Fw: [Asterisk-Users] Digium FXO Interfaces don't support groundstart???
Frank Cofer
fcofer at lifelabs.net
Tue Aug 3 05:47:32 MST 2004
A thought occurred to me to on how to further quantify the impact of "glare"
on a properly dimensioned trunk group and debunk the ground start glare
concern. A cursory traffic analysis clarifies:
1. Assume you have a two-way trunk group, dimensioned for average busy
hour, average busy season for P01 grade of service, Erlang B or Poison
distribution.
2. Further assume that the average holding time per call is the national
average of 300s (5 minutes).
3. Further assume that you have a worse case CPE incoming glare condition
by using the previously described brain dead arrangement of an ancient PBX
with ring detect as an incoming connect signal and a worse case CO that has
non-immediate ring (2s on 4s off cadence), yielding a horrific average glare
interval of 2 seconds instead of the typical of 10-50ms that would be normal
for change of state detection/immediate ring typical loop supervision line.
4. Further assume that the two-way hunt order is inverse (e.g., CO
ascending, CPE descending terminal hunt) as it should be.
What would be the expected incidence of glare in the busy hour?
By definition (1, above) the probability that any attempt arriving for the
assumed offered load, arriving at random, having exponentially distributed
holding times and having infinite sources will find any circuit busy would
be 1 out of 100 busy hour attempts for all calls (including those that
encounter glare).
Understand first that only 1 out of 100 attemps will find any facility
occupied at all. Then, for the busy event that is encountered, what is the
probability that it is the result of two calls, arriving at random, and
seize the same trunk from opposite ends during a concurrent 2 second window
for the assumed traffic intensity?
Intuitively, encountering a glare condition would certainly have to be far
less probable than encountering just any busy condition, which is the
traffic distribution equation's prediction.
In fact, since the average call holding time of 300 seconds (5 minutes) is
150 times more than the assumed average glare interval of 2 seconds, it is
far more likely to find a call in the non-glare state when the predicted
loss event occurs than finding it is the result of the assumed glare
condition of two calls, arriving at random from opposite ends, occuring
within a window of the 2 seconds. This is not just a simple ratio of the
average glare holding time versus the average call holding time (because the
distribution equation assumes exponential distribution of holding times),
but it is far less than 1 out of 1000 attempts for the assumed average
holding time and the average glare interval.
To put some real numbers on this for illustration, a trunk group for a full
T1 of 24 channels will carry 21.125 erlang (Poisson) in the busy hour. It
will have 253 attempts (from the typical average holding time assumed in 2,
above), of which only three will encounter a busy. Of the three that
encounter a busy, it is very unlikely any would encounter glare. Over time
(an interval of days), some nevertheless will.
Note that this is not what was previously described in the horror stories
related. Trunks were sticking all over the place and a rash of problems
were encountered, provoking panic. The situation was so awful that it
caused the ensuing foaming at the mouth, displacing it to a fault of Digium
being too cheap to put in required features, or "ignorant" engineers for not
having the wisdom and experience to insist on ground start to alleviate the
near certain disaster that would inevitably occur on both large or small
trunk groups.
Now let's take a look at a far more likely scenario that is supported by the
same traffic theory:
5. Assume that you have both ends hunting in the same order from both
directions, i. e., the same hunt order from both the central office and the
PBX (say both hunt in ascending order). The remaining assumptions (1
through 3, above) still are valid.
The traffic scenario drastically changes. Now the incoming/outgoing traffic
is always focused on the next idle line instead of the full dimension of all
of the servers in the group. In essence it behaves like a trunk group of 1
and the probability of failure now rises asymptotically. Two hundred
fifty-three busy hour attempts (from the previous example) on a trunk group
of one would raise the incidence of failure due to glare significantly and
the now trivial 2 seconds (or even 50ms) is of dire importance. Worse, the
glare interval is not over several days and confined to the busy hour, it
now raises its ugly head for just about any time of the day. But it becomes
massive during the busy hour. Trunks hang, subscribers complain, and the
situtation looks like it is going to hell in a hand basket.
Ground start is invoked and the problem appears to go away.
