MU9C8148
FUNCTIONAL DESCRIPTION
Referring to the Block diagram shown on Page 1, the
MU9C8148 consists of four functional blocks: the Transparent
Bridging (TB) block, the Source Routing Block (SRB), the
Instruction Buffer (IB), and the Arbiter. Four interfaces connect
the MU9C8148 to the Transceiver, the MAC controller, the Host
processor, and the LANCAM. For a detailed description of
Token Ring frames, refer to IEEE Standard 802.5, or the IBM
Token-Ring Network Architecture Reference.
be enabled on the four different frame types (MAC, LLC and
two types of reserved frames) individually. For each frame type,
a choice between two different learning routines can be made
in the Transparent Bridging register. Note that learning can only
take place when the RII bit of the frame is ZERO, or for every
frame when the TBO bit is HIGH. Thus, Routine 0 or Routine 2
is started after the RII bit is received and evaluated. Routine 1
is started by the TB block after the Frame Status field has been
received and the frame found to be error-free.
Transparent Bridging Block
Source Routing Block
If a frame does not contain an RIF, the TB block makes
decisions whether to copy or discard a frame based on the
Destination address (DA). If the bridge is set for Transparent
bridging only (the TBO bit in the Control register is HIGH,
disabling the SRB), the TB block also makes copy or discard
decisions for frames which do contain an RIF. The TB block
parses the data as received from the Transceiver off the Token
Ring network, and indicates to the MAC whether to assert the
XMATCH and/or XFAIL (/FLUSH) signals. For each frame, the
TB block examines the Token bit, the Frame Type bits, the
Destination address (DA), and the Source address (SA), which
contains the Routing Information indicator (RII).
The Token bit signals whether the current frame is a Token or a
regular frame. If the Token bit is LOW, a Token frame is being
received, and the TB block discards the frame. For a regular
frame, the Frame Type bits signify the type of frame (LLC,
MAC, or Reserved) being received, and the TB block decides
to copy or discard the frame, based on the settings in the
Frame Type Selection register and the results of filtering on the
Destination address (DA).
Positive or negative filtering on the DA can be done for frames
without an RIF, or for all frames when the bridge is set for
Transparent bridging only, based on the setting of the PONNE
bit in the Transparent Bridging register. Positive filtering implies
that a frame should be forwarded if its DA is found in the
LANCAM address database. Routine 0 in the instruction buffer
examines the DA field to determine whether a frame should be
copied or not. The results of this comparison are used to notify
the LAN controller to copy or discard the frame. Negative
filtering implies that a frame should be forwarded if its DA is not
found in the address database. In this case, the MU9C8148
checks the DA before routine 0 is started to differentiate
between MAC, Broadcast, Functional and Group addresses.
Based upon the settings of the Transparent Bridging register,
the TB block discards a frame whose DA is a Broadcast,
Functional and/or Group address before Routine 0 is started.
If the MU9C8148 is not used in a Transparent Bridging Only
mode and a frame containing an RIF is received, Routine 2 in
the Instruction buffer can be started. The results of this
comparison are combined with the output of the SRB to instruct
the LAN controller to copy or discard the frame. Routine 2 can
only perform positive filtering.
The Source address (SA) of a frame can be used to update the
database of addresses stored in the LANCAM. Routine 1 in the
Instruction buffer checks the SA of a frame against all the
entries in the database, and if the SA is not found, the address
is new, and if the frame received is error free, the address can
be learned by adding it to the LANCAM database. Learning can
The Source Routing Block (SRB) only decides to copy or
discard a frame if it contains an RIF, and the MU9C8148 is not
in TBO mode. The SRB (in combination with the results of
Routine 2 of the TB) notifies the MAC interface how to assert
its output flags .
When a frame is received, the SRB checks whether the Token
bit in the AC field is LOW, in which case a Token frame is being
received and no further processing is necessary. If the Token
bit is HIGH, a regular frame is being received and the SRB
examines the Frame Type bits in the FC field, which give the
type of frame being received. The frame types to be processed
by the SRB (those with an RIF) are selectable in the Frame
Type Selection register. If the RII bit is HIGH, the SRB signals
the MAC interface, based on the frame type and the settings in
the Frame Type Selection register, either to discard the frame
and flush the buffer, or to continue to check the RIF of the
frame, or to accept the frame. If the RII is LOW, the SRB is not
allowed to process the frame any further and waits for the next
frame to arrive.
If a copy/discard decision is to be made based on the RIF, the
SRB examines the information contained in the RCF. If the
length (LTH) bits of the RCF indicate a length equal to zero, or
contain an odd length, or if the length of the RIF is longer than
the allowed length stored in the RIF Length register, reception
of the frame is stopped, and the SRB indicates that the frame is
to be discarded. The D bit of the Routing Control Field (RCF)
indicates which direction the Routing Descriptors (RDs) of the
RIF should be interpreted.
The SRB provides for seven Ring(in)–Bridge–Ring(out)
combinations (LIN-BN-LOUT) stored in the Source Ring
Number register and Bridge/Destination Ring Number registers.
LIN is the LAN ID of the ring connected to that specific port,
while the BN(s) and LOUT(s) depend on the topology of the
network and the bridge design. The SRB provides for checks
between the LAN ring numbers and bridge numbers contained
in every RD with every LIN-BN-LOUT stored, allowing the user
to develop an SR(T) bridge with an internal virtual ring, or a
bridge with a Full Mesh design (IEEE P802.5M).
If the Routing Type (RT) bits are equal to 0XXB, a
Specifically-Routed Frame (SRF) is being received, and should
be forwarded on the conditions shown in Table 1. If the RT bits
are equal to 10XB, the frame is an All Routes Explorer (ARE)
frame, and should be handled as shown in Table 1. If the RT
bits are equal to 11XB, the frame is a Spanning Tree Explorer
(STE) frame, and should be dealt with as shown in Table 1
Also described in Table 1 are the conditions on which the Error
counters (Register 17H) are incremented.
Rev. 5.5 Draft
web
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