In normal packet or cell based communication, the header and contents are of the same bit rate and line codes etc. The only freedom the user has is in the actual choice of bits used in the cell and their meaning. Although this has some advantages, it restricts the use of the system. Some users may only require a low transmission capability and would wish to avoid transmission at the full line rate. Others may be quite capable of much higher rates and be frustrated at the slow speed available to them. This document discusses an alternative method for information transport which supports users with different transmission requirements. It uses time slices with attached addresses and is thus an addressed time slice (ATS) transport system.
The technique is now possible because of the advent of purely optical transmission and routing/ switching. It is no longer necessary to read a packet’s contents at a switch before routing it. This allows complete traffic transparency to be achieved, which has not been possible with previous technology.
2 ATS Mechanism
The ATS cell is comprised of a fixed duration time slice and an attached header. The time slice can be used in whatever way the user chooses. The user may send any signal subject to the bandwidth limitations of the system and the time slice duration. The header contains only an address, for routing purposes. There is no other header information. Such a cell is shown in figure 1. ATS cells are routed according to the address in the cell header so the address format and header rate must be the same throughout the network. This system has many advantages which are discussed below.
3 Contents Flexibility
As mentioned above, the time slice can be used for whatever purpose the user chooses. This could be a digital signal of any bit rate, analogue signal or just a chunk of optical spectrum. The user is no longer constrained to a fixed bit rate for transmission. He is not even forced to digitize a signal. The source simply fills the ATS time field with the signal. The transmission unit attaches an appropriate header. At the destination, the header is stripped off and the information field is relayed unchanged to the appropriate device. The format, information rate and meaning of the information field is entirely determined by the end devices and is completely independent of the carrier protocol. The only limitation on the signal is imposed by the system bandwidth.
4 Network Implications
The ATS technique has become possible because of the new optical technologies. It is possible to transmit and route an optical signal entirely optically, without ever converting it to the electrical domain. There is no need to have devices in the network which are linked with service categories. The signal itself need only be read at the destination. This allows the network to be made completely transparent to the data traffic types, digital and analogue. The user is constrained to use particular transmission times but is otherwise completely free. The network can be designed purely on the basis of the traffic weight, regardless of the form that the traffic takes.
Because the ATS does not have a fixed format information field, it is not possible to construct a network with fully active nodes which read and re-write data. Any system which affects the information field (except obviously at the destination) must be excluded. Thus regenerators, store and forward switches and any other modulation dependent devices are excluded, but appropriate amplifiers may be used. The system would work best on a network where the route is switched optically on the basis of the address without affecting the information field.
The system would also work well with optical transmission system which distribute all signals to all destinations, provided that there are no regenerators. In this case, the address would be read only by the terminals and the ATS would be completely ‘untouched’ on its way.
5 User Advantages
Because of the contents flexibility, the user can buy a transmission system which fits his personal needs in the most economic way because he is free to make the trade-off between cost of time and the cost of digitizing for fixed rate transmission. The user may wish to use the time slice to transmit a much lower data rate than bandwidth allows. Although he would not get the maximum value for the time bought, this may be outweighed by the saving through using a cheaper transmitter. Alternatively, a user may have a capability to transmit at a higher speed than that tolerable for the header readers in the routers. He can squeeze much more information into the ATS than would be allowed by a conventional system which limits the data rate to be the same as in the header.
6 ATS Routing
The ATS header contains only an address. This would be read optically and the ATS routed accordingly. There would not be any other information in the header. Whatever method is actually used for reading the address, the information field should be left alone so as not to affect the contents in any way. Optical routing would allow this to be done. The network could be made very simple indeed and the potential throughput would be very high.
7 ATM Compatibility
The ATS could be made to resemble an ATM cell in size and address structure. However, there would be no control or other information in the header except the address. Any ATM transport system which only reads the address and automatically routes the cell unaltered could be compatible with ATS.
8 WDM Migration
The technique would migrate well with WDM. It would allow wavelengths to be shared very easily, and could even be used to deal with multi-wavelength transmissions between nodes. If the problems of dispersion can be reduced to a tolerable level, an ATS could have a single wavelength header and be comprised of data using many wavelengths. This would reduce by orders of magnitude the switching problems caused by having to switch large numbers of packets at different wavelengths.
9 Multiplexing Options
Another extension of the technique would be to vary the size of the time slice at the different hierarchical levels in the network. Many local level ATSs could be multiplexed into large second level ATSs with others sharing the same second level route. This could be done again at the trunk level. A second level ATS is shown in figure 2.