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T3's normally occur on the downstream path after the modem has established communication with the CMTS (Cable Modem Termination System) during what is called station maintenance. The modem negotiates it's upstream power levels (transmit levels) and waits for a response from the CMTS. After 200ms if the modem has not received a response the modem encounters a T3 timeout. The modem then boosts its transmit levels again thinking that the CMTS cannot "hear" it until it can receive a response.

UCD's are Upstream Channel Descriptor. These messages are sent from the CMTS on the downstream path basically telling a modem what upstream frequency it needs to be on.

The next error "Received Response to Broadcast Maintenance Request,..." is known as a T4 timeout. This sort of timeout can sometimes indicate possible upstream issues such as contention (competition for bandwidth with other modems). Periodically, the CMTS will send out "keep-alive" messages to the modems during their maintenance cycles. The error indicates that it received that broadcast packet, but due to unknown reasons it was not able to transmit a response (But no Unicast opportunity received). Sometimes this can be due to the amount of traffic on the upstream channel, upstream utilization, things of that nature. There are numerous possibilities as to why the modem was not able to transmit back. Once a modem encounters 16 T4 timeouts in a row the modem will reset its connection.

The next message is a normal message regarding the Organizaional Identifier (OID).

ToD stands for Time of Day. When the modem has established communication with the CMTS it accesses a TFTP server (Trivial File Transfer Protocol server) where it will obtain an IP address from the DHCP (Dynamic Host Configuration Protocol) server and syncs its time with the CMTS, hence the Time of Day request.

The next log entry "DHCP WARNING - Non-critical field invalid response," sometimes will occur during a DHCP lease cycle. DHCP clients request an address on startup and when they are approaching the end of their address lease period. When a client does not have a current lease, it broadcasts a DHCP discover message on its local network segment. All DHCP servers on the network segment copy this message, and respond with a broadcast message announcing that they have an address available. The requesting client chooses one of these offers, usually the first one it receives, and broadcasts a DHCP request message indicating that it has selected that server's offer. While a server is awaiting the client's selection, it can (and should) reserve the offered address so that it is not offered to another client in the meantime. The DHCP server then replies to the request message with a DHCP acknowledgement message which includes the client's IP address, subnet mask, default gateway, and other configuration information.

DHCP provides option fields to allow the client to request and obtain vendor-specific configuration information from the server. This error indicates that one of those options fields were invalid.

The last log entry to address is the "SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing and FEC Synchronization Framing. The CMTS periodically broadcasts a basic set of instructions used by all modems. This instruction set must be used by the modem to enable any further communication with the CMTS. QAM (Quadrature Amplitude Modulation) is the form of digital modulation used primarily in transmitting downstream data signals. During its scan, the modem must locate and synchronize with the QAM-64 or QAM-256 carrier that contains data necessary for modem operation. This synchronization is commonly referred to as QAM lock.

FEC (Forward Error Correction) places overheard or parity bits in the data stream to help identify and correct errors that may occur when data is transmitted through the network. During the modem's initialization process it must synchronize with these FEC protocols so that the FEC system can correct errors. Cable receivers also use this method of error correction. This can also be associated with PreFEC BER (Bit Error Rate[or Ratio]) and PostFEC BER.

So to sum up many of these errors are common which you will see frequently. The main errors to watch out for in my opinion would be the T3, T4, QAM/QPSK symbol timing and FEC synchronization framing. These could potential point to some signal issues which most likely need to be addressed by a technician.

Descriptions, courtesy of AZHSISUPPRT2 See Profile

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by No_Strings See Profile
last modified: 2009-11-01 20:13:16