NOTE
Load Switching
3.Determine if a load switch is needed.
Table 2.1STDMA ACK Message
Table
Data | Size in | Description | Unit of | Notes: |
Type | Bytes | Measure | ||
|
|
|
|
|
IP | 4 | IP address of Remote | N/A | Used by remote to identify |
|
|
|
| itself |
|
|
|
|
|
Unsigned | 4 | Queued Bytes | Bytes | Total number of bytes queued |
|
|
|
| since last cycle (includes |
|
|
|
| possible buffer overflow) |
|
|
|
|
|
Unsigned | 4 | Bytes in Queue | Bytes | Number of bytes currently |
|
|
|
| queued |
|
|
|
|
|
Unsigned | 1 | Group Number | N/A |
|
|
|
|
|
|
Unsigned | 1 | Dropped Buffers | Packets | Number of packets dropped |
|
|
|
| (due to limited bandwidth) |
|
|
|
|
|
If there is adequate upstream bandwidth available, the values of these two metrics will be the same. However, if there is not enough bandwidth to satisfy the traffic requirements of the remote, or if the remote has exceeded the maxi- mum allocation, some data will be held for the next cycle. In this case, the number of Bytes in Queue will start to grow and will exceed the Queued Bytes. (In other words, the Bytes in Queue is the sum of the data not yet transmitted plus the new data received.)
If the condition is due to a short burst of data, the backlogged data will eventu- ally be transmitted and the system will return to a sustainable rate. However, if the overload condition is due to long term increased activity, then the backlog condition will continue to grow and eventually trigger an SCPC switch. If the overload condition lasts long enough, buffer capacity will eventually be exceeded and some data may have to be discarded.
Note: This is not necessarily bad, as it is often more effective to discard old data than transmit it after it has become ‘stale’.
The “Bytes in Queue” metric is used to determine the STDMA bandwidth allo- cated (slot size) for the next cycle; the goal being to keep the data backlog to zero. The hub uses this metric to compute the slot size for each remote in the next cycle as follows:
•Fixed Mode - All remotes get the same slot regardless of need; i.e. the metric is not used.
•Dynamic Cycle Mode - Available bandwidth is allocated to remotes proportionally based on current need. (The Bytes in Queue for each
Appendix B - Automatic Switching |
