Partial Service is a new term encountered in the DOCSIS 3.0 MULPI specification and realized in field deployments of DOCSIS 3.0 cable modems using upstream bonding. This was a topic that I touched on in this years SCTE Cable-Tec Expo, but will explore in greater detail in this article. Partial service can be considered a feature because the cable modem will stay online even when one or more upstream transmit channels goes offline.
The focus of this article will be on the mechanics of upstream channel bonding and how it works more from a DOCSIS protocol perspective. Much more detailed information can be found in the DOCSIS 3.0 MULPIv3.0 document located in the Library, but this will provide a high level overview for the layman who is curious about the basics. First lets understand that it is the cable modem that is doing the channel bonding, remember in the upstream the cable modem transmits data to the CMTS. Per DOCSIS 3.0, the CM can bond from one to four channels in the upstream as coordinated by the CMTS. The CM is always under control by the CMTS.
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.
This article will focus more specifically on DOCSIS 3.0 issues that will occur as you are deploying DOCSIS 3.0 or post -deployment.
The first part of this article (in CT's March 2009 issue) discussed downstream potential issues, while this one focuses on the potential issues associated with upstream deployments. In particular, this article covers the critical upstream areas that one should be aware of when getting ready to deploy or already deploying DOCSIS 3.0.
DOCSIS 1.0 enabled data over coax with a "best effort" service using a data request-grant methodology. DOCSIS 1.1 and subsequent specifications added guaranteed Quality of Service (QoS) by providing Unsolicited Grant Synchronization (UGS) which means that a cable modem does not have to send a data request in order to receive a bandwidth grant from the CMTS. The new UGS service is an enabling technology which has allowed cable operators to successfully deploy the highly revenue generating Voice-over-IP (VoIP) services. In the following sections I will illustrate the differences between best-effort (request-grant) and QoS (UGS) services.
Through DOCSIS tutorial seminars, I have found the most effective way to bring someone up to speed on DOCSIS communications is by teaching the cable modem registration process. During the registration process, we will cover the RF physical layer, theDOCSIS Media Access Layer (MAC) and the Internet Protocol (IP) layer. Yes, the cable modem exercises the first three layers of the Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model)...
