1--Field of the Invention
The present invention is generally concerned with time division multiple access and time division multiplexing in telecommunication networks. The invention is more particularly concerned with multirate frame structures, also known as multispeed frame structures, for tree-structured star telecommunication networks, especially fiber optic networks.
2--Description of the Prior Art
As described in U.S. Pat. No. 5,363,370 by the inventor, a multirate frame is defined as a frame obtained by time division multiplexing of or time division multiple access to data having line bit rates, i.e., bit rates in one transmission medium, that are different; accordingly, a time interval, such as a frame time cell, having a constant duration can be allocated to a number of bits different from that allocated to another time interval according to the bit rates of the sources from which the bits originate. A multirate frame of this kind differs from a conventional frame produced by time division multiplexing of data at different bit rates, by increasing these different bit rates into a same line bit rate.
In prior art shared optic fiber networks there are two main modes of transmission between two equipments, namely half-duplex mode, also called as a two-way alternate transmission, and full-duplex mode, also called as a two-way simultaneous transmission, which results from wavelength division multiplexing.
In half-duplex mode the transmission direction alternates. While one equipment is sending data the other equipment is receiving the data, and vice versa. In each equipment there are a send phase and a receive phase that do not overlap. For a predetermined data bit rate D between the two equipments the line bit rate is equal to at least 2 D, given the alternating time sharing of the transmission medium between the two equipments. In practise a propagation time and an interference suppressing guard time increase the line bit rate to a value substantially equal to 2.5 D.
In wavelength division multiplexing full-duplex transmission, each of two equipments connected to respective ends of an optical fiber sends to the other equipment using a respective wavelength in the optical fiber.
Each of these two transmission modes has its own advantages and disadvantages.
Half-duplex mode is particularly well suited to low bit rate data transmission. The doubling of the line bit rate does not impose any bit rate limitations on either the equipments or the transmission medium, and this technique can be implemented using reversible send/receive components which are not costly. In particular, a single diode can be used to send and receive by reversing the bias thereof. A diode of this kind has a range of operating frequencies enabling it to be used to send and receive at respective different frequencies.
Wavelength division multiplexing full-duplex mode is the only efficient transmission technique for high bit rate data. It uses costly optical duplexers to simultaneously enable reception at a first wavelength and transmission at a second wavelength.