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 4-57 Synchronization 
4
Rule 1:
Selec t the node with the lowest stratum c loc k as the network referenc e c loc k 
master and  have the rest of the nod es d erive slave timing  from it.
If there are one or more c loc ks in the network at the lowest stratum level, the 
following  step s c an b e taken to d etermine the c loc k that should  b ec ome the 
network referenc e c loc k sourc e.
The CDMs c an only b e used to p rovid e timing  when they are used  to emulate a 
D4-c hannel b ank. 
Example For Rule 1
Fig ure 4-18 is used  to exp lain the ap p lic ation of rule 1.
To d etermine the sync hronization p lan for Fig ure 4-18
 the following  tasks were 
p erformed . First, ac c ording  to rule 1, an attempt was mad e to find  a nod e with 
the lowest stratum c loc k. The attemp t failed  b ec ause all the nod es in the 
illustration p rovid e stratum-4 timing . Therefore, the step s und er rule 1 must be 
used . Step s 1 and  2 are not ap plic ab le b ec ause there is no d ig ital switc h in the 
network.
Und er Step  3, it was d etermined , ac c ord ing  to the ranking , that a CEM should  be 
the network sourc e c loc k. Next, rules 2 throug h 7 were used  to determine whic h 
of the two CEMs should  b e the network sourc e c loc k. For rules 2 throug h 6, b oth 
CEMs were eq ually q ualified . Therefore, an arbitrary d ec ision was mad e to 
c hoose the CEM on the left.  Step  1: If there is only one d ig ital switc h in the network, that switc h is to 
b e the network referenc e c loc k master.
Step  2: If there is more than one d ig ital switc h in the network, rules 2 
throug h 7 should  b e used  to d etermine the switc h that should  
b e the network referenc e c loc k (master).
Step  3: If there are no d ig ital switc hes in the network, c hoose a d ig ital 
terminal p rod uc t as the network referenc e c loc k master and  
have the other nod es d erive timing from it. Listed  in ord er of 
p referenc e, use a CEM, a D4-c hannel b ank, or a CDM as the 
network referenc e c loc k master. 
						

							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 4-58 Synchronization 
4
Figure 4-18. Nonpublic Network without Digital Switches
The d ig ital terminal p roduc ts d o not automatic ally switc h to their internal 
hig h-ac c urac y c loc k on sync hronization referenc e failure.
Figure 4-19. Proper Use of Backup Facilities
CEM CEM D4
D4 D4 D4
D4 D4
Digital transmission facilities
Primar
yfrequencyreference
cydfnnds RPY 060597
D C A
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfpubf RPY 060397
B 
						

							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 4-59 Synchronization 
4
Rule 2:
A nod e may use a fac ility as a b ac kup  only if the nod e at the other end  is not 
using  that same fac ility as a b ac kup .
Example For Rule 2
Fig ure 4-19 illustrates an app lic ation of rule 2 and  prop er use of a b ac kup  fac ility 
b etween nodes A and  C.
Fig ure 4-20
 shows the same network arrang ement b ut with imp roper use of the 
b ac kup  fac ility b etween nod es A and C. This imp rop er arrang ement would form 
a timing  loop  if nod e B should  fail (nod es A and  C would  switc h to their b ac kup , 
whic h is eac h other).
Rule 3:
The transmission fac ilities assoc iated with those nod es meeting  Rules 1 and  2 
and  having  the g reatest availab ility must b e selec ted  for p rimary and  sec ond ary 
sync hronization fac ilities.
Figure 4-20. Improper Use of Backup Facilities
B A
C D
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfopdr RPY 060397 
						

							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 4-60 Synchronization 
4
Availab ility is d efined  as the ratio of the mean time b etween failures (the averag e 
time b etween suc cessive system failures, ab breviated  
MTBFs) to the sum of the 
MTBFs and  the mean time to rep air (the total maintenanc e time d ivid ed  b y the 
total numb er of failures d uring  the same amount of time ab b reviated  
M TTR). Low 
availability (poor transmission performance) is used as a criterion to reject a 
fac ility as a referenc e c and id ate or to reassig n one alread y c hosen.
Charac teristic s that p rimarily d etermine the availab ility of a transmission fac ility 
are rank-ord ered  as follows:
1. History rec ord
2. Ac tivity
3. Fac ility leng th
4. System rate (that is, whether the fac ility sup p orts T1/E1 or fib er)
5. Protec tion switc hing
6. Physic al typ e (that is, whether the fac ility is p hysic ally installed  as an 
und erg round , b uried , or aerial p aired  c ab le, a c oaxial c ab le, or a rad io 
link). (Satellite fac ilities should  not b e used to sup p ly sync hronization.)
7. Numb er and  typ e of reg enerative rep eaters
8. Numb er of d ig ital terminal p rod uc ts and  other intermed iate offic e 
eq uip ment, if any
Fac ility rank ord ering  is g enerally b ased  on limited  tec hnic al and  operational 
information. It is rec ommend ed  that fac ility selec tions b e b ased on loc al field  
exp erienc e where availab le.
Rule 4:
Where p ossib le, the routes for all primary and  sec ond ary sync hronization 
fac ilities should  b e d iverse.
Example For Rule 4
Fig ure 4-21 exp lains the op timal c onfig uration when rule 4 is used . Fig ure 4-22 
shows a less than op timal ap p lic ation of rule 4.  
						

