342 Networking Explained, Second Edition
intended recipient, and returns the frame to the ring where it is transmitted to lobe 4. Lobe
4 grabs the frame off the ring and sees that it is data and not a token. Hence, it cannot
transmit its data. Lobe 4 also determines that it is not the intended recipient so it returns
the frame to the ring where it is transmitted to lobe 5. Lobe 5 receives the frame and ascer-
tains that its destination address matches that of the frame’s. The lobe then makes a copy
of the frame and stores it in memory. If the frame is valid and the data successfully copied,
lobe 5 then sets the A and C bits to 1 and returns the frame to the ring. (It also processes
the data it receives.) Lobe 1 receives the frame, examines it, and returns it to the ring. The
frame, having completed a full pass around the ring, is now received by lobe 2. Seeing that
the A and C bits were properly set, lobe 2 strips the data from the frame. If its token-hold-
ing time has elapsed, it then resets the token bit to 0 and places the token frame on the
ring. Otherwise, it transmits another frame of data. When lobe 2 returns the free token to
the ring, it is transmitted to lobe 3. Since lobe 3 does not have any data to transmit, it
returns the token to the ring for transmission to lobe 4. Since lobe 4 has data to transmit, it
transforms the token into a data frame and the data transmission process begins again with
lobe 1 being the intended recipient.
3. Could you explain the contents of the access control field?
Sure. The access control field consists of eight bits: three priority bits (P), a token bit
(T), a monitor bit (M), and three reservation bits (R). Priority bits can be set from 0 (low-
est) to 7 (highest). Thus, a token ring network has eight possible priorities (000 to 111) rel-
ative to data transmission. The use of these bits is discussed later in the chapter. As
indicated earlier, the token bit identifies the frame as a token (T = 0) or data (T = 1). If the
token bit is set to 0, the frame is considered a “free” or idle token. Only one free token is
permitted on the ring, and the lobe that has the free token controls the ring and is permitted
to transmit data. Thus, unlike Ethernet/802.3 networks, only one lobe at a time can trans-
mit data on a token ring network. The monitor bit is used by one lobe, called the monitor
station or active monitor, to oversee the status of the token. When a lobe transmits data, or
when the token is idle, M is set to 0. When the monitor station receives a data frame, it sets
M to 1. If the monitor station now receives a data frame that has M set to 1, it knows that
the transmitting lobe did not strip the data off the frame after the frame completed a full
pass around the ring. The monitor station then removes this “orphaned” frame from the
ring and issues a new token. The monitor station is discussed later in the chapter. The res-
ervation bits are used for reserving a token at a particular level of priority. These, too, are
discussed later in the chapter.
4. How do the priority and reservation bits in the token frame function?
As indicated earlier, a token can have several different priority levels, ranging from 0
(lowest) to 7 (highest). When a lobe receives a free token, it must first compare the priority
value contained within the token to the priority of the data it has to transmit. If a lobe’s
data has a priority level equal to or greater than that of the token’s, then the lobe may
transmit its data. If the token’s priority is higher than the lobe’s data, then the lobe cannot
transmit its data and must pass the token to its neighbor lobe. During a particular transmis-
sion, a lobe may reserve the token at a higher level than the token’s current level. By doing

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