chapter also provides network-level optical performance references.
Note
Unless otherwise specified, "ONS 15454" refers to both ANSI and ETSI shelf assemblies.
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Chapter topics include:
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10.1 Network Applications
Cisco ONS 15454 nodes can be provisioned for metro core DWDM network applications. Metro core networks often include multiple spans and amplifiers, so the optical signal-to-noise ratio (OSNR) is the limiting factor for channel performance.
Within DWDM networks, the ONS 15454 uses a communications protocol, called Node Services Protocol (NSP), to communicate with other nodes. NSP automatically updates nodes whenever a change in the network occurs. Each ONS 15454 DWDM node can:
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10.2 Network Topologies
The ONS 15454 DWDM network topologies include ring networks, linear networks, and mesh networks.
10.2.1 Ring Networks
Ring networks support hubbed, multi-hubbed, any-to-any, and mesh traffic topologies.
10.2.1.1 Hubbed Traffic Topology
In the hubbed traffic topology ( Figure 10-1), a hub node terminates all the DWDM channels. A channel can be provisioned to support protected traffic between the hub node and any node in the ring. Both working and protected traffic use the same wavelength on both sides of the ring. Protected traffic can also be provisioned between any pair of optical add/drop multiplexing (OADM) nodes, except that either the working or the protected path must be regenerated in the hub node.
Protected traffic saturates a channel in a hubbed topology, that is, no channel reuse is possible. However, the same channel can be reused in different sections of the ring by provisioning unprotected multihop traffic. From a transmission point of view, this network topology is similar to two bidirectional point-to-point links with OADM nodes.
For more information about hub nodes, see the "Hub Node" section.
Figure 10-1 Hubbed Traffic Topology
10.2.1.2 Multihubbed Traffic Topology
A multihubbed traffic topology ( Figure 10-2) is based on the hubbed traffic topology, except that two or more hub nodes are added. Protected traffic can only be established between the two hub nodes. Protected traffic can be provisioned between a hub node and any OADM node only if the allocated wavelength channel is regenerated through the other hub node. Multihop traffic can be provisioned on this ring. From a transmission point of view, this network topology is similar to two or more point-to-point links with OADM nodes.
Figure 10-2 Multihubbed Traffic Topology
10.2.1.3 Any-to-Any Traffic Topology
The any-to-any traffic topology ( Figure 10-3) contains only reconfigurable OADM (ROADM) nodes (with or without optical service channel [OSC] regeneration) or optical amplifier nodes. This topology potentially allows you to route every wavelength from any source to any destination node inside the network.
See the "ROADM Node" section for more information.
Figure 10-3 Any-to-Any Traffic Topology
10.2.1.4 Meshed Traffic Topology
The meshed traffic topology ( Figure 10-4) does not use hubbed nodes; only amplified and passive OADM nodes are present. Protected traffic can be provisioned between any two nodes; however, the selected channel cannot be reused in the ring. Unprotected multihop traffic can be provisioned in the ring. A meshed ring must be designed to prevent amplified spontaneous emission (ASE) lasing. This is done by configuring a particular node as an anti-ASE node. An anti-ASE node can be created in two ways:
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For more information about OADM nodes, see the "OADM Node" section. For more information about anti-ASE nodes, see the "Anti-ASE Node" section.
Figure 10-4 Meshed Traffic Topology
10.2.2 Linear Networks
Linear configurations are characterized by the use of two terminal nodes, east and west. The 32-channel terminal nodes can be equipped with a 32MUX-O card and a 32DMX-O card, or with a 32WSS card and a 32DMX or 32DMX-O card. The 40-channel terminal nodes can be equipped with a 40-MUX-C card and a 40-DMX-C card or a 40-WSS-C card with a 40-DMX-C card. OADM or line amplifier nodes can be installed between the two terminal nodes. Only unprotected traffic can be provisioned in a linear configuration. Figure 10-5 shows five ONS 15454 nodes in a linear configuration with an amplified and a passive OADM node.
Figure 10-5 Linear Configuration with an OADM Node
Figure 10-6 shows five ONS 15454 nodes in a linear configuration without an OADM node. See the "Terminal Node" section for more information.
Figure 10-6 Linear Configuration without an OADM Node
A single-span link is a type of linear configuration characterized by a single-span link with preamplification and post-amplification. A single-span link is also characterized by the use of two terminal nodes, east and west. Only unprotected traffic can be provisioned on a single-span link.
Figure 10-7 shows ONS 15454s in a single-span link. Eight channels are carried on one span. Single-span link losses apply to OC-192/STM-64 LR ITU cards. The optical performance values are valid assuming that the sum of the OADM passive node insertion losses and the span losses does not exceed 35 dB.
