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Packet Optical Network Overview


Packet optical devices, such as Juniper Networks PTX Series devices properly equipped with suitable line cards, are capable of placing packets directly onto optical transports and receiving packets the same way. In a packet optical network, the packets leave the router in an optical transport envelope at the correct wavelength and arrive the same way, bypassing much of the other external networking equipment needed to groom or otherwise process electrical or optical signals originating on the router.

The advantages to having a direct optical network interface on the device are many. Among them are having access to optical span performance parameters such as signal strength and noise, gathering trend information about corrected and uncorrected bit errors, and (in the case of copper tail-ends) eliminating the low-performance copper links at the ends of the span. Packet optical networks can record and analyze optical performance and even provide protection switching if the bit error rate (BER) exceeds a configured threshold.

However, the advantages of fiber optical networks extend to more than just error rates.

The full list of advantages that fiber optical networks hold over copper networks (or even networks with limited amounts of copper) is considerable:

  • Lower BERs—Fiber offers lower bit error rates (BERs) than any other transmission media. If the BER of a fiber link is 1000 times better than on copper (coax, twisted pair), which is not unusual, this is about the same ratio of 1 day to 3 years of 365 days each. So all the errors seen yesterday take about 3 years to occur (1000 to 1 ratio).

  • Higher bandwidths—Fiber offers unsurpassed bit rates. And, unlike copper media, fiber speeds are increasing constantly, in some cases by orders of magnitude. If the ratio of pre-fiber to fiber bandwidth is also 1000:1 (not unusual), that means you can send over this link tomorrow all the bits you have sent in the past 3 years.

  • Longer distances without repeaters—Fiber links require fewer repeaters than the same length of cable composed of other media. Digital repeaters, which detect and repeat the string of 0s and 1s, require power and add potential points of failure.

  • Immunity from interference—Noise interference is an issue in copper because such cables are long antennas. Fiber does not pick up electromagnetic noise from the environment.

  • Security—The act of tapping into a fiber optical cable adds some signal loss. Fiber networks can be monitored for sudden signal loss increases, which could indicate damage or tapping..

  • Fewer maintenance costs—Solid state fiber optic components need less maintenance than other devices used in older copper networks.

  • Small size and weight—Some copper cables weigh several pounds per foot. The same crew can install more fiber length in 1 day than any other cable media.

  • Bandwidth upgrades—It is possible to increase the bandwidth available on the fiber link by upgrading the sender and receiver components.

Initially, networks deployed single links of fiber between devices, usually surrounded by masses of legacy copper links. But, as can be seen from the previous list, the advantages of fiber optics can best be realized when all of the links between devices are fiber optical links. After all, if the end-to-end path includes one high bit-error link, then the overall bit error cannot be better than this “weakest” link. The same is true in some sense for bandwidth: throughput is limited by one slow link in the whole series.

Today, many packet optical networks conform to the optical transport network (OTN) standard. The international standard for OTNs is G.709. This standard includes wavelength values for wavelength-division multiplexing (WDM), a method that allows a single physical fiber to carry multiple bit streams, all using different wavelengths.

However, because single wavelength links can still be used between devices, some of the parameters for fiber optical links, such as G.709 wavelengths, are configured on Juniper Networks devices under OTN options and others are not.

It should be noted that copper multiplexing channels are traditionally distinguished by frequency, and WDM channels by the length of the waves, mainly because light was historically considered very different than other electromagnetic waves and have been traditionally distinguished by wavelength. This can be confusing, but wavelength is the reciprocal of frequency, so they are really closely related.

Configuration of Juniper Networks packet optical devices establishes the most important optical parameters for fiber optic links and OTNs. These include:

  • signal-degrade—This parameter establishes thresholds and intervals for monitoring link performance.

    • interval—This parameter establishes the interval at which the link is checked for conformance to performance parameters.

    • ber-threshold-signal-degrade—This parameter establishes the BER threshold for the link to be considered degraded.

    • ber-threshold-clear—This parameter establishes the BER threshold for the link to be considered recovered from a degraded condition.

  • preemptive-fast-reroute—This parameter establishes that when the link crosses the degraded performance threshold, the link is declared down (failed), and the system takes steps to build new paths for traffic.

    • signal-degrade-monitor-enable—This parameter establishes that link degradation is monitored for fast reroute opportunities.

    • backward-frr-enable—This parameter establishes that link degradation at the receiver is sent “backward” to the sender so that fast reroute can be invoked.