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
Physical Layer 
Page 4-67 Synchronization 
4
Conclusions on Synchronization
Make no assump tions reg ard ing  sync hronization. Reverify items suc h as the 
availability of a synchronization source, the clock stratum, and compatibility of 
every T1/E1 sp an. The b est g uarantee is written c onfirmation that the loc al 
exc hange c arrier, AT&T Communic ations, or other vend or will either sync hronize 
to a System 75, System 85, Generic  1, Generic  2, or Generic  3 or p rovide an 
ap prop riate sync hronization referenc e.
Develop a d iag ram showing  the network sync hronization p lan. Make multip le 
c op ies of this d iagram and  keep  a c op y at eac h switc h site. This d iag ram is 
essential for installing , ad ministering , and  
tuning  up a DS1/CEPT1 network. Suc h 
a d iagram c an also b e used b y maintenanc e p ersonnel to troub leshoot network 
p rob lems assoc iated  with sync hronization.
A p roperly d esig ned sync hronization plan will imp rove the q uality and  reliab ility 
of a d ig ital p rivate network. If d esig ning  a sync hronization p lan b ec omes too 
c omp lex or if many ap p lic ations have a c ruc ial d ep end enc y on the d ig ital 
facilities, then the AT&T Qualnet Synchronization Design Service should be 
c onsulted .
Use of G2.2 as a System Clock Reference
To p rovid e inc reased  reliab ility, it is rec ommend ed  that (for all c ases) the p rimary 
and  sec ond ary system c loc k referenc es be p lac ed  in d ifferent mod ules. Tip s on 
how b est to use a DS1 as a c loc k referenc e are p rovid ed  next. Misc onc ep tion 4 The OIU-2 is not req uired  in a D4-c hannel bank sinc e all 
DEFINITY mod els are D4 c omp atib le.
Fac t 4 DEFINITY is (when op erating  in D4 mod e) c omp atib le with 
D4-formatted  DS1 or T1 fac ilities. However, a DS1 was not 
d esig ned  to b e func tionally the eq uivalent of a c hannel unit. 
D4-c hannel b anks use line-p owered  osc illators to p rovid e 
their c loc king . Given the normal drift in an AC line, the 
transmit frequenc y of a D4-c hannel b ank c an vary 
sig nific antly. The OIU ad ds a stratum-4 c loc k to the 
common equipment of a D4, allowing the transmit and 
rec eive sec tions to b e c ommonly sync hronized  to:
nThe incoming line (loop timed)
nFree-running  (stratum 4 ac c urac y)
nWith external referenc es that c an b e ap p lied  to b oth 
DS1 sp ans, you c an selec t either DS1 sp an as the 
referenc e. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Physical Layer 
Page 4-68 Synchronization 
4
ISDN-PRI Trunk Facilities
I SD N -PRI  t r u n k s  m a y  t e r m in a t e  o n  a  4 ESS t o l l o ff i c e ,  5 ESS d i g it a l C O ,  Sy st e m  8 5  
R2V4, DEFINITY c ommunic ations systems, or c omp atib le vend or’s switc h. 
Dep end ing  on other c onsid erations, any of these terminating  c onnec tions may 
b e selec ted  as either a p rimary or sec ond ary sync hronization referenc e. 
ISDN-PRI c onfig urations may be estab lished  with an ANN35, with a TN464C, or 
with a TN767 with or without a TN555, d ep end ing  up on the D c hannel 
c onfig uration of the TN767. A b rief d esc rip tion of these board s and  their g eneral 
use for ISDN-PRI is g iven next.
The ANN11 is the DS1 b oard  used  with trad itional modules in Generic  2.2. The 
ANN35 is the ISDN-PRI b oard  used  with Generic  2.2 trad itional mod ules. The 
TN767 is the DS1/ISDN-PRI b oard  used  with Generic  3V2, and  Generic  2.2 
universal mod ules. The TN555 is the p ac ket ad junc t used  with Generic  2.2 
universal mod ules. If the TN767 has a D c hannel on it, it need s a TN555 loc ated  
in the adjac ent virtual slot. The TN464C is the universal DS1/CEPT1 b oard  used  
in the Generic  3r and  Generic  3i–Glob al for all DS1 and  CEPT1 ap p lic ations, 
includ ing  ISDN PRI.
