Understanding the network.pdf - Back to Home

Token Ring addresses are expressed in big-endian (most significant byte
first) form, and Ethernet addresses are expressed in little-endian (least
significant byte first) form.
• Source Address (48 bits)—This address has three subfields. The first bit is
known as the Routing Information bit (RI). The RI indicates if routing
information is contained in the data portion of the frame. The second bit
indicates if the address is locally or universally administered. The remaining
bits contain the OUI (or NULL address) and the sending station's unique
address.
• Data (1 byte to 4,500 bytes or 18,000 bytes)—The data field contains either
control information, which is made up of vectors and subvectors which relay
the Token Ring MAC control information, or a standard 802.2 LLC-PDU. In
addition, the LLC-PDU frame can also contain source routing information.
Source routing is an IBM transparent bridge variation that permits the use of
more than one bridge to connect a ring to other rings within the greater
network. Source routing works by having end-stations send out network
learning packets that send information back to the originator about the
available paths to reach destinations on the network. Up to 16 bytes of
routing information can be sent with a packet. If source route bridging is
enabled and a packet contains no routing information, the packet cannot be
delivered to destinations outside the local ring. Source routing is not part of
the IEEE standard.
• Frame Check Sequence (32 bits)—The FCS contains the CRC value of the
frame. When the destination host receives the frame, it performs a CRC
check on the frame and compares the results to the CRC in the FCS field. If
they match, the frame is processed further if it fails, the frame is discarded.
• End Delimiter (8 bits)—This field, like the Start Delimiter, uses non-data
symbols to indicate the end of the frame. This frame also contains
identification bits that indicate if the frame is part of a multiframe
transmission and if a frame error has been detected by another host.
• Frame Status (8 bits)—This field is used by the destination station to indicate
the status of the frame and whether it was successfully received by the
destination.
To provide the inter-station messaging needed to manage the ring management
processes, the IBM Token Ring architecture defines 25 different MAC frame types.
The IEEE 802.5 standard defines 6 MAC frame types. The MAC frame
types'identification (called a vector) is contained in the first 32 bits of the frame's
information field. The first 16 bits is the vector message's length. The following 16
bits contain the MAC's vector code (that is, MAC type). The remainder of the
information field contains the actual management data (known as sub-vectors).
One or more management messages can be contained in a single Token Ring frame.
The IEEE 802.5-defined MAC frame types are as follows: