10 Gigabit Ethernet Fiber Design Considerations
Key factors to consider in the design of 10 Gigabit Ethernet networks are:
- The network topology, including operating distances, splice losses and numbers of connectors (i.e. the link power budget).
- The fiber cabling type (i.e. single-mode or multimode fiber) and the performance at a specified wavelength. The performance is characterized by channel insertion loss (cabling attenuation), and modal bandwidth(for multimode fiber).
- The use of mode-conditioning patch cords if required. The 1310 nm WWDM solution, 10GBASE-LX4, requires the use of a mode-conditioning patch cord on multimode fiber to achieve its specified range of operating distances.
- The implementation of a cabling design, compatible with LED and laser-based Ethernet network devices, which will allow the integration of current LED based 10 Mbps and 100 Mbps networks and laser-based 1 Gbps and 10 Gbps networks.
When designing individual fiber links, the first step is the characterization of the link power budget. This value (expressed in dB) is specified in the 10GbE standard for each optical interface. Tables for all interfaces are shown in this section. The link power budget is calculated by taking the difference between the minimum transmitter power launched into the fiber, and the minimum receiver sensitivity (Figure 2). The receiver sensitivity is the minimum amount of power that is necessary to maintain the required signal-to-noise ratio over the specified operating conditions. The link power budget determines the amount of total loss due to attenuation and other factors that can be introduced between the transmitter and the receiver.
Figure 2: Link Power Budget
The link power budget is applied to account for the channel insertion loss and power penalty. Channel insertion loss is the key parameter and is defined to address the cable and connector losses (Figure 3). The channel insertion loss consists of the specified cable loss for each operating distance, splice losses and the loss of two connections. A connection consists of a mated pair of optical connectors. An allocation of 1.5 dB is budgeted for connector and splice losses for multimode fiber and 2 dB for single-mode fiber. For 10 Gigabit Ethernet applications a power penalty is allocated to the link power budget. This power penalty takes into account effects such as dispersion that may cause inter-symbol interference and therefore degrade an optical signal.
Figure 3: Fiber Optic Cabling Channel
The 10 Gigabit Ethernet operating distances provided in the tables below are limited by the channel insertion loss, the cable bandwidth for multimode fiber, and the optical transceiver characteristics (i.e., PMD types). 10GBASE-E distances greater than 30 km are considered �engineered links� because to support those distances the attenuation of the cable needs to be less than the maximum specified for standard single-mode fiber (Table 4). Therefore, distances greater than 30 km for installed cabling should be �field-tested� for verification of conformance to the 11 dB (Table 7) channel insertion loss specification. Insertion loss measurements of installed fiber cables are made in accordance with ANSI/TIA/EIA-526-14A/ method B and ANSI/TIA/EIA-526-7/method A-1.
Table 5: 10GBASE-S link power budget as per IEEE Draft P802.3ae/D5.0
| Parameters | 10BASE-S | Unit | ||||
|---|---|---|---|---|---|---|
| 62.5 micron MMF | 50 micron MMF | |||||
| Modal Bandwidth at 850nm | 160 | 200 | 400 | 500 | 2000 | Mhz*km |
| Link power budget | 7.3 | 7.3 | 7.3 | 7.3 | 7.3 | dB |
| Operating distance | 26 | 33 | 66 | 82 | 300 | m |
| Channel insertion point * | 1.6 | 1.6 | 1.7 | 1.8 | 2.6 | dB |
| Power penalty ** | 4.7 | 4.8 | 5.1 | 5.0 | 4.7 | dB |
* These channel insertion loss numbers are based on a wavelength of 850 nm
** These power penalties are based on a wavelength of 840 nm
Table 6: 10GBASE-L link power budget as per IEEE Draft P802.3ae/D5.0
| Parameter | 10BASE-L | Unit |
|---|---|---|
| Link power budget | 9.4 | dB |
| Operating distance | 10 | km |
| Channel insertion point * | 6.2 | dB |
| Power penalty ** | 3.2 | dB |
* These channel insertion loss numbers are based on a wavelength of 1310 nm
** These power penalties are based on a wavelength of 1260 nm
Table 7: 10GBASE-E link power budget as per IEEE Draft P802.3ae/D5.0
| Parameter | 10BASE-E | Unit | |
|---|---|---|---|
