Lucent Technologies 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 4-27 The Digital Loss Plan 
4
The d ig ital loss plan is sig nific antly d ifferent from the VNL. Sinc e the two loss 
p lans d o d iffer, it c annot b e assumed  that the p ort-to-p ort losses measured in an 
all-d ig ital network or in a c omb ined  d ig ital and  analog  network are the same as 
the loss measured  b etween the same p orts in an all-analog  network.
G2.2 and  G3V2 systems p rovid e for imp lementing the dig ital loss p lan sp ec ified  
b y ANSI/EIA/TIA-464-A-1989. There are two versions of the d ig ital loss p lan. The 
early version is c alled  
digital fixed loss plan and  the later version is c alled  ISL 
digital low loss plan
.  D i g i t a l  C O s ,  s u c h  a s  AT&T 5 ESSe s  o r  N o r t h e r n  Te le c o m  
DMS 100s, and  toll switc hes, suc h as AT&T 4ESSes, also imp lement p arts of the 
digital loss plan.
The d ig ital loss p lan req uires a 6-d B loss (c onnec tion loss) from the switc h p ort at 
one end  to the switc h p ort at the other end  for p rivate d ig ital networks. 
Transmission p erformanc e for p rivate networks using  this 6-d B loss sp ec ific ation 
is very g ood . The p ub lic -network and  d ig ital COs have a similar 6-dB 
req uirement. 
Private-network to pub lic -network c onnec tions result in a total c onnec tion loss of 
12 dB — possibly more depending on the public network switch and factors 
suc h as whether a c hannel b ank is used . Transmission p erformanc e for this typ e 
of c onnec tion is generally c onsid ered  marg inal, and  if another p rivate-network 
c onnec tion (another 6 d B of loss) is ad d ed , then the end -to-end  transmission 
p erformanc e b ec omes unac c ep tab le. These typ es of c onnec tions and  their 
unac c ep table transmission p erformanc e were the motivation for d evelop ing  the 
ISL digital low loss plan.
For G2.2 and  G3V2 systems, the ad ministration op tions of the ISL low loss p lan 
allows you to op timize transmission p erformanc e for those p rivate network to 
p ublic  network typ es of c onnec tions at the planning  and  installation stag e. With 
p rop er d esig n and  app lic ation, the ISL d ig ital low loss p lan makes p ossib le a 
sig nific ant imp rovement in transmission p erformanc e for c all-forward ed  c alls 
involving  off-network c onnec tions.
Some q uantity of c onnec tion loss is d esirab le and  d elib erately eng ineered  into 
most typ es of transmission links. The q uantity of loss (mag nitud e and  numb er of 
d Bs) d epend s on the loss p lan that is administered  and  the p artic ular type of 
fac ility involved . Eac h p artic ular set of p ort-to-p ort c onnec tion losses is known as 
a 
loss pad group.
Connec tion loss serves to eliminate or signific antly red uc e talker ec ho on 
long -d istanc e transmission links. User p erc eption of transmission performanc e is 
p rimarily a func tion of the network c onnec tions most freq uently used  and  the 
p artic ular loss p lan ad ministered  for those c onnec tions. Onc e a loss p lan is 
selec ted  and ad ministered , the transmission p erformanc e b ec omes a fixed  
func tion of c all routing . Reg ardless of the loss p lan that is used , the op eration of 
the other switc h features will not b e affec ted .
The d ig ital loss p lan p rovid es for the flexib le assig nment of loss-p ad  g roup s on a 
trunk-g roup  b asis that is ind epend ent of trunk typ e and also of the p ort c irc uit  
						

							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 4-28 The Digital Loss Plan 
4
p ac k typ e (DS1 or analog ). Before the availab ility of this ad ministration c ap ab ility, 
loss-p ad -g roup  assig nment had  b een fixed , transp arent, and  d ic tated  b y the 
trunk typ e and  p ort c irc uit p ac k typ e b eing  used . For examp le with fixed loss, a 
p ort on a G2.2 c irc uit pac k ad ministered  as trunk typ e 47 is automatic ally 
assig ned  the EIA d ig ital tie trunk p ad g roup . Conversely, with the low-loss op tion, 
a trunk g roup  ad ministered  as trunk typ e 47 or 70 c an b e assig ned  one of 
several loss-p ad  g roup s and  thus ap p ear on either an analog  or d ig ital p ort. 
