This invention relates to a method and apparatus for controlling the flow
of fiber channel (FC) data frames from a fiber channel link to a
metropolitan area link of lesser bandwidth (a synchronous optical network
(SONET)). The release of each data unit by a data source for transport
onto the first channel is governed by a credit-based protocol whereby one
data unit is released upon the provision of one credit unit. A first
marker identifies a high limit H of the buffer at which a first condition
occurs, H being less than N and the first condition applying when the
number of data units in the buffer is greater than H. A second marker
identifies a low limit L of the buffer at which a second condition occurs,
L being less than H and the second condition applying when the number of
data units in the buffer is less than L. A credit unit owed counter O
counts the number of credit units which become owed to the data source
when the second condition applies. A third marker Max identifies a maximum
number of credit units that may be provided to the data source when the
first condition applies and the value of the credit unit counter is
greater than zero. Through this invention, effective means to control the
flow of data from a fiber channel link to a metro area transport link
without loss of frames or delay is provided.
