The document discusses network synchronization in BSNL India's telecom network. It proposes having 21 highly accurate atomic clocks located at major nodes to serve as primary reference clocks (PRCs). PRC clocks would be distributed throughout the network via synchronization supply units (SSUs) located at convergent nodes. To ensure stable clock quality, it is proposed that no more than 10 SSUs be supported per PRC, no more than 20 network elements between SSUs, and no more than 60 total network elements per PRC chain. The plan is for 321 SSUs total to distribute PRC clocks and synchronize all transmission, switching and access elements across BSNL India's telecom network.

1. E4-E5 (E&WS) Rev. Date 16.03.11
SYNCHRONIZATION
OF
TELECOM NETWORK
BSNL India For Internal Circulation Only 1

2. E4-E5 (E&WS) Rev. Date 16.03.11
Synchronization
Telecom traffic is constantly increasing both in fixed as well as in mobile (including new
3G systems) telephone network. More and more information is being transported at
higher and higher data speeds and therefore the Synchronization of the traffic in the
networks is becoming increasingly important. Telecommunications’ reliability is based
on the data signals being synchronized and clocked using the same clock everywhere in
the network. Network synchronization is one important part in network planning,
especially on SDH network.
The basic clock in the SDH networks is called E1, and it must be exactly 2.048 MHz. In a
fully synchronized SDH network, there is a very stable and accurate atomic (caesium)
clock of 2.048 MHz that functions as a "master clock" or primary reference clock (PRC).
The quality of PRC clock always accuracy better than 10exp-12. The PRC is distributed
throughout the network using the transmission backbone comprising of DWDM and SDH
transport network. The clock is regenerated in the network’s nodes in “slave clocks”
called as synchronization supply units (SSUs). The SSUs are clock generator and
distributor of the accuracy better than 10exp-11. The SSUs can also temporarily (up to
hold over time) be used as PRCs if the input connection from the master clock is
interrupted. The SSUs regenerate and distribute the clock thus received from PRC
through a chain of SECs (SDH Equipment Clocks). The SECs are the clocks in the
network elements (see Figure below).
This clock regeneration is never completely perfect; rather, each regenerated clock will
have variations in frequency and phase. The more nodes passed "en route", the less stable
the clock will be. The same is shown as given below:
BSNL India For Internal Circulation Only 2

3. E4-E5 (E&WS) Rev. Date 16.03.11
Synchronization Planning: while extending PRC clock from one node to other
node through transmission network of SDH and DWDM few guidelines are required to
be kept in mind. This is essential as the clock quality keeps on getting deteriorated as and
when it crosses number of nodes. Following is the salient planning rules:
• From one PRC maximum 10 numbers of SSUs can be supported.
• Between two SSUs maximum 20 NE can be placed.
• In total maximum 60 numbers of NEs can be supported in one PRC chain.
However, the above figures are theoretical figures and as such they may be adjusted as
per the quality of transmission network, so that the quality of clock in the network always
remains comparable to PRC clock i.e. better than 10exp-12 accuracy.
BSNL India For Internal Circulation Only 3
PRC: Primary Resource Clock
SEC: SDH Equipment Clock

4. E4-E5 (E&WS) Rev. Date 16.03.11
BSNL Synchronization Plan:
It is proposed to have 21 numbers of PRCs with two caesium tubes (mostly at all Level-1
TAX locations) for geographical diversity and for ensuring the minimum nodes slippage.
However, in view of the requirement for the orderly distribution of the PRC clock
throughout the transmission network, it is proposed to locate the SSUs at the convergent
nodes of the existing DWDM backbone rings. Overlapping with DWDM network
enables to transport the clock on a number of SDH rings to as many district head
quarters, where SSUs are proposed to be located. Most of these nodes are co-located with
the Level –I TAX stations. Thus these 21 PRCs are expected to supply clock for all the
Transmission, Switching and Access elements in the transmission geographic area, also
called as the PRC timing Island. The SSU at the PRC location depends on the PRC itself
for the first master clock input. In case of failure or deterioration of the PRC, the SSU
depends on the other three PRC clock inputs received from the nearest three PRCs. In the
event of complete failure of the PRC inputs or in the event of the deterioration of clock
quality all the PRCs below the GPS standard, the SSU draws the clock from the GPS
BSNL India For Internal Circulation Only 4
SS
U
K=1
PRS
SSU
SSU
Rule 1
K < 10 SSUs
N < 60 SECs
SEC (NE with SONET/SDH Equipment Clock)
Rule 2
N < 20 SECs
K=2
K=3
SSU
Co-located PRS & TSG
SSUK=1
1
20
1
2
0 PR
S
1
2
0
SSU K=1
1
2
0
1
20
201
2
0
1
1
2
0
SSU K=2
PRS

5. E4-E5 (E&WS) Rev. Date 16.03.11
clock, which shall be in-built in SSU. It is proposed to place at least one SSU at each of
the SSA Headquarter. Thus at least 322 SSUs shall be required for distributing the clock.
In the cities, where more than 3 lakh DELs are available, there will be at least 3 SSUs in
the same city for distributing the clock across the city network. Similarly in all the cities
with more than one Lakh DELs but less than 3 Lakh DELs two SSUs are proposed in the
same city for distributing the clock.
BSNL India For Internal Circulation Only 5

6. E4-E5 (E&WS) Rev. Date 16.03.11
clock, which shall be in-built in SSU. It is proposed to place at least one SSU at each of
the SSA Headquarter. Thus at least 322 SSUs shall be required for distributing the clock.
In the cities, where more than 3 lakh DELs are available, there will be at least 3 SSUs in
the same city for distributing the clock across the city network. Similarly in all the cities
with more than one Lakh DELs but less than 3 Lakh DELs two SSUs are proposed in the
same city for distributing the clock.
BSNL India For Internal Circulation Only 5