Figure 10-7 Single-Span Link
10.2.3 Mesh Networks
A mesh network can be native or multiring. In a native mesh network ( Figure 10-8), any combination of four-degree and eight-degree mesh nodes can work together. Four-degree mesh nodes transmit an optical signal in four directions, while an eight-degree mesh node transmits an optical signal in eight directions. For additional information about mesh nodes, see the "Configuring Mesh DWDM Networks" section. The intermediate nodes are ROADM nodes. In a mesh node, all wavelengths can be routed through four (four-degree mesh node) to eight (eight-degree mesh node) different optical line termination ports using a 40-WXC-C card without any optical-electrical-optical (OEO) regeneration. It is possible to combine 40-WSS-C, 40-WXC-C, and 32WSS cards in the same mesh network without impacting system performance. For nodes equipped with 32WSS cards, the maximum system capacity is 32 channels. Terminal sites are connected to the mesh network as a spur.
Figure 10-8 mesh Network
In a multiring mesh network ( Figure 10-9), several rings are connected with four-degree or eight-degree mesh nodes. The intermediate ROADM nodes are equipped with MMU cards. All wavelengths can be routed among two or more rings using a 40-WXC-C card without any O-E-O regeneration. As in a native mesh network, is possible to combine 40-WSS-C, 40-WXC-C, and 32WSS cards in the same multiring network without impacting system performance. For nodes equipped with 32WSS cards, maximum system capacity is limited to 32 channels. A terminal node is connected to a multiring node as a spur.
For information on node configurations for both native mesh and multiring networks, see the "Configuring Mesh DWDM Networks" section.
Figure 10-9 Multiring Network
10.3 Optical Performance
This section provides optical performance information for ONS 15454 DWDM networks. The performance data is a general guideline based upon the network topology, node type, client cards, fiber type, number of spans, and number of channels. The maximum number of nodes that can be in an ONS 15454 DWDM network is 16. The DWDM topologies and node types that are supported are shown in Table 10-1.
Table 10-1 Supported Topologies and Node Types
32 channels
SMF-281
E-LEAF2
TW-RS3
Ring
Linear
Linear without OADM
Hub
Active OADM
Passive OADM
Terminal
Line
OSC regeneration
16 channels
SMF-28
Ring
Linear
Linear without OADM
Hub
Active OADM
Passive OADM
Terminal
Line
OSC regeneration
8 channels
SMF-28
Linear without OADM
Terminal
Line
1 SMF-28 = single-mode fiber 28.
2 E-LEAF = enhanced large effective area fiber.
3 TW-RS = TrueWave reduced slope fiber.
10.4 Automatic Power Control
The ONS 15454 automatic power control (APC) feature performs the following functions:
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Amplifier software uses a control gain loop with fast transient suppression to keep the channel power constant regardless of any changes in the number of channels. Amplifiers monitor the changes to the input power and change the output power proportionately according to the calculated gain setpoint. The shelf controller software emulates the control output power loop to adjust for fiber degradation. To perform this function, the TCC2/TCC2P needs to know the channel distribution, which is provided by a signaling protocol, and the expected per-channel power, which you can provision. The TCC2/TCC2P compares the actual amplifier output power with the expected amplifier output power and modifies the setpoints if any discrepancies occur.
10.4.1 APC at the Amplifier Card Level
In constant gain mode, the amplifier power out control loop performs the following input and output power calculations, where G represents the gain and t represents time.
Pout (t) = G * Pin (t) (mW)
Pout (t) = G + Pin (t) (dB)
In a power-equalized optical system, the total input power is proportional to the number of channels. The amplifier software compensates for any variation of the input power due to changes in the number of channels carried by the incoming signal.
Amplifier software identifies changes in the read input power in two different instances, t1 and t2, as a change in the traffic being carried. The letters m and n in the following formula represent two different channel numbers. Pin/ch represents the input power per channel.
Pin (t1)= nPin/ch
Pin (t2) = mPin/ch
Amplifier software applies the variation in the input power to the output power with a reaction time that is a fraction of a millisecond. This keeps the power constant on each channel at the output amplifier, even during a channel upgrade or a fiber cut.
The per-channel power and working mode (gain or power) are set by automatic node setup (ANS). The provisioning is conducted on a per-side basis. A preamplifier or a booster amplifier facing Side i are provisioned using the Side i parameters present in the node database, where i - A, B, C, D, E, F, G, or H.
Starting from the expected per-channel power, the amplifiers automatically calculate the gain setpoint after the first channel is provisioned. An amplifier gain setpoint is calculated in order to make it equal to the loss of the span preceding the amplifier itself. After the gain is calculated, the setpoint is no longer changed by the amplifier. Amplifier gain is recalculated every time the number of provisioned channels returns to zero. If you need to force a recalculation of the gain, move the number of channels back to zero.
