Transmission system with improved synchronization

DETAILED DESCRIPTION The object of the present invention is to provide a transmission system according to the preamble in which it is possible to transfer accurate timing information in said packets.
To achieve this object the present invention is characterized in that the second node comprises timing extraction means for extracting a timing signal when a predetermined symbol in a packet is transmitted in a predetermined position in the frame.
The present invention is based on the recognition that it is possible to extract accurate timing information from the signal if this timing signal is extracted when a predetermined symbol in a packet appears at a predetermined position in the frame.
In the example of carrying ATM packets in G.
704 frames, 53 bytes ATM packets are carried in a plurality of frames, each of them carrying 30 bytes.
In this case the first byte of an ATM cell coincides with the first byte of a G.
704 frame each 53 frames.
With a frame period of 125 .
mu.
s, said first byte of an ATM packet will coincide with the first byte of a frame each 53.
multidot.
125 .
mu.
s 6.
625 ms.
Consequently an accurately defined timing signal can be obtained from the ATM signal by generating a timing signal at the instant on which the first byte of an ATM frame corresponds to the first byte of a G.
704 frame.
An embodiment of the invention is characterized in that the first node comprises timing information transmission means for transmitting a clock reference value to the second node, and in that the second node comprises synchronization means for synchronizing at arrival of the timing signal a local clock with the clock reference value.
In this embodiment of the invention it is possible to synchronize a local clock in the second node with a clock in the first node.
Both clocks can have a frequency which is unrelated to the clock frequencies involved with the signals transmitted over the transmission path.
A further embodiment of the present invention is characterized in that the timing reference value corresponds to an expected clock value at arrival of the next timing signal at the second node