E3Switch DS3 Operating information manual - page 14
Chapter 5: Operating Modes and Configuration
1000Mbit/s is generally preferred over 100Mbit/s on the PPPoE port only, though it generates
significantly more power-requirements, heat, and radiated electromagnetic noise even in the absence of
packet flow. 1000Mbit/s may slightly reduce path latency, as an incoming LAN packet must be fully
received before being forwarded to an outgoing port. The latency savings to receive or transmit a 1500-
byte packet at 1000Mbit/s vs 100Mbit/s speed is 0.108 milliseconds (1500bytes/packet x 8bits/byte /
(100Mbits/s) - 1500bytes/packet x 8bits/byte / (1000Mbits/s)).
1000Mbit/s LAN port speed may be desirable when one LAN port is configured to monitor the other LAN
port in addition to receiving incoming DS3/E3 data. In such a case, the data rate that the LAN port is
expected to transmit (the sum of all ports that could be a data source for the LAN port) may be greater than
100Mbit/s. The HTTP management statistics screen will show overflow errors if a port's data rates are
exceeded.
Setting more than one LAN port to 1000Mbit/s is not recommended and may result in
underflow/overflow errors in certain high packet load, memory-intensive cases.
Autonegotiation Problems
There are rare cases with older LAN equipment in which it may be necessary to disable autonegotiation. If
crc-errors or short packet errors are seen in the management statistics of the LAN port, the attached LAN
equipment has probably configured itself to half-duplex mode and colliding packets are being lost. In such
a case, autonegotiation should be disabled on both the converter and the attached LAN equipment, with
both forced to 100BaseTX full-duplex. Autonegotiation interoperability and standards were not well
understood by the industry at the inception of 100BaseTX, resulting in some older LAN equipment not
understanding the converter's autonegotiation advertisement of strictly full-duplex capability.
SFP Second LAN Port
The SFP LAN Port 1 hardware exists on all converters shipped and enables out-of-band management,
through either LAN port, or fiber-optic LAN connections of 10km or more. Refer to interoperability
section of this document for compatible SFP transceivers.
Dedicated Management/PPPoE-Data LAN Ports
Either LAN port may be configured to pass all packets to DS3/E3 or, selectively, to pass only management
or only PPPoE data packets.
If a LAN port is configured for PPPoE-only packets, the unit will drop incoming management packets
destined for an E3Switch MAC address. This provides a moderate level of security. These packets and
management broadcast/multicast packets may not be forwarded to the second LAN if LAN-to-LAN traffic
is configured.
VLAN
As shipped, the unit will accept management packets with any VLAN tags and attempt to respond to the
same. For more robust performance, specific VLAN tag settings can be configured. These settings only
apply to packets to and from the converter's management entity.
VoIP / Video or High-CoS Priority Frames
In certain firmware, receive queue space is reserved in the converter to allow frames with high 802.1p class-
of-service (CoS) priority settings to bypass existing frames waiting to be transmitted to the DS3/E3. This
allows voice, video and other high-priority traffic to experience low-latency transmission. Firmware allows
the “high” CoS level to be configured. Most VoIP traffic is tagged at CoS 5 or 6, so level 5 is typically a
good setting for the high-CoS value.
Port Auto-Disable and Return-to-Service Delay
In addition to manually configuring a port as disabled, the converter has the ability to delay a DS3/E3 port's
return to service for a specified period of time after it has failed or disable a LAN port if both telecom links
are down.
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