Therefore, the d ig ital loss p lan op tions that are ad ministered  d ep end  on the 
ap plic ation and  the c onfig uration of the transmission fac ilities. For examp le, 
ap plic ation may inc lud e suc h items as whether the fac ility is a DMI-to-host link, 
and  whether the c onnec tion is on-p remises to on-p remises, or on-p remises to 
off-p remises. Config uration of the transmission fac ilities may inc lud e:
nWhether the c onnec tion is c omp leted  via a p rivate network or a 
c omb ination of p rivate and  p ub lic  networks
nWhether the end -to-end  c onnec tion is c omp leted  via all dig ital or a 
c omb ination of analog  and  d ig ital switc hing fac ilities
Loss-Plan Implementation
The d ig ital loss plan op tions vary b etween G3V2 and  G2.2 as desc rib ed  next.
Generic 2.2 Loss Plan Implementation
The d ig ital loss plan op tions are ad ministered by sp ec ifying an enc ode that 
c orresp ond s to the d esired pad  g roup. For line ap p lic ations, this information is 
translated in field 8 of procedure 000, word 1. For trunk applications, this 
information is translated  in field  13 of proc ed ure 101, word  1. The p ermitted  
enc odes and  their c orresp ond ing  loss-p lan func tion for trunks are listed  in Ta b l e  
4-9:
Table 4-9. Digital Loss Plan Encodes 
Encode Loss-Plan FunctionANSI/EIA/TIA-464-A-1
989 Designator
0 The digital fixed-loss plan pad loss is determined 
b y the trunk typ e ad ministered  in proc ed ure 100, 
word 1—
1 ISL tie trunk S/DTT
2 EIA tie trunk (rec ommend ed for ISDN) D/TT
3 ISL d ig ital CO D/CO -3/3 loss
4 EIA d ig ital CO D/CO 0/6 loss
5 d ig ital toll offic e D/TO
Continued on next page 
						

							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 4-29 The Digital Loss Plan 
4
Althoug h the d ig ital loss p lan p rovid es for the assig nment of a loss-p ad  g roup  
ind ep end ent of p ort typ e (analog  or d ig ital), there are c ertain restric tions. 
Ad ministration d oes not p rovid e for alternate p ort ap p earanc e on DS1 p ort c irc uit 
p ac ks for the following  lab eled  p ad  g roup s: analog  tie trunk, analog  CO trunk 
(terminal b alanc ed ), analog CO trunk (not b alanc ed). The TSI arithmetic  log ic  
unit (ALU) und er mod ule p roc essor c ontrol ac tually inserts (d ig itally) the 
ad ministered  q uantity of loss in the c onnec tion. The TN380D mod ule p roc essor 
(or later) is req uired  for p rovid ing  the full d ig ital low loss p lan. Therefore, to 
imp lement any p ortion of the low loss p lan, it is essential to have b oth the 
ap prop riate hard ware (TN380D) and  software issue. A network c onsisting  of 
some nod es that imp lement the fixed -loss p lan and  other nodes that implement 
the low loss p lan are p ermitted . However, their transmission performanc e is the 
same as for an all fixed -loss network.
