Link-Level Flow Control and Buffering - Microarchitecture of Network-on-Chip Routers - page 21

free slots
counter

a

ready

update

valid

data

sender

receiver

free slots
counter

b

ready

update

valid

data

sender receiver

c

free slots
counter

ready

update

valid

data

sender receiver

Fig. 2.12 An abstract model of a ready-valid flow-controlled link including a free-slot counter ( a )
at the receiver's side, ( b ) in the middle and ( c ) at the sender

(ready signal that shows buffer availability and valid signal that declares the arrival
of a new data item at the receiver's buffer) do not change.
Equivalently, we could move this counter at the sender's side (see Fig. 2.12 c).
Then, the valid signal from the sender and the ready signal from the counter are
both local to the sender. The read signal tells to the sender which it can send a new
data item, while the receiver sends to the counter only the necessary status update
signals that denote the removal of a data item from the receiver's buffer. Assuming
that the counter reflects the number of empty slots at the receive side, it should be
incremented when it receives a new status update from the receiver and decremented
when the sender wants to transmit a new data item. Also, the receiver is ready to
accept new data when the value of the free slots counter is larger than zero.

2.4 Credit-Based Flow Control

When the counter is attached to the sender, the derived flow control policy is called
credit-based flow control and gives to the sender all the necessary knowledge to
start, stop, and resume the transmission (Kung and Morris 1995 ; Dally and Towles
2004 ). In credit-based flow control, the sender explicitly keeps track of the available