Lucent Technologies DEFINITY Communications System Generic 2.2 And Generic 3 V2 DS1/CEPT1/ISDN PRI Reference Manual

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-15 ISDN PRI Layer 3 
5
1. Assign interface identifier 0 to each board containing a D 
c hannel.
2. Assign interface identifier 1 to each board containing a 
b ac kup  D c hannel.
3. For eac h b oard  c ontaining  all B c hannels, assig n an 
interfac e id entifier. Assig n id entifiers sequentially starting  
with 2 if the D c hannel g roup  has a b ac kup  D c hannel, or 1 if 
it d oes not. In other word s, all b oard s in a D c hannel g roup 
should  b e numb ered sequentially starting  with 0 and  not 
skip p ing  any numb ers.
Both end s of a trunk must terminate on b oards with the same 
c hannel ID. For examp le, if a c all from a G3V2 g oes out a trunk on 
a n  N FA S b o a rd  t o  t h e  4 ESS,  t h e  4 ESS kn o w s  f ro m  t h e  i n f o r m a t i o n  in  
the D c hannel that a c all is c oming  in on c hannel 10, say. But, sinc e 
t h is  i s  a n  N FA S c a ll ,  t h e  4 ESS m u st  k n o w  w h i c h  c h a n n e l 1 0  t h e  c a ll  
is inc oming  on. It g ets this information from the interfac e ID. 
Ob viously, the two sides must agree at p rovisioning  time to numb er 
their interfac es the same on both sides. In other word s, if trunk n on 
one system is on interfac e m, trunk n must also b e on interfac e m 
on the other system.
NOTE:
A DS1/CEPT1 board is defined as being a member of a 
sig naling  g roup  if any of the B c hannels on the b oard are in 
the sig naling g roup . For examp le, if c hannels 1-10 on a DS1 
b oard  are assig ned  to sig naling  g roup 1 and  c hannels 11-24 
are assig ned  to sig naling  g roup  2, then the board  is a 
memb er of both sig naling  g roup s 1 and  2. This is important 
b ec ause an interfac e id entifier c annot b e d up lic ated  within a 
sig naling  group . Thus, if another b oard  in the ab ove examp le 
c ontained  all its B c hannels from sig naling  g roup  2, it c ould  
not have an interfac e id entifier of 1 b ec ause that id entifier has 
alread y b een used . For this reason, and in g eneral for ease of 
ad ministration and  maintenanc e, you should  not mix B 
c hannels from d ifferent sig naling  g roups on a b oard .
nSome c ountries sup p ort the field  Trunk Group for Channel 
Selection
. You c an enter a valid  ISDN PRI trunk g roup  here. If the 
c ountry sup p orts this feature, a B c hannel from this trunk g roup  will b e 
selec ted  when a SETUP messag e is rec eived  that does not exp lic itly 
ind ic ate the B c hannel to be used  for the c all.
nIn the Group Type
 field  of the trunk g roup  form enter the g roup  typ e 
isdn-pri
. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-16 ISDN PRI Layer 3 
5
nIn the Group Memb er Assig nments p ag e of the ISDN PRI trunk g roup  
form, p ut the d esired  sig naling  g roup of the trunk into the Sig Grp
 field s. 
The entry in this field must be a signaling group containing the board 
c ontaining  this trunk member. If the memb ers on a b oard  are in only one 
signaling  g roup , this information is filled  in automatic ally b y the system.
Call Control Signaling
The layer 3 p rotoc ol d esc rib ing  the p roc ed ures for estab lishing, maintaining , and  
c learing  c irc uit switc hed  c alls is c alled  c all c ontrol. The c all states, p roc ed ures, 
messag es, and  information elements for this p rotoc ol are d esc rib ed  in the 
various spec ific ations. The DEFINITY systems imp lement this protoc ol b ased  on 
these sp ec ific ations.
User-Network and Peer Protocols
The DEFINITY systems emp loy two typ es of layer 3 p rotoc ols, user-network and  
p eer. A user-network p rotoc ol means that one sid e of the interfac e is d esig nated  
as the user and  the other sid e as the network. A p eer p rotoc ol means that b oth 
sid es of the interfac e ac t as user. Connec tions b etween a DEFINITY system and 
another PBX, a p ub lic  network switc h, or a host c an all use a user-network 
p rotoc ol. A p eer p rotoc ol is d esigned  for use b etween two PBXs. The G2.2 and  
G3V2 systems employ varying protocols, as follows:
nG2.2 systems emp loy a p eer p rotoc ol as d efined  in the AT&T ISDN PRI 
sp ec ific ation.
nG3V2 systems employ both the user-network protocol (with variations for 
each country protocol option) and the peer protocol specified by the 
Europ ean Comp uter Manufac turers Assoc iation (ECMA). The op tion exists 
in ad ministration to use either p rotoc ol.
