выдержка из ietf драфта:
....
3. Clock Recovery
TDM networks are inherently synchronous. In the public switched
telephone network and in SONET / SDH networks one node, called the
clock master provides a time reference to the other, called the
slave. Somewhere in the network there will always be at least one
extremely accurate primary reference clock, with long-term accuracy
of one part in 1011. This node, whose accuracy is called stratum 1,
provides the reference clock to secondary nodes with lower stratum 2
accuracy, and these in turn provide reference clock to stratum 3
nodes. This hierarchy of time synchronization is essential for the
proper functioning of the network as a whole.
Packets in IP networks reach their destination with delay that has a
random component, known as jitter. When emulating TDM on an IP
network, it is possible to overcome this randomness by using a
"jitter buffer" on all incoming data, assuming the proper time
reference is available. The problem is that the original time
reference information is no longer available.
Anavi, Stein, Schwartz [PAGE 4]
TDM over IP February, 2001
In principle there are two different levels of integration of TDMoIP
into a T1/E1 network. In the "bypass" scenario a one party might
want to transport TDM T1/E1 traffic over another party's network. In
such applications both TDMoIP devices SHALL receive time reference
from the central offices to which they connect.
In the "whole network" scenario, major portions of the primary
infrastructure are replaced with TDMoIP networks, and a method of
time synchronization is required. IP networks also disseminate clock
information through NTP (RFC 1305), but unless the IP network is
completely private and dedicated to the TDMoIP link, there will be
no connection between the NTP clock and the desired TDM one. In such
cases independent time standards MAY be provided to all TDMoIP
devices, thus relieving the IP network of the need to send
synchronization information. The use of time standards less accurate
than stratum 3 is NOT RECOMMENDED since it may result in service
impairments.
When the provision of accurate local time references is not
practical then clock recovery SHOULD be performed based on the rate
of arrival of incoming packets using an appropriate `averaging'
process that negates the effect of the zero mean random jitter.
Conventionally a phase locked loop (PLL) is used for this purpose.
......
подробнее (даже с картинками) см. например :
http://pdfserv.maxim-ic.com/en/an/AN4115.pdfили набор доков по теме:
http://www.dspcsp.com/tdmoip/index.html
xenomai (linux) + ARM9, сбор информации о сабже
Apr 4 2008, 09:29
Хотя, если случай симметричный, то это не требуется (раскодирование-кодирование).
