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4 . 7. 2 Optional LAN port Configuration Items
The following items are not necessary to enable basic operation but may be configured to
give a more reliable and manageable system.
4.7.2.1 Flow Control
The MetroLAN provides IEEE 802.3x pause frame based flow control to prevent buffer
overflow when operating with a reduced bandwidth connection. By default this is enabled
and is only active when auto negotiation is selected.
4.7.2.2 MDI/MDIX
The MetroLAN LAN ports by default will auto sense the cable type attached and will
automatically configure as MDI ( End Station e.g. PC for connection to a switch port ) or as
MDIX ( Cross Over, for direct connection to an end station such as a PC ). This may be
disabled, in which case the MetroLAN will appear as an MDIX or switch port.
4.7.2.3 MAC Address
When Flow Control is enabled, the MetroLAN will output pause frames. These pause frames
are formatted according to IEEE 802.3x and will have this MAC address as the source
address. The options are 0.0.0.0.0.0 or the unique unit address. Selecting the unit address
may help when trying to identify where pause frames are coming from, however since pause
frames are not used by switches when building their MAC address tables there is no
requirement for a unique source address.
4.7.2.4 FCS Generation
An Ethernet MAC frame has a four byte CRC-32 Frame Check Sequence appended to
provide error detection. With FCS Generation disabled the entire MAC frame including FCS
is transmitted end to end. However, to save bandwidth the FCS may be stripped on ingress
and recalculated and appended on egress at the remote end. In this way bandwidth is
reduced since all MAC frames are effectively 4 bytes smaller. It is important that this
configuration option matches that of the far end otherwise frames will end up either 4 bytes
to small ( no FCS ) or 4 bytes to large ( double FCS ).
4.7.2.5 Max Frame Size
For most common LAN protocols 1536 bytes is more than adequate. For some protocols
however, larger frame sizes may be required. MetroLAN can support frames up to 4096
bytes in length.
4.7.2.6 GFP Payload FCS
The GFP protocol allows for transmission of frames with or without protection of an FCS. If
enabled a 4 byte CRC-32 is appended to GFP payload. Since most SDH networks have
very low error rates, errors introduced on the network are rare and will be dealt with by the
higher layer protocols, so the bandwidth gains are beneficial. If however, a large number of
errors are being detected, the payload FCS should be enabled to allow for the detection of
errors in the network.
Summary of Contents for MetroLAN-1000
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