10.4.2 APC at the Shelf Controller Layer
Amplifiers are managed through software to control changes in the input power caused by changes in the number of channels. The software adjusts the output total power to maintain a constant per-channel power value when the number of input channel changes.
Changes in the network characteristics have an impact on the amplifier input power. Changes in the input power are compensated for only by modifying the original calculated gain, because input power changes imply changes in the span loss. As a consequence, the gain to span loss established at amplifier start-up is no longer satisfied, as shown in Figure 10-10.
Figure 10-10 Using Amplifier Gain Adjustment to Compensate for System Degradation
In Figure 10-10, Node 1 and Node 2 are equipped with booster amplifiers and preamplifiers. The input power received at the preamplifier on Node 2 (Pin2) depends on the total power launched by the booster amplifier on Node1, Pout1(n) (where n is the number of channels), and the effect of the span attenuation (L) between the two nodes. Span loss changes due to aging fiber and components or changes in operating conditions. The power into Node 2 is given by the following formula:
Pin2 = LPout1(n)
The phase gain of the preamplifier on Node 2 (GPre-2) is set during provisioning in order to compensate for the span loss so that the Node 2 preamplifier output power (Pout-Pre-2) is equal to the original transmitted power, as represented in the following formula:
Pout-Pre-2 = L x GPre-2 x Pout1(n)
In cases of system degradation, the power received at Node 2 decreases due to the change of span insertion loss (from L to L'). As a consequence of the preamplifier gain control working mode, the Node 2 preamplifier output power (Pout-Pre-2) also decreases. The goal of APC at the shelf controller layer is simply to detect if an amplifier output change is needed because of changes in the number of channels or to other factors. If factors other than changes in the number of channels occur, APC provisions a new gain at the Node 2 preamplifier (GPre-2') to compensate for the new span loss, as shown in the formula:
GPre-2' = GPre-2 (L/ L') = GPre-2 + [Pout-Pre-2 -Exp(Pout-Pre-2)]
Generalizing on the above relationship, APC is able to compensate for system degradation by adjusting working amplifier gain or variable optical attenuation (VOA) and to eliminate the difference between the power value read by the photodiodes and the expected power value. The expected power values are calculated using:
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Channel distribution is determined by the sum of the provisioned and failed channels. Information about provisioned wavelengths is sent to APC on the applicable nodes during circuit creation. Information about failed channels is collected through a signaling protocol that monitors alarms on ports in the applicable nodes and distributes that information to all the other nodes in the network.
ASE calculations purify the noise from the power level reported from the photodiode. Each amplifier can compensate for its own noise, but cascaded amplifiers cannot compensate for ASE generated by preceding nodes. The ASE effect increases when the number of channels decreases; therefore, a correction factor must be calculated in each amplifier of the ring to compensate for ASE build-up.
APC is a network-level feature that is distributed among different nodes. An APC domain is a set of nodes that is controlled by the same instance of APC at the network level. An APC domain optically identifies a portion of the network that can be independently regulated. An optical network can be divided into several different domains, with the following characteristics:
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APC Domain Node_1 Side A, Node_4 Side B
+ Span 1: Node_1 Side A, Node_2 Side B
+ Span 2: Node_2 Side A, Node_3 Side B
+ Span 3: Node_3 Side A, Node_4 Side B•
Inside a domain, the APC algorithm designates a master node that is responsible for starting APC hourly or every time a new circuit is provisioned or removed. Every time the master node signals APC to start, gain and VOA setpoints are evaluated on all nodes in the network. If corrections are needed in different nodes, they are always performed sequentially following the optical paths starting from the master node.
APC corrects the power level only if the variation exceeds the hysteresis thresholds of +/- 0.5 dB. Any power level fluctuation within the threshold range is skipped since it is considered negligible. Because APC is designed to follow slow time events, it skips corrections greater than 3 dB. This is the typical total aging margin that is provisioned during the network design phase. After you provision the first channel or the amplifiers are turned up for the first time, APC does not apply the 3 dB rule. In this case, APC corrects all the power differences to turn up the node.
To avoid large power fluctuations, APC adjusts power levels incrementally. The maximum power correction is +/- 0.5 dB. This is applied to each iteration until the optimal power level is reached. For example, a gain deviation of 2 dB is corrected in four steps. Each of the four steps requires a complete APC check on every node in the network. APC can correct up to a maximum of 3 dB on an hourly basis. If degradation occurs over a longer time period, APC compensates for it by using all margins that you provision during installation.
If no margin is available, adjustments cannot be made because setpoints exceed the ranges. APC communicates the event to CTC, Cisco Transport Manager (CTM), and TL1 through an APC Fail condition. APC clears the APC fail condition when the setpoints return to the allowed ranges.