NOTE:
The Generic  3iV2 c ommunic ates with the D c hannel via a TN765 (PI) 
interfac e c irc uit and does not use the TN555.
Line-Only Mode DS1/DMI-BOS (ANN11 or 
TN767)
When using  a line-only mod e interfac e for sync hronization p urp oses, the 
following  fac ts should  b e known:
nSinc e the D4-c hannel b ank (or eq uivalent) at the far end  will b e timed  to 
the sig nal rec eived  from a DS1, no slips should  oc c ur on a line-only mod e 
DS1 fac ility. Bec ause of this, slip  c ounts from line-only mod e DS1s should  
not b e used  to determine the health of a c loc k referenc e (that is, they 
would  tend  to make the referenc e ap p ear to b e in b etter c ond ition than it 
may b e).
nLine-only mod e DS1s should  not b e used  as system c loc k referenc es 
unless it is known for certain that the incoming DS1 is locked to the public 
network referenc e freq uenc y. Normally, a DS1 fac ility terminating  d irec tly 
on a far-end  D4-c hannel b ank is not loc ked  to the AT&T referenc e 
freq uenc y. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Physical Layer 
Page 4-69 Synchronization 
4
Line+Trunk Mode DS1/DMI-BOS (ANN35 or
TN767 with TN555)
Reg ard ing  the use of a line+ trunk mod e DS1/DMI-BOS for sync hronization, the 
following  p oint should  be noted . Sinc e it would  b e exp ec ted  that a line+ trunk 
mod e DS1 would  terminate at a c lass-5 or hig her CO or at another switc h, the 
line+ trunk interfac e should  b e suitab le for use as a primary or sec ond ary c loc k 
referenc e (or to be the timing  master for another switc h). 
The slip  c ount p rovid ed  b y the interfac e should  also b e used  in the p roc ess of 
c hoosing  a healthy c loc k referenc e. However, when the interfac e terminates on a 
D4-c hannel b ank (or equivalent) that is not loc ked  to the AT&T referenc e 
freq uenc y for its timing , then the interfac e should  not b e used  for 
sync hronization.
Use of G3V2 as a System Clock Reference
Tip s on how best to use DS1/CEPT1 as a c loc k referenc e are p rovid ed  next.
Trunk-Mode ISDN-PRI 
I SD N -PRI  t r u n k s  m a y  t e r m in a t e  o n  a  4 ESS t o l l o ff i c e ,  5 ESS d i g it a l C O ,  Sy st e m  8 5  
R2V4, Generic  1, Generic  2.2, Generic  3V2, or c omp atible vend or’s switc h. 
Dep end ing  on other c onsid erations, any of these terminating  c onnec tions may 
b e selec ted  as a sync hronization referenc e, either p rimary or sec ond ary.
Trunk-Mode Interface (ISDN-PRI + Robbed Bit) 
Sinc e it is exp ec ted  that a trunk mod e DS1 would  terminate at a c lass-5 or hig her 
CO or at another switc h, the trunk interfac e should  b e suitable for use as a 
p rimary or sec ond ary c loc k referenc e (or to b e the timing  master for another 
switch).
The slip  c ount p rovid ed  b y the interfac e should  also b e used  in the p roc ess of 
c hoosing  a healthy c loc k referenc e. However, when the interfac e terminates on a 
D4-c hannel b ank (or equivalent) that is not loc ked  to the AT&T referenc e 
freq uenc y for its timing , then the interfac e should  not b e used  for 
sync hronization.
Line-Only Mode DS1/DMI-BOS
When using  a line-only mod e interfac e for sync hronization p urp oses, the 
following  fac ts should  b e known:
nSinc e the far-end  D4-c hannel b ank (or equivalent) is timed  to the sig nal 
rec eived  from a DS1/DMI, no slips should  oc c ur on a line-only mod e 
DS1/DMI fac ility. Bec ause of this, slip  c ounts from line-only mod e 
DS1/DMI should  not b e used  in the p roc ess of d etermining the health of a 
c loc k referenc e (that is, they would  tend  to make the referenc e ap p ear to 
b e in b etter c ond ition than it may b e). 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Physical Layer 
Page 4-70 Synchronization 
4
nLine-only mod e DS1/DMI should  not b e used  as system c loc k referenc es 
unless it is known for c ertain that the inc oming  DS1/DMI sig nal is loc ked  to 
the AT&T referenc e freq uenc y. Normally, a DS1/DMI fac ility terminating  
d irec tly on the far-end  D4-c hannel b ank is not loc ked  to the AT&T 
referenc e freq uenc y.