G3V2 Loss Plan Implementation
G3V2 systems imp lement the full d ig ital loss p lan. The ap prop riate loss value is 
ad ministered , for the p artic ular trunk g roup , b y entering  #
 in the Name
 field  for 
sc reens on p ag es 2 thoug h 5 of the trunk-g roup  memb er assig nments. Noted  
b elow are the minor differenc es in names (terminology) used  for G3V2 and  those 
used  for G2.2. The following  list id entifies the op tion values: 6 analog  toll offic e A/TO
7NI2A (AUTOPLEX
® NETWORK Interfac e V)—
8 N I 2 B  (A U TO PL EX N ETWO RK  I n t e rf a c e  Y) —
Loss p lanpbx-eia
 — for p rivate-network-only ap p lic ations that use 
analog  tie trunks or d ig ital tie trunks
pbx-low
 — for use with c omb ination tie trunks (p rivate 
networks tand emed  with p ub lic  networks)
toll
 — for use with c onnec tions to an analog  toll offic e or d ig ital 
toll offic e
Dig ital c onn 
lossnormal
 — same as the EIA op tions used  with G2.2
low
 — same as the ISL op tions used  with G2.2; rec ommend ed  
for c omb ination tie-trunk ap p lic ations where low sp eec h 
volume is a p rob lem b ut ec ho is not a p rob lem
Table 4-9. Digital Loss Plan Encodes  — Continued
Encode Loss-Plan FunctionANSI/EIA/TIA-464-A-1
989 Designator
Continued on next page 
						

							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 4-30 The Digital Loss Plan 
4
Port-to-Port Loss Values
The p ort-to-p ort loss values shown in Tab le 4-10, can be used to determine the 
c orrec t loss b etween two p rop erly terminated  p orts of a d ig ital switc h.
To d etermine the c orrec t end-to-end  loss for a tand em c onnec tion (throug h b oth 
an analog  and dig ital switc h), simp ly add  up  the losses in eac h leg . 
Comb ination tie trunks are freq uently enc ountered when station-to-station c alls 
are being c ompleted. Depending on the partic ular facilities involved, some 
unexp ec ted  losses may b e enc ountered . For the network shown in the fig ure, if a 
c all is mad e from an on-p remises station (ONS) on one of the analog  switc hes 
tand eming throug h the d igital switc h to an ONS on the other analog  switc h, an 
end -to-end  loss of 6 d B should  be measured  in eac h d irec tion. However, if a c all 
is mad e from an ONS on the d ig ital switc h to an ONS on either one of the analog  
switc hes, an end -to-end  loss of 9 d B should  b e measured  in eac h d irec tion. This 
3-d B d ifferenc e is a result of analog and  d ig ital loss p lan differenc es and  should  
be expected.
Before sp ec ific  loss information is g iven, the following  imp ortant p oints should  b e 
rememb ered :
nThere are no ad justab le attenuators assoc iated  with DEFINITY ports. 
Port-to-p ort losses are c omp osed  of d istrib uted  losses in the p orts and  in 
the internal c onnec tion mad e b etween the p orts. The loss for the internal 
c onnec tion is a func tion of the p ort typ e and  the numb er of p orts on the 
c onnec tion. Different p ort typ es (suc h as, d ig ital CO, d ig ital tie trunk, 
digital off-premises station) use the same DS1/CEPT1 hardware.
Table 4-10. Digital Loss Plan (Port-to-Port Losses)
Transmit DirectionReceive Direction
(Values in dB loss)
ONS-
 Line 
Tr kOPS-
 LineANA
L Tie 
Tr kCOMB
 or 
DTTAnalog CO 
Tr u n kEIA 
DCO
 Tr kISL 
CTTAT O
 Tr kDTO
 Tr kISL 
DCO
BAL NBAL
On-p remises station 
(ONS)6 3 3 3 00 3 363 3
Off-p remises station 
(OPS)3 0 2 0 00 0 230 0
Analog  tie trunk (A/TT) 3 2 0 -3 0 2 2 0 0 -3 2
Comb ination or dig ital 
tie trunk (D/TT)9 6 3 0 36 6 630 3
Analog  
CO trunk 
(A/CO)BAL 0 0 0 -3 00 0 020 0
NBAL 0 0 2 0 02 2 220 2
EIA d igital CO trunk 
(D/CO 0/6 loss)3 0 2 0 02 0 030 0
Continued on next page 
						

							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 4-31 The Digital Loss Plan 
4
NOTE:
A terminal b alanc ed  trunk is d efined  as meeting  an ERL of g reater than or 
eq ual to 18 dB and  an SRL of g reater than or eq ual to 10 dB, when 
measured  into a q uiet termination at the CO.
