Doing the (High) Splits – Signal Leakage in High-Split Cable Networks
Cable operators globally recognize that tight networks simply work better, and signal leakage is the best way to detect, find, and fix the shielding weaknesses which let undesirable upstream ingress into networks. Leakage monitoring has adapted with the evolving HFC to remain feasible and maximize effectiveness, but the migration to a 204MHz upstream with high-split networks presents the most daunting challenge that leakage tools have faced to date.
High-Split Leakage Challenge
Plant leakage systems really took off in popularity after, as legend has it, leakage from a cable system interfered with Air Force One navigation – driving the FCC to crack down on networks leaking signal into the aeronautical band. 138MHz has been the default monitoring frequency near the aero band,
This is where high splits throw a wrench into existing tools/techniques. By moving the upstream split up to 204Mhz, the typical 138MHz aero band monitoring frequency shifts from the downstream to the upstream. No longer can signal taggers transmit tags to be combined into the downstream for detection by field meters – tools and methods must adapt.
The Solution – OUDP Test Burst Detection
Leakage detection has typically consisted of detecting signal tags combined into HFC downstreams or OFDM pilots escaping from shielding weaknesses in cable plants. With the move of the aeronautical band to the upstream, the only practical solution to enable continued monitoring of this band is to use signals transmitted in the upstream. Using cable modems to transmit traditional signal tags is theoretically possible, but this approach would result in loss of upstream spectrum available for revenue-generating traffic, would degrade CPE bandwidth capacity, and would require extensive R&D and specification changes to operationalize.
The clever alternative that the industry is gravitating toward is the use of existing upstream test signals called OFDMA Upstream Data Pilot (OUDP) test bursts as signal tags. These bursts are generally used to determine the maximum Interval Usage Code (IUC) that modems can support, but the OUDP bursts with the highest pilot density have been proven to be detectable by advanced leakage meters. OUDP usage is already specified in the DOCSIS 3.1 specification, so all deployed DOCSIS 3.1 hardware should support these bursts – no spec changes and extensive R&D required!
How To Prepare for and Launch High-Split Leakage
From a Tech Ops perspective, the goal of any technology migration plan is to make it as transparent as possible to the folks and reuse existing tools and processes whenever possible. Swapping out test gear every time network technology changes is expensive and a logistical nightmare. This is why software-defined radio-based leakage detectors with full-frequency agility are taking off in popularity. Operators can buy these detectors today and clean up their nodes targeted for future high split upgrades, focusing on FM ingress which is much easier to find/fix as a downstream issue vs in the high-split upstream. When the time comes to migrate nodes to high split, these meters can then have a license key and new frequency plan deployed to them via a mobile app and begin OUDP monitoring also without the meters ever leaving technicians trucks.
High split architectures are coming whether we are ready or not, the 500% increase in usable spectrum vs 42MHz plants is just too powerful of a value proposition to pass up. While switching from downstream tag of OFDM pilot detection to upstream OUDP test burst detection sounds challenging, the change can be seamless with the right preparation and equipment selection. Whether you want to monitor leakage in high-split upstreams to maintain FCC compliance or just to prevent ingress issues, you can rest assured that solutions will be ready when you are to make this transition.