G2.2 Synchronization Implementation
To imp lement sync hronization on a G2.2, you must d o the following :
nWire the TN463 c orrec tly d ep end ing  on whether the system is sing le 
module or multi module and depending on whether the DS1 board to be 
used  as a sourc e is on a trad itional or universal mod ule as follows:
— The g roup  334 c ab le is wired  from the sync hronization sourc e DS1 
b oard  to the TN463 in a multimod ule system when the DS1 board  is 
in a trad itional mod ule.
— The g roup  361 c ab le is wired  from the sync hronization sourc e DS1 
b oard  to the TN463 in a sing le mod ule system when the DS1 b oard  
is in a trad itional module.
— The g roup  503 c ab le is wired  from the universal bus interfac e to the 
TN463 in a multimod ule system when the DS1 board  is in a 
universal mod ule.
— If the sourc e is on a universal mod ule in a sing le mod ule system, a 
sync hronization c ab le is not required .
nAd minister the b oard  as a timing sourc e in p roc ed ure 260, word  1, field s 
12 and  13, as follows:
— Enter a 1
 in field  12 if this b oard  is to b e used  as a timing  sourc e 
(meaning  it is d eriving  timing  from the far end sourc e c onnec ted  to 
the b oard , suc h as a CO or toll switc h).
— Enter a 1
 in field  13 if this b oard is the p rimary sourc e or 2
 if it is the 
sec ond ary sourc e.
NOTE:
Remote modules c annot serve as timing  sourc es. 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Physical Layer 
Page 4-71 Synchronization 
4
G3V2 Synchronization Implementation
The following  list summarizes some of the imp lementation issues involved  with 
sync hronization:
nIn a network of DEFINITY systems c onnec ted  by DS1/CEPT1 fac ilities, one 
master timing  sourc e exists. This timing  sourc e, if on a G3V2 system, is 
one of the following :
— A DS1/CEPT1 c irc uit p ac k c onnec ted  to a T1/E1 fac ility (suc h as a 
4ESS). If this is the c ase, this is the most d esirab le sourc e for 
master sync hronization.
— If a T1/E1 fac ility is not availab le, the next b est master sourc e is a 
stratum 3 c loc k c onnec ted  to a p ort network.
— If neither of the ab ove two op tions is availab le, the master timing  
sourc e d efaults to the tone/c loc k b oard  on one of the systems in the 
network.
Sync hronization is imp lemented  in the sync hronization p lan form, as 
follows:
— All slave systems in the network must have at least a p rimary 
sourc e ad ministered  from whic h it d erives timing . This is d one b y 
assig ning  the loc ation of a DS1/CEPT1 c irc uit pac k to the p rimary 
referenc e in the form. This means that the system d erives timing  
from the system or fac ility c onnec ted  to that c irc uit p ac k. If the 
c onnec tion fails, then that system will d erive timing  from the 
free-running c loc k on the c irc uit p ac k, whic h is a stratum 4 or 4E 
clock.
— If a system is to b e a master to the rest of the network, you must 
ad minister either a stratum 3 c loc k on that system or let a 
tone/c loc k b oard serve as the master sourc e.
nTo ad minister a stratum 3 c loc k you must enter the stratum 
and  the loc ation of the TN780 in the form.
nTo ad minister a tone/c loc k as the master sourc e, you c an 
leave the form b lank. This will c ause the tone/c loc k in the 
p ort network that was last used  as a timing  sourc e to 
become the timing source. If no other port network has been 
used  as a timing  sourc e, the tone/c loc k in the PPN is the 
d efault. You c an also ad minister the tone/c loc k of any p ort 
network to b e the timing  sourc e b y d oing  the following:
1. disable sync
2.set sync
 
[location of tone/clock]
3. enable sync
4.stat sync
 (to d isp lay the loc ation of the new sync hronization 
source) 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
Physical Layer 
Page 4-72 Synchronization 
4
Following  are rec ommend ations for c hoosing  a sync hronization sourc e in G3V2 
systems:
nIf you have a c hoic e, use a TN767 or TN464C b efore using  a TN722 as a 
timing source.
nIf at all possible, the p rimary and sec ond ary sourc es should  be loc ated  in 
the PPN.
nYou c annot loc ate a timing sourc e in a remote p ort network in G3rV2. 