APC can be manually disabled. In addition, APC automatically disables itself when:
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The APC state (Enable/Disable) is located on every node and can be retrieved by the CTC or TL1 interface. If an event that disables APC occurs in one of the network nodes, APC is disabled on all the other nodes and the APC state changes to DISABLE - INTERNAL. The disabled state is raised only by the node where the problem occurred to simplify troubleshooting.
APC raises the following minor, non-service affecting alarms at the port level in CTC, TL1, and Simple Network Management Protocol (SNMP):
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After the error condition is cleared, the signaling protocol enables APC on the network and the APC DISABLE - INTERNAL condition is cleared. Because APC is required after channel provisioning to compensate for ASE effects, all optical channel network connection (OCHNC) and optical channel client connection (OCHCC) circuits that you provision during the disabled APC state are kept in the Out-of-Service and Autonomous, Automatic In-Service (OOS-AU,AINS) (ANSI) or Unlocked-disabled,automaticInService (ETSI) service state until APC is enabled. OCHNCs and OCHCCs automatically go into the In-Service and Normal (IS-NR) (ANSI) or Unlocked-enabled (ETSI) service state only after APC is enabled.
10.4.3 Managing APC
The APC status is indicated by four APC states shown in the node view status area:
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You can view the APC information and disable and enable APC manually on the Maintenance > DWDM > APC tab. See Figure 10-11 for an example of how the information is displayed.
Caution
Figure 10-11 Automatic Power Control
The APC subtab provides the following information:
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10.5 ROADM Power Equalization Monitoring
ROADM nodes allow you to monitor the 32WSS, 40-WSS-C, and 40-WXC-C card equalization functions on the Maintenance > DWDM > ROADM Power Monitoring > Optical Side n-n tab, where n-n is A-B, C-D, E-F, or G-H ( Figure 10-12). The tab shows the input channel power (Padd), the express or pass-through power (Ppt) and the power level at output (Pout).
Figure 10-12 ROADM Power Monitoring Subtab
10.6 Span Loss Verification
Span loss measurements can be performed from the Maintenance > DWDM > WDM Span Check tab ( Figure 10-13). The CTC span check compares the far-end OSC power with the near-end OSC power. A Span Loss Out of Range condition is raised when the measured span loss is higher than the maximum expected span loss. It is also raised when the measured span loss is lower than the minimum expected span loss and the difference between the minimum and maximum span loss values is greater than 1 dB. The minimum and maximum expected span loss values are calculated by Cisco MetroPlanner for the network and imported into CTC. However, you can manually change the minimum and expected span loss values.
CTC span loss measurements provide a quick span loss check and are useful whenever changes to the network occur, for example after you install equipment or repair a broken fiber. CTC span loss measurement resolutions are:
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For ONS 15454 span loss measurements with higher resolutions, an optical time domain reflectometer (OTDR) must be used.
Figure 10-13 Span Loss Verification
10.7 Network Optical Safety
If a fiber break occurs on the network, automatic laser shutdown (ALS) automatically shuts down the OSCM and OSC-CSM OSC laser output power and the optical amplifiers contained in the OPT-BST, OPT-BST-E, OPT-BST-L, OPT-AMP-L, and OPT-AMP-17-C cards. The Maintenance > ALS tabs in CTC card view provide the following ALS management options for OSCM, OSC-CSM, OPT-BST, OPT-BST-E, OPT-BST-L, OPT-AMP-L, and OPT-AMP-17-C cards:
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10.7.1 Automatic Laser Shutdown
When ALS is enabled on OPT-BST, OPT-BST-E, OPT-BST-L, OPT-AMP-L, OPT-AMP-17-C, OSCM, and OSC-CSM cards, a network safety mechanism will occur in the event of a system failure. ALS provisioning is also provided on the transponder (TXP) and muxponder (MXP) cards. However, if a network uses ALS-enabled OPT-BST, OPT-BST-E, OPT-BST-L, OPT-AMP-L, OPT-AMP-17-C, OSCM, and OSC-CSM cards, ALS does not need to be enabled on the TXP cards or MXP cards. ALS is disabled on TXP and MXP cards by default and the network optical safety is not impacted.
If TXP and MXP cards are connected directly to each other without passing through a DWDM layer, ALS should be enabled on them. The ALS protocol goes into effect when a fiber is cut, enabling some degree of network point-to-point bidirectional traffic management between the cards.
If ALS is disabled on the OPT-BST, OPT-BST-E, OPT-BST-L, OPT-AMP-L, OPT-AMP-17-C, OSCM, and OSC-CSM cards (the DWDM network), ALS can be enabled on the TXP and MXP cards to provide laser management in the event of a fiber break in the network between the cards.