							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 4-61 Synchronization 
4
Figure 4-21. Optimal Diverse Routing
Fig ure 4-22
 shows nod e C d eriving  b oth p rimary and sec ond ary timing from 
nod e D, via the two sep arate T1/E1 fac ilities. If nod e D should  fail, nod e C would  
no long er rec eive timing  that is trac eab le to node A. Here, nod e C would lose 
sync hronization and  b eg in to introd uc e slip s into the network.
B A
C D
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydflodr RPY 060397 
						

							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 4-62 Synchronization 
4
Figure 4-22. Less Than Optimal Diverse Routing
Rule 5:
Ob taining  b oth p rimary and  sec ond ary sync hronization fac ilities from within the 
same transmission c ab le should  b e minimized .
Rule 6:
The total numb er of c asc ad e nod e c onnec tions from the referenc ed  nod e should  
be minimized.
Example For Rule 6
Fig ure 4-23 shows exc essive c asc ad ing  in that nod e B d erives timing  from a 
sourc e three transmission fac ilities away.
Fig ure 4-24
 shows less c asc ad ing  in that node B d erives timing  from a sourc e 
two transmission fac ilities away. Exc essive c asc ad ing  is und esirab le b ec ause of 
intermediate link vulnerab ility. For examp le, an intermed iate link failure in nod e C 
of Fig ure 4-23
 would  c ause nod e B to lose timing . Suc h a failure inc reases the 
numb er of slip s b etween nod es A and  B. However, the same failure in Nod e C of 
Fig ure 4-24
 d oes not affec t nod e B’s sync hronization.
A
C
DB
Digital transmission facility
Primary frequency reference
cydfexcs RPY 060397 
						

							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 4-63 Synchronization 
4
Figure 4-23. Excessive Cascading
Figure 4-24. Minimized Cascading
A
C
DB
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfminc RPY 060397
A
E
B
DC
F
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfx1nd RPY 060597 
						

							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 4-64 Synchronization 
4
Rule 7:
The numb er of nod es rec eiving  sync hronization referenc e from any g iven node 
should  b e minimized .
Example For Rule 7
Fig ure 4-25 shows an exc ess of sync hronization from one nod e. If transmission 
fac ility linking  nod es A-B fails, then nod es B, D, E, and  F will lose their 
synchronization. However, if this same facility fails in Figure 4-26
 only nodes B, D, 
and  E will lose sync hronization. Nod e F remains sync hronized  to the network.
With few exc ep tions, it is not p ossib le to eng ineer a sync hronization network that 
will satisfy all seven rules. In general, rules 1 throug h 3 ap p ly in all c ases. For 
rules 4 throug h 7, use those rules that ap p ly to your spec ific  network and  try to 
minimize outages based on the cases presented in the examples for each of 
these rules.
Figure 4-25. Excessive Synchronization from One Node
A
E
B
DC
F
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfm1nd RPY 060397 
						

							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 4-65 Synchronization 
4
Figure 4-26. Minimized Synchronization from One Node
.PIC /ks6/lath/multiw/app b .p 22 5.0i
External-Reference Selection Rules
There are seven rules for selec ting  external sync hronization referenc es. These 
rules are d esc rib ed next.
Rule 1:
Nod es within the externally referenc ed  sub level may not use as a referenc e 
sourc e a fac ility from a nod e within the internally referenc ed  sub level
 (refer to 
Fig ure 4-17
 for an exp lanation of these terms).
Rules 2 Through 7:
Rules 2 through 7 are the same for both internal- and  external-referenc e 
selections.
A
E
B
DC
F
Digital transmission facility
Primary frequency reference
Secondary (backup) frequency reference
cydfm1nd RPY 060397 
						

							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 4-66 Synchronization 
4
Availability of Synchronization Sources
Misc onc ep tion 1 The loc al exc hang e c omp any (LEC) c an always p rovid e the 
sync hronization sourc e.
Fac t 1 The LECs are not always sub sc ribers to the AT&T referenc e 
freq uenc y.
Many end  offic es still use analog  switc hes and  D4-c hannel 
b anks. In the p ast, these c hannel b anks have not had  to b e 
sync hronized  and  p rob ab ly have not b een eq uip p ed with an 
offic e interfac e unit (OIU-2); therefore, they d o not p rovid e a 
lower- (3, 2, or 1) stratum referenc e.
Even if the CO/PTT p rovid es a lower stratum referenc e, a 
T1/E1 fac ility must b e terminated in eq uipment that is 
c loc ked  b y the lower stratum referenc e to ac c ess this lower 
reference.
Misc onc ep tion 2 AT&T c ommunic ations c an provid e the sync hronization 
sourc e.
Fac t 2 In the United  States, AT&T Communic ations is b ound  b y 
FCC tariffs that c urrently d o not req uire the use of framing  
b its in the c ustomer’s inc oming  T1 b it stream. Until framing  
b its are req uired , you should  not arb itrarily assume that the 
c arrier’s T1/E1 fac ilities p rovid e a sync hronization sourc e.
Misc onc ep tion 3 Sync hronization is not nec essary for DS1/CEPT1 networks 
that only transmit voic e.
Fac t 3 Even thoug h voic e is fairly insensitive to slip s, a DS1/CEPT1 
network that is imp rop erly sync hronized  c an c asc ad e a 
small number of slip s into a larg e numb er of slip s. This will 
c ause voic e d egrad ation and  DS1 alarms. In fac t, this 
c asc ad ing  effec t may even c ause the DS1/CEPT1 fac ility to 
b e taken out of servic e. Therefore, p rop er sync hronization is 
nec essary for all DS1/CEPT1 networks.