WE CLAIM:
1. A transfer apparatus having an input port and an
output port, an input line speed of the input port and an
output line speed of the output port being capable of being
set to differ from each other, the transfer apparatus
comprising, for each output port:
a low-latency frame transmission buffer to store a low-latency frame received at the input port;
a normal-latency frame transmission buffer to store a normal-latency frame that is a frame received at the input port, the normal-latency frame being permitted to be transferred with higher latency than the low-latency frame;
a time calculation unit to calculate output start time of the low-latency frame by using information on a frame length of the low-latency frame, the input line speed, and the output line speed when the low-latency frame is received; and
an output contention control unit to control transfer of the low-latency frame and the normal-latency frame by using the output start time.
2. The transfer apparatus according to claim 1, wherein
when the output line speed is higher than the input
line speed,
the time calculation unit calculates the output start time at which the low-latency frame is transferable without error, and
the output contention control unit starts transfer of the low-latency frame on a basis of the output start time.
3. The transfer apparatus according to claim 2, wherein
in a case where the normal-latency frame is being
transferred, when transfer of the normal-latency frame is

completable before the output start time, the output contention control unit completes the transfer of the normal-latency frame, and when the transfer of the normal-latency frame is not completable before the output start time, the output contention control unit divides the normal-latency frame with a divisible frame length being left, and suspends the transfer of the normal-latency frame.
4. The transfer apparatus according to claim 3, wherein
in interrupting the transfer of the normal-latency
frame, the output contention control unit transfers the normal-latency frame until the output start time or time of completion of transfer of a frame length obtained by excluding a divisible minimum frame length from an untransferred portion of the normal-latency frame, whichever comes earlier.
5. The transfer apparatus according to claim 2, wherein
in a case where the normal-latency frame is not
transferred but the normal-latency frame is stored in the normal-latency frame transmission buffer, the output contention control unit starts the transfer of the normal-latency frame and completes the transfer of the normal-latency frame when the normal-latency frame is transferable by the output start time, and the output contention control unit does not start the transfer of the normal-latency frame when the normal-latency frame is not transferable by the output start time.
6. A frame transfer method in a transfer apparatus having
an input port and an output port, an input line speed of
the input port and an output line speed of the output port
being capable of being set to differ from each other,

when the transfer apparatus comprises, for each output port: a low-latency frame transmission buffer to store a low-latency frame received at the input port; a normal-latency frame transmission buffer to store a normal-latency frame that is received at the input port, the normal-latency frame being permitted to be transferred with higher latency than the low-latency frame; a time calculation unit; and an output contention control unit, the method comprising:
a calculation step of the time calculation unit calculating output start time of the low-latency frame by using information on a frame length of the low-latency frame, the input line speed, and the output line speed, when the low-latency frame is received; and
a control step of the output contention control unit controlling transfer of the low-latency frame and the normal-latency frame by using the output start time.