QoS and QoS Policies
34
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
3.1.6.3 Buffer Unit Allocation and Buffer Chaining
Packetization buffers and queues are supported in the packet memory of each
adapter card or platform. All adapter cards and platforms allocate a fixed space for
each buffer. The 7705 SAR supports two buffer sizes: 512 bytes or 2304 bytes,
depending on the type of adapter card or platform.
The adapter cards and platforms that support a buffer size of 2304 bytes do not
support buffer chaining (see the description below) and only allow a 1-to-1
correspondence of packets to buffers.
The adapter cards and platforms that support a buffer of size of 512 bytes use a
method called buffer chaining to process packets that are larger than 512 bytes. To
accommodate packets that are larger than 512 bytes, these adapter cards or
platforms divide the packet dynamically into a series of concatenated 512-byte
buffers. An internal 64-byte header is prepended to the packet, so only 448 bytes of
buffer space is available for customer traffic in the first buffer. The remaining
customer traffic is split among concatenated 512-byte buffers.
Table 4 shows the supported buffer sizes on the 7705 SAR adapter cards and
platforms. If a version number or variant is not specified, this implies all versions of
the adapter card or variants of the platform. Adapter cards and platforms that support
buffer chaining have 512 byte buffer size (“Yes”); those that do not support buffer
chaining have 2304 byte buffer size (“No”).
Note: The 8-port Ethernet Adapter card (version 2) uses an additional 64 bytes at the end
of the first buffer for packets larger than 452 bytes (448 bytes plus 4 bytes for FCS); for
example, when 449 bytes need to be buffered, the available space in the first buffer is 384
bytes, whereas all other adapter cards that support buffer chaining will have 448 bytes of
buffer space available in the first buffer.
Table 4 Buffer Support on Adapter Cards and Platforms
Adapter Card or Platform Buffer Space per
Card/Platform (Mbytes)
Buffer Chaining
Support
2-port 10GigE (Ethernet) Adapter card 268
201 (for L2 bridging domain)
Yes
Yes (each buffer unit is
768 bytes)
2-port 10GigE (Ethernet) module 201 (for L2 bridging domain) Yes (each buffer unit is
768 bytes)
2-port OC3/STM1 Channelized Adapter card 310 No
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