10.7.2 Automatic Power Reduction
Automatic power reduction (APR) is controlled by the software and is not user configurable. During amplifier restart after a system failure, the amplifier (OPT-BST, for example) operates in pulse mode and an APR level is activated so that the Hazard Level 1 power limit is not exceeded. This is done to ensure personnel safety.
When a system failure occurs (cut fiber or equipment failure, for example) and ALS Auto Restart is enabled, a sequence of events is placed in motion to shut down the amplifier laser power, then automatically restart the amplifier after the system problem is corrected. As soon as a loss of optical payload and OSC is detected at the far end, the far-end amplifier shuts down. The near-end amplifier then shuts down because it detects a loss of payload and OSC due to the far-end amplifier shutdown. At this point, the near end attempts to establish communication to the far end using the OSC laser transmitter. To do this, the OSC emits a two-second pulse at very low power (maximum of 0 dBm) and waits for a similar two-second pulse in response from the far-end OSC laser transmitter. If no response is received within 100 seconds, the near end tries again. This process continues until the near end receives a two-second response pulse from the far end, indicating the system failure is corrected and full continuity in the fiber between the two ends exists.
After the OSC communication is established, the near-end amplifier is configured by the software to operate in pulse mode at a reduced power level. It emits a nine-second laser pulse with an automatic power reduction to +8 dBm. This level assures that Hazard Level 1 is not exceeded, for personnel safety, even though the establishment of successful OSC communication is assurance that any broken fiber is fixed. If the far-end amplifier responds with a nine-second pulse within 100 seconds, both amplifiers are changed from pulse mode at reduced power to normal operating power mode.
For a direct connection between TXP or MXP cards, when ALS Auto Restart is enabled and the connections do not pass through a DWDM layer, a similar process takes place. However, because the connections do not go through any amplifier or OSC cards, the TXP or MXP cards attempt to establish communication directly between themselves after a system failure. This is done using a two-second restart pulse, in a manner similar to that previously described between OSCs at the DWDM layer. The power emitted during the pulse is below Hazard Level 1.
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10.7.3 Fiber Cut Scenarios
In the following paragraphs, four ALS scenarios are given:
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10.7.3.1 Scenario 1: Fiber Cut in Nodes Using OPT-BST/OPT-BST-E Cards
Figure 10-14 shows nodes using OPT-BST/OPT-BST-E cards with a fiber cut between them.
Figure 10-14 Nodes Using OPT-BST/OPT-BST-E Cards
Two photodiodes at Node B monitor the received signal strength for the optical payload and OSC signals. When the fiber is cut, an LOS is detected at both of the photodiodes. The AND function then indicates an overall LOS condition, which causes the OPT-BST/OPT-BST-E transmitter, OPT-PRE transmitter, and OSCM lasers to shut down. This in turn leads to an LOS for both the optical payload and OSC at Node A, which causes Node A to turn off the OSCM, OPT-PRE transmitter, and OPT-BST/OPT-BST-E transmitter lasers. The sequence of events after a fiber cut is as follows (refer to the numbered circles in Figure 10-14):
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When the fiber is repaired, either an automatic or manual restart at the Node A OPT-BST/OPT-BST-E transmitter or at the Node B OPT-BST/OPT-BST-E transmitter is required. A system that has been shut down is reactivated through the use of a restart pulse. The pulse is used to signal that the optical path has been restored and transmission can begin. For example, when the far end, Node B, receives a pulse, it signals to the Node B OPT-BST/OPT-BST-E transmitter to begin transmitting an optical signal. The OPT-BST/OPT-BST-E receiver at Node A receives that signal and signals the Node A OPT-BST/OPT-BST-E transmitter to resume transmitting.
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10.7.3.2 Scenario 2: Fiber Cut in Nodes Using OSC-CSM Cards
Figure 10-15 shows nodes using OSC-CSM cards with a fiber cut between them.
Figure 10-15 Nodes Using OSC-CSM Cards
Two photodiodes at the Node B OSC-CSM card monitor the received signal strength for the received optical payload and OSC signals. When the fiber is cut, LOS is detected at both of the photodiodes. The AND function then indicates an overall LOS condition, which causes the Node B OSC laser to shut down and the optical switch to block traffic. This in turn leads to LOS for both the optical payload and OSC signals at Node A, which causes Node A to turn off the OSC laser and the optical switch to block outgoing traffic. The sequence of events after a fiber cut is as follows (refer to the numbered circles in Figure 10-15):
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When the fiber is repaired, either an automatic or manual restart at the Node A OSC-CSM card OSC or at the Node B OSC-CSM card OSC is required. A system that has been shut down is reactivated through the use of a restart pulse. The pulse indicates the optical path is restored and transmission can begin. For example, when the far-end Node B receives a pulse, it signals to the Node B OSC to begin transmitting its optical signal and for the optical switch to pass incoming traffic. The OSC-CSM at Node A then receives the signal and tells the Node A OSC to resume transmitting and for the optical switch to pass incoming traffic.
10.7.3.3 Scenario 3: Fiber Cut in Nodes Using OPT-BST-L Cards
Figure 10-16 shows nodes using OPT-BST-L cards with a fiber cut between them.
Figure 10-16 Nodes Using OPT-BST-L Cards
Two photodiodes at Node B monitor the received signal strength for the optical payload and OSC signals. When the fiber is cut, an LOS is detected at both of the photodiodes. The AND function then indicates an overall LOS condition, which causes the OPT-BST-L transmitter and OSCM lasers to shut down. This in turn leads to an LOS for both the optical payload and the OSC at Node A, which causes Node A to turn off the OSCM OSC transmitter and OPT-BST-L amplifier lasers. The sequence of events after a fiber cut is as follows (refer to the numbered circles in Figure 10-16):
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When the fiber is repaired, either an automatic or manual restart at the Node A OPT-BST-L transmitter or at the Node B OPT-BST-L transmitter is required. A system that has been shut down is reactivated through the use of a restart pulse. The pulse indicates the optical path is restored and transmission can begin. For example, when the far end, Node B, receives a pulse, it signals to the Node B OPT-BST-L transmitter to begin transmitting an optical signal. The OPT-BST-L receiver at Node A receives that signal and signals the Node A OPT-BST-L transmitter to resume transmitting.
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10.7.3.4 Scenario 4: Fiber Cut in Nodes Using OPT-AMP-L or OPT-AMP-17-C (OPT-LINE mode) Cards
Figure 10-17 shows nodes using OPT-AMP-L or OPT-AMP-17-C (in OPT-LINE mode) cards with a fiber cut between them.
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Figure 10-17 Nodes Using OPT-AMP-L Cards
Two photodiodes at Node B monitor the received signal strength for the optical payload and OSC signals. When the fiber is cut, an LOS is detected at both of the photodiodes. The AND function then indicates an overall LOS condition, which causes the OPT-AMP-L amplifier transmitter and OSCM OSC lasers to shut down. This in turn leads to an LOS for both the optical payload and OSC at Node A, which causes Node A to turn off the OSCM OSC and OPT-AMP-L amplifier lasers. The sequence of events after a fiber cut is as follows (refer to the numbered circles in Figure 10-17):
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When the fiber is repaired, either an automatic or manual restart at the Node A OPT-AMP-L transmitter or at the Node B OPT-AMP-L transmitter is required. A system that has been shut down is reactivated through the use of a restart pulse. The pulse indicates the optical path is restored and transmission can begin. For example, when the far end, Node B, receives a pulse, it signals to the Node B OPT-AMP-L transmitter to begin transmitting an optical signal. The OPT-AMP-L receiver at Node A receives that signal and signals the Node A OPT-AMP-L transmitter to resume transmitting.
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10.7.3.5 Scenario 5: Fiber Cut in Nodes Using DCN Extension
Figure 10-18 shows a fiber cut scenario for nodes that do not have OSC connectivity. In the scenario, references to the OPT-BST cards refers to the OPT-BST, OPT-BST-L, OPT-BST-E, and the OPT-AMP-L and OPT-AMP-17-C cards when provisioned in OPT-LINE mode.
Figure 10-18 Fiber Cut With DCN Extension
Two photodiodes at Node B monitor the received signal strength for the optical payload. When the fiber is cut, an LOS is detected on the channel photodiode while the other one never gets a signal because the OSC is not present. The AND function then indicates an overall LOS condition, which causes the OPT-BST amplifier transmitter to shut down. This in turn leads to a LOS for the optical payload at Node A, which causes Node A to turn off the OPT-BST amplifier lasers.
The sequence of events after a fiber cut is as follows (refer to the numbered circles in Figure 10-18):
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When the fiber is repaired, a manual restart with 90 sec restart pulse time (MANUAL RESTART FOR TEST) at the Node A OPT-BST-E transmitter and at the Node B OPT-BST-E transmitter is required. A system that has been shut down is reactivated through the use of a 90 sec restart pulse. The pulse indicates that the optical path is restored and transmission can begin.
For example, when the far end, Node B, receives a pulse, it signals to the Node B OPT-BST-E transmitter to begin transmitting an optical signal. The OPT-BST-E receiver at Node A receives that signal and signals the Node A OPT-BST-E transmitter to resume transmitting.
Note
10.8 Network-Level Gain—Tilt Management of Optical Amplifiers
The ability to control and adjust per-channel optical power equalization is a principal feature of ONS 15454 DWDM metro core network applications. A critical parameter to assure optical spectrum equalization throughout the DWDM system is the gain flatness of erbium-doped fiber amplifiers (EDFAs).