nPort-to-p ort losses in Generic  2.2 and  Generic  3V2 c onform to the ANSI 
digital loss plan standard. Tab le 4-10
 is an exc erp t from this stand ard . If 
other vendors’ switc hes are used  in the same network, p ort-to-p ort loss 
measurements throug h suc h a switc h will have value only after verific ation 
of this switc h’s p ort-to-port loss sp ec ific ation. Conformanc e with ANSI 
stand ard s g reatly simp lifies this p roc ess and  red uc es the likelihood  of 
c omp atib ility prob lems.
nThe loss b etween switc hes is 0 d B over d ig ital fac ilities, 1 d B for 
c omb ination fac ilities, and VNL for analog  fac ilities.
nIf the losses in a switc h network have b een verified  c orrec t and  sp ec ific  
p rob lems assoc iated  with the loss p lan remain, the ap p rop riate 
transmission engineering  organization should  b e c onsulted .
DS1/CEPT1 Port Losses
All DS1/CEPT1 c irc uit-p ac k c hannels, reg ard less of the typ e of p ort for whic h 
they are used , are transparent (zero loss) and  p reserve d igital b it integ rity. All 
c onnec tion losses are inserted  in the switc h network in c onformanc e with the 
d ig ital loss p lan and  d ep end ent on switc h ad ministration op tions.
ISL d ig ital tie trunk 
( S/ D TT)3 2 0 0 02 0 030 0
Analog  toll offic e trunk 
(A/TO)6 3 0 -3 22 3 30-33
Dig ital toll offic e trunk 
(D/TO)9 6 3 0 66 6 630 6
ISL dig ital CO trunk 
(D/CO -3/3 loss)3 0 2 -3 02 0 030 0
Table 4-10. Digital Loss Plan (Port-to-Port Losses) — Continued
Transmit DirectionReceive Direction
(Values in dB loss)
ONS-
 Line 
Tr kOPS-
 LineANA
L Tie 
Tr kCOMB
 or 
DTTAnalog CO 
Tr u n kEIA 
DCO
 Tr kISL 
CTTAT O
 Tr kDTO
 Tr kISL 
DCO
BAL NBAL
Continued on next page 
						

							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 4-32 The Digital Loss Plan 
4
Terminating a DS1 at a Channel Bank
The DS1 c hannels that leave the switc h in d ig ital form and  are c onverted  to 
analog  form b y a c hannel b ank are c alled  \E2
c omb ination c hannels. Three b asic  
types are described below.
Tie Trunk Ports
To ob tain the req uired  c hannel unit losses for c omb ination tie trunks, 
extend ed -rang e E&M c hannel units (or their equivalent) should  b e used . The 
attenuators on this c hannel unit are set as follows: the transmit (A-to-D) attenuator 
should  b e set to (15.5-Lc ) d B, where Lc  is d efined as the loss in the 
interc onnec ting  c ab le b etween the c hannel b ank and  the analog switc h, and the 
rec eive (D-to-A) attenuator should  b e set to (12.5-Lc ) d B, where Lc  is as ab ove. 
Setting  the c hannel unit attenuators to these values will meet the req uirement for 
c omb ination tie trunks. This requirement sp ec ifies a net g ain of 2 d B in the A to D 
d irec tion and  a net loss of 4 d B in the D to A direc tion.
CO DID Trunk Ports
Dig ital g round-start CO, loop -start CO, and  Direc t Inward  Dial (DID) trunks may 
(op tionally) terminate on c hannel b anks loc ated  at a CO. If this is the c ase, it is 
the CO’s resp onsib ility to set the c hannel unit losses. As a g uid eline, the net loss 
should  rang e from 1 to 3 d B. Also, the loss should  b e the same in b oth d irec tions.
