BusWorks
®
900EN-S005 Ethernet Switch User’s Manual Ethernet I/O
__________________________________________________________________
_______________________________________________________________________________________
Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:
http://www.acromag.com
52
Rate Limiting Controls…continued
CONTROL REGISTER
FUNCTION
Port Control 11 bit 6
Low Priority Receive Rate Control Enable: Set
to enable the port low priority receive rate
control. Clear to disable port low priority receive
rate control.
Port Control 11 bit 7
Receive Differential Priority Rate Control: Set to
enable receive rate control on port for low priority
packets at low priority rate (if bit 6 is also set), or
to enable receive rate control on high priority
packets at high priority rate (if bit 5 is also set).
Clear to perform receive rate control using the
low priority rate for all packets on this port.
Port Control 5 bits 7-0
and Port Control 7
bits 3-0.
Transmit HIGH Priority Rate Control: A 12-bit
number that sets how many “32Kbps” high
priority blocks can be transmitted (in a unit of
32K bits or 4K bytes) in a one second period.
Port Control 6 bits 7-0
and Port Control 7
bits 7-4.
Transmit LOW Priority Rate Control: A 12-bit
number that sets how many “32Kbps” low priority
blocks can be transmitted (in a unit of 32K bits or
4K bytes) in a one second period.
Port Control 8 bits 7-0
and Port Control 10
bits 3-0.
Receive HIGH Priority Rate Control: A 12-bit
number that sets how many “32Kbps” high
priority blocks can be received (in a unit of 32K
bits or 4K bytes) in a one second period.
Port Control 9 bits 7-0
and Port Control 10
bits 7-4.
Receive LOW Priority Rate Control: A 12-bit
number that sets how many “32Kbps” low priority
blocks can be received (in a unit of 32K bits or
4K bytes) in a one second period.
The Acromag 900EN-S005 can discern priority via three different means at
its input port: on a per-port basis, via the 802.1p frame tag, and by viewing
the DSCP (TOS) field in the IPv4 header. Note however, in order for any
priority control to be effective, the high and low priority queues must be
separately enabled at the input point or destination port.
Priority Using The Per-Port Method
Each port has a general Enable Port Priority bit that when set, specifies that
the port has high-priority traffic and all traffic from this switch port is
considered high-priority in the destination queue. This is most useful for IP
phone applications mixed with other types of traffic data where the IP phone
is connected to a specific port. In this case, the IP phone traffic would be
high priority (outgoing) to the WAN (Wide Area Network). The incoming
phone traffic is all the same priority to the IP phone.
Priority Using The 802.1p Method
Each port has an “Enable Port 802.1p Priority” bit that when set, directs that
the 802.1p priority tag (3 bits) be used to determine a received frame’s
priority. A 3-bit value is defined in bits 6..4 of Global Control 0 register which
is used to classify the priority of incoming packets. In this method, the
inbound traffic port examines the priority field in the tag, compares it to the 3-
bit value, and determines if it is of high or low priority.
Rate Limiting
Priority Controls