The ground start "cure" simply masks the problem. If a identical hunting
arrangement is used at both ends, PBX and CO 2nd trial failures abound, but
calls do go through or are blocked from double connection because the ground
start recovers from the collision. It doesn't reduce the incidence of glare
mathematically predicted. It justs copes with it by invoking an
(unnecessary) 2nd trial. The very fact that the "pre-seize" test describe
by the previsous contributor is used at all makes this evident. Think about
it: Properly dimensoned, only three busy events will occur at all in the
busy hour for a 24 server trunk group. Why is the "pre-seize" so vital?
The problem is still there, it is beating the switchgear to death, but it is
no longer causing subscriber complaints.
6. This is why I just can't believe the terrible incidence of "glare"
described by the previous contributors. It either has to be a drastically
under dimensioned trunk group, a colliding hunting order, or cockpit trouble
in the hardware configuration of the trunks.
7. Analogously, it is better to avoid an accident than to drive recklessly
and rely on air bags.
Hope this helps.
> ----- Original Message -----
> From: "Bill Church" <bill.church at bsius.com>
> To: "'Frank Cofer'" <fcofer at lifelabs.net>
> Sent: Sunday, August 01, 2004 5:19 PM
> Subject: RE: [Asterisk-Users] Digium FXO Interfaces don't support
> groundstart???
>
>
> > I got a kick out of this. There is always an "expert" who thinks their
10
> > years of experience running cable has any relevance.
> >
> > -Bill
> >
> > -----Original Message-----
> > From: asterisk-users-admin at lists.digium.com
> > [mailto:asterisk-users-admin at lists.digium.com] On Behalf Of Frank Cofer
> > Sent: Sunday, August 01, 2004 3:23 PM
> > To: asterisk-users at lists.digium.com
> > Subject: Re: [Asterisk-Users] Digium FXO Interfaces don't support
> > groundstart???
> >
> > He who knows not that he knows not, knows little; he who knows that he
> knows
> > not, knows a lot.
> >
> > 1. Glare is the simultaneous seizure of a common facility using two
> > separate independent universes whereby a decision at one end is not
> > immediately know to the other. As mentioned previously, because any two
> > points that are not the same in space are separated in distance, they
are
> > necessarily separated in time. The possibility of seizure of a given
> > circuit from both ends simultaneously is minimized by careful design,
but
> it
> > is never reduced to zero.
> >
> > Thus the original statement, "...ground start eliminates glare..." is
> simply
> > false.
> >
> > You can tell it is false if you have any rudimentary knowledge of
physics,
> > which (currently) holds that the maximum speed any information can
travel
> is
> > the speed of light. Electrical circuits are much slower. Thus the
means
> to
> > communicate an independent decision made at one end, that is separated
by
> a
> > distance, is also necessarily separated by time to the other end. Due
to
> > this propagation delay, "glare" cannot be eliminated on any two-way
> circuit.
> > This does not take craft credentials to recognize, it merely requires a
> > modicum of logic and perception.
> >
> > 2. Without maligning your comparative lack of telephony experience of
a
> > scant ten years, it would be useful to bring up how the central office
or
> > the PBX "know" what the other end is doing.
> > On older central offices that had line and cut off relays, ground start
> > service was effected by simply removing the tip ground of a bifalor (two
> in
> > hand) wound line relay. There was a line relay for every line in the
> > central office. Doing this in effect preventing it from operating
unless
> > the ring conductor (the -48vdc power feed to the line) was grounded,
> closing
> > the loop would no longer do. Until either the line relay or the cutoff
> > relay (busy at central office) was operated, the central office was
> > "ignorant" of the change of status of the distant end. So to operate a
> line
> > relay if the ground was removed from it, the PBX had to place an earth
> > ground on the ring side of the line for a short interval that would not
> only
> > be safely more than the time to cause the line relay to operate, but
also
> to
> > hold it there until a circuit had been established and the associated
> cutoff
> > relay operated by the central office to hold the connection. Typically
> this
> > occurs in 20-80ms but, to be safe, the time for the PBX to ground the
ring
> > is set nominally to 150ms or so, or until ground is seen on the tip
> > conductor at the PBX end. The latter is not seen until the call set up
is
> > nearly complete at the central office, since it is first extended from
the
> > inter- or intra-office trunk through the cut off relay.