NOTE:
The ECMA protocol does not support the following information 
elements:
— Network Sp ecific Facilities
—Display
— Transit Network Selec tion
— Distrib uted  Communic ation System
— DCS AUDIX Messag ing
— Lookahead  Interflow
— Traveling  Class Mark
Some of these IEs c an b e sup pressed b y ad ministration.For examp le, in the trunk 
group form if Send Name
 is set to no
, the d isp lay IE will not b e sent. If 
ad ministration allows these IEs to b e sent, however, no harm will oc c ur — the far  
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-17 ISDN PRI Layer 3 
5
end  will simp ly d rop  them. If you need  these IEs, however, you should  use the 
user-network protocol. 
For more information on these information elements and  how they are used , see 
the sec tion ‘‘Call Control Messages’’
 later in this c hap ter.
Bec ause the p eer protoc ol d oes not sup p ort these information elements and  for 
other reasons, some features availab le with the user-network p rotoc ol are not 
available with the ECMA peer protoc ol. Tab le 5-2
 shows the availab ility of 
imp ortant features using  the p eer p rotoc ol:
NOTE:
When c onnec ting  two DEFINITY systems via ISDN PRI, you must use the 
user-network p rotoc ol and  c ountry op tion 1 if you want the extra features 
supported by option 1.
The d etails of these p rotoc ols and  their d ifferenc es are larg ely transp arent to 
users. Their primary purpose is to define the call states each end should be in 
d epend ing  on the messag e rec eived . One asp ec t of these p rotoc ols the user 
d oes see, however, is the way in whic h g lare is hand led . Glare is the 
1. ECMA sup p orts Name Delivery as a supp lementary servic e. Althoug h 
D92 sup p orts Name delivery, it d oes not sup port the feature using  the 
ECMA imp lementation. 
Table 5-2. Feature Availability using the Peer Protocol
Feature Functional over Peer Interface?
DCS over PRI no
Traveling  Class Mark no
Uniform Dial Plan yes
Main/Satellite yes
Wideband no
NFAS no
D-Channel Bac k-up no
LookAhead  Interflow no
Name Delivery no
1
Calling  Party Numb er 
Deliveryyes
Connec ted Numb er Delivery yes 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-18 ISDN PRI Layer 3 
5
simultaneous seizing  of a trunk b y b oth end s. In g eneral, g lare c an b e hand led  in 
one of the following  ways:
nOne sid e wins and  seizes the trunk while the losing  side c all fails. In this 
c ase, the losing  system c an take ac tions suc h as automatic ally retry the 
c all or simp ly ap p ly reord er tone.
nOne sid e wins and  seizes the trunk and  send s a messag e to the losing  
side req uesting it to negotiate its c all to a d ifferent B c hannel. This is 
c alled  c hannel neg otiation on g lare. The losing  system must sup p ort this 
c ap ab ility by hold ing  the c all long  enoug h to rec eive the messag e 
req uesting  it to neg otiate the c all to another B c hannel. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-19 ISDN PRI Layer 3 
5
G2.2 Protocol and Glare Handling
In the G2.2 p eer p rotoc ol, the G2.2 is always the user sid e at layer 3 and  is not 
c ontrollab le b y ad ministration. In a g lare situation the G2.2 uses the layer 2 
network-user b it setting  in p roc ed ure 262, word  1, field  6 to d etermine if it wins or 
loses. If it is set to network, it wins and attemp ts to neg otiate the other sid es c all 
to another B c hannel. If this b it is set to user, it loses and  sup p orts the far end s 
attemp t to neg otiate the c all to another B c hannel. This is true only if the c all was 
d estined  for a p referred c hannel, whic h is always the c ase unless the c all is a 
DCS c all. If the c all is a DCS c all or if no B c hannels are availab le, the c all will fail 
unless p roc ed ure 100 word  3 field  4 is set to retry the c all.
