Specifications

9-3
Cisco ONS 15454 Installation and Operations Guide, R3.2
March 2002
Chapter 9 Ethernet Operation
G1000-4 Card
The G1000-4 card can carry over a SONET network any layer three protocol that can be encapsulated
and transported over Gigabit Ethernet, such as IP or IPX. The data is transmitted on the Gigabit Ethernet
fiber into the standard Cisco Gigabit Interface Converter (GBIC) on a G1000-4 card. The G1000-4 card
transparently maps Ethernet frames into the SONET payload by multiplexing the payload onto a SONET
OC-N card. When the SONET payload reaches the destination node, the process is reversed and the data
is transmitted from the standard Cisco GBIC in the destination G1000-4 card onto the Gigabit Ethernet
fiber.
The G1000-4 card discards certain types of erroneous Ethernet frames rather than transport them over
SONET. Erroneous Ethernet frames include corrupted frames with CRC errors and under-sized frames
that do not conform to the minimum 60-byte length Ethernet standard. The G1000-4 card forwards valid
frames unmodified over the SONET network. Information in the headers is not affected by the
encapsulation and transport. For example, packets with formats that include IEEE 802.1Q information
will travel through the process unaffected.
9.1.2 802.3x Flow Control and Frame Buffering
The G1000-4 supports 802.3x flow control and frame buffering to reduce data traffic congestion. To
buffer over-subscription, 512 kb of buffer memory is available for the receive and transmit channels on
each port. When the buffer memory on the Ethernet port nears capacity, the ONS 15454 uses 802.3x flow
control to send back a pause frame to the source at the opposite end of the Gigabit Ethernet connection.
The pause frame instructs that source to stop sending packets for a specific period of time. The sending
station waits the requested time before sending more data. Figure 9-1 illustrates pause frames being sent
from the ONS 15454s to the sources of the data. The G1000-4 card does not respond to pause frames
received from client devices.
This flow-control mechanism matches the sending and receiving device throughput to that of the
bandwidth of the STS circuit. For example, a router may transmit to the Gigabit Ethernet port on the
G1000-4. This particular data rate may occasionally exceed 622 Mbps, but the ONS 15454 circuit
assigned to the G1000-4 port may be only STS-12c (622.08 Mbps). In this example, the ONS 15454
sends out a pause frame and requests that the router delay its transmission for a certain period of time.
With a flow control capability combined with the substantial per-port buffering capability, a private line
service provisioned at less than full line rate capacity (STS-24c) is nevertheless very efficient because
frame loss can be controlled to a large extent.
Some important characteristics of the flow control feature on the G1000-4 include:
• The G1000-4 card only supports asymmetric flow control. Flow control frames are sent to the
external equipment but no response from the external equipment is necessary or acted upon.
• Received flow control frames are quietly discarded. They are not forwarded onto the SONET path,
and the G1000-4 card does not respond to the flow control frames.
• On the G1000-4 card, you can only enable flow control on a port when auto-negotiation is enabled
on the device attached to that port. For more information, see the “G1000-4 Port Provisioning”
section on page 9-7.
Because of the above characteristics the link auto-negotiation and flow control capability on the attached
Ethernet device must be correctly provisioned for successful link auto-negotiation and flow control on
the G1000-4. If link auto-negotiation fails, the G1000-4 does not use flow control (default). Without flow
control, traffic loss can occur if the input traffic rate is higher than the bandwidth of the circuit for an
extended period of time.