						

							Layers 2 and 3 
Page 5-1 Bit Oriented Signaling Layers 2 and 3 
5
DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
555-025-107  Issue 1
July 1993
5
5Layers 2 and 3
This c hap ter d isc usses how various asp ec ts of layer 2 and  layer 3 p rotoc ols have 
b een d esig ned  into the DEFINITY systems. It d isc usses these sub jec ts as they 
relate to narrowb and  (64 kb p s) switc hing . 
For tec hnic al information on wid eb and  switc hing  in DEFINITY systems, refer to 
AT&T DEFINITY Wid eband  Tec hnic al Referenc e, issue 1, 555-230-230.
In the c ases where a relationship  exists, this c hap ter d isc usses how layer 2 and  
layer 3 sub jec ts relate to system ad ministration. For a c omp lete g uid e to 
ad ministering  narrowb and  DS1 and  CEPT1 c ap ab ilities, however, you should 
refer to Chapter 6, ‘‘System Administration’’
. 
Chap ter 4, ‘‘Physic al Layer’’
, shows that at the p hysic al layer, DS1 and  CEPT1 
trunks use two b asic  typ es of sig naling  to c ontrol c alls — b it oriented  sig naling  
and  ISDN PRI sig naling  (a typ e of messag e oriented  sig naling ). The p hysic al 
layer d oes not c over how these sig naling  sc hemes ac tually c ontrol c alls. Call 
c ontrol req uires hig her p rotoc ol layers. The International Stand ards Org anization 
(ISO) has d efined  a mod el c alled  the Op en Systems Interc onnec t (OSI), whic h 
d efines layers of p rotoc ols that c ommunic ate with eac h other. The lowest layer is 
the p hysic al layer and  the hig hest layer is the ap plic ation layer. Basic  c all c ontrol, 
whic h d efines how c alls are set up  and  torn d own, c onforms roug hly to three 
layers of p rotoc ol, the physic al layer (layer 1), the link layer (layer 2), and  the 
network layer (layer 3).
Bit Oriented Signaling Layers 2 and 3
The d isc ussion of b it-oriented  sig naling  in Chap ter 4, ‘‘Physic al Layer’’ described 
how the signaling  b its are multip lexed  into either a c ommon sig naling  c hannel or 
rob b ed  from the trunk c hannel. It d oes not d isc uss, however, the p rotoc ol that 
c ontrols c alls, whic h c an b e thoug ht of in the Op en Systems Interc onnec t (OSI)  
						

							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 5-2 Bit Oriented Signaling Layers 2 and 3 
5
mod el as layers 2 and  3 p rotoc ols. ISDN PRI sig naling , for examp le, b y d efinition 
uses the layers 2 and  3 c onc ep ts to d efine its c all c ontrol sig naling . Althoug h 
muc h less c omp lic ated , b it-oriented  sig naling  has its analog ous c all c ontrol 
struc ture (althoug h the c onc ep t of the link layer (layer 2) is hard  to fit into b it 
oriented  sig naling  sinc e the OSI mod el was d evelop ed  for messag e-oriented  
sig naling ). For examp le, in ISDN PRI sig naling, the layer 3 p rotoc ol send s a 
SETUP messag e to seize a trunk and  send  d igits, thus initiating  a c all. Similarly, 
b it-oriented  sig naling  relies on a p rotoc ol that seizes a trunk and  send s d ig its to 
initiate a c all. To illustrate this p oint, the following  list shows the seq uenc e of 
events used  b y b it-oriented  sig naling  to initiate a c all on a wink-start tie trunk 
using dual tone multifrequency (DTMF) digit sending:
1. Both end s of the trunk are initially on hook, meaning the A and  B b its b eing  
transmitted from both ends are 0.