G3V2 Protocol and Glare Handling
The G3V2 systems c an be ad ministered  for either the user-network p rotoc ol or 
the ECMA p eer p rotoc ol. In G3V2 systems, when you ad minister the 
user-network p rotoc ol, b oth the layer 2 and  layer 3 user-network p rotoc ol is 
ad ministered . For examp le, in a G3V2 system, if you ad minister the system as 
user, it is user at both layer 2 and  layer 3. 
You c an also ad minister the ECMA p eer p rotoc ol in G3V2 systems. At layer 3 in 
the ECMA p eer p rotoc ol, the G3V2 is either sid e A or sid e B. For purp oses of 
resolving  glare, sid e A is roug hly eq uivalent to the network sid e in a user-network 
p rotoc ol and  sid e B is roughly eq uivalent to the user sid e. Thus, ad ministering  
p eer-master sid e A is roug hly eq uivalent to ad ministering  network at layer 3 and 
network at layer 2. Administering  p eer-master sid e B is roug hly eq uivalent to 
ad ministering  network at layer 3 and  user at layer 2. Ad ministering  p eer-slave 
sid e A is roug hly equivalent to administering  user at layer 3 and  network at layer 
2. Administering  p eer-slave sid e B is roug hly eq uivalent to ad ministering  user at 
layer 3 and  user at layer 2.
In the user-network p rotoc ol in a g lare situation, the outgoing c all from the 
network sid e always wins. In the p eer p rotoc ol, the winning  sid e d ep end s on 
whether it is sid e A or B and also whether the outg oing  c all has req uested  a 
p referred or exc lusive c hannel in the c hannel ID IE. The following tab le 
summarizes whic h outg oing  c all wins in a p eer p rotoc ol:
In b oth the user-network p rotoc ol and  the p eer p rotoc ol, onc e the winner has 
b een d etermined , the following events take p lac e:
1. If the G3V2s outg oing  c all wins and  the inc oming  (losing ) c all is c od ed  as 
p referred, the G3V2 attemp ts to neg otiate the inc oming  c all to an alternate 
B c hannel as follows:Side ASide B
preferred exclusive
p referred A wins B wins
exc lusive A wins A wins 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-20 ISDN PRI Layer 3 
5
— If the G3V2 find s an alternate B c hannel it send s a resp onse to the 
inc oming  SETUP message ind ic ating  the alternate B c hannel.
— If the G3V2 c annot find  an alternate B c hannel it drop s the inc oming  
call.
2. If the G3V2s outg oing  c all wins and  the inc oming  (losing ) c all is c od ed  as 
exc lusive, the G3V2 drops the incoming call.
3. If the G3V2s outg oing  c all loses, even if the winning  sid e has neg otiated  
the c all to an alternate B c hannel, it will d rop the c all and  retry the c all on 
another B c hannel.
NOTE:
When c onnec ted  to a G2.2 system and  using  the user-network p rotoc ol, the 
G3V2 system must b e ad ministered  as network and  the G2.2 system as 
user. This is b ec ause the user/network administration on the G2.2 is only 
implemented  for layer 2. The layer 3 user/network relationship in G2.2 is 
user b y default, whic h means the layer 3 user/network p rotoc ol must b e 
network in the G3V2. If ad ministered  as user on b oth sid es, the G3V2 
system will tear d own c alls d uring  the c all setup  p hase if it d etec ts an 
inap p rop riate c all state d uring  an aud it of the G2.2s c urrent c all state 
status. In G3rV2 the freq uenc y of the aud its is muc h g reater than in G3iV2 
and  servic e d isrup tion will b e ap p arent if b oth sid es are user. Thus, sinc e 
the G3V2 must b e ad ministered  as network, the layer 2 user/network is also 
network, whic h in turn means the layer 2 user/network on the G2.2 must b e 
user. If both sides are ad ministered  as network or as user at layer 2, the c all 
will always fail b ec ause a layer 2 c onnec tion c annot b e mad e.
For information on the layer 2 p rotoc ol, see the sec tion ‘‘Layer 2 User-Network 
and Master-Slave Protocol’’ earlier in this c hap ter.