> >
> > Since a subscriber line relay, modified as mentoned above for ground
> start,
> > has only one half the ampere-turns of current to operate as loop start,
> its
> > speed of operation is decreased greatly and the mark of busy to the
> central
> > office (from the time the line is seized by the PBX at the distant end
> until
> > such time the cut off relay operates) is markedly increased. From (1),
> > above, it logically and inescapably follows that the time that the
central
> > office "knows" that the PBX has seized the line is increased over that
of
> > loop start because, in the latter, both coils are aiding and the relay
> > operates much faster (its speed of operation is a function of the flux
> > density which is a function of the ampere turns).
> >
> > Thus, for a seizure from the PBX to the central office, "glare" (the
time
> of
> > not "knowing" what the other end is doing) is increased when ground
start
> is
> > used. Though I want bother to describe it here, the same is true of
> modern
> > electronic offices using any form of earth ground signaling because they
> are
> > arranged to emulate the older offices above.
> >
> > 2. Now for the other end, for a call from the central office via a
> two-way
> > trunk to a PBX, the same applies. It is true that some older PBX's
> > (Stromberg-Carlson X-Y and some really old SXS versions come to mind)
were
> > somewhat brain dead and could not recognize an incoming connect from the
> > central office using loop supervision until ringing was applied. For
> these
> > PBXs, they relied merely on the presence of ringing (AC on tip to ring >
> 65
> > volts and wait for a sufficient time to ignore transient spikes) as an
> > incoming signal. This is the fault of using ringdown detection instead
of
> > change of state for an incoming connect when loop supervision is
employed.
> > Severely aggravating this, many older central offices did not have
> > multi-phased "immediate" ring, thus the combination of "ring detect" and
> > non-immediate ringing could delay up to four seconds (due to the ringing
> > cadence of two seconds on and four seconds off) before the associated
PBX
> > would "know" the line was seized by the central office. For ground
start,
> > these older PBX's were arranged to look for presence of ground on the
tip
> > (which was normally absent when the facility was idle in the central
> office
> > except for call setup).
> >
> > However, few PBX's (unless they are ancient or sloppily configured by
the
> > installer) wait for ringing to be applied as an incoming signal in loop
> > start anymore (they recognize change in state immediately when the
> > superimposed DC is applied) and even for those that do nearly all
> electronic
> > offices have immediate ring. But even in these cases, the time that the
> > line is seized by the central office until such time the PBX "knows" it
> has
> > happened is discrete and glare nevertheless occurs, be it looking for
> ground
> > on the tip (ground start) or looking for a change in state (loop start).
> >
> > 3. There are numerous schemes to inspect the line for latent seizure as
> the
> > call setup process goes on to try to minimize glare. The central office
> > routinely does this after call setup is mostly complete, as a final
check
> > before 2nd trial failure on all lines (both loop and ground start). To
my
> > knowledge, this is not universally known as "pre-seize" in your limited
> > vernacular and it certainly is not restricted to ground start lines,
> though
> > it may be described as such on the machines that you have worked on.
> >
> > 4. Though your other arguments are patently irrelevant, I will address
a
> > few of them. Wink supervision is independent of loops (it can be
effected
> > by E&M, reverse battery, loop, etc.) and merely refers to a brief change
> of
> > state (from on hook to off hook to on hook); hence "wink" at the remote
> end
> > after the call has been set up. It is nominally 130ms. It is primarily
> > used as a channelized signal between common control switches to indicate
> > that the receiving end is ready to receive digits. The first wink
causes
> > the distant sender to send the called number and a second wink
(optionally
> > employed) causes the distant end to send the calling number for use for
> > Automatic Number Identification (ANI). Other than the similarity of the
> > time interval, it has as much to do with the discussion of ground start
as
> a
> > pig knows about Sunday.
> >
> > It is also somewhat academic, because most central offices and a growing
> > number of PBXs that provide Direct Inward Dialing (DID) now use some
> common
> > channel form of supervision (e. g., SS7 or ISDN D channel, though it is
> > still widely employed on channelized DID for some switches (Mitel).
