Constructing a distributed boundary clock over a dedicated communication channel
Implementing a distributed boundary clock in situations where book-end devices such as microwave TX/RX pairs must collaborate in achieving PTP on-path support is described. A dedicated channel, generally low-speed compared to the main channel is used to transfer timing from the master side to the slave side using framing and super-framing applied to the digital channel. Time-stamps of events such as super-frame boundaries are communicated between the two sides to enable timing transfer.
1. A method, comprising synchronizing a first clock located at a receiving side to a second clock located at a sending side including aligning the first clock to the second clock by adjusting the first clock using forward and reverse error estimates ε F (i) and ε R (i) where
ɛ
F
(
i
)
-
Δ
MS
=
-
1
N
·
∑
n
=
0
N
-
1
(
T
2
(
n
)
-
T
1
(
n
)
)
ɛ
R
(
i
)
+
Δ
SM
=
1
N
·
∑
k
=
0
N
-
1
(
T
4
(
k
)
-
T
3
(
k
)
)
where i is an index that represents an iteration number, each iteration representing N estimates of one-way delay from time-stamps that are averaged to reduce measurement noise,
where Δ MS is forward delay from the second clock to the first clock,
where Δ SM is reverse delay from the first clock to the second clock,
where subscripts F and R represent forward and reverse directions, respectively,
where T 1 is a first time of departure measured according to the second clock,
where T 2 is a first time of arrival measured according to the first clock,
where T 3 is a second time of departure measured according to the first clock and
where T 4 is a second time of arrival measured according to the second clock.
2. The method of claim 1 , wherein Δ MS =Δ SM and an estimate of clock error ε(i) is developed by combining forward and reverse error estimates as
ɛ
(
i
)
=
ɛ
F
(
i
)
+
ɛ
R
(
i
)
2
=
1
N
∑
k
=
0
(
N
-
1
)
[
T
4
(
k
)
-
T
3
(
k
)
]
-
1
N
∑
n
=
0
(
N
-
1
)
[
T
2
(
n
)
-
T
1
(
n
)
]
2
.
3. The method of claim 2 , wherein the estimate of clock error ε(i) is used in a closed loop clock correction step.
4. The method of claim 3 , further comprising delivering a timing reference over a dedicated framed digital channel.
5. The method of claim 4 , further comprising building a distributed boundary clock.
6. The method of claim 5 , further comprising deploying a radio relay link for backhaul of cellular traffic between base-stations and a wireless network, the radio link having a primary wide-band channel and a secondary narrow band channel that is used to transfer timing.
7. A non-transitory computer readable media, comprising computer or machine readable program elements translatable for implementing the method of claim 1 .