Alcatel Omni Switch/Router, User Manual

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Translating the Frame
Page 18-3
Translating the Frame
In order to discuss these issues independent of particular media and protocols, consider that
every frame, of any protocol, on any media, consists of the following parts.
The Essential Parts of Frame
MAC Header
Consists of a source and destination address specifying the transmitting station in the broad-
cast domain and the intended recipient(s), as well as other media specific fields. For exam-
ple, AC and FC fields in Token Ring, FC in FDDI, etc.
RIF (Router Information Field)
If present, it is defined by the source routing standard and is only found on Token Ring and
FDDI media.
Encapsulation
Defined by the various standards for the media, many of which reference common standards.
For example, on Ethernet media, as defined by Ethernet II, this is a 16 bit type field. On
Ethernet media, as defined by the IEEE 802.3 committee, this is a length field together with
any encapsulation defined by the IEEE 802.2 Logical Link Control (LLC) committee. On Token
Ring and FDDI, it is any encapsulation defined by the IEEE 802.2 LLC committee.
Network Header
Defined by the organization responsible for the particular routing protocol whose data is
being carried within the frame. The values of fields defined in the Encapsulation area allow
the recipient to identify which protocol standard to use to decode the Network Header part of
the frame.
Data
The payload being carried between the end-stations.
In a routing implementation the first three fields (i.e., MAC header, RIP, and Encapsulation)
are the ones stripped and rebuilt when the frame is forwarded. These are the three areas that
have to be manipulated. The next sections examine each of these frame packet areas further
to see the media and protocol dependencies. We can also examine their interactions.
MAC Header RIF Encapsulation Network Header Data