Introduction to Network-on-Chip Design - Microarchitecture of Network-on-Chip Routers - page 4

connections as is the mainstream approach followed so far, several other research
efforts try to provide a better communication medium for the on-chip connections
utilizing on-chip optical connections (Bergman et al. 2014 ).

1.2 Flow Control

At the system level, using only a set of data wires in a communication channel
between two peers (a sender and a receiver module) is not enough. The receiver
should be able to distinguish the old data sent by the sender from the new data
that it sees at its input. Also, the sender should be informed if the data that has
sent has been actually accepted by the receiver or not. Therefore, some additional
form of information needs to be conveyed across the sender and the receiver
that would allow them to understand when a transaction between them has been
completed successfully. Such information is transferred both in the forward and
in the backward direction, as depicted in Fig. 1.3 , and constitutes the flow-control
mechanism.
The flow control mechanism can be limited at the borders of a single wire (called
link-level flow control) or it can be expanded between any source and destination
possibly covering many links and thus called end-to-end flow control (Gerla and
Kleinrock 1980 ). Figure 1.3 tries to explain graphically the difference between
the local and the global flow control mechanisms. While link-level flow control
is explicitly implemented by the additional flow control wires of the link, end-
to-end flow control can be either explicitly or implicitly implemented in a NoC
environment. Explicit implementation requires several flow control wires arriving
at each node from different destinations, that each one would describe the status
of the corresponding connection. Implicit implementation means that any source or
destination node has a mechanism to understand the status of the other side using the
normal or special messages transmitted between them. Message transmission in this
case, would have used all the intermediate links between the source and destination
pair.

end-to-end data

Link-Level
Flow Control

SRC DST

data

data

data

notify

notify

notify

end-to-end notification

Fig. 1.3 Link-level and end-to-end flow control