Today's fiber optic installations are quickly increasing in number and in bandwidth. These increases are meant to alleviate the throughput bottlenecks on the backbone networks where traffic from multiple workstations aggregates. This article examines the latest developments in high-speed Ethernet transmission over fiber optic media and the new field testing issues associated with emerging standards.

Background-the need for more speed

The two-year old Institute of Electrical and Electronics Engineers (IEEE) gigabit Ethernet standard [1] for LAN backbone signaling can no longer meet the exploding demand for bandwidth. The number of LAN users is increasing so fast that the backbones are once again ready for a tenfold boost in throughput. As a result, in March the IEEE approved development of the 10 gigabit Ethernet standard for backbone communications. The new IEEE working group responsible for the 10 gigabit Ethernet specification is 802.3ae. (Figure 1 shows the hierarchy of Ethernet physical layer (PHY) standards, including the 10 gbps specification under development.)

Figure 1: Chart of Ethernet physical layer standards. Shaded boxes represent fiber optic physical layers and clear boxes represent twisted pair physical layers. The new specification for fiber optic backbone signaling is 10 Gbps Ethernet.

Ethernet data rates have traditionally been a power of 10 increasing exponentially from one generation of products to the next. The 10 Gbps Ethernet standard is the latest and the fastest generation. With the release of this new standard, the LAN backbone data rates will increase from 1 to 10 Gbps. (See figure 2.)

Figure 2: Typical LAN. Backbone connections will be migrating from 1 to 10 Gbps Ethernet. It is not clear whether work area connections will need to be upgraded above 100 Mbps. The bandwidth shortage on the backbone is mostly attributable to the growing number of stations.

The bandwidth shortage is even more acute in the Wide Area Network (WAN) environment than on the LAN backbones as more and more Web traffic flows outside the LAN and over the Internet. For this reason the IEEE is planning to develop a 10 Gbps Ethernet PHY that can operate either in the LAN or in the WAN mode.

The emerging IEEE 10 Gigabit Ethernet standard

The IEEE 802.3ae working group is in the process of narrowing down the numerous proposals for 10 Gbps signaling schemes. The most promising proposals are as follows.

- 1,310 nm Wide Wavelength Division Multiplexing (WWDM)
- Supports 300 m over installed 62.5 µm and 50 µm fiber
- Supports at least 10 km over SMF
- Requires the use of an offset patch cord just like 1000Base-LX [1]

- 850 nm Vertical Cavity Surface Emitting Laser (VCSEL)
- Supports 300 m over new 2200 MHz-km 50 µm fiber but less than 100 m over installed 62.5 µm fiber
- No SMF support

- 1310 nm distributed feedback (DFB) laser
- Cooled version supports 40 km
- Uncooled version supports 10 km
- Candidate for supporting dual data rate communications-10 Gbps for the LAN environment and OC-192 data rates for the WAN environment

At least one of the PHY schemes is expected to support both LAN and WAN data rates. This LAN/WAN PHY will be able to operate either at exactly 10 Mbps on an Ethernet LAN or at a multiple of 51.84 Mbps OC-1 (Optical Carrier 1) data rate of 9.95328 Gbps1 over a WAN. The most likely candidate for the dual data rate PHY is the 1,310 nm DFB laser proposal.

The most broadly applicable proposal is the 1,310 nm WWDM proposal. This scheme is similar to the 1000Base-LX flavor of the currently deployed gigabit Ethernet in that it supports both MMF and SMF and requires an offset patch cord for multimode fiber (MMF) operation. The WWDM PHY supports at least 300 meters over all types of MMF and 10 km over single-mode fiber. Table 1 shows the different types of fiber to be supported by 10 gigabit Ethernet.

Table 1: Fiber types to be supported by 10 Gbps Ethernet
Bandwidth
at 850 nm at 1300 nm
MMF 62.5 mm 160 MHz·km 500 MHz·km
MMF 62.5 mm 200 MHz·km 500 MHz·km
MMF 50 mm 500 MHz·km 500 MHz·km
MMF 50 mm 2200 MHz·km 500 MHz·km
SMF

While it may be too early to speculate on which PHY proposals will be selected for the 802.3ae standard, it is clear that several schemes, optimized for different types of environments, may be standardized. All of this translates into increased complexity when it comes to field-testing.

