Examining DOCSIS 3.0 and 3.1

Examining DOCSIS 3.0

The increase in content and number of applications on the Internet increased the demand for higher data throughputs. Subsequently, CableLabs introduced channel bonding in DOCSIS 3.0 to meet these demands and to remain competitive with fiber-to-the-premises (FTTP) network architectures. Channel bonding effectively multiplies the data throughput of a single channel by the number of bonded channels. Figure 1 illustrates the basic concept of channel bonding with regard to four QAM channels in to achieve a maximum downstream data throughput of 152 Mbps. Using whole numbers (to simplify the explanation), each QAM channel has a maximum downstream data throughput of 38 Mbps, but the cable modem receives the cumulative data for a total of 152 Mbps (4 × 38 = 152).

Figure 1: Channel bonding in the downstream.

Originally, DOCSIS 3.0 specified that up to eight QAM carriers could be bonded, for a maximum data throughput of more than 300 Mbps. Cable modem manufacturers have since developed modems that can receive and process up to 32 (32 × 38 = 1,216 Mbps) downstream channels. In addition, the bonded channels can be spread across the spectrum instead of having to be adjacent to one another.

Channel bonding can also be used for upstream data transport, yielding data throughput up to 120 Mbps (30 × 4 = 120) when four upstream channels are bonded together, as originally specified. Cable modems that can transmit up to eight bonded channels, for a maximum upstream throughput of 240 Mbps (30 × 8 = 240), have also been developed.

When the Internet was in its early stages, the scheme used to assign an Internet protocol (IP) address to all connected devices was Internet protocol version 4 (IPv4). The 32-bit numbering scheme of IPv4 allows 4,294,967,296 (approximately 4.2 × 109) addresses, which was ample when users of the Internet were mostly researchers and scientists at learning institutions. 

After the Internet became available to the general public, it became apparent that the number of IPv4 addresses would become insufficient to support the increasing numbers of Internet connected devices. The Internet Engineering Task Force (IETF) began development of a scheme to replace IPv4 in the mid-1990s. Because Internet protocol version 5 (IPv5) had already been used for another project, the replacement IP address system was called Internet protocol version 6 (IPv6). 

For comparison, Figure 2 shows an example of an IPv4 address and an IPv6 address. The 128-bit numbering scheme of IPv6 yields approximately 3.4 × 1038 addresses, which is substantially more than the number of people on the planet (estimated to be a little over 7,125,000,000 or 7.125 × 109).

IPv4 was globally deployed in hardware and well established with websites and Internet service providers (ISP) when IPv6 became available in 1999. Despite warnings that the number of IPv4 addresses would be soon exhausted, there was government and industry reluctance to transition over to IPv6. Following the recommendations of the IETF, DOCSIS 3.0 was designed to support IPv4 and IPv6, as well as an orderly transition from IPv4 to IPv6. As a result, the broadband cable industry has been a leader in the transition from IPv4 to IPv6, first by replacing earlier versions of DOCSIS modems with DOCSIS 3.0 modems. However, because there is huge number of customer-owned IPv4 devices, it will be years before the entire transition to IPv6 is complete.

Figure 2: Comparison of IPv4 address to IPv6 address.

Introduction to Examining the Different Versions of DOCSIS

DOCSIS 3.0 has proven to be a powerful and versatile product, capable of delivering data at rates that are comparable to competing FTTP architectures. However, DOCSIS 3.0 is at risk of becoming outdated as more FTTP networks are built by the competition and as new breakthroughs emerge in optical technology. DOCSIS 3.1 was developed and brought to market so that the broadband cable industry would have an answer for impending competition from FTTP networks. To increase downstream and upstream data throughput, DOCSIS 3.1 utilizes orthogonal frequency division multiplexing (OFDM) and low-density parity-check (LDPC).

OFDM is a scheme in which several independent narrowband carriers (subcarriers) are modulated at a low data rate. To avoid interference, the peak power of each subcarrier aligns with the minimum power of the adjacent subcarrier, or orthogonal. Figure 1 illustrates this concept—the peak of SC-1 coincides with the point where the lowest sideband of SC-2 is at zero. When SC-2 reaches its peak, the sidebands of SC-1 and SC-3 are at zero and the relationship continues for the remaining subcarriers.

Any noise or interference from the network can impair only a small number of subcarriers, leaving the majority intact and error-free. Any lost data can be recovered using error correction. This resilience to noise and interference, along with spectrum efficiency, is why OFDM is used in many wireless applications. Some of these applications include cellular, Wi-Fi®, and Europe’s digital video broadcast (DVB) standard. A single DOCSIS 3.1 OFDM channel has a bandwidth of 24 to 192 MHz, departing from the concept of using 6 MHz wide television (TV) channels to transport DOCSIS data (Figure 2).

Figure 2: OFDM subcarriers.

FEC is a process that adds coded data during modulation to enable the receiver to identify and then correct errors that occur during signal transport (Figure 3). The versions preceding DOCSIS 3.1 used Reed-Solomon (RS) FEC. Advancements in computer computational power have enabled DOCSIS 3.1 to utilize LDPC, a more robust and efficient FEC coding scheme. Under equal network conditions, the high computational complexity of LDPC enables orders of modulation that are twice that of RS coding. If LDPC were available with DOCSIS 3.0, the order of modulation could be increased from 256 QAM (38.8 Mbps) to 1024 QAM (48.5 Mbps) without any performance upgrades to the network, yielding a 25% increase in data throughput.

Figure 3: Coding error correction data during modulation.

An under-utilized function in DOCSIS 3.0 was Proactive Network Maintenance (PNM), which enabled cable modems to be used as network probes that technicians can use to collect device and network parameters from the customer premises. DOCSIS 3.1 enhanced the PNM management information base (MIB) data to obtain more useful information from the modems. For example, DOCSIS 3.1 modems can be accessed to create a timeline of how the modem’s internal upstream equalization coefficients adjust to network conditions. The modems reporting a mutual impairment can be grouped together so that a technician who knows the network can quickly identify a common point to begin troubleshooting.