2. The near end  PBX g oes off hook. This results in the near end  send ing  1s 
on the A b it and  1s on the B b it to the far end .
3. The far end  returns a wink, whic h means send ing  1s on the A bit for a 
sp ec ific  p eriod  of time then switc hing  b ac k to 0s on the A bit. The B b it is 
ig nored .
4. If the near end  rec eives the c orrec t wink, it means the far end  is read y to 
rec eive d ig its. The near end  then outp ulses the dig its in the form of DTMF 
tones over the trunk c hannel.
5. After rec eiving the dig its, the far end  answers by g oing  off hook and  
returning  1s on the A b it. The B b it is ig nored .
This seq uenc e d esc rib es only one asp ec t of a c all, the outg oing  c all initiation, on 
one sp ec ific  typ e of trunk, a wink start tie trunk with DTMF send ing . In g eneral, a 
call using bit oriented signaling requires the following sequences to be 
described by a protocol:
nOutg oing  call initiation seq uence
nIncoming call initiation sequence
nDisc onnec t seq uenc e
To c over all b it-oriented  p rotoc ols, these sequenc es must b e d esc rib ed  for c alls 
on the following  types of trunks:
nTie trunks with various c omb inations of start-d ial seq uenc es (wink start, 
delay dial etc.) and DTMF or dial pulsing.
nCO trunks with either loop  start or g round  start sig naling  and  DTMF or d ial 
p ulsing .
nDID trunks, with either wink start or immed iate start sig naling  and  DTMF or 
dial pulsing.
In all of these c ases, the c all c ontrol signaling  p rotoc ol emulates the historic al 
analog  c ase. For examp le, a wink on a d ig ital trunk is a stream of 1s on the A b it 
for a p eriod  of time eq ual to the time a c ertain voltag e level is maintained  d efined  
						

							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 5-3 Bit Oriented Signaling Layers 2 and 3 
5
b y the wink on a 4-wire E&M analog  trunk. When dig ital trunks first ap p eared , the 
d ec ision was mad e to d efine the sig naling  suc h that it emulated all the existing  
analog  c ases. Though d esc rib ing  the d etailed  p rotoc ols are beyond  the sc op e of 
this document, you can order the following documents that describe these 
p rotoc ols:
nPrivate Branc h Exc hange (PBX) Switc hing  Eq uipment for Voic eb and  
Ap p lic ation, EIA/TIA-464-A
, February 1989. Published  b y Elec tronic s 
Ind ustry Assoc iation, Eng ineering  Dep artment, 1722 Eye Street, N.W., 
Washing ton, D.C. 20006.
nAT&T Tec hnic al Pub lic ation TR41458 Sp ec ial Ac c ess Connec tions to the 
AT&T Network for Nodal Service Applications
.
On all DEFINITY systems, the call control p rotocols for b it-oriented  sig naling  (as 
well as for ISDN PRI sig naling) are imp lemented  p rimarily in a c omb ination of 
trunk g roup  ad ministration and  firmware on the DS1/CEPT1 interfac e b oard. The 
firmware on the b oard  is c ap ab le of p erforming  all the various typ es of winks, 
d elay d ials, DTMF, d ial p ulsing , etc . All it requires is a c ommand  from the 
software telling  it what kind  of trunk it is. Thus, the b asic  instruc tions for 
b it-oriented  c all c ontrol sig naling  c ome from trunk g roup  ad ministration.
G2.2 Implementation
The G2.2 imp lements b it-oriented  signaling  c all c ontrol in p roc ed ure 100, word  1, 
field  6 and  in p roc ed ure 100, word  3, field  2. Word  1, field  6 d efines the trunk 
typ e, whic h for the most p art are d ifferent flavors of the CO, Tie and DID stand ard  
typ es. For examp le, if you ad minister trunk typ e 41 on a trunk g roup , the DS1 
b oard  will sig nal ac c ord ing  to the EIA tie trunk standard  for that c hannel. Sinc e 
d efault sig naling  typ e for a typ e 41 trunk is sig naling  typ e 26, whic h is wink start, 
the b oard  will use the standard  d efinition of a wink to know when to b eg in 
outp ulsing  d ig its. Word  3, field  2 allows you to c hang e the sig naling  typ e from its 
d efault if you want. For examp le, if you want c alls on a trunk to be treated  b y the 
feature software in the system as CO c alls b ut the trunk is ac tually a 4-wire E&M 
typ e of interfac e, you c an ad minister the trunk typ e as CO b ut c hang e the d efault 
sig naling  to type 29, whic h is an E&M wink start sig naling . Not all sig naling  typ es 
are valid  with all trunk typ es, however. The G2.2 ad ministration manual lists the 
c omp atib le sig naling -to-trunk typ es in p roc ed ure 100, word  3.
