REVIEW DRAFT—CISCO CONFIDENTIAL
14-3
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter14 Ethernet Operation
14.1.3 IEEE 802.3z Flow Control and Frame Buffering
The G-Series card discards certain types of erroneous Ethernet frames rather than transport them over
SDH. Erroneous Ethernet frames include corrupted frame s with cyclic redundancy checking (CRC)
errors and under-sized frames that do not conform to the minimum 64-byte length Ethernet standar d. The
G-Series card forwards valid frames unmodified over the SDH network. Information in th e 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.
14.1.3 IEEE 802.3z Flow Control and Frame Buffering
The G-Series supports IEEE 802.3z flow control and frame buffering to red uce data traffic congestion.
To prevent 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 SDH uses
IEEE 802.3z flow control to transmit a pause frame to the source at the opposite end of the Gigabit
Ethernet connection.
The pause frame instructs the source to stop sending packets for a specific perio d of time. The sending
station waits the requested time before sending more data. Figure14-1 on page 14-2 illustrates pause
frames being sent and received by ONS 15454 SDHs and attached switches.
The G-Series card proposes symmetric flow control when auto negotiating flow control with attached
Ethernet devices. Symmetric flow control allows the G-Series to respond to pause frames sent from
external devices and to send pause frames to external devices. Prior to Software R4.0, flow control on
the G-Series card was asymmetric, meaning the card sent pause frames and discarded received pause
frames.
This flow-control mechanism matches the sending and receiving device throughput to that of the
bandwidth of the STM circuit. For example, a router might transmit to the Gigabit Ethernet port on the
G-Series card. This particular data rate may occasionally exc eed 622 Mbps, but the ONS 15454 SDH
circuit assigned to the G-Series port might be only VC4-4c (622.08 Mbps). In this example, the
ONS 15454 SDH sends out a pause frame and requests that the router delay i ts transmission for a certain
period of time. With flow control and a substantial per-port buffering capability, a private line service
provisioned at less than full line rate capacity (VC4-8c) is efficient because frame loss can be controlled
to a large extent.
The G-Series has flow control threshold provisioning, which allows a user to select one of three
watermark (buffer size) settings: default, low latency or custom. Default is the best setting for general
use and was the only setting available prior to Software R4.1. Low latency is good for sub-rate
applications, such as VoIP. For attached devices with insufficient buffering, best effort traffic or long
access line lengths, set the G-Series card to a higher latency.
The custom setting allows you to specify an exact buffer size threshold for Flow Ctrl Lo and Flow Ctrl
Hi. The flow control high setting is the watermark for sending the “Pause On” frame to the attached
Ethernet device; this frame signals the device to temporarily stop transmitting. The flow control low
setting is the watermark for sending the “Pause Off” frame, which signals the device to resume
transmitting.
Note External Ethernet devices with auto-negotiation configured to interoperate with G-Series cards running
releases prior to R4.0 do not need to change auto-negotiation settings when interoperating with G-Series
cards running R4.0 and later.
Note With a G-Series card, you can only enable flow control on a p ort if auto negotiation is enabled on the
device attached to that port.