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Hardware Blocking

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Hardware blocking is a condition for preventing use of an SS7 inter-machine trunk circuit - that is, a voice timeslot in an E1 or T1 digital transmission facility.

ITU Q.764 Section 2.8.2 and Q.763 Section 3.13 describe maintenance oriented blocking and hardware failure oriented blocking. Maintenance oriented blocking is an controlled and intentional circuit block put in effect by command. Hardware failure oriented blocking is the normally uncontrolled and unintentional blocking of a circuit resulting from some failure of a system or a component of a system.

Local Hardware Blocking


In ss7boost, a circuit is hardware blocked for one or more of four causes. Those causes are:
  1. Loss of communication with ss7box (the MTP3 signaling server); detected by loss of heartbeat over N periods
  2. Loss of communication with sangoma_mgd (the call engine); detected by loss of heartbeat over N periods
  3. Loss of the E1/T1 transmission line; detected and reported by sangoma_mgd
  4. Loss of the SS7 signal route; detected and reported by ss7box

Each circuit has four status fields associated with it, one field for each of the hardware blocking causes listed above. All four status fields must be cleared for the circuit to be hardware unblocked.

A method must be devised for sending either a CGB/CGU with a Circuit Group Supervision Message indicator of "hardware failure" (ref. Q.763 Section 3.13), or a BLO/BLA when a circuit becomes hardware blocked or unblocked respectively. It remains to be decided if ss7boost will optionally send CGB/CGU with hardware failure indications, or always send BLO/BLA for both hardware and maintenance blocking conditions. Current thinking leans toward the latter method.

With five blocking reasons (four hardware and one maintenance) is it necessary or desirable to send a blocking message each time one of the conditions is set? Or should only one blocking message be sent on the occurrence of the first condition and the unblock sent when all blocking conditions are cleared? If the latter method is chosen then there will need to be a state machine developed and a state variable maintained in the CDR for circuit.

Remote Hardware Blocking


Only the CGB/CGU carries the Circuit Group Supervision Message field. So only when a node is capable of sending such messages with the `hardware failure oriented value in the CG Supervision Message field will ss7boost be able to set/clear the rhb bit in the CDR entry for a circuit. If a node does not send the CGB/CGU then there will be no distinct indication of remote hardware blocking. If the node only sends BLO/BLA then the remote maintenance block bit for a circuit will be set/cleared.

CLI --ckt-report changes


The ss7boost_cli program ckt-report function will be enhanced to report the four hardware blocking status of each circuit in a report.

Development Plan

  1. convert comms with ss7box to SCTP (plan 6h, actual 2h)
  2. convert local hardware block bit in t_cdr to 4 bits (plan 1h)
  3. ss7box heartbeat and loss detection; set/clear lhb_ss7box bit all ckts (plan 6h)
  4. sangoma_mgd h/b and loss detect; set/clear lhb_mgd bit all ckts (plan 3h)
  5. E1/T1 status
    1. new sangoma_mgd load (plan 2h)
    2. set/clear lhb_facility for affected ckts (plan 8h)
  6. signal route avail/unavail
    1. DUNA/DAVA affect route table (plan 8h)
    2. set lhb_route bits for affected ckts (plan 4h)
  7. modify ss7boost_cli --ckt-report (plan 8h)
  8. devise and build a method for sending hardware block messages to the node sharing the affected blocked circuit (plan TBD)
  9. handle CGB/CGU with CG supervision message containing "hardware failure oriented" message type (plan 4h)

Created by: mike last modification: Thursday 30 of August, 2007 [19:54:48 UTC] by mike


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