OPS Ports
Analog  off-p remises station (OPS) fac ility req uirements sp ec ify a loss not to 
exc eed  4 d B in eac h d irec tion. To p rovid e for transmission stab ility (eliminate 
sing ing  and  ec ho) with D4-c hannel units, at least 1 d B of loss in b oth d irec tions is 
req uired  from the 2-wire analog  hyb rid terminals. Foreig n exc hang e sub sc rib er 
end  c hannel units (FXSs) p rovid e losses ad justab le from 1.0 throug h 1.8 d B. This 
c ap ab ility p ermits their use with analog  fac ilities having  losses as g reat as 3 d B, 
g iving  a net loss of 4 d B. FXS end with g ain transfer c hannel units (FXS/GTs) 
make availab le an ad d itional 2-wire g ain of 6 d B, p ermitting their use with analog  
fac ilities with losses as g reat as 9 d B. Attenuator, eq ualizer, and  b alanc e setting s 
should  b e made with the eng ineering data listed  on the c irc uit d etail rec ord .
Refer to 
D4-Channel Bank Channel Units — Ap p lic ation Eng ineering /Carrier 
Engineering
 855-351-105 for eng ineering  d esig n information as well as switc h 
setting s and  ap p lic ation notes on D4-c hannel units. 
						

							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 4-33 Synchronization 
4
Synchronization
The DS1/CEPT1 transmit and  rec eive buffers for DEFINITY systems operate from 
a sing le external or internal c lock. Each d ig ital switc h c an accommod ate multiple 
DS1/CEPT1 or T1/E1 sp ans that link multip le switc hes. These may inc lud e b oth 
ISDN-PRI and  DS1/CEPT1 links. Sinc e eac h switc h c an transmit at a rate 
d etermined  b y its internal c loc k, information will b e lost if the d igital network is not 
sync hronized  to a sing le c loc k. Furthermore, one switc h should  b e selec ted  as 
the master and all others should  ob tain slave-timing  from it. Fi g u re  4 -5
 shows 
various DS1/CEPT1 sync hronization ap p lic ations.
Sync hronization issues affec t all network nod es. Compatib ility d etails must b e 
ad dressed, and  a network sync hronization p lan must b e d evelop ed , d ep loyed , 
and  verified  to b e installed  c orrec tly.
The Need for Synchronization
The term sync hronization refers to an arrang ement whereb y d ig ital fac ilities 
op erate from a c ommon c loc k. Whenever d ig ital sig nals are transmitted  over a 
c ommunic ations link, the rec eiving  end  must b e sync hronized  with the 
transmitting  end  to read  the d ig ital sig nals p rop erly. This arrang ement is c alled  
link sync hronization. When d ig ital sig nals are transmitted  over a network of d igital 
c ommunic ations links, switc hing  nod es, multip lexers, and  transmission 
interfac es, all entities in this network must b e sync hronized  tog ether. This is 
known as 
network synchronization. With d ig ital transmission, information is 
c od ed  into d isc rete p ulses. When these p ulses are transmitted  over a 
c ommunic ations link, there must b e at least three d ifferent levels of 
sync hronization. For transmitting  d ata, these levels are known as 
bit, c harac ter, 
and 
messag e sync hronization. For pulse-code modulation (PCM) voice 
transmission, the levels are 
bit, time-slot, and frame sync hronization.
Bit sync hronization refers to the req uirement for the transmitter end  and  the 
rec eive end  to op erate at the same c loc k rate so that b its are not lost. Other 
levels of sync hronization refer to the need  for the transmitter and  rec eiver to 
ac hieve p rop er p hase alig nment so that the b eg inning  and  the end of a 
c harac ter, messag e, time slot, or frame c an b e identified . 
For sync hronous transmission, d ata is transmitted  at a fixed rate. Eac h b it 
oc c upies a fixed -unit interval. All sig nific ant transitions must c orresp ond  to 
multip les of the fixed -unit interval. Messag e and  frame sync hronization are 
ac hieved  b y using  sp ec ial c harac ters at the b eg inning  and  end  of the messag e, 
and  b y knowing  the numb er of bits c ontained  in eac h frame.
Fig ure 4-5
 shows the exc hang e of d ig ital b it streams b etween various elements 
that req uire some form of sync hronization. The role of sync hronization is 
examined  in eac h of the three c onfig urations.
