KS8995E Micrel Inc, KS8995E Datasheet - Page 22

• Manufacturer Part Number: KS8995E
• Manufacturer: Micrel Inc

Specifications of KS8995E

• Number Of Primary Switch Ports: 5
• Fiber Support: Yes
• Integrated Led Drivers: Yes
• Phy/transceiver Interface: MII/SNI
• Power Supply Type: Analog/Digital
• Package Type: PQFP
• Data Rate (typ): 10/100Mbps
• Vlan Support: Yes
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Pin Count: 128
• Mounting: Surface Mount
• Jtag Support: No
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Not Compliant

KS8995E
SNI Interface (7-wire) Operation
The SNI (Serial Network Interface) is compatible with some controllers used for network layer protocol processing. KS8995E
acts like a PHY device to external controllers. This interface can be directly connected to these types of devices. The signals
are divided into two groups, one being for transmission and the other being the receive side. The signals involved are described
in the table below.
This interface is a bit wide data interface and therefore runs at the network bit rate (not encoded). An additional signal on the
transmit side indicates when data is valid. Likewise, the receive side has an indicator that conveys when the data is valid.
For half-duplex operation there is a signal that indicate a collision has occurred during transmission.
8995E Improvements
Priority Schemes
The KS8995E can determine priority through three different means. The first method is a simple per port method, the second
is via the 802.1p frame tag and the third is by viewing the DSCP (TOS) field in the IPv4 header. Of course for the priority to
be effective, the high and low priority queues must be enabled on the destination port or egress point.
Per Port Method
General priority can be specified on a per port basis. In this type of priority all traffic from the specified input port is considered
high priority in the destination queue. This can be useful in IP phone applications mixed with other data types of traffic where
the IP phone connects to a specific port. The IP phone traffic would be high priority (outbound) to the wide area network. The
inbound traffic to the IP phone is all of the same priority to the IP phone.
802.1p Method
This method works well when used with ports that have mixed data and media flows. The inbound port examines the priority
field in the tag and determines the high or low priority. Priority profiles are setup in the Priority Classification Control in the
EEPROM.
IPv4 DSCP Method
This is another per frame way of determining outbound priority. The DSCP (Differentiated Services Code Point–RFC#2474)
method uses the TOS field in the IP header to determine high and low priority on a per code point basis. Each fully decoded
code point can have either a high or low priority. A larger spectrum of priority flows can be defined with this larger code space.
More specific to implementation, the most significant 6 bits of the TOS field are fully decoded into 64 possibilities, and the
singular code that results is compared against the corresponding bit in the DSCP register. If the register bit is a 1, the priority
is high and if 0, the priority is low.
Other Priority Considerations
When setting up the priority scheme, one should consider other available controls to regulate the traffic. One of these is Priority
Control Scheme (register 3 bits 7-6) which controls the interleaving of high and low priority frames. Options allow from a 2:1
ratio up to a setting that sends all the high priority first. This setting controls all ports globally. Another global feature is Priority
Buffer Reserve (register 5 bit 7). If this is set, there is a 12KB (10%) buffer dedicated to high priority traffic, otherwise if cleared
the buffer is shared between all traffic.
On an individual port basis there are controls that enable DSCP, 802.1p, port based and high/low priority queues. These are
contained in registers 3-7 bits 5-3 and 0.
KS8995E
SNI Signal
TXEN
TXD
TXC
COL
CRS
RXD
RXC
Description
Transmit enable
Serial transmit data
Transmit clock
Collision detection
Carrier sense
Serial receive data
Receive clock
Table 2. SNI Signal
KS8995E SNI Signal
MTXEN
MTXD[0]
MII_CLK
MCOL
MRXDV
MRXD[0]
MII_CLK
22
KS8995E Input/Output
Input
Input
Output
Output
Output
Output
Output
August 2003
Micrel