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Chapter 10


Virtual Circuits and Channels 


How to Interconnect OmniBoxes

A Virtual Channel is one  that is not associated with a digital span timeslot or a loop current device (Analog line) at loading time, but one that is created at runtime assuming either the physical parameters of a real channel or those of a Virtual Circuit. A Virtual Circuit is anything that can convey a voice bit stream between to distant points, these Virtual Circuits will allow to connect Virtual channels in different OmniBoxes.


Virtual channels are not just for remote routing, they can also perform Call-Back in the inbound channel that received the trigger, do inbound/outbound through the same channel and blind test calls in any channel.

Remote Connection of  Two Real Channels

The SC (or H.100) Bus

Before we go any further, it is important to understand how two channels in two different spans are connected through SC Bus because the principles involved in the connection of virtual channels in different OmniBoxes are very much the same.


The SC bus, that is the 26 lead gray ribbon cable that connects all the ISA Dialogic boards, has 1024 timeslots.  The H.100 Bus, used by the newer PCI boards,  has 4096 timeslots. The difference is just capacity, but in both cases you may regard the timeslots in these busses as a being virtual wires through which you may send sound bit streams. For simplicity, we will refer to any of these busses as the H.100 Bus.


OmniBox deals with three types of bus devices: digital channels (DTI) , loop current (LC) and voice resources (VOX). Each of these devices has a transmitting and a receiving port that can connect to other devices through the H.100 bus.


 The Dialogic driver connects a timeslot to each device’s transmitting port when the service starts. These assignments are fixed and can’t be changed by an application at runtime, however, the Receiving ports may be set at runtime to “listen” to any timeslot (TS from now on) in the H.100 bus.


Assume a system with a D/240SC-2T1 (48 DTI and 24 VOX) with ID 0 and a D/41ESC.(4 LC and 4 VOX) with ID 1 The fixed assignments will go as follows:


Lets consider the routing of a successful call from the third channel in the first T1 (Ch103) to, say, the fifth channel in the second T1 (Ch 205) .  To receive the digits, dial out and to monitor the call progress the 7th voice resource will be used. Ch 103 transmits in TS 30, Ch 205 in TS 56 and the VOX in the TS 6.

*in this example the D/240SC-2T1 is an ISA legacy board that uses the SC Bus, not the H.100.

Virtual Channels

Virtual Channels (VC’s) may “assume” real channels physical parameters (the TS among others).  You are going to need that for Call-Back services and two-way traffic where calls are required to be placed in channels defined as inbound


A VC may also represent one in a remote OmniBox connected through virtual circuits. Hereafter, we will refer to this type of VC as a Remote Channel or RM. In the case of Remote Channels there is no “assuming”. If you need to route a real channels into a remote one, you will need to assign one of the free timeslots in the H.100 bus to this virtual channel the same way the Dialogic driver assign timeslots for real devices. You may not create more remote channels that there are free timeslots in the H.100 bus. Parameter 19 for Set_ID = 0 in the sys_Parameter table, sets the Virtual Circuit Array size, it must be less than the bus size minus total device count or:


ArraySize <= TS count – CTI channel count + LC channel count + VOX count


For the example above ArraySize <= 1024 – 48 – 4 – 28 =  1024 – 80 = 944 . Of course, if all the virtual channels are to be remote, even if all the real channels were defined as inbound, you’ll never need more than 52, so there’s no need to make virtual channel array greater than this.

The Remote Connection

There are a few technologies that can establish virtual circuits (VC’s), between switches, namely: ATM, VoIP, Fiber optic TDM, Ethernet packets, etc. All these technologies, no mater how different in principle of operation, have one common feature, there is always a capacity or max VC count. Lets call this limit VCMAX and, no matter what technology is to be used, the virtual circuits available will be numbered from 0 – (VCMAX-1).


To route a call from a remote Virtual Channel in OmniBox1 to another in OmniBox2, you will need one VC each to transmit into. This VC’s must not be used by any other connection until the call is over, so a means must be provided to organize the use of these virtual circuits.


