MDS212 Datasheet, PDF (23/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
(that is, stops transmitting) for a specific period (defined by the IEEE 802.3 Binary Exponential Backoff Algorithm)
before sending the jam sequence and rescheduling transmission. A frame with a size of no less than 96 bits (64 bits
of preamble and 32 bits of jam pattern), is sent to guarantee that the duration of the collision is long enough to be
detected by the transmitting ports involved.
3.5 Auto-negotiation
The default value of the MDS212 MAC enables Auto-negotiation. The default value is overwritten if the PHY lacks
the ability to support Auto-negotiation, which is ascertained through its respective management interface, RMII. The
Auto-negotiation process detects the different modes of operation (i.e. speed selection, duplex mode) supported by
the system at the other end of the link segment. Upon power on/reset, the PHY generates a special sequence of
fast link pulses (FLPs) to begin Auto-negotiation. The MDS212 MAC, supporting Auto-negotiation, reads the results
of the operation from the MAC Configuration Registers.
3.6 VLAN Support
Virtual Local Area Networks (VLANs) assemble a group of independent ports (and/or MAC addresses) to
communicate as if they were on the same physical LAN segment, without being restricted by the physically
connected hardware. The ports are logically grouped together by VLAN Identifiers (VLAN IDs). The MDS212
implements a MAC Address-based classification that associates each VLAN ID with its MAC address in the Switch
Database Memory (SDM) for purposes of aging out, or replacing, old VLANs.
The MDS212 MAC recognizes VLAN-Tagged frame formats. During transmission the MAC inserts (or extracts) the
4-byte VLAN Tag and regenerates the Frame Check Sequence for the transmitted frame. VLAN support requires an
increase in the maximum legal frame size, which is set in the Device Configuration Register (DCR2), from 1518 to
1522 bytes. During transmission, if the MAC is required to remove the VLAN Tag from a 64-67 byte Rx frame, the
MAC will append extra bytes (pad) to form a 64 byte frame.
3.7 MAC Control Frames
MAC Control Frames, as defined by the IEEE, are used for specific control functions within the MAC Control
sublayer “MAC Control.” Similar to data frames, control frames are also encapsulated by the CSMA/CD MAC,
meaning that they are prepended by a Preamble and Start of Frame delimiter and appended by a Frame Check
Sequence. These frames may be distinguished from other MAC frames by their length/type field identifier (88.08h).
The control functions are distinguished by an opcode contained in the first two bytes of the frame. Upon receipt,
MAC control parses the incoming frame and determines, by looking at the opcode and the MAC address, whether it
is destined for the MAC (a data frame) or for a specific function within MAC Control. After performing the specified
functions, the MDS212 discards all MAC control frames it receives, regardless of the port configuration. These
control frames are not forwarded to any other port and are not used to learn source addresses.
3.8 Flow Control
Flow control reduces the risk of data loss during long bursts of activity, by saturating the buffer memory with
backlogged frames. The MDS212 supports two types of Flow Control: Collision-based for half-duplex mode and
IEEE 802.3x Flow Control for full duplex mode. In both cases, the MDS212 recognizes congestion by constantly
monitoring available frame buffer memory. When the amount of free buffer space has been depleted, the MDS212
initiates the flow control mechanism appropriate to the current mode of operation. Setting the Flow Control
(FC_Enable) bit in the MAC Port Configuration Register (ERC1) turns this operation on, thereby initiating PAUSE
frames or applying back pressure flow control when necessary.
3.9 Collision-based Flow Control
Collision-based Flow Control, also referred to as Backpressure Flow Control, inhibits frame reception for ports
operating in half-duplex mode by “jamming” the link. When the free buffer space drops below a user-defined buffer
memory threshold, the MDS212 sends a jam sequence to all non transmitting ports, after approximately eight bytes
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