Two items, gain tilt and gain ripple, are factors in the power equalization of optical amplifier cards such as the OPT-BST and OPT-PRE. Figure 10-19 shows a graph of the amplifier output power spectrum and how it is affected by gain tilt and gain ripple.
Figure 10-19 Effect of Gain Ripple and Gain Tilt on Amplifier Output Power
Gain ripple and gain tilt are defined as follows:
•
•
Gain tilt is the only contribution to the power spectrum disequalization that can be compensated at the card level. A VOA internal to the amplifier can be used to compensate for gain tilt.
An optical spectrum analyzer (OSA) is used to acquire the output power spectrum of an amplifier. The OSA shows the peak-to-peak difference between the maximum and minimum power levels, and takes into account the contributions of both gain tilt and gain ripple.
Note
10.8.1 Gain Tilt Control at the Card Level
The OPT-BST and OPT-PRE amplifier cards have a flat output (gain tilt = 0 dB) for only a specific gain value (Gdesign), based on the internal optical design (see Figure 10-20).
Figure 10-20 Flat Gain (Gain Tilt = 0 dB)
If the working gain setpoint of the amplifier is different from Gdesign, the output spectrum begins to suffer a gain tilt variation.
In order to compensate for the absolute value of the increase of the spectrum tilt, the OPT-BST and OPT-PRE cards automatically adjust the attenuation of the VOA to maintain a flat power profile at the output, as shown in Figure 10-21.
Figure 10-21 Effect of VOA Attenuation on Gain Tilt
The VOA attenuator automatic regulation guarantees (within limits) a zero tilt condition in the EDFA for a wide range of possible gain setpoint values.
Table 10-2 shows the flat output gain range limits for the OPT-BST and OPT-PRE cards, as well as the maximum (worst case) values of gain tilt and gain ripple expected in the specific gain range.
Table 10-2 Flat Output Gain Range Limits
OPT-BST
G < 20 dB
0.5 dB
1.5 dB
OPT-PRE
G < 21 dB
0.5 dB
1.5 dB
If the operating gain value is outside of the range shown in Table 10-2, the EDFA introduces a tilt contribution for which the card itself cannot directly compensate. This condition is managed in different ways, depending the amplifier card type:
•
•
10.8.2 System Level Gain Tilt Control
System level gain tilt control for OPT-PRE cards is achievable with two main scenarios:
•
•
10.8.2.1 System Gain Tilt Compensation Without ROADM Nodes
When an OPT-PRE card along a specific line direction (Side A-to-Side B or Side B-to-Side A) is working outside the flat output gain range (G > 21 dB), the unregulated tilt is compensated for in spans not connected to ROADM nodes by configuring an equal but opposite tilt on one or more of the amplifiers in the downstream direction. The number of downstream amplifiers involved depends on the amount of tilt compensation needed and the gain setpoint of the amplifiers that are involved. See Figure 10-22.
Figure 10-22 System Tilt Compensation Without an ROADM Node
The proper Tilt Reference value is calculated by Cisco MetroPlanner and inserted in the Installation Parameter List imported during the node turn-up process (see the "Turn Up a Node" chapter in the Cisco ONS 15454 DWDM Procedure Guide). For both OPT-PRE and OPT-BST cards, the provisionable Gain Tilt Reference range is between -3 dB and +3 dB.
During the ANS procedure, the Tilt value for the OPT-BST or OPT-PRE card is provisioned by the TCC2/TCC2P card (see Figure 10-23). The provisioned Tilt Reference Value is reported in the CTC OPT-PRE or OPT-BST card view (in the Provisioning > Opt. Ampli. Line > Parameters > Tilt Reference tab).
Figure 10-23 Cisco MetroPlanner Installation Parameters
10.8.2.2 System Gain Tilt Compensation With ROADM Nodes
When a ROADM node is present in the network, as shown in Figure 10-24, a per-channel dynamic gain equalization can be performed. Both gain tilt and gain ripple are completely compensated using the following techniques:
•
•
Figure 10-24 System Tilt Compensation With an ROADM Node
10.9 Optical Data Rate Derivations
This section discusses the derivation of several data rates commonly used in optical networking.
10.9.1 OC-192/STM-64 Data Rate (9.95328 Gbps)
The SONET OC-1 rate is 51.84 Mbps. This rate results from a standard SONET frame, which consists of 9 rows of 90 columns of 8-bit bytes (810 bytes total). The transmission rate is 8000 frames per second (125 microseconds per frame). This works out to 51.84 Mbps, as follows:
(9) x (90 bytes/frame) x (8 bits/byte) x (8000 frames/sec) = 51.84 Mbps
OC-192 is 192 x 51.84 Mbps = 9953.28 Mbps = 9.95328 Gbps
STM-64 is an SDH rate that is equivalent to the SONET OC-192 data rate.
