Time slot scheduling for shared-medium communications networks
View Patent ↗A method and apparatus for allocating time slots in a communications network, comprising a headend, a plurality of outstations, and a shared medium connecting each outstation to the headend. The method involves scheduling timeslots for each outstation responsive to the measures of distance of each outstation from the headend. The method reduces the requirement for guard bands between timeslots, since it obviates allocation of a guard band between consecutive first and second timeslots where the outstation allocated the second timeslot is located no closer to the headend than the outstation allocated the first time slot. Ideally, the timeslots are allocated to outstations cyclically in ascending order of distance of the outstations from the headend, with the closest outstation following the farthest to complete the cycle, so that guard bands are rendered unnecessary between the majority of timeslots. In preferred embodiments distance may be round-trip delay, and the outstations and head end may be configured to form a ring network.
1. A method of allocating guard bands between time slots in a communications network comprising a headend, a plurality of outstations, and a shared medium connecting each outstation to the headend, the method comprising the steps of:
establishing measures of distance of each outstation from the headend;
allocating a guard band between successive first and second timeslots allocated to the outstations only where the outstation allocated the second timeslot is closer to the head end than the outstation allocated the first timeslot.
2. A method according to claim 1 in which the measure of distance is a measure of round-trip delay between headend and outstation.
3. A method according to claim 1 additionally comprising the step of:
scheduling timeslots for the outstations responsive to the measures of distance.
4. A method according to claim 3 wherein the step of scheduling comprises the step of:
scheduling the timeslots to outstations cyclically in ascending order of distance of the outstations from the headend.
5. A method according to claim 1 wherein the step of establishing comprises the steps of:
for each outstation forming a measure of the time interval between the headend issuing an interrogation and the headend receiving a response from that outstation;
for each outstation, forming the measures of the distance responsive to the intervals.
6. A method according to claim 1 wherein the step of establishing comprises the steps of:
for each outstation, measuring the intervals between sending at least one interrogation message from the headend to the outstation and receiving respective replies from the outstation;
for each outstation, forming the measures of the distance responsive to the respective minima of the intervals.
7. A method according to claim 1 in which a timeslot is periodically allocated for purposes of re-establishing the measures of distance of each outstation from the head end.
8. A method according to claim 1 wherein the shared medium is one of an optical fibre medium, a copper medium, and a wireless medium.
9. A method according to claim 1 in which all outstations are substantially equidistant from the head end.
10. A method according to claim 1 in which the head end, shared medium, and outstations are configured as a loop.
11. A method according to claim 10 in which separate shared media are provided for upstream and downstream communication between the head end and the outstations.
12. A method according to claim 1 in which the outstations and head end are configured in a ring network, in which differential distance between each outstation and the headend is substantially eliminated by causing both downstream and upstream traffic to travel around the ring in the same direction.
13. A Method according to claim 1 in which the step of establishing measures is performed prior to installation of the network.
14. A method according to claim 3 in which the step of scheduling timeslots comprises the step of scheduling timeslots according to demand.
15. A line termination unit for a telecommunication network comprising a plurality of outstations and a shared medium connecting each outstation to the headend, the line termination unit comprising:
a guard band allocator arranged to allocate guard bands between successive first and second timeslots allocated to the outstations only where the outstation allocated the second timeslot is closer to the head end then the outstation allocated the first timeslot
a timeslot scheduler arranged to schedule timeslots for each outstation responsive to measures of distance of each outstation from the headend.
16. A line terminating unit according to claim 15 additionally comprising:
a timeslot scheduler arranged to schedule timeslots for each outstation responsive to measures of distance of each outstation from the headend.
17. A line terminating unit according to claim 16 additionally comprising:
a ranging system arranged to derive the measures of distance of each outstation from the head end.
18. A line termination unit according to claim 15 in which the scheduler is arranged to schedule timeslots to outstations cyclically in ascending order of distance of the outstations from the headend.
19. A line termination unit according to claim 15 in which the shared medium is one of an optical medium, a copper medium, and a wireless medium.
20. A communications network comprising a line termination unit according to claim 15 .
21. A communications access network comprising a line termination unit according to claim 15 .
22. A communications network according to claim 20 further comprising:
at least one outstation,
the line terminating unit and the at least one outstation being configured to form a ring network.
23. A communications network according to claim 22 being an optical communications network.
24. A communications network according to claim 23 in which wave division multiplexing is employed to separate upstream and downstream traffic.
25. A program for a computer stored on a computer readable medium, for allocating guard bands between time slots in a communications network, comprising a headend, a plurality of outstations, and a shared medium connecting each outstation to the headend, the program being arranged to perform the steps of:
storing measures of distance of each outstation from the headend;
allocating a guard band between successive first and second timeslots allocated to the outstations only where the outstation allocated the second timeslot is closer to the head end than the outstation allocated the first timeslot.