IEEE 802.5 Token Ring

IEEE 802.5 Token Ring
The IEEE 802.5 standard specifies a 4 and 16Mbps token ring LAN. The first token ring network
was developed by IBM in the 1970s, and then IEEE released the 802.5 specification
based on IBM’s work. Because of the IBM orientation of Token Ring, you’ll find that most
Token Ring networks in facilities deploy IBM mainframes and servers.
Today, IBM Token Ring and IEEE 802.5 networks are compatible, although there are minor
differences. For instance, IBM’s Token Ring network specifies a star configuration, with all
end stations attached to a device called a multistation access unit (MSAU). IEEE 802.5 does
not specify a topology, but most 802.5 implementations are based on a star configuration similar
to the UTP version of ethernet. Also, IEEE 802.5 does not specify a media type, but IBM
Token Ring identifies the use of UTP wire (see the section “Unshielded Twisted-Pair (UTP)
Wire,” earlier in this chapter).  Token Ring protocols ensure that only one station transmits at a time through the use of a
token. The token, which is a distinctive group of bits, circulates the ring. If a station wants to
transmit data, it must first wait its turn to receive the token, and then it can transmit its data.
The capturing of the token ensures that no other station will transmit. The data circulates the
ring, and the appropriate destination senses its address and processes the data. When finished,
the sending station forwards the token to the next station downline.
Because of the token-passing mechanism, 802.5 operates with more stability under heavier
traffic than 802.3 ethernet. The predictable access method of 802.5 enables it to handle
synchronous-type information transfers. IEEE 802.5 is the second most popular LAN medium
access technique and is slightly more expensive to implement than ethernet.
Figure 6.9 illustrates the token ring frame formats, and the following lists explains the purpose
of each field.
The following list describes the fields of an 802.5 token:
• Start Delimiter—The start delimiter alerts each station that a token (or data frame) is
arriving. This field includes signals that distinguish the byte from the rest of the frame by
violating the encoding scheme used elsewhere in the frame.
• Access Control—The access control byte contains the priority and reservation fields, as
well as a token bit that is used to differentiate a token from a data or command frame.
The monitor bit is used by the active monitor to determine whether a frame is endlessly
circling the ring. End Delimiter—The end delimiter identifies the end of the token or data/command
frame. It also contains bits to indicate a damaged frame, as well as the last frame in a
logical sequence
The following list describes the field of an 802.5 MAC frame:
• Frame Control—The frame control byte indicates whether the frame contains data or
control information. In control frames, the frame control byte specifies the type of control
information.
• Destination and Source Address—As with IEEE 802.3, the destination and source
address are 6 bytes long and designate the source and destination stations.
• Data—The Data field contains the data being sent from source to destination. The
length of this field is limited by the ring token holding time, which defines the maximum
time that a station may hold the token.
• Frame Check Sequence (FCS)—The 4-byte FCS field contains a cyclic redundancy
check (CRC) value so that the receiving device can check for errors.
• End Delimiter—The end delimiter identifies the end of the data/command frame. It also
contains bits to indicate a damaged frame, as well as the last frame in a logical sequence.