> >
> > It is important to understand that the DC supervision labels used by
> > two-wire and four-wire switches were carried over as a label for channel
> > banks, much in the same way the old cord board "tip", "ring" and
"sleeve"
> > now carries the labels for transmission lines. Since you only have ten
> > years of experience, you are probably unaware of this. These signals
were
> > converted at channel banks and encoded as "A" and "B" signaling bits in
> the
> > digital stream; their meaning is simply binary at the lowest level, but
> > their interpretation differs based on the signaling schemes of the
> attached
> > equipment. "E&M" signaling is a means to communicate supervision is a
> > very old (>50 years) outband method of signaling (the "E" means "ear
> piece"
> > or receive and the "M" means "mouthpiece" or transmit). Originally,
> battery
> > was placed on the "M" lead to transmit an offhook, and ground received
on
> > the "E" lead was interpreted as an offhook from the remote end. It was
> > devised on open wire before there was any carrier. E&M has evolved into
> > several so-called "types" whereby same source battery and ground is used
> > (each connecting unit provides its own battery and ground reference and
> the
> > opposing end merely provides an isolated relay contact). Ironically,
the
> > E&M Type II and Type III were developed to avoid ground loops between
> > equipment, which is very undesirable for digital equipment, and is again
> > discussed below.
> >
> > My point that the carry over label used in channel bank terminology as
> > "ground start" has advantage because it start with A and B as 11 (an
idle
> > line) and then changes only one bit at a time (Grey code) which relieves
> it
> > from ambiguity. (Normal binary count going from 01 to 10 must pass
> through
> > either 11 or 00 before it gets to 10). The supervision used on T1
should
> > always be "ground start" because of that reason. (There is no delay in
> > connect, however, if loop or E&M supervision is used instead). Your
> comment
> > that Feature Group D somehow made ground start available is mistaken and
> > misleading. Ground start has been available for ages and was introduced
> > with No. 1 SXS PBX's over fifty years ago and also was used with N, ON
as
> > well as with T carrier beginning in the early 1950s.
> >
> > I am unfamiliar with your "funky" description (which seems to lack any
> > precision) as to what it means but it may have something to do with
> > connotated ignorance.
> >
> > 6. The fact remains that any analog balanced pair transmission line is
> > severely compromised by use of earth ground connections and its use
should
> > be avoided. One rarely hears of a 2500 set being struck by lightning
and
> > it is no accident. It is immune from longitudinal voltages since it is
> > entirely transformer coupled and is totally isolated from earth ground.
> > This also applies to circuit cards. As soon as ground start is added as
a
> > feature, earth ground must be extended to the equipment and through to
the
> > connecting circuits. And as soon as that is done, all of the circuits
are
> > exposed to longitudinal voltages to that earth ground from the
> transmission
> > line. Believe me, this is a very bad idea. It is also unnecessary.
> >
> > 7. Digium is not alone in not offering ground start. Avaya, Nortel,
and
> > other major manufactures of key and PBX's have eliminated ground start
> other
> > than accommodating diehards on some older equipment. You might argue
> that
> > their reason is more value engineering rather than to get away from FEMF
> > failures that are intrinsic to any such design. Or, you may argue that
> they
> > did so because they no longer care about the benefit to the central
office
> > any more, or because they are no longer controlled by the Bell System
and
> > can do what's singly best for their customer. Or, you might argue that
> Bell
> > Labs (Avaya) was unaware of the dangers of glare that are present when
you
> > don't have ground start as simply lacks the wisdom of your ten years
> > experience. I don't know what there reasons were for abandoning ground
> > start, but I think the reasons I have given above should certainly
> suffice.
> >
> > 8. How do you prevent (or minimize) glare on a two way circuit via
fiber
> > optic, since there is no earth ground? Your answer, if your head firmly
> is
> > not implanted firmly in the sand, should make you come to terms that
glare
> > is independent of ground and, indeed, is independent of any supervision
> > method. It is merely a function of time. No amount of signaling
schemes
> > can eliminate it for two-way circuits. One can only reduce it by
speeding
> > up the communication of the state at each end to the other end or
> lessening
> > the probability that the event will occur by changing the hunt order.
> Even
> > ISDN PRI using common channel signaling is still susceptible to glare on
> > 2-way circuits and still employs inverse hunt order to minimize glare.
