/mcpThe T1 or E1 controllers at both origination and termination gateways are typically error free. If errors occur, repeat the show controller (T1, E1, and 1/0 varies) command several times within the call to see if the number of errors increases. The most common problem of slips is a synchronization problem that results in clocking errors. In packet voice networks, it is usually sufficient to confirm that the router clocks from the line. If it does not, ensure the clock source line command is entered at the controller level, but in VoATM or TDM networks, where a clocking hierarchy is established and the routers need to pass clock through the network, additional considerations need to be made. The Clocking Plan document provides more information about synchronous clocking. On 26xx / 366x routers, when you use the AIM VOICE card, the controller shows "controlled slips" unless you add the network-clock-participate and network-clock-select commands. On the Cisco MC3810 platform, you need to configure the network-clock-select command and issue the show network-clock command to make sure the configuration has taken effect. On the Cisco 7200VXR platform, the frame-clock-select command is required for the voice cards. This command is particularly important for the 7200VXR voice gateways because, by default, the internal TDM bus is not driven by the local oscillator. Because the E1 trunks are normally synchronized to the telephony network, the result is hidden clocking errors and intermittent fax transmission problems. More detail is available in Cisco bug ID CSCdv10359. On the C4224 MFT cards, when they are to accept the clock from the line, under the controller t1 x/y you need to issue the clock source loop-timed command. This setting decouples the controller clock from the system-wide clock. It is then required to set the network-clock-select command. In this case, it would be network-clock-select 1 t1 x/y. 4. Check Fax Interface-type On some platforms, which include the Cisco 3660, 5300, 5350, 5400, and 5800, the router defaults to fax interface-type modem. The fax interface-type modem global configuration command forces fax calls to a modem (usually for T.37 Store and Forward fax) and not to a DSP. For Cisco fax relay to work, the fax call must be sent to a DSP, which means it must be configured with the fax interface-type vfc command. fax interface-type Command