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S. G. Kandlikar and A. Ganguly
schemes require precise synchronization between the transceivers to avoid interchannel interference by preserving the orthogonality of the code-channels. Such
a synchronization is difficult to achieve in transceivers distributed across multiple
layers.
Similarly, synchronized Time Division Multiple Access (TDMA) is difficult to
adopt for the same reason. Therefore, Asynchronous TDMA (A-TDMA) based
on token passing [30] or Carrier Sense Multiple Access/Collision Detection
(CSMA/CD) are proposed. However, CSMA based A-TDMA does not perform well
in the presence of the high traffic density due to exponential back-off. Therefore,
to access the wireless channel in a distributed fashion, without the need for precise
synchronization or centralized arbitration while avoiding collision a token based
medium access mechanism is proposed in [30] for Wireless NoCs (WiNoCs). In a
token-based medium access mechanism, the access to the wireless medium is granted
by the possession of a token. Only the WI possessing the token can transmit via the
wireless medium. No separate request mechanism or priority is considered as a part
of the token passing scheme to avoid the need for a central grant or arbitration unit.
However, in such a MAC only whole packets are transmitted to other WIs, to maintain
the integrity of the wormhole switching [29]. This increases the buffer requirement
and hence static power consumption in the WIs. Therefore, we propose a MAC
mechanism that allows partial packet transmission from a WI while maintaining the
integrity of the wormhole switching.
In the proposed MAC, instead of circulating a token at the end of each transmission, each WI broadcasts a control packet at the beginning of its transmission.
The control packet consists of a header for identification and differentiation of data
packets. In addition, to enable partial packet transmission and correct routing, the
control packet has 3-tuples: (DestWI, PktID, NumFlits) for every partial packet that
it will transmit. Each 3-tuple contains the information about the number of flits (i.e.,
NumFlits) to be transmitted from the WI to a particular destination (i.e., DestWI)
along with the packet ID (i.e., PktID) of the packet, to which the flits belong. The
PktID enables the destination WI to identify the Virtual Channel (VC) number at the
destination WI to put the flits, thus maintaining wormhole switching.
In case the PktID does not exist at the destination WI, the WI reserves an unoccupied VC. The number of output VC of the transmitting WI limits the number of
3-tuples in a control packet. The control packet is broadcast to all WIs. Therefore,
the next WI in sequence computes the duration of the current transmission from the
information in the control packet and transmits its control packet when the current
transmission is completed. For this purpose, the WIs are numbered in a sequence.
Thus, contention between WIs in accessing the channel is avoided.
We have chosen non-coherent on-off keying (OOK) modulation, as it allows
relatively simple and low-power circuit implementation. The WI transceiver circuitry
has to provide a very wide bandwidth as well as low power consumption. The OOK
transceiver design is adopted from [30] where low power design considerations are
taken into account at the architecture level. While this transceiver is designed for
planar NoCs, it is also suitable for the 3D NoCs due to similar wireless path losses.
S. G. Kandlikar and A. Ganguly
schemes require precise synchronization between the transceivers to avoid interchannel interference by preserving the orthogonality of the code-channels. Such
a synchronization is difficult to achieve in transceivers distributed across multiple
layers.
Similarly, synchronized Time Division Multiple Access (TDMA) is difficult to
adopt for the same reason. Therefore, Asynchronous TDMA (A-TDMA) based
on token passing [30] or Carrier Sense Multiple Access/Collision Detection
(CSMA/CD) are proposed. However, CSMA based A-TDMA does not perform well
in the presence of the high traffic density due to exponential back-off. Therefore,
to access the wireless channel in a distributed fashion, without the need for precise
synchronization or centralized arbitration while avoiding collision a token based
medium access mechanism is proposed in [30] for Wireless NoCs (WiNoCs). In a
token-based medium access mechanism, the access to the wireless medium is granted
by the possession of a token. Only the WI possessing the token can transmit via the
wireless medium. No separate request mechanism or priority is considered as a part
of the token passing scheme to avoid the need for a central grant or arbitration unit.
However, in such a MAC only whole packets are transmitted to other WIs, to maintain
the integrity of the wormhole switching [29]. This increases the buffer requirement
and hence static power consumption in the WIs. Therefore, we propose a MAC
mechanism that allows partial packet transmission from a WI while maintaining the
integrity of the wormhole switching.
In the proposed MAC, instead of circulating a token at the end of each transmission, each WI broadcasts a control packet at the beginning of its transmission.
The control packet consists of a header for identification and differentiation of data
packets. In addition, to enable partial packet transmission and correct routing, the
control packet has 3-tuples: (DestWI, PktID, NumFlits) for every partial packet that
it will transmit. Each 3-tuple contains the information about the number of flits (i.e.,
NumFlits) to be transmitted from the WI to a particular destination (i.e., DestWI)
along with the packet ID (i.e., PktID) of the packet, to which the flits belong. The
PktID enables the destination WI to identify the Virtual Channel (VC) number at the
destination WI to put the flits, thus maintaining wormhole switching.
In case the PktID does not exist at the destination WI, the WI reserves an unoccupied VC. The number of output VC of the transmitting WI limits the number of
3-tuples in a control packet. The control packet is broadcast to all WIs. Therefore,
the next WI in sequence computes the duration of the current transmission from the
information in the control packet and transmits its control packet when the current
transmission is completed. For this purpose, the WIs are numbered in a sequence.
Thus, contention between WIs in accessing the channel is avoided.
We have chosen non-coherent on-off keying (OOK) modulation, as it allows
relatively simple and low-power circuit implementation. The WI transceiver circuitry
has to provide a very wide bandwidth as well as low power consumption. The OOK
transceiver design is adopted from [30] where low power design considerations are
taken into account at the architecture level. While this transceiver is designed for
planar NoCs, it is also suitable for the 3D NoCs due to similar wireless path losses.