G.hn and CATV Coexistence – MDU Gigabit Speeds Without Compromise

Unlike Data Over Cable Service Interface Specifications (DOCSIS), G.hn uses baseband spectrum from 5-200 MHz and dynamically allocates each tone to carry either upstream or downstream traffic to reflect the demand from the subscribers, achieving gigabit speeds in both directions over coaxial cable networks.

The base definition of DOCSIS 3.1 allocates the 5-42 MHz range to upstream traffic, resulting in “up to” 100 Mbps of shared bandwidth across all subscribers on a Cable Modem Termination System (CMTS) port. DOCSIS vendors have recently introduced solutions to provide more upstream bandwidth by extending the spectrum for the upstream bands by reducing the spectrum used for the downstream direction. Two split levels are defined as per Figure 1.

Figure 1 shows the Mid-Split approach extends the upstream spectrum to 5-85 MHz, resulting in about 500 Mbps. The High-Split approach extends the spectrum to 5-204 MHz for about 1.5 Gbps of throughput. A DOCSIS network is engineered to serve an average of 150 subscribers on the premises in which the peak bandwidth levels per subscriber is well below 5 Mbps downstream and below 350 Kbps (not Mbps) upstream. With the increase in bandwidth consumption for teleworking, remote learning, video streaming, online gaming and related services, this no longer reflects the real-time traffic demand patterns we experience today.

When it comes to the question of bandwidth availability, unlike DOCSIS High-Split, G.hn avoids spectrum overlap with Community Antenna Television (CATV) channels.

With DOCSIS High-Split, the Community Antenna Television (CATV) channels are allocated from 258 MHz (channel 30), and the total downstream spectrum available and bandwidth achievable is a function of the number of channels offered to subscribers. The more channels, the less spectrum remains for the downstream traffic.

The very important difference and benefit of G.hn is that it does not use any spectrum that overlaps with the CATV content. Therefore, the total available bandwidth of G.hn is not impacted by the number of channels offered to the subscribers. For instance, 20 CATV channels, each using 6 MHz of spectrum, reduce the downstream bandwidth of DOCSIS by about 1 Gbps. Like the DOCSIS High-Split, G.hn allows CATV channels starting at 258 MHz.

When deploying a GAM in environments such as an MDU or a hotel, it is very easy to overlay the G.hn and CATV signals over the same coaxial cabling using standard splitters-combiners as shown in Figure 3.

Looking at Figure 3, we see that the CATV signal needs to be overlaid with G.hn for each port of a GAM device. This combined G.hn + CATV signal can then feed up to 16 subscribers per GAM port using standard splitter devices that are typically already in place. A single coaxial drop per apartment or guest room is then used to bring the G.hn signal to a G.hn endpoint (Ethernet bridge) and to a set-top-box or TV.

Although the specific channel(s) may vary depending on the Pay-per-View (PPV) solution in use, PPV services rely on return channels located below the 50 MHz range. To make room for such signals, a G.hn signal can be “notched” to begin at a higher frequency than the default 5 MHz defined by the G.hn standard (ITU-T G.9660), via a simple configuration setting in the GAM. While this reduces the available spectrum, it does not prevent G.hn from delivering peak bandwidth of 1 Gbps in any direction.

For example, a GAM can configure the G.hn signal to begin at 60 MHz. Looking at Figure 4, we clearly see that the Power Spectral Density (PSD) for the G.hn signal is from 60 MHz to 200 MHz on the G.hn port using the coaxial cable.

G.hn permits a seamless migration to Internet Protocol television (IPTV) services, allowing subscribers to switch at their own pace, through a 2-step approach.

Step 1. Inject the IPTV feed via the uplink port of a GAM. The GAM provides full support for Internet Group Management Protocol (IGMP) Multicasting, allowing multiple subscribers to share the same IPTV feed, which is very convenient for serial TV. At the same time, the GAM supports unicast IPTV which allows for more advanced user experiences (pause, rewind) without additional complexity.

Step 2. Whenever a subscriber is ready to migrate to IPTV, use the second Gigabit Ethernet port of the G.hn endpoint and connect it to the set-top-box or smart TV. You can then disconnect the coax cable from that previously used for the CATV signal.

Once there are no remaining CATV subscribers on a coaxial segment, remove the spectrum notch (5-60 MHz in the example earlier) to reclaim the full G.hn spectrum to deliver the full 1.7 Gbps of aggregate bandwidth of G.hn.

For building owners or property managers who wish to provide gigabit coverage to multiple users in apartment buildings, condominiums, hotels and other MDUs without a costly fiber retrofit, G.hn offers an elegant solution. Through the use of existing infrastructure, G.hn achieves superior upstream and downstream bandwidth than DOCSIS, and it permits the easy integration of other advanced services, such as pay-per-view and IPTV.

See the Broadband Forum TR-419 document for more information about the concept of Fiber to the Extension point.

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