There’s the static way, you assign them in the same manner Dialogic driver does, for instance, virtual circuits for OmniBox1 will take VC’s from 0 to 199, OmniBox2 from 200 to 399…and so on. It’s simple but makes an inefficient use of the capacity since in no switch all the channels are being used all the time.


The best use of the capacity is when you assign the VC’s dynamically. For this you need a central VC distributor, when you need to place a call in a remote channel, a VC is requested to the VC distributor and it will give you a free VC, that from this moment on will be “In Use”. When the call is over, the VC is return to the distributor and its status is set to “Free” again. This way it is guaranteed that if there’s no VC for a call, it is that they are all “in use” and not just “assigned” as with the static way.

The Remote Call Protocol

To be able to handle calls through Virtual Circuits, there must be a way of setting them up and controlling its progress. This problem is very similar to that Common Channel Signaling or CCS, like ISDN or SS7. Since the problem is similar, the solution should be similar too, hence, OmniBox uses a protocol in the same spirit of the one used for ISDN described in the ANSI specification Q.931, only simpler.


As in the ISDN protocol the Remote Call Protocol (RCP) will have a Call setup message and also the call Proceeding, alerting connect, disconnect and release complete. Also drop causes will follow the Q.931 specs but there’s where the similarity ends. The messages and Elements IE. are not binary but textual and are sent over IP an not a DS0. The port number and the IP address of the socket to send and receive these protocol messages must be set in the ParamNumValue and ParamStrValue fields of the sys_Parameter table respectively with ParamID = 20.


Here is a detailed description of the messages:


Call setup is the “O” message (OFFERED) :

<Message Size>O o:<outbVCh> t:<VC num> r:<Local Range>

i:<Orig Domain> d:<DNIS> a:<ANI>

This message is set from originator to terminator, the IE’s are:   

<Msg Size>      Three numerical digits expressing is the character count of the message

 without including those three. This IE will be found leading all messages.

<Outb VCh>    Virtual channels get a channel number as they are created, this

information element is the number of the outbound VCh originating the

remote call. It will do as a CRN (Call Reference Number) for the

originating OmniBox, in other words, every message sent back to the

originating OmniBox must carry this reference.

<VC Num>      This is the Virtual Circuit number that the VC distributor gave to the

originating OmniBox to transmit into. The OmniBox receiving this

message must create an inbound Virtual Circuit and have this VC talk

into the H.100 bus timeslot assigned to it.

<Local Range>This is the outbound trunk group that connects to the desired destination,

The receiving OmniBox must find a free outbound channel in that range

To place the requested call.

<Orig Domain>This is a string that will identify the originating inbound trunk group

                        and the originating OmniBox as well. I has the format:

 <Eng>_<Domain Name> for example Domain “Client1” in OmniBox

number 3 will be expressed in this information element as “3_Client1”.

This string will show in the call detail information text box of the

OmniMonitor to the right of the remote inbound channel number that

will show as negative to highlight its virtual nature.

<DNIS> <ANI> The DNIS will be used as  the number to be dialed (no further digit

processing will be done) and the ANI is the originating phone number to

be passed along to the setup of the call in the real outbound channel.


The “P” message or call PROCEEDING.

            <Msg Size>P o:<outb VCh> i:<inb VCh> t:<VC num>

This message is sent from terminator to originator as a call setup acknowledge, the IE’s are:

<Msg Size>      Same as above

<Outb VCh>    The CRN, the reference so that the originator can figure out which call is

this message referencing.

<Inb VCh>      The CRN that will reference this call in the terminating OmniBox (the one

sending this message) Any further messages from the originator regarding

this call must refer to this number. The inbound virtual channel number is used as the call CRN.

<VC num>       Is the Virtual Circuit number given by the VC distributor to the

inbound virtual channel. This VC will be listening to the Tx timeslot of the

real outbound channel. The originating Virtual channel, upon reception of

this message, must route this VC into its assigned H.100 timeslot.


The “A” message (ALERTING.)

