Information Technology Reference
In-Depth Information
In a dual leaky bucket model, if the cell is compliant in the second bucket, the cell is sent to the
network. If the cell is noncompliant in the second bucket, it can be discarded, or it can have its
CLP bit set to 1 depending on the configuration of the policing.
As a cell passes through the core of the network, it is held in various buffers. Time sensitive
traffic, such as AAL1 traffic, is guaranteed a maximum latency end-to-end through the network.
CBR and RT-VBR pass through buffers that are serviced at a guaranteed rate. ABR and UBR
traffic pass through different buffers.
When a cell is discarded because of policing or congestion in the network, the traffic type is
considered. Video and voice traffic is more tolerant of the loss of a single cell. However, if a
discarded AAL5 cell is part of a larger frame of data, the remainder of the frame is useless. For
this reason, ATM implements the Frame Based Generic Cell Rate Algorithm (FGCRA).
With FGCRA, the switch can implement partial packet discard (PPD). If a cell is discarded,
PPD ensures that all subsequent cells in the frame are discarded, except the last cell. The last
cell is required for the ES to recognize that the frame has been sent. The third bit in the PTI field
of the cell header indicates if the cell is the last cell in a frame.
With FGCRA, the switch can implement early packet discard (EPD). If the first cell of a frame must
be discarded, the entire frame, including the last cell, is discarded when EPD is implemented.
PNNI
When a SVC, and in some cases a SPVC, is established across an ATM network, the PNNI
protocol identifies the route for the connection. PNNI is a link-state routing protocol similar to
the Open Shortest Path First (OSPF) protocol. PNNI allows the ATM switches within a private
network to exchange routing information and defines the network-to-network signaling
between switches to support QoS requirements.
The PNNI routing protocol provides the following functions:
•
Distributes topology information between switches
•
Exchanges QoS metrics between switches
•
Provides a hierarchy of peer switch groups for large scale networks
PNNI nodes broadcast their ATM addresses to all other nodes in their peer group. PNNI
distributes routing information across a well-known PVC that uses the VCI of 18.
The PNNI signaling protocol provides the following functions:
•
Messages to establish point-to-point and point-to-multipoint connections
•
Source routing, crank back (connection request not accepted), and alternate routing for
connection setup
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