User`s guide

5 – Managing Ports
Configuring Ports
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5.2.7
Extending Port Credits
Each port is supported by a data buffer with a 12 credit capacity; that is, 12
maximum sized frames. For fibre optic cables, this enables full bandwidth service
over a distance of 20 kilometers at 1 Gbps (0.6 credits/Km), or 10 kilometers at 2
Gbps (1.2 credits/Km). Longer distances can be spanned at full bandwidth by
borrowing credits from designated donor ports thus pooling the buffer capacities.
This is called credit extension. Each donor port contributes 11 credits to the pool
from which the recipient ports can draw. For example, one donor port contributes
11 credits to the pool from which a recipient draws for a total of 23 credits (11+12).
This provides approximately 38 Km at 1 Gbps (23÷0.6) or 19 Km at 2 Gbps
(23÷1.2).
To extend port buffer credits, do the following:
1. In the faceplate display, select the ports that are to serve as donor ports.
Open the Port menu and select Port Properties. In the Port Properties
window, choose the Donor radio button and choose OK.
2. In the faceplate display, select the recipient ports. Recipient ports must be
configured as G_Ports or F_Ports. Open the Port menu and select
Extended Credits. This opens the Extended Credits window shown in
Figure 5-3.
3. Distribute the borrowed credits by incrementing the Ext Credits Requested
field for the recipient ports up to the total credits in the pool. Choose OK. If
you exceed this total, a message will be entered in the alarm log indicating
that some recipient ports did not receive the requested credits.
Note: Donor ports cannot transmit or receive data.