> >
> > 9. I have a hard time correlating anything but an overloaded or
> > misconfigured trunk group that would be responsible for the severe
> problems
> > given as horror examples you and the previous writer have related, since
> any
> > glare interval is relatively short if the system is properly configured
> > (inverse hunting sequence) and the probability that a circuit would be
> > simultaneously seized would be relatively infrequent unless it is
severely
> > underdimensioned. How large was the trunk group you had, what was the
> > traffic carried and what was the engineered grade of service? Did you
> check
> > the time of disconnect of the associated central office trunk? What
was
> > the guard interval employed? Did you have immediate ring from the
central
> > office? Did you use change of state for an incoming call with loop
> > supervison? Did you have reverse battery disconnect supervison from the
> CO?
> >
> > Finally, there are two times one should mention their credentials:
> shortly
> > before they're hired and shortly before they should be fired. Lawyers
> argue
> > credentials, scientists argue verifiable facts. Those that mention their
> > credentials to persuade, don't even believe themselves.
> >
> > Tone down your rhetoric, forget your credentials, and think.
> >
> > ----- Original Message -----
> > From: "Bruce Ferrell" <bferrell at baywinds.org>
> > To: <asterisk-users at lists.digium.com>
> > Sent: Saturday, July 31, 2004 1:27 PM
> > Subject: Re: [Asterisk-Users] Digium FXO Interfaces don't support
> > groundstart???
> >
> >
> > > First of all I use this in humor:
> > >
> > > Frank you ignorant slut!
> > >
> > >
> > > I have to disagree on your analysis. I worked in telephone COs
(DMS250,
> > > Stromberg/Carlson) and with PBXes for over a decade. Glare can and is
> > > controlled by ground start signaling. It does so because the ground
is
> > > tested for (or supposed to be) prior to dialing. It's called the
> > > pre-seize condition. On a T1 using robbed bit signaling, tip and ring
> > > conditions are converted into A/B signaling states in the channel
> > > modules of a channel bank.
> > >
> > > Ground start was the prefered signaling system for what was called
> > > Feature Group D trunks between Other Common Carriers and the RBOCs.
> > > Before FGD was available, we used loop start. We had incoming and
> > > outgoing trunk groups, hence no glare... Needless to say expensive.
> > > Because FGD had ground start, to cut interconnect costs, we went there
> > > as soon as it was made available.
> > >
> > > The 150ms pulse you described is called wink start, which was funky.
I
> > > most commonly say it on systems using E&M signaling. Gawd I hated
> those!
> > >
> > > YA know, the asterisk list has been for a lot of walks down memory
lane
> :)
> > >
> > > Frank Cofer wrote:
> > > > Glare cannot be prevented on two way trunks (it is physically
> impossible
> > > > because the two ends are separated in distance and therefore
separated
> > in
> > > > time and any independent decision to use it at one end is never seen
> > > > instantly at the other end).
> > > >
> > > > Ground start does not decrease glare at all (it actually increases
it)
> > and
> > > > use of "ground start to eliminate glare" is a common myth. This is
> > because
> > > > use of ground start (which uses only one side of the pair to earth
> > ground to
> > > > "start" a request for service) increases the time to mark a central
> > office
> > > > line busy when it is seized from the Customer Premise Equipment
(CPE),
> > owing
> > > > to its clunky signaling (150ms earth ground on the ring of the line)
> and
> > the
> > > > fact that it uses only one half of the current to start the line as
> loop
> > > > start. Since it increases the time to signal the distant end, it
> > increases
> > > > glare.
> > > >
> > > > Its only benefit is to the central office because it stops a second
> > seizure
> > > > to the central office when a call disconnects from the central
office
> > end
> > > > first, which would otherwise find a request (loop) as soon as the
> > disconnect
> > > > was effected. This is why "ground start" was introduced by the Bell
> > System
> > > > (when they owned both the PBX and the CO) since it would reduce the
> > attempt
> > > > load on the central office from large business users by 25% or more
> > saving a
> > > > lot of central office gear for a relatively small expenditure on the
> PBX
> > > > end. Ground start has some ugly drawbacks, since it reduces
signaling
> > > > range, requires the normally isolated floating pair to be referenced
> to
> > > > earth ground (which exposes the circuit to longitudinal spikes,
noise
> > and
> > > > lightning) and requires the circuit to be muted during the
imbalanced
> > > > condition that occurs when the ring conductor is momentarily
grounded
> to
> > > > draw dial tone. Digium is right to leave it out. Most other
> informed,
> > > > modern manufacturers do likewise.