NOTE:
The trunk typ e administered  in word  1 imp lements more than just the 
b it-oriented sig naling c all c ontrol p rotoc ol. System features are also b ased  
up on this trunk type ad ministration. For examp le, to g ain ac c ess to ETN 
features, you must ad minister a tie trunk typ e. 
						

							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 5-4 ISDN PRI Layer 2 
5
G3V2 Implementation
Imp lementing the bit-oriented  c all c ontrol p rotoc ol in the G3V2 systems is 
analog ous to that of G2.2. The p rotoc ol is imp lemented  in the trunk g roup  form. 
The g roup  typ e is analog ous to the trunk typ e of G2.2. Thus, you c an enter types 
suc h as tie, CO, and  DID. Like the G2.2, you c an ad minister many variations of 
the typ es listed  in the EIA stand ard . For examp le, g roup  typ e tand em is a 
variation of a tie trunk that allows ac c ess to sp ec ial system networking  features. 
The trunk typ e in G3V2 systems is analog ous to the signaling  typ e in G2.2. If you 
enter wink/wink
, for examp le, the b oard  is told  to sig nal like a 4-wire E&M wink 
start trunk on c hannels b elonging  to that trunk g roup . Unlike the G2.2, however, 
some of the b it-oriented  p rotoc ols imp lemented  c onform to a p artic ular c ountrys 
stand ard  rather than the EIA stand ard . For examp le, if the c ountry p rotoc ol on the 
DS1/CEPT1 b oard  is 4
 (Italy), CO sig naling  will c onform to Italian standard s. 
Whereas in the U.S., only the A and  B b its are d efined  for trunk sig naling , in other 
c ountries, the A, B, C, and  D b its might all b e used . 
ISDN PRI Layer 2
Layer 2 in ISDN PRI uses the link ac c ess c ontrol proc edure for the D c hannel 
(LAPD), whic h is the link layer p rotoc ol used  to transp ort the D c hannel 
end-to-end. The primary func tions of LAPD are as follows:
nISDN PRI layer 3 messag e p ac kets are sent in LAPD layer 2 frames.
nError c orrec tion and  retransmission is p rovid ed  b y LAPD.
nLAPD p rovid es a method  to multip lex sig naling  on many log ic al c hannels 
on the single D channel.
The DEFINITY systems imp lement layer 2 ac c ord ing  to the CCITT Q.921 
rec ommend ation. The layer 2 imp lementation is mostly transparent to anyb od y 
imp lementing  or sup p orting  ISDN PRI in the DEFINITY systems. 
Sig nific ant d ifferenc es exist in how the LAPD p rotoc ol is terminated  among  the 
systems, however. In G2.2 trad itional mod ules, the DS1 interfac e (the ANN35) 
terminates the LAPD p rotoc ol. This is why this b oard  c an only b e used  for ISDN 
PRI sig naling . In G2.2 universal mod ules, the mod ule p roc essor terminates the 
LAPD p rotoc ol. In G3iV2 the ISDN PRI LAPD protoc ol is terminated  b y the 
p roc essor interfac e b oard . In G3rV2, however, the ISDN PRI LAPD p rotoc ol is 
terminated  b y the p ac ket interfac e b oard , whic h also terminates all LAPD links in 
the system, inc lud ing  internal system links to the p orts. In all c ases, an internal 
p rotoc ol is used  to c ommunic ate b etween the b oard  terminating  the LAPD and 
the p roc essor, whic h terminates the level 3 p rotoc ol. Differenc es also exist 
b etween G2.2 and  G3V2 in the p rotoc ol d etermining  whic h end  of an interfac e is 
user and  whic h one is network (or slave and  master in a p eer p rotoc ol) and  also 
in the way layer 2 timers are ad ministered .