Flow Control

Flow Control 

The Generic Flow Control (GFC) field can be used for control of cell flow at
the local user-network interface. The details of its application are for further study.
The field could be used to assist the customer in controlling the flow of traffic for
different qualities of service. One candidate for the use of this field is a multiplepriority
level indicator to control the flow of information in a service-dependent
manner. In any case, the GFC mechanism is used to alleviate short-term overload
conditions in the network.
The Virtual Path Identifier (VPI) field constitutes a routing field for the network.
It is 8 bits at the user-network interface and 12 bits at the network-network
interface, allowing for more virtual paths to be supported within the network. The
Virtual Channel Identifier (VCI) field is used for routing to and from the end user.
Thus, it functions much as a service access point.
The Payload Type (PT) field indicates the type of information in the information
field. Table 3.1 shows the interpretation of the PT bits. A value of 0 in the first
bit indicates user information (that is, information from the next higher layer). In
this case, the second bit indicates whether congestion has been experienced; the
third bit, known as the service data unit (SDU)l type bit, is a one-bit field that can
be used to discriminate two types of ATM SDUs associated with a connection. The
term SDUrefers to the 48-octet payload of the cell.A value of 1 in the first bit of the
payload type field indicates that this cell carries network management or maintenance
information. This indication allows the insertion of network-management
cells onto a user's VCC without impacting the user's data. Thus, the PT field can provide
inband control information.
The Cell Loss Priority (CLP) field is used to provide guidance to the network
in the event of congestion. A value of 0 indicates a cell of relatively higher priority,
which should not be discarded unless no other alternative is available. A value of

1 indicates that this cell is subject to discard within the network. The user might employ
this field so that extra information may be inserted into the network, with a CLP of 1,
and delivered to the destination if the network is not congested. The network may set
this field to 1 for any data cell that is in violation of a agreement between the user and
the network concerning traffic parameters. In this case, the switch that does the setting
realizes that the cell exceeds the agreed traffic parameters but that the switch is capable
of handling the cell.At a later point in the network, if congestion is encountered, this cell
has been marked for discard in preference to cells that fall within agreed traffic limits.
The Header Error Control (REC) field is an 8-bit error code that can be used to
correct single-bit errors in the header and to detect double-bit errors. In the case of most
existing protocols, the data that serve as input to the error code calculation are in general
much longer than the size of the resulting error code. This allows for error detection.
In the case ofATM, the input to the calculation is only 32 bits, compared with 8 bits
for the code.The fact that the input is relatively short allows the code to be used not only
for error detection but also, in some cases, for actual error correction. This is because
there is sufficient redundancy in the code to recover from certain error patterns.
The error protection function provides both recovery from single-bit header
errors and a low probability of the delivery of cells with errored headers under bursty
error conditions. The error characteristics of fiber-based transmission systems appear
to be a mix of single-bit errors and relatively large burst errors. For some transmission
systems, the error correction capability, which is more time consuming, might not
be invoked.