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