{"id":772,"date":"2014-12-03T09:37:58","date_gmt":"2014-12-03T07:37:58","guid":{"rendered":"https:\/\/excentiscom.webhosting.be\/?p=772"},"modified":"2022-06-24T08:31:55","modified_gmt":"2022-06-24T06:31:55","slug":"ranging-feeling-the-2-bpm-docsis-heartbeat","status":"publish","type":"post","link":"https:\/\/www.excentis.com\/blog\/ranging-feeling-the-2-bpm-docsis-heartbeat\/","title":{"rendered":"Ranging \u2013 Feeling the 2 bpm DOCSIS heartbeat"},"content":{"rendered":"

While working with DOCSIS systems it is important to understand the\u00a0upstream periodic ranging<\/strong>\u00a0process. Ranging can be seen as the\u00a0DOCSIS heartbeat<\/strong>\u00a0between Cable Modem (CM) and Cable Modem Termination System (CMTS). As many people tend to struggle with the details of this process, the goal of this article is to\u00a0visualize<\/strong>\u00a0the periodic ranging\u00a0process.\u00a0<\/strong>Lets take a closer look at that ranging process \u2013 and let\u2019s try to understand where things can go wrong during this DOCSIS heartbeat.<\/p>\n

Periodic Ranging, the DOCSIS heartbeat<\/h2>\n

The upstream periodic ranging process (the initial ranging process is not part of this discussion) can be seen as the DOCSIS heartbeat, meaning it is very critical to the DOCSIS system! When this heartbeat is failing, the real data transmissions (usually using a less robust modulation) will also be subject to packet loss. The modem might even end up losing the upstream channel and\/or in worst case reinitializing itself.<\/p>\n

More specific, the ranging process is the\u00a0periodic tuning<\/strong>\u00a0of the CM transmit parameters, such as\u00a0timing,<\/strong>\u00a0frequency<\/strong>\u00a0and\u00a0power.<\/strong>\u00a0The goal is\u00a0to ensure a perfectly aligned<\/strong>\u00a0and reliable\u00a0DOCSIS communication<\/strong>\u00a0between CM and CMTS. This tuning is maintained per CM and for all of its available upstreams.
\nThe process consists of the CMTS providing a \u2018\u2019ranging poll\u2019\u2019 to the CM (unicast ranging opportunity in a DOCSIS MAP message), triggering the CM to send a DOCSIS Ranging Request message (RNG-REQ). Based on the reception of this message, the CMTS provides the CM with the necessary transmit signal adjustments using a Ranging Response message (RNG-RSP). This process, commonly known as \u201cStation Maintenance\u2019\u2019, is repeated periodically to keep the CM within acceptable CMTS reception tolerances. Serious deviation of any of the transmit parameters will cause improper CMTS reception, hence will result in data transmission loss.<\/p>\n

Ranging visualized: successful or failing?<\/h2>\n

The picture below visualizes this ranging process, it shows how successful ranging takes place (green cases), but also where the communication might fail (red cases) and how the system tries to deal with<\/p>\n

\"\"<\/p>\n

2 bpm?<\/strong>\u00a0Default DOCSIS timing requires that Periodic Ranging must occur at least once every\u00a030 seconds<\/strong>\u00a0(also called the DOCSIS\u00a0T4-timer<\/strong>) for each CM. Usually this\u00a0ranging interval<\/strong>\u00a0can be configured on the CMTS; typically this is set to about 10 to 20 seconds. When all goes well, it is expected to see the RNG-REQ \u2013 RNG-RSP in every ranging interval (\u201dgreen\u201d cases 1, 3 and 5 on the figure).<\/p>\n

However! Due to downstream and\/or upstream RF impairments following situations can show up:<\/p>\n