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GT-96100A Advanced Communication Controller
Revision 1.0
253
12. 10/100MB ETHERNET UNIT
12.1
Functional Overview
The 10/100Mb Ethernet unit handles all functionality associated with moving packet data between local memory
or PCI and the Ethernet ports. The unit in the GT-96100A is designed to support two independent 10/100Mb
Ethernet ports.
Each 10/100 Mbit port is fully compliant with the IEEE 802.3 and 802.3u standards and integrates the MAC
function and a dual speed MII interface. The port’s speed (10 or 100Mb/s) as well as the duplex mode (half or
full duplex) is auto-negotiated through the PHY and does not require user intervention. The port also features
802.3x flow-control mode for full-duplex and backpressure mode for half duplex.
Integrated address filtering logic provides support for up to 8K MAC addresses. The address table resides in
memory and a proprietary hash function is used for address table management. The address table functionality
supports Multicast as well as Unicast address entries.
An important feature related to the address recognition is IGMP packet trapping mode. In this mode layer 3 hard-
ware analysis is performed in order to check if a packet being received is an IGMP packet. Each packet identified
as IGMP is queued in the high priority queue of the port from which it was received. The IGMP analysis is per-
formed on the fly, so it does not impact bandwidth capability.
Another important feature related to priority queues is the Type of Service queuing algorithm. This algorithm is
based on the decoding in layer 3 of the DSCP field from the IP header. If enabled, the decoded field indexes a 64
entry IPT Table in the Ethernet register space. The 2-bit output of this table sets the priority queue of the received
packet.
The Ethernet unit integrates powerful DMA engines, which automatically manage data movement between
buffer memory and the ports, and guarantee wire-speed operation on all ports (even when all ports are in 100Mb
full-duplex mode). There are two DMA engines per port - one dedicated for receive and the other for transmit.
The DMA logic handles multiple priority queues per port, providing support for priority sensitive data in both
directions. There are four receive priority queues and two transmit priority queues per port. Priority information
for received packets is either extracted from the packet tag (if the packet is VLAN tagged) or from the destina-
tion address entry in the address table (if the packet is not tagged) of the IP parameters as described above. The
priority function is MAX (ip_parameters, vlan_tag, address_entry).