What Is Frame Relay - Digi PortServer II User Manual

What is Frame Relay?

Frame Relay was adapted from the Link Access Protocol D-Channel (LAPD) of the
ISDN model. It is a switched digital service, in which several virtual or logical cir-
cuits share a common physical circuit. Each physical circuit connects to a Frame
Relay service provided by the service provider; this service is sometimes referred to as
the "cloud". This arrangement allows a server to make a "logical" connection to sev-
eral other servers that are also connected to the service, but utilizing only one physical
circuit.
User Terminal
User Terminal
Each logical connection is referred to as a Permanent Virtual Circuit (PVC). Each
PVC has a Data Link Channel Identifier (DLCI) associated with it. The DLCI is used
locally to identify the connection, but does not traverse the network. The DLCI is
converted and mapped between its origin and its destination, according to a scheme
specified by the network service provider, and may change each time it passes through
a network switch. A DLCI differs from an address because each data frame only
identifies a circuit in each direction, not a source and destination.
Frame Relay networks incorporate error detection, but not error recovery, to permit
faster data throughput. Because this scheme does not guarantee delivery of packets,
several flow control mechanisms are provided to minimize the possibility of network
overflow. (If the network overflows due to buffer pools being full, it may drop
packets because of the lack of error recovery.) Three mechanisms are available to
control flow:
1. The network service provider specifies the Committed Information Rate (CIR),
which is the rate in bytes-per-second that data should be successfully delivered by
the network, under normal conditions.
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FR Switch
Figure 21 Typical Frame Relay Configuration
Configuring Frame Relay
FR Switch
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DLCI
Frame Relay
Service
PortServer
FR Switch
Fr Switch
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90030500B