            <Msg Size>A o:<outb VCh>

This message is sent from terminator to originator to signify call acceptance (not answer supervision), this normally after a ring is detected or an ALERTING message is received, the IE’s are:

<Msg Size>      Same as above

<Outb VCh>    The CRN, the reference so that the originator can figure out which call is

this message referencing.


The “C” message (CONNECT)

            <Msg Size>A o:<outb VCh>

This message is sent from terminator to originator as Answer Supervision notification, the IE’s are:

<Msg Size>      Same as above

<Outb VCh>    The CRN, the reference so that the originator can figure out which call is

this message referencing.

The “D” message (DISCONNECT)

            <Msg Size>D o:<outb or inb VCh> c:<Cause>

This message may be sent by either the terminator or originator to notify a hang up, the IE’s are:

<Msg Size>      Same as above

<Outb or inb VCh>The CRN that suits the notified. If the called party hung-up, it is the

terminator that must notify the originator and thus, the CRN is the

originator’s outbound virtual channel number  In case of the calling party

hanging up, the notification goes originator to terminator and the CRN

would be the inbound virtual channel number in the terminator.

<Cause>          References the Q.931 standard causes for disconnection. (See appendix A)


The “R” message (RELEASE COMPLETE)

            <Msg Size>R o:<outb or inb VCh> c:<Cause>

This message may be sent by either terminator or originator to acknowledge a DISCONNECT message , the IE’s are:

<Msg Size>      Same as above

<Outb or inb VCh>The CRN that suits the notified, same as above

<Cause>          Returns the same cause as received in the DISCONECT message

Virtual Trunk Groups

Every OmniBox that is to be involved in remote connections must have one virtual inbound trunk group or Virtual Domain (VD), to which all inbound remote channels will belong. Also, when a call is setup in a real channel, but its dialed string corresponds to a destination that is only available through another OmniBox, the routing will point to a Virtual Range (VR). Domains and Ranges have properties that are common to all its member channels, many of these properties don’t make sense for virtual ones and also there are properties for virtual channels that can’t be applied to real ones. Virtual trunks group parameters will be found in the same tables of the real ones, dia_InCh and dia_OutCh. They must also be listed in the dia_GroupDefs table with ChCount field set to 0 (The value of ChMin must be 0 too for the Virtual Domain).


In the case of the Virtual Domain, you may simply ignore the properties related to digit processing, routing, the watchdog or the hung-trunk detector, also, the accounts issue is irrelevant, you may use any account or a bogus one, if you like to be consistent with its irrelevancy. The only fields in the dia_InCh table that play a roll are: DomainID; Description (Trunk Name) and Function that must be set to 4.


The Virtual Range case is slightly more complicated. First, there can be any number of VR’s in an OmniBox and second is that not only the watchdog, digit processing, idle time, mode mask and Provider fields won’t apply, but that there are remote specific properties that must me stored in the same dia_OutCh table, so a different set of meanings to the fields must be defined for the virtual case.


Field Meanings for the dia_OutCh table


Real Ranges

Virtual Ranges

Field 0



Field 1



Field 2



Field 3



Field 4



Field 5



Field 6

Outbounf Mode Mask

Remote Port

Field 7


Remote Range ID

Field 8


Remote Engine

Field 9



Field 10



Field 11



Field 12



Field 13



Field 14




Explanation of the new meanings:

IP1.IP2.IP3.IP4 :        The 4 components of the IP address of the OmniBox where the

Remote Range is.

Remote port :             Is the port number of the socket that will attend the in the Remote

Call protocol in the remote OmniBox

Remote Range ID:     The value of the RangeID field in the remote OmniBox.

Remote Engine         The Engine number of the Remote OmniBox.


The utility OmniBoxSetup.mdb provides special entry form for virtual Ranges:


Following a Remote Call


A real inbound channel is seized and the dialed digits collected. The digits happen to correspond to a route with its top priority being a Virtual Remote Range (VRR), a Remote Call (RC) must be placed through a Virtual Channel (VCh) of that VRR. A VCh doesn’t exist until it is created so, we must request the VRR to create a VCh that inherits its properties (IP, Remote port, Remote Range etc.). This creation process also involves the assignment of a free H.100 timeslot and a Virtual Circuit (VC) from the VC distributor. The real inbound channel is then routed full duplex to this virtual channel, in other words, the inbound channel will be set to listen o the free TS just assign to the VCh and the given VC set to listen  to the Tx TS of the real inbound channel.


