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16 Introduction to LonWorks Routers
message. See Configuring a Router 5000 Half-Router on page 53 and the
example Neuron C code in Example Neuron C Source on page 56, for a
description of how to change the size and count of buffers. You can also use the
NodeUtil Node Utility, which you can download from the Echelon Web site.
However you allocate the buffer sizes and counts, the total memory required by
the three buffer queues must not exceed 25 K bytes.
The general buffer configuration shown in
Table 3 balances the buffers between
the input and output queues of the router. For systems with large bursts of
traffic, you could specify additional non-priority output buffers. Priority packets
are sensed and forwarded through the router’s priority output buffers, so that
priority packets are processed as quickly as possible, rather than allowing them
to be delayed behind non-priority packages in a large input queue.
Router Performance
A major criterion of router performance is network throughput. An optimal
router would be able to forward traffic at the wire-rate, with zero packet loss and
minimal delay. Thus, an optimal router would forward traffic from 9.8 kbps to
1.25 Mbps, depending on the router’s transceiver type.
A real router typically does not perform at the wire-rate because of latency within
the router, including the time to receive and buffer the incoming packet at the
near side, the time to forward the packet between the halves, and the time to
buffer and transmit the packet at the far side. You should measure your router
device’s latency to determine if its design meets your system’s needs.
Example : For a 20 MHz Router 5000 device (where both halves use the Router
5000 chip), a measured data transfer rate for sending a service-pin message
between the router halves was approximately 1.2 μs per byte ( or 830 kbytes/sec).
Some additional latency was also seen for the time between the beginning of the
original packet transmission and the beginning of the forwarded packet
transmission.
For slower channel types, this router latency is not significant, but could become
significant for faster channel types.
The latency between router halves is relatively invariant, with respect to router
configuration, whereas overall router latency depends on the router type and
configuration. For an RTR-10 device, the maximum data transfer rate between
router halves is approximately 2.4 μs per byte (or 416 kbytes/sec). For a Router
5000 device (where both halves use the Router 5000 chip), the maximum data
transfer rate between router halves is approximately 300 ns per byte (or 3.3
Mbytes/sec, assuming an 80 MHz system clock for both halves; this rate scales
with the system clock setting).