FDDI Management
6-16 Configuring FDDI Frame Translation Settings
If the frame is exiting the FDDI ring through another FDDI/Ethernet bridge, the FDDI
frame must be converted back into an IEEE 802.3/Ethernet frame. As there are four
potential Ethernet frame types to which the two FDDI frame types can be translated, you
must determine which translation options you want in effect — depending on which
network protocols and applications are being run on the destination network.
In addition, there are frame size differences between FDDI (which allows a maximum
frame size of 4500 bytes) and Ethernet frames (1518 byte maximum, excluding
preamble), so FDDI frames may need to be fragmented before being bridged onto an
Ethernet network.
The Frame Translation window lets you set the parameters for frame translation and
fragmentation when Ethernet traffic needs to traverse an FDDI ring. The frame types that
you select for translation will depend on which higher-layer communications protocols
and software you are running on the network segments connected to your
Ethernet-to-FDDI bridge. Eframesach frame type and its us age is described below.
Ethernet Frames
The HSIM-F6 supports translation of the following four Ethernet frame types:
Ethernet II
Ethernet II is the Novell® NetWare™ designation for the basic Ethernet frame type (also
commonly referred to as Ethernet or Ethernet DIX). This frame format has an Ethernet II
MAC header with a two byte Ethernet Type field. The Type field contains a protocol ID
which indicates the upper layer protocol (e.g., XNS, DECnet, TCP/IP, etc.) used in the
Data field of the packet. Most current transmission protocols, including TCP/IP, use the
Ethernet II frame format, as do networks running Apple’s AppleTalk 1 protocol and
Digital’s DECnet™ protocol.
Note that the Type field of an Ethernet II frame will always have a decimal value greater
than 1500, so that it can be differentiated from the Length field of Ethernet 802.2 frames
(described below).
Ethernet 802.2
The Ethernet 802.2 frame format is the IEEE 802.3 formalization of the original Ethernet
frame format. This frame format is similar to the Ethernet II frame format, except that the
two byte Type field is eliminated and replaced with a two byte Length field, and an 802.2
LLC header is encapsulated within the 802.3 frame. This LLC header contains the
destination and source addressing information for the LLC frame (DSAP and SSAP), and
a one byte Control field (the LSAP – or LLC Service Access Point value) which provides
the frame’s protocol ID. Ethernet 802.2 packets are differentiated from Ethernet II packets
because the Length field will always have a decimal value of 1500 or less (since the data
field in Ethernet frames cannot be greater than 1500 bytes), and the Ethernet II Type field
will always be greater than 1500 decimal.
This is the default frame type for Novell NetWare software version 3.12 and beyond; it is
also used for OSI packets on IEEE 802.x LAN networks.