For the sake of the example, lets put some numbers here, starting with the inbound call lets assume that:


The VCh just created say, is the 5th in that OnmiBox, which lets say, is Engine number 4. The VCh’s are numbered on a 0 basis, so this one will be VCh(4).  The VRR happened to be the one named Ivory Coast with group ID=3 and its properties were:


Finally assume the given VC was number 20. From all these numbers, we can figure that channel 103 Tx TS is 30, the free TS assigned to the VCh, being the 5th, is number 84, so channel 103 will be set to listen to TS 84.


Now the VC is requested to make the RC (Remote Call, as you had probably forgotten), that starts by sending the following  O” message to port 5001 at IP 123.465.789.001 :


050O o:4 t:20 r:9 i:4_Client1 d:41234567 a:7185671234


Now, Engine 8, receives the “O” message, next, it must requests its Virtual Domain to create an inbound VCh. Lets assume it is the 17th or that VCh(16) is created. If we further assume, for simplicity, that both OmniBoxes have the same type and count of boards, the VCh(16) will have the H.100 TS 96 to have it listening to VC(20). VCh(16) will also need a VC to transmit into, so it must request one from the VC distributor. It has not been long since Engine 4 requested its VC, so, assuming its free, it is likely that the VC distributor assigns the next VC in line, namely VC(21).

Provided all the data from the “O” message has been properly received, the procedure to place a call in the requested range is started and the “O” message is acknowledged by replying with the following “P” message.


015P o:4 i:16 t:21


When Engine 4 receives the “P“ message, the routing is completed. The H.100 TS 84 will be set to listen to VC(21) and VC(20) will be set to listen to H.100 TS 30 (remember? The one channel 103 transmits into). Now it waits for further call progress messages from Engine 8.

Back in Engine 8, an available channel, say the 205, was found in Range 9, VC(21) is set to listen to H.100 TS 56 (205 transmits into TS 56)  and the number 41234567 has been dialed, VOX(7) is waiting for a ring. Got a ring!, actually, the caller in channel 103  heard that ring too (Ch(205)->ts56->VC(21)->ts84->Ch(103)). Now the following “A” message is sent to Engine 4 to stop the “dead air” timer and start the “no answer” one.


005A o:4


VOX(7) is waiting again, this time for a voice, a FAX or a recording… Got a voice!, Caller at Ch(103) heard “Hello…”, next the reverse sound path is established, Ch(205) is set to listen to H.100 TS 96 that’s listening VC(20) that was listening to H.100 TS 30 where Ch(103) transmits. From this moment on, the called party will hear when the callers says “Linda, Hi!, long time no talk…”.  Also, a “C” message must be sent to Engine 4 to “officially” notify that it can start charging Mr. Caller in channel 103.


005C o:4

Back again in Engine 4, when the C message is received the answer supervision signal is sent through Ch(103) back to the seizing switch. Now all the channels, Engine 4 and 8, virtual and real are in a connect state.


This particular example call is not going to be a long one and though it will have a normal ending it is not be happy one. What Mr. Caller hears next is Linda saying “…and you have the nerve of calling me after all this time…” and then a busy signal, after a couple of useless “Linda…Linda?”, he must hang up too.


This is what went on in the OmniBoxes:Ch(205) gets a DISCONNECT event when Linda hangs up, lets assume that it is set up, not to truncate by sending a DISCONNECT message to engine 4 but just to play a Busy Signal. This is accomplished  by having VC(21) listen to H.100 TS 0 (the voice resource that plays the busy in an OmniBox can be either VOX(1) or VOX(2) depending on what was played first, a ring or a busy, we have just assumed it was VOX(0) that transmits in TS 0).

