MB86967 Datasheet, PDF (31/129 Pages) Fujitsu Component Limited. – LAN Controller with PC Card, ISA Bus, and General-purpose Bus Interfaces

MB86967
and field synchronization for the receiver; the bit immediately following the end of preamble is the first bit of the
first byte of the packet’s destination address field.
When packet transmission is unflawed, carrier sense remains asserted for the duration of the packet, negating
just after the last bit of the CRC field is received, when the transceiver detects the end-of-packet symbol at the
end of the packet. Loss of carrier sense at any other time may also result from a collision or other network
problems.
9.2 Address Filtering
A receive packet can be filtered by applying selectable criteria to the contents of its Destination Address field,
which is the first data-bearing field following immediately after the Preamble. There are several control bits in
the MB86967’s registers which provide programmablility of the filter criteria. The contents of the Destination
Address field can be of three basic types. The first data type which can occur in this 48-bit field is a single node
address, the unique, single-node address, globally registered with the IEEE. This data type is indicated by a “0”
in the first bit position of the address. The second type is the multicast address, an address for a pre-defined
group of nodes. The multicast address is indicated by a “1” in the first bit position of the address. The MB86967
filters multicast addresses using a hash function and a 64-bit hash table. Thirdly, a broadcast address is defined
as a special case of the multicast address which addresses all nodes on the local network. This address value
consists of all “1”s. The MB86967 provides programmable address filtering logic for each of these address types.
Among the address filtering selections possible with the MB86967 are the following examples:
1. All Pass (no filtering)
2. Node address and broadcast packets only
3. Node address xxx.xxx, multicast addresses yyy.yyy and zzz.zzz and broadcast packets only
9.3 Hash Table
The Hash Table provides a way to filter incoming multicast packets so the host processor need not process
packets that are not of interest. The principle behind this filtering process is based on the arrangement of a large
number of elements of an array, or database, to facilitate searching for elements associated with a given key or
datum. The hash function is a mathematical or logical function that maps all elements in a domain onto a smaller
domain called the hash table.
Assume this hashing function as an example: treat the multicast address as a nonnegative 48-bit integer, divide
this number by 64 and take the remainder. This function maps all multicast addresses into a 64-element hash
table because the remainder must be an integer between 0 and 63. Applying this hashing function results in
taking the least-significant six bits of the multicast address as an integer. In the hash table, for each element, 0
through 63, a single bit is stored to indicate if the address is accepted (1) or rejected (0). If, for example, the
node belongs to three multicast groups, only three or fewer of the hash table elements store ones, and the rest
store zeroes. The scheme allows the acceptance of any number of addresses, including all of them. However,
while this filters out most nonspecific addresses, there may be addresses not of interest used on the network
that also fall into the accept elements, so filtering may be imperfect.
The actual hashing function used in the MB86967 is to calculate the CRC on the multicast address and to store
the most-significant six bits of this calculation in a register . The six bits are used to address the elements of the
hash table: the three MSBs are used as the Hash Table register address, and the three LSBs are used as the
bit address within a register byte. If the addressed Hash Table element yields a ‘1’ and the packet is a multicast
packet (first bit of destination address equals 1), and it passes the error filters, the packet will be accepted. The
hash filter criteria are only used on multicast addresses, which all start with a one. Node IDs that start with a
zero are not filtered by the hash filter. The broadcast address, a special case of the multicast set wherein all the
bits are ones, is accepted anyway unless the Reject All Packets mode is selected.
31