Programmer’s Guide BCM5722
10/15/07
Broadcom Corporation
Document 5722-PG101-R Transmit Data Path Page 18
ISOCHRONOUS SEND RING (BCM5906, BCM5906M ONLY)
In addition to the normal (or best-effort) data send ring (known as the Data SBD), a second send ring, the isochronous (ISO)
send ring (known as the ISO SBD), is available on the BCM5906/BCM5906M. The ISO SBD supports Residential Ethernet
applications such as Voice Over IP (VOIP) and video streaming, and offers the following features:
• Isochronous packets with a standard Ethernet packet size of 64 bytes to 1518 bytes
• Short voice packets with a minimum scheduling interval of 125 µs
• To minimize end-to-end delay and effects from lost packets, packetization intervals must be limited. In combination with
efficient media encodings, this leads to short packet sizes, typically on the order of 150 bytes, depending on the exact
coding and application. Video packets are typically much larger.
• Large MPEG video packets with scheduling interval of 2 ms
• 250 µs per hop latency
• 250 µs worst-case instantaneous delivery jitter. Delivery jitter is the time variance (early or late) around the optimal
delivery time of a packet.
• The Data SBD handles best-effort traffic and operates identically to other members of the NetXtreme/NetLink product
family.
• The ISO SBD provides traffic shaping support and is processed whenever the ISO SBD queue is non-empty and the
local “network time” clock exceeds the value of the “launch time” field of the head-of-queue ISO SBD packet.
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