Minimizing the Occurrence of Glare
The c hanc es that two sid es attemp t to seize the same trunk simultaneously are 
muc h less in ISDN PRI trunks than with other trunks. Thus, it is usually not 
imp ortant to try to minimize the oc c urrenc e of g lare. The one c ase where 
minimizing the oc currence of glare is essential, however, is on DCS trunks. When 
a c all is made on a DCS trunk g roup, the trunk numb er is p ut into the DCS 
messag e. If the trunk numb er in the messag e d oes not matc h the trunk c arrying  
the c all, DCS transp arenc y will fail. Thus, a DCS c all c annot b e neg otiated  to 
another c hannel. For this reason, you c an d o the following  to minimize g lare on 
DCS trunk g roup s that use ISDN B c hannels, or to minimize g lare on any ISDN 
PRI trunk g roup  if it is a c onc ern:
nDivid e the ISDN PRI trunk g roup  into two trunk g roup s.
nAssig n the two trunk g roup s to two sep arate p referenc es in a p attern on 
one system.
nReverse the order of the p referenc es on the other system. For examp le if 
the first p referenc e on one system is trunk group  1, then the first 
p referenc e on the other system is trunk g roup  2. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-21 ISDN PRI Layer 3 
5
Another b etter method  to minimize the oc c urrenc e of g lare on ISDN PRI trunks is 
availab le only in G3V2. In G3V2 you c an ad minister the trunk hunting  d irec tion for 
the ISDN PRI trunk group  as asc end ing  on one system and  as d esc end ing  on the 
other system. This will make it unlikely that the same trunk (B c hannel) will b e 
c hosen for the two c alls.
Digit Collection
The intention of the ISDN PRI sp ec ific ations is that all d ig its b e c ollec ted  and sent 
in the SETUP messag e for a c all. This is not an ab solute req uirement, however. 
Two method s exist for c ollec ting  d ig its on ISDN PRI c alls, as follows:
nISDN PRI c alls routed  b y networking  software c ollec t all d ig its and  send 
them in the SETUP messag e.
nISDN PRI c alls mad e b y d ialing  a trunk ac c ess c od e d o not c ollec t the 
d ig its. Instead , the SETUP messag e is sent without the d ig its. The d ig its 
are then sent in-b and  as with non-ISDN c alls.
G2.2 d oes not sup p ort the use of trunk ac c ess c od e d ialing  on ISDN PRI trunks. 
The p rimary p rob lem with TACs on G2.2 systems is that all d ig its must be d ialed  
b efore expiration of the T310 layer 3 timer (10 sec ond s) otherwise the c all will b e 
torn d own. Thus, this limits the numb er of switc hes you c an c ut throug h within the 
allotted time. G2.2 systems bloc k ISDN PRI c alls that try to use a trunk ac c ess 
c od e to d ial the c all.
G3V2 systems sup p ort trunk ac c ess c ode d ialing  on ISDN PRI trunks that are not 
c all b y c all trunks or nod al servic es trunks. When a G3V2 system rec eives a 
SETU P m e s s a g e  w i t h  n o  d i g it s ,  i t  re t u r n s a  PRO G RESS m e s sa g e ,  w h i c h  
sup p resses the layer 3 timers. Thus, for c alls d ialed  with trunk ac c ess c od es to 
b e g uaranteed  to work, all switc hes in the c all p ath must b e G3V2s (or G1s or 
G3V1s).
For c alls routed with AAR or ARS, G3V2 systems imp lement d igit c ollec tion b y 
automatic ally c ollec ting  all d ig its before trunk seizure for all c alls in a p attern 
starting  with the first ISDN p referenc e enc ountered . For examp le, if the third  
p referenc e in a p attern is the first oc c urrenc e of an ISDN trunk g roup , all dig its 
will not b e c ollec ted  b efore trunk seizure for c alls on the first two p referenc es. All 
d ig its will b e c ollec ted  b efore trunk seizure starting  with c alls on the third 
p referenc e and c ontinue for all sub seq uent p referenc es reg ard less of whether 
they are ISDN trunk g roup s. Thus, if you want to c ollec t all d ig its b efore trunk 
seizure on some non-ISDN c alls, you c an administer the first p referenc e of a 
p attern as an ISDN trunk group  c ontaining  no memb ers. All c alls on sub seq uent 
p referenc es will have all d ig its c ollec ted  b efore trunk seizure. This ad ministration 
is d one in the routing  p attern form. There are b oth ad vantag es and  
d isad vantag es to ad ministering  the p atterns this way, however, as follows: 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-22 ISDN PRI Layer 3 
5
nAdvantages
— No resourc es are tied  up  for c alls not c omp letely d ialed  
(ab and oned  c alls).
