Reducing homes per node

As detailed in Section 4.2 earlier, channels can carry either broadcast or narrowcast transmissions. Each transmission stream is multiplexed (i.e. compressed and combined with other streams) and modulated onto a single RF carrier, using up one available channel. This combination of broadcast and narrowcast channels occurs at the optical node, with a single uniform signal transmitted over coaxial cable to the final households served by that optical node.

Given the limited number of channels available with this uniform signal, the bandwidth per household depends on the number of available narrowcast channels (having removed channels taken up by broadcast transmissions) divided by the number of households that need to be addressed at any point in time. Reducing the number of households served by a single combined spectrum catchment area (by reducing the size of the optical node‟s catchment area) will increase the capacity available for each household, as shown in Figure 7.6. However, given the use of statistical multiplexing of the available capacity (i.e. optimising for the fact that not all households use capacity at the same time), the relationship between capacity and households is non-linear.

Figure 7.6: Illustration of increased capacity due to reduced RF spectrum catchment areas [Source: Analysys Mason, 2014]

In general, narrowcast (i.e. DOCSIS and VoD) services are added to broadcast components at the local head end, with a unique combined stream formed for each optical node. The data capacity of the available DOCSIS channels is therefore shared among all the subscribers below this point. Under a typical HFC network architecture, as shown in Figure 7.7, the optical node generally serves between 250 and 1500 households.

a+x+y

a+x a+y a+z

a+z Coa x ia l Fi bre Coa x ia l a+x+y+z

a+x a+y a+z

Coa

x

ia

l

RF carriers (on coaxial) need to carry up to 4 streams at any point

If the node is split but the whole path remains on coaxial, then the

maximum number of streams to be carried drops to 3 …

… but if fibre extended deeper into network, then maximum number of streams per coaxial

cable drops to 2 Service:

Figure 7.7: HFC architecture prior to node splitting [Source: Analysys Mason, 2014]

The best way to reduce the number of houses per optical node is to split the node. This is achieved by replacing a single optical node with two nodes, each of which serves a subset of the original node‟s houses, as shown in Figure 7.8. Each of these two new paths has the same equivalent number of available DOCSIS channels as the original path, but this bandwidth serves far fewer households.

Generally, when further splitting a node, the new optical nodes are moved deeper into the network (such as to an RF splitter location). This is a natural point to create the two separate coaxial cable paths, though this may incur some additional cost to replace a passive component with an active optical node (i.e. the addition of a street cabinet and power source).

Figure 7.8: HFC architecture following node splitting [Source: Analysys Mason, 2014]

Core fibre rings Local fibre RF amplifier Coaxial splitter Optical node Coaxial Fibre Fibre Coaxial cable Key: Core fibre rings Local fibre Optical amplifier Optical splitter

Optical node split into two, often at the point of a previous coaxial cable splitter

Coaxial Fibre Fibre Coaxial cable Key:

A summary of the benefits and barriers to node splitting is provided in Figure 7.9. Figure 7.9: Features associated with node splitting upgrade [Source: Analysys Mason, 2014]

Description

Benefits  Doubling the number of nodes roughly doubles the sustainable bandwidth per end user for both downlink and uplink

 The upgrade is independent of DOCSIS versions, so can be implemented on both DOCSIS 3.0 and DOCSIS 3.1 networks

 Does not require any additional CPE or changes to consumer households (reducing truck-roll costs)

Barriers to introduction  Requires new optical nodes, and may require both cable replacement and new power sourcing if optical nodes are at splitter sites

 May require a CMTS capacity upgrade (and potentially some head- end capacity) to support the increased number of optical nodes Characteristics of likely

operators to deploy upgrade

 Operators with a large number of households per node, or a large number of subscribers per node

 Operators which are restricted from applying technology or spectrum upgrades for some reason

The process of node splitting can continue further down into the access network, addressing smaller and smaller groups of households. Generally these split nodes occur at points in the network where previous active electronic components were present, due to the pre-existence of a power source and street furniture (cabinet, manhole chamber, etc.) at these locations. The logical end point for node splitting occurs when the fibre path is extended to the last active piece of equipment within the coaxial chain, i.e. the last repeater prior to any group of households. This is referred to as „deep fibre‟, or node plus zero (N+0), as shown in Figure 7.10, with optical nodes serving approximately 50–150 households.

Figure 7.10: HFC architecture following N+0 deep fibre deployment [Source: Analysys Mason, 2014]

Core fibre rings Local fibre No active RF equipment left in chain

Coaxial Fibre Fibre Coaxial cable Key:

The roll-out of fibre deeper into the network leads to a reduction in complexity of the HFC network, often producing power and maintenance cost savings. This is because greater signal attenuation occurs in coaxial cable than in fibre, requiring a larger number of amplifiers over a set distance. Furthermore, each amplification of the RF signal adds new noise to the signal, and more significantly, amplifies any existing noise which has accumulated over its prior journey. Therefore, a deeper fibre network can lead to better SNR properties, potentially allowing for increased QAM rates (especially on the upstream) and reducing consumer connectivity issues related to noise. A summary of the benefits of, and barriers to, N+0 fibre deployment is shown in Figure 7.11. Figure 7.11: Features associated with deep fibre (N+0) deployment upgrade [Source: Analysys Mason, 2014]

Description

Benefits  Significantly reduces the number of households per node, allowing additional benefits from technologies such as switched digital video (SDV)

 Reduces SNR, enabling the use of higher modulation schemes under DOCSIS 3.1, and increased reliability of connections

 Reduces opex and maintenance costs by removing all RF amplifiers

 Simplifies the process of moving the frequency split as it reduces the number of amplifiers that need to be modified

Barriers to introduction  Requires new optical nodes and significant replacement of coaxial cable with fibre, depending on how close to the households fibre is deployed

 Requires a CMTS upgrade (including potentially buying new D-CMTS or EQAM) to support the increased capacity

Characteristics of likely operators to deploy upgrade

 Operators intending to undertake a significant overhaul of their existing network

 Operators which are restricted from applying technology or spectrum upgrades for some reason

 Operators with a large amount of ducting, or overhead cabling, for easy cable replacement