| Link power budget | 15.0 | dB | |
| Operating distance | 30 | 40 *** | km |
| Channel insertion point * | 10.9 | 10.9 | dB |
| Power penalty ** | 3.6 | 4.1 | dB |
* These channel insertion loss numbers are based on a wavelength of 1550 nm
** These power penalties are based on a wavelength of 1565 nm and other penalties
*** Greater than 30 kilometers distance mandates an "engineerd link" requiring "field testing" for verification of conformance to the 11 dB channel insertion loss specification. Insertion loss measurements of installed fiber cables are made in accordance with ANSI/TIA/EIA-526-14A/method B and EANSI/TIA/EIA-526-7/Method A1
Table 8: 10GBASE-LX4 link power budget as per IEEE Draft P802.3ae/D5.0
| Parameter | 10BASE-LX4 | Unit | |||
|---|---|---|---|---|---|
| 62.5 micron MMF | 50 micron MMF | SMF | |||
| Modal bandwidth as measured at 1300 nm (minimum, overfilled launch) | 500 | 400 | 500 | - | Mhz*km |
| Link power budget | 7.5 | 7.5 | 7.5 | 8.2 | dB |
| Operating distance | 300 | 240 | 300 | 10000 | km |
| Channel insertion point * | 2.0 | 1.9 | 2.0 | 6.2 | dB |
| Power penalty ** | 5.0 | 5.5 | 5.5 | 1.9 | dB |
* These channel insertion loss numbers are based on a wavelength of 1300 nm for multimode and 1310 for single mode. An offset launch pad cord is assumed. The total insertion loss, when including the attenuation of the offset launch patch cord is allowed to be 0.5 dB higher than shown in the table.
** These power penalties are based on a wavelength of 1269 nm and other penalties
Table 9: 10GbE supported fiber and distances
| Fiber | 62.5 micron MMF | 50 micron MMF | SMF | |||
|---|---|---|---|---|---|---|
| Mhz*km | 160 * | 200 | 400 | 500 | 2000 * | - |
| SR/SW 850 nm | 26m | 33m | 66m | 82m | 300m | - |
| LR/LW 1310 nm | - | - | - | - | - | 10 km |
| ER/EW 1550 nm | - | - | - | - | - | 40 km |
| LX4 1310 nm | 300m @ 500Mhz * km (***) | 240m | 300m | - | 10 km | |
* Commonly referred to as "FDDI Grade Fiber"
** Sometimes referred to as "10 Gigabit Ethernet Multimode Fiber", detailed in TIA-492AAAC
*** 62.5 micron multimode fiber has a model bandwidth of 500 Mhz*km at 1300 nm as opposed to 160 or 200 Mhz*km at 850nm
When designing 10GBASE-E links greater than 30 km (i.e., the cable is not already installed) a cabling link-loss calculation, which is a simple arithmetic process, is used to make sure the combined loss of the cabling components in the link does not exceed the 11 dB channel insertion loss allocated for 10GBASE-E (Table 7). The cabling link-loss is calculated by adding the connector and splice loss to the cable loss. The cable attenuation for the link is calculated by multiplying the link distance by the loss per unit distance specified for the fiber (e.g., dB/km).
As shown in Table 10 (scenario 1) given a cable attenuation of 0.225 db/km, the cable attenuation for a 40 km link is 9 dB (40 km x 0.225 = 9 dB). Assuming 2 dB for single-mode fiber connector and splice losses the link-loss is 11 dB (9 dB + 2 dB = 11 dB); which is an allowable channel insertion loss for 10GBASE-E (Table 7) and would insure that this link can achieve 40 km. A similar calculation can be done for scenario 2 and 3.
Table 10: 10GBASE-E link-loss calculation examples
| Parameter | Scenario 1 | Scenario 2 | Scenario 3 |
|---|---|---|---|
| Channel insertion point | 11dB | 11dB | 11dB |
| Required attenuation loss | 0.225 dB/km | 0.225 dB/km | 0.3 dB/km ** |
| Connector and splice loss | 2 dB | 2 dB | 2 dB |
| Maximun distance | 40 km | 35 km | 30 km |
* The 10BASE-E channel shall have attenuation between 5 and 11 dB. If required an attenuator can be added to comply with this specification
** This is the maximum fiber attenuation allowed for standerd single mode fiber at 1550 nm as per IEC 60793-2. See table 4 for details.
Conclusion
As with previous generations of Ethernet, 10 Gigabit Ethernet requires a network designer to thoroughly understand the capabilities of his/her fiber infrastructure. With 10GbE new challenges and considerations have emerged such as the effects of chromatic and polarization mode dispersion on signal integrity. In addition, decisions may have to be made regarding whether to use single-mode or multimode fiber. This paper has introduced some basic fiber related concepts and outlined some of the key points to understand and consider when designing a 10 Gigabit Ethernet network.
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