— Call hold ing  times are shortened.
nDisadvantages
— Call setup  times (the time from the last d ialed  d ig it to alerting on the 
far end ) will leng then for non-ISDN PRI c alls, esp ec ially over 
d ial-p ulse trunks.
— Trunk queuing will not be possible because the first preference is 
the only trunk group  used for q ueuing and  it has no memb ers. 
Digit Receiving
Althoug h DEFINITY systems c ollec t all dig its and  send  them in a sing le SETUP 
messag e, not all systems are c onstrained  to this method . Some vend ors 
eq uip ment use overlap  send ing  p roc ed ures in whic h the d ig its are sent as they 
are rec eived  starting  with the first d ig its in the c alled  p arty numb er IE in the 
SETUP messag e and  the rest of the d ig its in the c alled  p arty numb er IE in 
sub seq uent INFORMATION messag es. Althoug h the DEFINITY systems d o not 
support overlap sending, the G3V2 systems do support receiving digits from 
systems using  overlap send ing. This new c ap ab ility is c alled  overlap  rec eiving  
and is available only on G3V2 systems. You administer overlap receiving on 
p age 1 of the trunk g roup  form.
At the p resent time in the U.S., p ub lic  network ISDN d oes not emp loy overlap  
send ing . Thus, in the U.S. you should  not use this op tion. If you set overlap  
rec eiving  and  the d ig its are all d elivered  in the SETUP messag e, the c all 
completion might be delayed while the system waits for added digits in the 
INFORMATION message.
Call Control Messages
ISDN PRI call c ontrol messag es are those messag es involved  in estab lishing , 
maintaining , and  terminating  c irc uit switc hed  c onnec tions. These messag es are 
c omp osed of a series of oc tets (b ytes) of information. One or more of these 
oc tets c omp ose an information element (IE). In any g iven c all c ontrol message, 
some of these IEs are used  for c all c ontrol sig naling  while others are used  for 
user-to-user information signaling. For example, within the SETUP message, 
whic h is a c all c ontrol messag e, is the c hannel id entific ation IE, whic h is used  for 
c all c ontrol signaling , and  the low layer c omp atib ility IE, whic h is used  for 
user-to-user information signaling.
Some IEs not defined as user-to-user information IEs can also be used to convey 
user-to-user data. The calling party number, for example, which is defined as a 
c all c ontrol IE, is often used  b y the end  user for ap p lic ations suc h as terminal  
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-23 ISDN PRI Layer 3 
5
d isp lays. The IEs used  for user-to-user information sig naling  are d esc rib ed later 
in the sec tion ‘‘User-to-User Information Sig naling ’’
.
The first three information elements of all ISDN PRI messages are the same — 
the p rotoc ol d isc riminator IE followed  b y the c all referenc e value IE, followed  b y 
the message typ e IE. The remaining  IEs in any messag e vary, d ep end ing  on the 
messag e. All messag es, IEs, and  their p urp oses are d efined  in the ISDN PRI 
sp ec ific ations. 
Some information elements are req uired  b y the p ub lic  network (suc h as the 4ESS 
toll switc h, for examp le) while other IEs have meaning  only to the end users. For 
e xa m p le ,  t h e  4 ESS m u st  kn o w  t h e  i n f o r m a t io n  t r a n s f e r  c a p a b i li t y ( re st ri c t e d  o r  
unrestric ted ) so that it c an route the c all over the p rop er fac ilities. The 4ESS d oes 
not have to know the c ontents of the d isp lay information element, however, sinc e 
it is used  only to d isp lay information on the end  users terminal. In ord er to 
d isting uish b etween information need ed  or not need ed  b y the p ub lic  network, the 
c onc ep t of c od esets exists.
Information elements c oded  as b eing  p art of c od eset 0 is information need ed  b y 
the p ub lic /international network and is d efined  b y the CCITT stand ard . 
Information elements in c od esets 1-4 are reserved  for future stand ard s. Cod eset 
5 is used  for the p ub lic  network in the U.S. Cod eset 6 is used  b y p rivate 
networks. Cod eset 7 is information need ed  b y the end user. In prac tic e, the 
d ivision b etween c od esets 6 and  7 is not always c lear b ec ause c od eset 6 
information often looks like end  user information.
