There was a major development in Frame
Relay's history in 1990, when Cisco Systems, StrataCom, Northern
Telecom, and Digital Equipment Corporation formed a group to focus on
Frame Relay technology development and accelerate the introduction of
interoperable Frame Relay products. This group developed a
specification conforming to the basic Frame Relay protocol, but
extended it with features that provide additional capabilities for
complex internetworking environments. These Frame Relay extensions are
referred to as LMI (local management interface).
The main functions of LMI process are:
- To determine the operational status
of the various PVCs that the router knows about
- To transmit keepalive packets to
ensure that the PVC stays up and does not shut down due to
inactivity
- To tell the router what PVCs are
available
Three LMI types can be invoked by
the router: ansi, cisco, and q933a.
LMI extensions
In addition to the basic Frame Relay
protocol functions for transferring data, the Frame Relay
specification includes LMI extensions that make supporting large,
complex internetworks easier. Some LMI extensions are referred to as
common and are expected to be implemented by everyone who adopts the
specification. Other LMI functions are referred to as optional. A
summary of the LMI extensions follows:
- Virtual circuit status messages
(common) -- Provide communication and synchronization between the
network and the user device, periodically reporting the existence
of new PVCs and the deletion of already existing PVCs, and providing
general information about PVC integrity. Virtual
circuit status messages prevent the sending of data over PVCs that
no longer exist.
- Multicasting (optional) -- Allows a
sender to transmit a single frame but have it delivered by the
network to multiple recipients. Thus, multicasting supports the
efficient conveyance of routing protocol messages and address
resolution protocols that typically must be sent to many
destinations simultaneously.
- Global addressing (optional) -- Gives
connection identifiers global rather than local significance,
allowing them to be used to identify a specific interface to the
Frame Relay network. Global addressing makes the Frame Relay
network resemble a local-area network (LAN) in terms of
addressing; address resolution protocols therefore perform over
Frame Relay exactly as they do over a LAN.
- Simple flow control
(optional) -- Provides for an XON/XOFF flow control mechanism that
applies to the entire Frame Relay interface. It is intended for
devices whose higher layers cannot use the congestion notification
bits and that need some level of flow control.
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