Q

Switch transmitters, what are they?

What is the transmitter in a switch? I am reading a reference to switch transmitters in an IBM Redbook. The McData switch has 4 ports per ASIC and one ASIC per card. Validated Fibre frames are put into the receive buffers of each ASIC. Ports on the same card can share the buffers but not those of other cards. However, all transmitters in a switch can read from any buffer. Is this referencing the GBIC transceivers?

Editor's note: Click here to view updated answer.

In a McData ED6064 Director, each interface card (I call them port cards) is configured with 4 Fibre ports. You can have up to 16 port cards for a total of 64 ports. Each port contains a GBIC (Gigabit interface converter) which houses most of the logic that takes the incoming and outgoing optical pulses, and converts it to a digital format. The GBIC contains the actual lasers that "transmit" data through the optical SAN network as pulses of light.

The director uses a "central memory" architecture so data coming into any port (receiver) can go out over any other port (transmitter). So, any transmitter in the switch can read from any receiver through shared memory.

As an ASIC port receives and validates a frame, it stores the frame in one of its receive buffers in the central memory and forwards a routing request to the appropriate destination ports. When the destination port is capable of transmitting the frame, it reads the frame contents from central memory and forwards the frame to its transmit interface.

Each port has 16 buffers assigned by default. Those ports configured for extended distances (10 - 100Km) are assigned 60 buffers. The shared memory resides in the CMM (controller cards). The receiving port stores the received data in a buffer in the shared memory and signals the connected port to retrieve that data by a message handled by the MPC Card.

The Director supports all ports connected at the same time at full speed, 1.0625 Gb/s bi-directional.

So, the short answer is the "transmitter" is the laser located inside the GBIC of the port cards in the director.

Chris

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This was first published in June 2002

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