> > > >
> > > > "Ground start" signaling referred to in T1 (which is an absurd label
> > since
> > > > there is no ground placed on a T1) is really after the Grey Code
(only
> > one
> > > > signaling bit transitions at a time) and has nothing to do with
glare
> or
> > > > ground start signaling and is just a carry over label.
> > > >
> > > > Glare can be reduced by changing the hunt order from either end and
to
> > > > employ faster signaling. The former method decreases the likelihood
> > that
> > > > both ends will compete for the circuit at the same time and the
latter
> > > > reduces the window that a commitment has been made at one end and is
> > still
> > > > not known by the other end. Typically, the CO is set to hunt
> ascending
> > and
> > > > the CPE descending and this is still employed even in ISDN circuits.
> > This
> > > > is a "terminal hunt" and NOT a "round robin" hunting sequence. If
you
> > want
> > > > to absolutely eliminate glare, use one way (incoming/outgoing only)
> > > > circuits. I believe asterisk has a feature to set the hunt order
> > > > preference.
> > > >
> > > > The disconnect problems you experienced with your Agilent PBX may be
> > more
> > > > likely related to the "guard interval" that a circuit is left alone
at
> > your
> > > > end after it is used. Though "ground start" will appear to fix it,
> > there
> > > > are some issues of CO message rate three way calling that have
caused
> > grief
> > > > (the CO interprets the next call as a flash for a three way call and
> > holds
> > > > the circuit rather than disconnecting it). This phenomena may have
> been
> > > > misdiagnosed as glare, since the message unit 3-way calling was
> imposed
> > as a
> > > > default feature in certain jurisdictions. Increasing the guard
> interval
> > to
> > > > 2 or 3 seconds will suffice, or specify to the carrier that the
3-way
> > > > calling is to be denied for your lines.
> > > >
> > > > Hope this helps.
> > > >
> > > > ----- Original Message -----
> > > > From: <ghost at silcon.com>
> > > > To: <asterisk-users at lists.digium.com>
> > > > Sent: Thursday, July 08, 2004 4:55 AM
> > > > Subject: [Asterisk-Users] Digium FXO Interfaces don't support
> > groundstart???
> > > >
> > > >
> > > >
> > > >>Hi All,
> > > >>
> > > >>I was surprised to be told by a Digium support person today that
> > Digium's
> > > >>FXO interfaces (X100P, TDM400P FXO modules) don't support
groundstart
> > > >>signalling. This surprises me because as far as I know in a typical
> PBX
> > > >>configuration with analog trunk lines, groundstart signalling is the
> > only
> > > >>way to prevent Glare.
> > > >>
> > > >>I just purchased two TDM400P's for a system I'm building to replace
> our
> > > >>office PBX (Altigen). Since there are no statements anywhere on
> > Digium's
> > > >>website about lack of groundstart support (Actually, to the contrary
> > they
> > > >>boast about all the signalling support in their sales slick), I now
> need
> > > >>to decide if I want to return the products and switch to a T1 /
> channel
> > > >>bank configuration.
> > > >>
> > > >>I remember when we setup our current Altigen PBX, we had problems
with
> > > >>glare and disconnect detection and so I went through the process of
> > > >>figuring out what was going on and learning about groundstart.
After
> we
> > > >>switched to groundstart everything worked great.
> > > >>
> > > >>In a high use system, it's highly likely that a trunk will
experience
> > > >>glare, which is annoying for incoming callers and system users.
I'm
> > just
> > > >>a bit baffled as to why Digium wouldn't support groundstart on cards
> > > >>designed to be PBX trunk lines.
> > > >>
> > > >>Someone please tell me I'm missing something.
> > > >>
> > > >>Mark
> > > >>
> > > >>
> > > >>
> > > >>_______________________________________________
> > > >>Asterisk-Users mailing list
> > > >>Asterisk-Users at lists.digium.com
> > > >>http://lists.digium.com/mailman/listinfo/asterisk-users
> > > >>To UNSUBSCRIBE or update options visit:
> > > >> http://lists.digium.com/mailman/listinfo/asterisk-users
> > > >>
> > > >
> > > >
> > > > _______________________________________________
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