When Mr. Caller, after losing all hope, hangs up, a disconnect event is received in Ch(103), the call is truncated, this is the inbound call is dropped and the VCh(4) sends a “D” message:


011D o:16 c:16


The o:16 is addressing  VCh(16) in Engine 8 while c:16 is the Q.931 cause code that means NORMAL CLEARING.  At Engine 8, when receiving this, the call is be dropped at Ch(205) and after sending the “R” message and releasing VC(21) with the VC distributor, the VCh(16) is destroyed.

010R o:4 c:16


Back at Engine 4,  when the “R” message is received VC(20) is released with the VC distributor and VCh(4) is destroyed and this is the end of Mr. Caller call to Linda

Virtual Channels in a Call-Back function

To set up a Call-Back service, a DNIS service with as many phone-numbers as clients will be needed. Each client will be given one of the mentioned phone-numbers to which he or she must call to get a call back from the service. In the OmniBox, the Domain doing the Call-Back service will have its function set to dynamic by DNIS or -1 (See dia_InCh table description in Chapter 4). It is in the DNISLookup table where the Call-Back ServFunction number (13) must be set, together with the Call-Back phone number in Str2. This Call-Back phone-number, must match the Account of the Call-Back client’s account registry in the Balances table (See Balances table description in Chapter 4). The cost rate of the call-back call in field Float1 and the mode mask (See dia_OutCh table description in Chapter 4) for the outbound call in the Num2 field. An example of a Call-Back entry in the DNISLookup table:


















OmniBox perfoms the Call-Back in the same channel that attends the trigger call.  As it is the normal way in Call-Back, the trigger call will never be answered, the program will wait until the caller hangs up or for a 20 second timeout to expire, whatever happens first.


Now, a Virtual Outbound Channel (the VCh) will be created by a Virtual Range (VR) with the same group ID as the Domain to which the inbound channel attending the call belongs. This VCh will be an outbound but will assume the physical properties of the inbound. Exactly, the assumed properties are those specified in the span’s cdp file and also the assuming VCh inherits the ownership of any voice resource that the assumed channels might have had.


Assuming the example case, OmniBox will make up to three attempts to 5821473456 at 10 seconds interval, if a connection is achieved, the Virtual Channel will “Un-assume” the inbound channel, in other words, everything will behave from that moment on, as if the call-back had been an inbound call. The Long Distance IVR state machine (See Chapter 4) will take it from there prompting the caller for a destination phone number…

Etc. After the Call-Back client is done with service, the VCh resources are freed.


Virtual Channels that Place Calls on Real Inbound Channels or Two Way Trunks

A Virtual Range with ChCount = 0 and ChMin = 0, as we have seen is a VRR or a one to be used in Remote Calls. If ChMin is <> 0, then is not remote but one “Associated” to a Domain, the one with its ID equal to the ChMin value. OmniBox recognizes logical channels only as inbound or outbound, but this virtual channel concept allows physical channels to work both as inbound and outbound, thus allowing channels to act as two way trunks.


When a call needs to be routed to a VR with association to a Domain, OmniBox will search for a free channel in it, then have the VR create a VCh and make it assume the physical properties of the free inbound channel (same as with the Call-Back) but in this case, even the channel number will be assumed, so that the monitor can better display the details of the call that will be placed in this VCh. When the call is over, the VCh will be destroyed.


For Two Way trunks the original meanings of the parameters in the dia_OutCh table, will make sense again. Everything will operate the same, only that physical channels belong to a REAL inbound trunk group or Domain.


WARNING: Since the dia_GroupDefs table may have more than one record per trunk group, it is possible to enter ChCount = 0 in one, making it Virtual, and then make it different than zero in another making it also Real. This is not acceptable for OmniBox, trunk groups must be either REAL or VIRTUAL. A trunk group can not have a part that is virtual and also a few channels that are real.



APPENDIX A: Q.931 Cause values


* NOTE: Not all cause values are universally supported across switch types.

              Before a particular cause value is used, its validity should be

              compared with the appropriate switch vendor specs.


0x01  /* Cause 01 */






















































































*The Cause value greater than127 is allowed to define here. There is

No conflict with those universal defines which ranges 0-127


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