RfoG taps into cable
RF over glass taps into cable FTTH opportunities
A growing number of cable and multiple system operators (MSOs) are looking to strengthen their broadband fibre access capabilities with investments in radio frequency over glass (RFoG) systems, writes John Williamson.
there is the added bonus of cable operators being able to re-use existing head-end, back office and customer premises equipment when they implement RFoG. Also relevant here, from the CapEx perspective, is the potential that RFoG offers to be rolled out in an incremental fashion. OBI: downbeat However, notwithstanding the considerable advantages of RFoG technology, and the adoption of the standards five years ago, overall market uptake of RFoG has so far been distinctly modest. Why could this be? According to Cornel Ciocirlan, EMEA Chief Technology Officer at ARRIS, one explanation is that early operator RFoG experiences have been mixed, with some reporting that they
proposals were approved by the American National Standard Institute (ANSI) the following year. RFoG: the attractions There are several reasons why RFoG is an attractive technology for cable companies and MSOs. In general, service providers of all stripes are under intensifying competitive market and marketing pressures to provide fibre connections to their end users. In the RFoG context, the use of RFoG optical networking units (R-ONUs) as micronodes enables multiple subscribers to take optical signals from one R-ONU, and moves the optical/electrical conversion process much closer to the customer premises than is the case with conventional HFC networks. Furthermore,
were experiencing unhelpful packet error rates. The reason for the latter, says Ciocirlan, is the phenomenon known as Optical Beat Interference (OBI), a chronic signal degradation issue caused by multiple optical transmitters communicating with closely- spaced wavelengths at the same time over the same fibre. Ciocirlan likens the OBI phenomenon to the “tremolo effect” encountered in certain audio systems. Unfortunately, the potential for OBI degradation is increased in the micronode scenario, outlined above. With multiple users being serviced off each R-ONU, the probability of multiple R-ONU lasers being active simultaneously is higher. The introduction of the Data Over Cable Service Interface Specification 3.1 (DOCSIS 3.1) also raises the likelihood of OBI effects occurring by virtue of the increased upstream throughput it supports. With up to 1 Gbits/s upstream throughput available, many cable modems will transmit simultaneously, thus increasing the probability of R-ONU laser wavelengths colliding. The DOCSIS 3.1 specification, introduced by the CableLabs cable telecommunications research and development organisation in October 2013, is designed to enable a new generation of cable services and help operators continue to meet consumer demand for high speed connections and sophisticated applications (see panel story DOCSIS 3.1 speeding things up, right).
John Williamson
I mplementing a Radio Frequency over Glass access networks involves replacing the coax in hybrid fibre coax (HFC) systems with passive optical network (PON) technology. The Society of Cable Telecommunications Engineers proposed a set of RFoG standards in late 2010. These included the use of AM or FM wavelengths in the upstream direction, a downstream wavelength of 1550 nm, and an upstream wavelength of either 1310 nm or 1610 nm. The SCTE
1550 nm
DOCSIS Set-Top DOCSIS CM/EMTA
• CMTS allows upstream bursts on different frequencies to overlap • Such overlap creates no issues in traditional HFC
1610 nm RFoG CPE
85 MHz - 1 GHz
1550 nm
DOCSIS Set-Top DOCSIS CM/EMTA
Burst
Burst
1610 nm RFoG CPE
WDM Mux
//
Splitter
Burst
Burst
5-65MHz
• With RFoG, upstream bursts from different homes are nominally on the same optical wavelength • In reality each laser is slightly different due to manufacturing tolerances. Different wavelengths could have minimal interaction • However bursts on identical wavelengths that are coincident in time (overlap) create strong interaction in the receiver • This is called Optical Beat Interference (OBI)
1550 nm
DOCSIS Set-Top DOCSIS CM/EMTA
1610 nm RFoG CPE
Background of optical beat interference (OBI)... “Upstream optical collisions”. Copyright ARRIS International.
ISSUE 6 | Q1 2016 16
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