CMTS vendors configure their equipment to send RNG-RSP messages much faster than 200 msec to avoid CMTS-related T3 timeout errors. If the cable modem does not receive the RNG-RSP within this interval a T3 timeout error will be generated. Once the CMTS receives a RNG-REQ from a cable modem it must send a Range Response (from here on out I will use RNG-RSP) within 200 msec per the DOCSIS specification. The referenced changes will now be discussed in the following section on Range Response. Units are in unsigned centiseconds (10 msec). Note that only equalization can be deferred. If there is a non-zero value, then this is time estimated to complete changing new parameters from the previous Range Response. The Pending Till Complete field is also required, since it has a value of zero (0) in this example it indicates that all previous RNG-REQ changes have been completed before this RNG-REQ was transmitted. As seen in the message, the RNG-REQ is only 28 bytes long, so this a very short message, which is important since it is sent by every cable modem at least once every 30 seconds (CMTS vendors normally reduce this time to 20 seconds or less to eliminate T4 timeouts – more on this shortly). the name of downstream DOCSIS channel it is receiving data). The RNG-REQ must also contain its permanent SID (temporary SID if it is just coming online) and the Downstream Channel ID (i.e. The source and destination MAC address so that the CMTS knows how to route the message and from which cable modem the message came. Clearly it must contain the type of message, which is a RNG-REQ. Range Request (RNG-REQ)ĭownstream Channel ID (HEX): 03 Pending Till Complete: 0Īfter examining the Range Request (from here on out I will use RNG-REQ per the DOCSIS specification) you will find that it contains very little information. For example, Google “OUI 002040” and find out what cable modem manufacturer sent this Range Request message. Note: If you Google “ OUI” and then the first six digits of any MAC address you will get a “MAC Address Lookup” hit that will tell you the vendor name of the equipment that you are using. The following is an example of what a Range Request message looks like coming from a cable modem Source MAC Address ( PDU SA) of 00204066A0AE to the CMTS Destination MAC Address ( PDU DA) of 00070DAEC8A8: This “handshake” between every cable modem and the CMTS must occur once every 30 seconds as dictated by the DOCSIS specification. The CMTS analyzes the signal quality of the Range Request message and sends back any necessary RF adjustments in the Range Response message. A station maintenance session consists of a Range Request sent from a cable and a Range Response sent by the CMTS. While the UCD provides the language of the DOCSIS network, the DOCSIS Station Maintenance messaging is the proverbial “heartbeat” of the DOCSIS network.
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