Fig ure 4-5
A shows one p ossib le c onnec tion between a pair of D4-c hannel 
b anks. Suc h a c onnec tion (using  D4-c hannel b anks) c an typ ic ally b e found  with  
						

							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 4-34 Synchronization 
4
a p air of analog  switc hing  systems c onnec ted b y T1-c arrier fac ilities. For this 
arrang ement, the transmitting  portion of eac h c hannel b ank ind ep end ently 
d etermines the c loc k rates. The rec eiving  p ortion of eac h c hannel b ank d erives 
its c loc k from the inc oming  d ig ital b it stream. In this arrangement, the c hannel 
b anks c onvert the rec eived  d ig ital sig nal d irec tly to analog, and  there is no 
req uirement that the two c loc k freq uenc ies p rec isely matc h as the c hannel bank 
d oes not interfac e to another dig ital system.
Figure 4-5. Options for Synchronization
Fig ure 4-5
B shows a d ifferent c onnec tion b etween a c hannel b ank and  a d ig ital 
switc hing  system. For this c onfig uration, the d ig ital switc h transmits a d ig ital b it 
stream at the rate (F0) that is d etermined  b y its internal c loc k. The d igital switc h 
must rec eive the inc oming  d ig ital b it stream at this same rate (F0). Otherwise, the 
switc h’s rec eiving  b uffer may eventually overrun or und errun.
NOTE:
Switc hing  is d one b y p lac ing the sig nals from individ ual time slots on one 
link into the time slots on other links using  a p roc ess c alled  
time-slot 
interc hang e
 (TSI). For this p roc ess to work p rop erly, b it sync hronization 
must b e maintained  on all links terminating  on the d ig ital switc hing  nod e, no 
matter where the links originate.
If the averag e transmit rate is faster than the averag e rec eive rate, the rec eive 
buffer will eventually overrun. If the average receive bit-c loc k rate is faster than 
Channel
bankChannel
bank
Channel
bank Digital bit stream
Digital bit stream
Digital bit streamA: Not synced
B: Loop timed
C: Exter nally synced Tr a n s m i t
F0
Tr a n s m i t Digital
switching
system
F
0
Digital
switching
system
F
0
Digital
switching
system
F
0
Tr a n s m i tF
0
F0
F0Tr a n s m i t
F
0
F0
F0
Transmit
F
1
Transmit F
1Receive
Receive
ReceiveReceive
Receive
ReceiveAnalog
Digital
DigitalAnal og
Anal og
Anal og 
						

							DEFINITY Communications System Generic 2.2 and Generic 3 V2 
DS1/CEPT1/ISDN PRI Reference  
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Page 4-35 Synchronization 
4
the average transmit bit-clock rate, the receive buffer will eventually underrun. It 
is nec essary to p revent overruns (d eletions) and  und erruns (repetitions) b y 
sync hronizing  the network p roperly. Imp rop er sync hronization results in b uffers 
rep eating  or d eleting b its in 1-frame inc rements.
NOTE:
The d eletion or rep etition of a sing le frame is termed  a slip or a c ontrolled  
slip
. For an ind ivid ual d ig ital bit stream, slip s are serious imp airments sinc e 
d ig ital switc hing  systems with imp roperly sync hronized  c loc ks will 
eventually suffer slip s on every rec eived  d ig ital b it stream.
Notic e in Fi g u re  4 - 5
B that the overrun or und errun p rob lem c an b e p revented  b y 
forc ing  the c hannel b ank transmitter to op erate at the same c loc k rate as the 
rec eive p ortion of the c hannel b ank. This arrang ement is c alled 
c hannel-b ank 
loop  timing
. This b ec omes more c omp lex when two d ig ital switc hes terminate a 
d ig ital transmission fac ility. Fi g u re  4 -5
C shows this configuration.