Existing and emerging fiber field testing standards

The IEEE typically references ISO and TIA cabling standards for field testing requirements. However, while these specifications cover field measurement methodology, the IEEE still specifies the loss and length limits for each application.

TIA 568B.3 [3] and ISO 11801 [2] specifications include generic loss limits based on wavelength and fiber type. (Tables 2 and 3 show the loss limits for fiber cables, connectors, and splices currently specified in draft TIA 568B.3 document being developed by the TIA TR42.8 committee.)

Table 2: TIA 568B.3 Fiber optic cable loss limits

Optical fibercable type Wavelength(nm) Maximum attenuation(dB/km)
50/125 mm 850 3.5
1,300 1.5
62.5/125 mm 850 3.5
1,300 1.5
single-mode 1,310 1.0
inside plant cable 1,550 1.0
single-mode 1,310 0.5
outside plant cable 1,550 0.5

Table 3: TIA-568B.3 Connector and splice loss limits

Attenuation (dB)
Splice 0.3
Connection, TIA 0.75
Connection, ISO 0.5

A field tester can evaluate the measured fiber losses against the generic limits shown in tables 2 and 3 provided the test technician specifies the length of fiber and the number of connectors or splices. (See figure 3.)

Figure 3: A screen shot of the WireScope 350 field tester, which allows the user to enter the number of connectors and splices and to specify the loss for each.
However, testing to these generic limits does not guarantee that the applications would work. It is important to select a field tester that can automatically produce pass/fail limits for different networks.

Already, we have seven different sets of length and loss limits specified by IEEE for the existing variants of gigabit Ethernet. (See table 4.)

Table 4: Maximum length and attenuation specifications for different versions of gigabit Ethernet over various types of fiber optic media

Gigabit Ethernet Specification Type of Fiber Wave-length (nm) Fiber Core Size (microns) Modal Bandwidth (MHz · km) Maximum Distance (m) Attenuation(dB)
1000Base-SX MMF 850 50 400 500 3.37
500 550 3.56
62.5 160 220 2.38
200 275 2.60
1000Base-LX MMF 1,310 50 400,500 550 2.35
62.5 500 550 2.35
SMF 1,310 10 5,000 4.57

The new 802.3ae 10 Gbps Ethernet standard will likely require at least as many different sets of limits which will considerably add to field testing complexity. With so many different limits, it becomes virtually impossible to certify fiber optic installation with old-fashioned loss meters and still guarantee that all the backbone technologies will work over a given installation.

The loss and length limits for different networks are a function of cable type and transceiver operating wavelength. Because of the vast number of different applications and, in many cases, several different sets of limits for each application, the field tester should automatically keep track of the application test limits. (See figure 4.) The test report should document the pass/fail result for each network and the pass/fail result with respect to generic TIA or ISO limits. (See figure 5.)

Figure 4: An example of fiber optic network test limits programmed into a field tester.

Figure 5: A sample test report displaying application-specific pass/fail results for each fiber optic network in addition to the generic TIA and ISO pass/fail results.
The new generation field testers typically test two fibers simultaneously (See figure 6.) and automatically record the test results at both wavelengths on both fibers. At each end, a transmitter is connected to one fiber and a receiver to another.

Figure 6: Screen shot from the WireScope 350 field tester showing the test configuration for two-way testing.

The testing can be performed from both ends at once or in a loopback mode by connecting the two fibers with a jumper at the far end. (See figure 7.)

Figure 7: Screen shot from the WireScope 350 field tester showing the test configuration double-ended testing or single-ended testing.

Summary

The emerging IEEE 802.3ae 10 Gbps Ethernet will complicate field testing considerably. This specification is expected to support several different transceivers operating over five different types of fiber. There have already been seven different sets of test limits for IEEE 802.3z gigabit Ethernet; the new 10 Gbps Ethernet is expected to at least double this number.

Increasingly complex field testing makes it more important than ever to use a tester that automates the pass/fail determination for different networks.
MLINARSKY is a research and development (R&D) manager at Agilent Technologies, Marlborough, Mass. An active participant in developing networking and cabling standards at IEEE, ANSI, ISO/IEC, and TIA, she can be reached at fanny_mlinarsky@agilent.com.