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OSI Layer 2: Data-Link Layer Protocols and Applications

The physical connectivity of Token Ring/IEEE 802.5 is also fault-tolerant.

If a machine on a ring becomes unreachable, the machine is electrically dis-
connected from the ring. The MAU in which the Token Ring/IEEE 802.5
cabling is located shuts down the port that is unreachable, thus healing the
ring. Physical connectivity is tested by using beaconing, which locates the
fault and reports back with the nearest active upstream neighbor, or NAUN.
This information helps to isolate network failures and shortens trouble-
shooting time.

Token Frame Format

We now look at the two frame types used by Token Ring/IEEE 802.5.
Remember that while referencing ., we will move from right to left. The two
principal frame types used in Token Ring and IEEE 802.5 are the token
frame and the data, or command, frame formats.

F I G U R E 2 . 2 6

Token and data/command frames

Figure 2.26 shows that both frames start with a start delimiter and

access control

. These two fields are each only one byte long. Start

delimiter

alerts each station on the ring that the frame is a data or com-

mand frame, not a simple token frame. The Access control field is used to
assign priority to the frame and to help distinguish what frame type it is. Pos-
sible frame types are token, data/command, and a monitor frame.

Here is where the difference starts. The data frame contains additional

fields that the token frame does not have. All of the station's data is inserted
into the frame following the access control field. The data segment commences
with a one-byte field that holds information regarding frame control. This is
an indicator of whether the frame contains data or command information.

The next two fields are six bytes long and contain the packet's destination

and source addresses. The Data field, of variable length, carries all of the
data that is to be encapsulated. The length is determined by how long the
sending station can hold a token.

Data/command frame

Frame
status

End

delimiter

FCS

Data

Source address

Destination

address

Frame

control

Access
control

Start

delimiter

End

delimiter

Access
control

Start

delimiter

Token frame

byte

Variable

size

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