MDS212 Datasheet, PDF (24/111 Pages) Zarlink Semiconductor Inc – 12-Port 10/100Mbps Ethernet Switch

MDS212
Data Sheet
of payload data has been received, to generate a collision. The jam sequence is a predefined serial data stream
sent to all ports to indicate that there has been a collision on the network. These ports will delay (defer) the
transmission of data onto the network until the sequence has been completed.
3.9.1 IEEE 802.3x Flow Control
IEEE 802.3x Flow Control reduces network congestion on ports that are operating in full duplex mode using MAC
Control PAUSE frames and is managed by the Flow Control Management Registers. The full-duplex PAUSE
operation instructs the MAC to enable the reception of frames with a destination address equal to a globally
assigned 48-bit reserved multicast address of 01-80-C2-00-00-01. These PAUSE frames are subsets of MAC
Control frames with an opcode field of 0x0001 and are used by the MAC Control to request that the recipient stops
transmitting non-control frames for a specific period. The PAUSE Timer is loaded from the PAUSE frame and is
started upon the reception of a PAUSE frame. It will request a length of time for which it wishes to inhibit data frame
transmission.
In general, the IEEE standard allows pause frames longer than 64 bytes to be discarded or interpreted as valid. The
MDS212 recognizes all MAC Control frames (PAUSE frames) between 64 and 1518 bytes long. Any PAUSE
frames presented to the MAC outside of these parameters are discarded.
4.0 Frame Engine Description
The Frame Engine is the heart of the MDS212. It coordinates all data movements, ensuring fair allocation of the
memory bandwidth and the XPipe bandwidth.
When frame data is received from a MAC port, it is temporarily stored in the MAC RxFIFO until the Frame Engine
moves it to the chip’s external memory one granule (128-byte-or-less fragment of frame data) at a time. The Frame
Engine then issues the Search Engine a switching request that includes the source MAC address, the destination
MAC address, and the VLAN tag. After the Search Engine has resolved the address, it transfers the information
back to the Frame Engine via a switching response that includes the destination port and frame type (e.g. unicast or
multicast).
When the destination port is idle, the frame data is fetched from the memory and is written to the destination port’s
MAC TxFIFO. However, when the destination port is busy transmitting another frame, the Frame Engine writes a
transmission job that includes a frame handle for future identification. These transmission jobs are stored in the
destination port’s transmission scheduling queues (TxQ). There are four TxQs per port, one for each priority class.
When the destination port is ready, the Frame Engine selects the head-of-line job from a TxQ. The frame, specified
by the job, will be fetched from the memory and will be written to the MAC TxFIFO.
For unicast frames, if the destination device is local (i.e., the destination port is located in the same device), the
Frame Engine writes a job into the destination port’s transmission scheduling queue (TxQ). The Transmit DMA
(TxDMA) moves the frame data to the MAC TxFIFO once the frame’s transmission job is selected for transmission.
If the destination device is remote (i.e., the destination port is located on another device, and can only be reached
through the XPipe), all signalling between the two devices are sent as XPipe messages. The Frame Engine sends
a scheduling request message via the XPipe to the destination port. This message asks the remote Frame Engine
to write a job into the destination port’s TxQ. When that job is selected, the remote Frame Engine sends a data
request message via the XPipe to the local Frame Engine. Reception of a data request message triggers the
forwarding engine module to forward the frame data to the destination port, one granule at a time through the XPipe
until the end of file (EOF) safely arrives at the remote port’s MAC TxFIFO.
For multicast frames, the process is slightly different. The Frame Engine uses the VLAN index, which is part of the
search result, to identify the destination ports. For local destination ports, the Frame Engine writes a job to each
port’s TxQ. When a transmission job is selected, the TxDMA moves data from the memory to the MAC TxFIFO.
Multicast frame data is sent multiple times, until all local destination ports’ requests are satisfied.
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