At p resent, the DEFINITY systems automatic ally p ut all IEs exc ep t for three into 
their p rop er c od esets. The three exc ep tions are the d isp lay, lookahead  interflow, 
and  traveling  c lass mark IEs. By d efinition, IEs sent in c od esets 6 or 7 are used  
for user-to-user information sig naling  and are thus d isc ussed  later in that sec tion 
of this document. All DEFINITY systems allow some degree of codeset mapping, 
whic h is the c ap ab ility to sp ec ify whic h c od eset an IE will b e sent in. G3V2 
systems limit this mapp ing  to user-to-user information elements. G2.2 systems, 
however, allow any IEs to b e map p ed  to any c od eset. In theory, this c ould  b e 
d one for b oth c all c ontrol and  user-to-user information sig naling . In p rac tic e, 
however, it is used  to map  user-to-user information elements. Thus, c odeset 
mapping is discussed later in the section ‘‘User-to-User Information Sig naling ’’
.
NOTE:
Pub lic  network switc hes in many c ountries d o not p ass c od eset 6 and  
codeset 7 IEs.
The following  sec tions d isc uss the c all c ontrol IEs having  relevanc e to p ersonnel 
imp lementing  or sup p orting  ISDN PRI c ap ab ilities on the DEFINITY systems.
Cause IE
Cause IEs are information elements used  p rimarily to c onvey the reasons for c all 
failures. For examp le, if the rec eiving switc h rejec ts a voic e c all b ec ause it is 
inc oming  on a trunk ad ministered  for mod e 1 d ata only, the rec eiving  switc h  
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Layers 2 and 3 
Page 5-24 ISDN PRI Layer 3 
5
c ould  return the c ause value for bearer capability not implemented
 in 
the DISCONNECT messag e. Cause values are valid  in only some messag es. The 
list of c ause values and their meaning s c an b e found  in the Q.931 sp ec ific ations. 
Note that the titles of the c ause values sug g est the situations in whic h they 
should  b e used , b ut the ultimate d ec ision as to whic h c ause value to use for a 
c ertain situation lies with the d esigners of eac h system. In other word s, the 
sp ec ific ations d o not g ive rig orous rules for whic h c ause values must b e used  for 
all possible situations.
Cause information elements are imp ortant to anyone using  a messag e trac ing  
tool to troub leshoot ISDN PRI p rob lems. They are also imp ortant to the d esig n of 
features. For examp le, d ep end ing  on the c ause value rec eived , the rejec ted c all 
mig ht b e sent to interc ep t or to an attend ant.
Bearer Capability IE
The b earer c apab ility IE is sent b y the near end  switc h to ind ic ate that a c all with 
c ertain c haracteristic s is being made. For example, if the endpoint on the switch 
makes a d ig ital d ata c all, the b earer c ap ability IE c onveys information used  b y 
the switc h at the other end  of the link to let it know it need s to sup p ort a d ig ital 
d ata c all. If the d estination is a voic e terminal, for examp le, rec eiving  the b earer 
c ap ab ility IE will tell the rec eiving  switc h that it will not b e ab le to sup p ort the c all 
and  to take app rop riate ac tion, suc h as b loc k the c all.
The b earer c ap ab ility IE is used  to ind ic ate the following  c harac teristic s of a c all:
nWhether the c all is a c irc uit-switc hed c all or a p ac ket mod e c all
nWhether the c all is sp eec h/voic e-g rad e d ata (3.1 KHz), or d ig ital d ata
nThe information transfer c ap ab ility (restric ted  or unrestric ted )
nOther information either transp arent to users or not p resently used  b y 
DEFINITY systems
The b earer c ap ab ility IE is used  b y DEFINITY systems in c omb ination with the 
low layer comp atib ility IE for the following  p urp oses:
nBloc king  inc omp atib le c alls
nRouting calls to desired trunk groups
nDetermining  c ompatib ility of end p oints
nDetermining when modem pool resources should be inserted
These d ec isions are mad e b y c omp aring  the information from the BC and  LLC 
IEs with the ad ministered  c all hand ling  c ap ab ilities of end p oints, trunk g roup s, 
and  routing  p referenc es. These administered  c ap ab ilities are c alled  b earer 
c ap ab ility c lasses (BCCs) and  information transfer c apab ilities (ITCs). A BCC 
has elements of b oth the BC IE and  the LLC IE. For examp le, the Mod e 2 BCC 
inc lud es the d igital d ata c harac teristic  of the BC IE and  the user rate 
c harac teristic  of the LLC IE.