10.9.2 10GE Data Rate (10.3125 Gbps)
10.3125 Gbps is the standard 10 Gbps Ethernet LAN rate. The reason the rate is higher than 10.000 Gbps is due to the 64-bit to 66-bit data encoding. The result is 10 Gbps x 66/64 = 10.3125 Gbps. The reason for 64-bit to 66-bit encoding is to ensure that there are adequate data transitions to ensure proper operation of a clock and data recovery circuit at the far end. Additionally, the encoding assures a data stream that is DC balanced.
10.9.3 10G FC Data Rate (10.51875 Gbps)
The Fibre Channel rate is based on the OC-192 rate of 9.95328 Gbps, with the addition of 64-bit to 66-bit encoding and WAN Interconnect Sublayer (WIS) overhead bytes.
The rate is derived from the basic 9.95328 Gbps OC-192 rate. First, it has the 64-bit to 66-bit encoding added, which brings it to the 10.3125 Gbps rate (10 Gbps x 66/64 = 10.3125 Gbps). Beyond that, the WIS overhead is added, which is an additional two percent on top of the 10.3125 Gbps. This yields:
10.3125 Gbps x .02 = 0.20625 Gbps
10.3125 Gbps + 0.20625 Gbps = 10.51875 Gbps
10.9.4 ITU-T G.709 Optical Data Rates
To understand optical networking data rates, an understanding of the ITU-T G.709 frame structure, shown in Figure 10-25, is needed.
Figure 10-25 ITU-T G.709 Frame Structure
Each of the sub-rows in Figure 10-25 contains 255 bytes. Sixteen are interleaved horizontally (16 x 255 = 4080). This is repeated four times to make up the complete ITU-T G.709 frame.
The Reed Solomon (RS) (255,239) designation indicates the forward error correction (FEC) bytes. There are 16 FEC, or parity, bytes. The ITU-T G.709 protocol uses one overhead byte and 238 data bytes to compute 16 parity bytes to form 255 byte blocks—the RS (255,239) algorithm. Interleaving the information provides two key advantages. First, the encoding rate of each stream is reduced relative to the line transmission rate and, second, it reduces the sensitivity to bursts of error. The interleaving combined with the inherent correction strength of the RS (255,239) algorithm enables the correction of transmission bursts of up to 128 consecutive errored bytes. As a result, the ITU-T G.709 contiguous burst error correcting capability is enhanced 16 times above the capacity of the RS(255,239) algorithm by itself.
ITU-T G.709 defines the Optical Transport Unit 2 (OTU2) rate as 10.70923 Gbps. ITU-T G.709 defines three line rates:
1.
2.
3.
The OTU2 rate is higher than OC-192 because the OTU2 has to carry overhead and FEC bytes in its frame; the bits must be sent faster to carry the payload information at the OC-192 rate.
The ITU-T G.709 frame has two parts. Two are similar to a SDH/SONET frame:
1.
2.
In addition, the ITU-T G.709 frame also includes FEC bytes.
10.9.4.1 OC-192 Packaged Into OTU2 G.709 Frame Data Rate (10.70923 Gbps)
In this case, an OC-192 frame is being transported over a OTU2 G.709 frame, which adds the benefit of FEC. The OC-192 data rate (9.95328 Gbps) must increase in order to transport more bytes (OC-192 plus ITU-T G.709 overhead plus ITU-T G.709 FEC bytes) in the same amount of time. In an OTU2 transmission, 237 of the 255 bytes are OC-192 payload. This means the resultant data rate is:
9.95328 x 255/237 = 10.70923 Gbps
10.9.4.2 10GE Packaged Into OTU2 G.709 Frame Data Rate (Nonstandard 11.0957 Gbps)
Encapsulating Ethernet data into an OTU2 G.709 frame is considered non-standard. The goal is to add the benefit of ITU-T G.709 encapsulation to achieve better burst error performance. However, this means adding overhead and FEC bytes, so more bytes must be transmitted in the same amount of time, so the data rate must increase. The new date rate is:
10.3215 x 255/237 = 11.0957 Gbps
10.9.4.3 10G FC Packaged Into OTU2 G.709 Frame Data Rate (Nonstandard 11.31764 Gbps)
Encapsulating Fibre Channel in an OTU2 frame is considered non-standard. The rate is higher than the 10.51875 rate because OTU2 includes FEC bytes. The bits must run at a faster rate so that the payload is provided at the standard Fibre Channel rate. The rate is:
10.51875 x 255/237 = 11.31764 Gbps
Posted: Mon Oct 22 05:46:59 PDT 2007
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