In Fi g u re  4 - 5
C eac h switc hing system transmits at a rate d etermined  b y a 
referenc e c loc k. Unless the rec eived d ig ital bit stream arrives at the same c loc k 
rate as its internal c loc k rate, slip s will oc c ur. To p revent or minimize slip s, it is 
nec essary to forc e b oth switc hing  systems to sync hronize on a c ommon 
referenc e c loc k rate (F0). Both will then b e p art of a sync hronized  network, and  
will read  and  write their b uffers as they should .
The p rimary ob jec tive of network sync hronization is to minimize the slip  rate. This 
is d one b y sync hronizing  the c loc ks assoc iated  with the switc hing  nod es so that 
all transmissions from these nod es have the same average line rate. For 
short-term intervals, the switc hing -system rec eive b uffers ab sorb  the d ifferenc e 
b etween the line rate and  the averag e rate. These short term variations are c alled  
jitter. A long -term d ifferenc e will result in a slip .
The imp ac t of slip s on a c ustomer varies with the servic es used. For examp le, 
voic e servic es are fairly insensitive to slip s. Slip  rates as hig h as 20 p er sec ond  
are b arely p erc ep tib le; therefore, sp eec h is not c onsid ered a limiting  fac tor in 
setting  slip -rate stand ard s. However, d ata servic es are muc h more sensitive to 
slip s sinc e eac h b it of data is a d isc rete p iec e of information. For d ata 
ap plic ations, a slip  (at any slip  rate) req uires retransmission and will ad versely 
affec t the throug hp ut and  d eg rad e performanc e of d ata transmission fac ilities.
Slip s c an oc c ur for two b asic  reasons:
1. Lac k of or loss of freq uenc y sync hronization among  the network nod es 
(for example, when slips occ ur at a constant, regular rate)
2. Phase mod ulation of the transmitted  d ig ital b it streams d ue to 
environmental variations of the transmission fac ilities (suc h as temp erature 
variations that affec t the elec tric al leng th of a transmission line)
Therefore, even if all network nod es are sync hronized , slip s c an still oc c ur d ue to 
transmission imp airments. 
						

							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 4-36 Synchronization 
4
Synchronization Hierarchy
Within North Americ a, all p rivate d igital telec ommunic ations fac ilities that 
c onnec t to the p ub lic  dig ital network must ob tain sync hronization b y the 
hierarc hic al method. This method  is based  on:
1. Controlling the slip  rate to provid e satisfac tory servic e to the c ustomer
2. Maximum reliab ility
3. Minimum c osts
4. Ease of administration
5. Ease of maintenanc e
For the hierarc hic al method , a nod e c ontaining  a very stable referenc e freq uenc y 
is id entified  as the sourc e or 
master referenc e. The master referenc e is 
transmitted  to another nod e that is sync hronized  (slaved ) to this master 
referenc e. A network imp lementing  this method  is hierarc hic al in nature with 
nod al c loc ks sup plying  the sync hronization freq uenc y to c ertain other nod es, 
whic h in turn sup p ly the referenc e to still other nod es. Fi g u re  4 -6
 shows a 
hierarchical network configuration.
With the hierarc hic al method , the existing  d ig ital transmission fac ilities are used  
to d istrib ute the referenc e freq uenc y. For examp le, with a T1-c arrier, the slave 
nod es c an d erive their referenc e c loc k from the DS1 b it rate. Sinc e the c loc k 
freq uenc y is d erived  from the d ig ital b it stream, the traffic -c arrying  c ap ac ity of a 
c arrier system is not d iminished  (that is, the b and wid th is not used  to c arry a 
sep arate cloc k signal).
Reliable op eration is an imp ortant c onsid eration for all p arts of a 
telec ommunic ations network. So, the sync hronization network should  c onsist of 
both 
primary and  secondary sync hronization fac ilities. In ad d ition, eac h nod e 
must b e eq uip p ed  with an internal c loc k that (with automatic  switc hing ) c an 
bridge short disruptions of the primary and secondary synchronization reference. 
Therefore, if sync hronization is d isrup ted , the internal c loc k will assume c ontrol. 
The internal c loc k will d rift at a rate d etermined  b y its stab ility (also c alled  
free run 
ab ility
 or ac c urac y).