Downstream Channel Bonding is perhaps the ball bearings of DOCSIS 3.0, enabling subscriber data speeds in excess of 160 Mbps (4 times that of previous DOCSIS versions). While conceptually simple, the principle of combining multiple downstream DOCSIS channels together to carry the same user data must have tight constraints in order to preserve the integrity of the data and have the data arrive at the correct subscriber's device and in sequence. This article will cover both the physical layer aspects and DOCSIS protocol aspects that enable channel bonding.
Channel bonding simply means that the CMTS knows that there are four or more RF signals within a 60 MHz passband (greater if more than four channels are bound). The 60 MHz window is defined in section 6.3 of the DOCSIS 3.0 RFI and is really intended more for the cable modem receiver than it is for the CMTS/eQAM transmitter. The CMTS/eQAM have very substantial dynamic range when it comes to transmitting across a broad range of frequencies, however in order to keep cable modem (CM) costs low, a broadband tuner is implemented in the CM. It was determined that a 60 MHz bandwidth would be reasonable for cost effectiveness based upon existing hardware at the time of the specification. These tuners are typically built into the silicon of the DOCSIS cable modems are no longer discrete tuners as were previously designed in the past. If more than four downstream channels are tranmistted as part of the Downstream Bonding Group (DBG), then more than 60 MHz is permitted, but the cable modem must be able to tune to at least four channels in a 60 MHz bandwidth, support of additional channels outside of the 60 MHz bandwidth is optional and now becoming more of a necessity for cable operators who are commonly using eight channels in their DBGs - Note, remember the term Downstream Bonding Group or DBG as it is common lingo for DOCSIS 3.0.
Another change in DOCSIS 3.0 from DOCSIS 1.x and 2.0 is the downstream operational frequency range. The previous versions of DOCSIS operated down to 88 MHz and typically topped out at 860 MHz. DOCSIS 3.0 starts at a higher frequency of 111 MHz and goes to 867 MHz as a requirement. Additionally, the specification has a recommendation that 999 MHZ should be the high end frequency of DOCSIS 3.0, fully utilizing a 1 GHz plant. In a competitive market, these recommendations are usually taken as "we had better do it to stay in business", which has been the case. The reason that DOCSIS 3.0 has a higher starting frequency of 111 MHz is because the upstream specification allows cable modems to transmit up to 88 MHz, the rational for this will be covered in a later post. Otherwise, other physical characteristics of DOCSIS 3.0 are similar to DOCSIS 1.x and 2.0. Any channel in the DBG can operate in either 64-QAM or 256-QAM mode. BER must be equal to or better than 1x10-8 and codeword error rate (CER) must be less than or equal to 9x10-7. Oh, maybe you have not heard about the new spec. for code error rate before? Well this is new to the DOCSIS 3.0 specification and is something that is quite over due since BER is nearly impossible to measure in a live DOCSIS plant while CER can be obtained right from the CMTS. CER can be computed as follows:
Keep an eye on CER to be a new metric for equipment manufacturers and test vendors to be using moving forward in addition to BER and MER.
In a DOCSIS 3.0 network implementing downstream channel bonding, the DOCSIS CMTS dynamically balances the data across the Downstream Bonding Group (DBG), which can consist of four or more downstream channels. The reason this is done is to offer subscribers the best quality of service across downstream channels with changing impairments and changing congestion at the receive side. Each outgoing packet from the CMTS is tagged with a sequence number. The sequence number becomes important for a number of reasons. Packets can be dispersed across different downstream channels and can have different time delays in arriving at the receiving cable modem. It is then the cable modem's responsibility to re-synchronize the incoming packets based upon the sequence numbers. TCP/IP windowing acknowledgments will take care of any lost packets at Layer 3, however for UDP flows, such as voice and video, those packets will be forever lost. Further, by dynamically distributing the packets across downstreams, the CMTS can take advantage of statistical gains of many cable modems connected to the DBG. This becomes especially critical when your system has a mixture of legacy DOCSIS 1.x, 2.0 and 3.0 cable modems. The 1.x and 2.0 modems will all be receiving data from only the Primary Channel of the DBG which the DOCSIS 3.0 modems will be able to receive data from all four+ downstream channels in the DBG. So this dynamic prioritization is in effect acting like upstream load balancing in the downstream.
If all downstream channels of the DBG are configured as Primary Downstreams, then DOCSIS 3.0 has another capability to load balance all legacy cable modems across the DBG. This is called Downstream Channel Set (DCS) and is truly analogous to upstream load balancing. It is highly recommended that when you are first turning on a DOCSIS 3.0 network with few DOCSIS 3.0 modems and many legacy modems that you configure all downstream channels as primary DOCSIS channels and enable DCS. There is a secondary benefit to configuring all downstreams as primaries and that is you will be able to fully test each DOCSIS primary channel using your legacy DOCSIS 2.0 hand-held test meters. What is the drawback then? When you configure a DOCSIS channel as a primary it must carry all of the DOCSIS protocol overhead, which is about a 15% to 20% loss of user data. So when you have the ability to later configure a DOCSIS downstream channel to a secondary channel, the secondary channel no longer carriers the excessive DOCSIS overhead, except for some minimal synchronization information, and so you can now transport significantly more data to your end subscribers.
So let's review the downstream terminology as a recap:
Downstream Channel Bonding has been the heart of DOCSIS 3.0 and is what has made it a huge success. There have been a number of deployment issues along the way, but that is for another post. From the physical layer standpoint, DOCSIS 3.0 is very similar to DOCSIS 1.x and 2.0. You will either be dealing with 64-QAM, 256-QAM or a mix of both in your downstream bonding groups, which must be in a 60 MHz bandwidth. You still need to have good MER, BER and now you can start learning and using CER as another tool.
Just as you may have been getting acquainted with upstream load balancing, this is a new a valuable feature in DOCSIS 3.0. Not only are cable modems load balanced in 3.0, but the very data packets themselves. At first this was done from a statistical standpoint to take advantage of which modems may be pulling more traffic than others. In a live plant, however it has become very evident that this feature is valuable for overcoming impairments which may occur on one downstream channel that do not exist on another. Consider it a self-healing mechanism that can come into play that gives you an opportunity to resolve a problem on that particular frequency. It does not fix the plant, because your codeword errors and thus CER will go up considerably on that channel, your overall data rate will drop for the DBG, but data will still go through on the other channels at a maximum rate.