Ex 11:
Token ring priorities range from 000 to 111, with 111 highest.   Suppose a station has data with priority 011.   Show all possible incoming priority bit patterns and the changes that result.   Show the decision as each bit arrives.   Repeat for priority 101.

A token consists of 3 bytes that must circulate continuously on the network until captured by a station that has data ready to send. The AC byte's 8 bits are represented as PPPTMRRR. The P bits specify a priority associated with this token and determine which station will gain the token. The T bit is set to 1 to indicate that this is a token, 0 to indicate that it is a frame. The R bits provide a reservtion facility.

A station that has a frame ready to send can grab a token if frame PPP >= token PPP. It sets T=0 and sends data.
If frame PPP < token PPP, the station can attempt to reserve a future token by setting token RRR to match frame PPP (only if frame PPP > token RRR).

If incoming bits are part of a frame (T=0), the data must be sent. If waiting PPP > incoming RRR, then RRR is changed to PPP value.
When the station that sent the data regenerates the token, it will set the token PPP to the value currently in token RRR.

The new token will be grabbed by the first station with data PPP > token PPP.

Waiting PPP = 011
Incoming-TR-AC     Resulting-TR-AC
0001MRRR               0001M011
0011MRRR               0011M011
0101MRRR               0101M011
0111MRRR               0110MRRR
1001MRRR               1000MRRR
1011MRRR               1010MRRR
1101MRRR               1100MRRR
1111MRRR               1110MRRR

Incoming-TR-AC     Resulting-TR-AC
PPP0M000               PPP0M011
PPP0M001               PPP0M011
PPP0M010               PPP0M011
PPP0M011               PPP0M011
PPP0M100               PPP0M100
PPP0M101               PPP0M101
PPP0M110               PPP0M110
PPP0M111               PPP0M111

Waiting PPP = 101
Incoming-TR-AC     Resulting-TR-AC
0001MRRR               0001M101
0011MRRR               0011M101
0101MRRR               0101M101
0111MRRR               0111M101
1001MRRR               1001M101
1011MRRR               1010MRRR
1101MRRR               1100MRRR
1111MRRR               1110MRRR

Incoming-TR-AC     Resulting-TR-AC
PPP0M000               PPP0M101
PPP0M001               PPP0M101
PPP0M010               PPP0M101
PPP0M011               PPP0M101
PPP0M100               PPP0M101
PPP0M101               PPP0M101
PPP0M110               PPP0M110
PPP0M111               PPP0M111

The first bit (I/G bit - individual or group address) of the addresses in all the 802.x MAC protocols is 1 if the address refers to a group of destinations, and 0 if the destnation is a single station. Since this is a single station, the only possible value for the I/G bit is 0.

E=1: Error
A=1: Valid destination address
C=1: Data frame copied to destination buffer

Possible combinations of EAC and meaning:
EAC     Meaning
000       No error, invalid dest addr, data frame not copied.
010       No error, valid dest addr, data frame not copied.
011       No error, valid dest addr, data frame copied.
100       Error, invalid dest addr, data frame not copied.
110       Error, valid dest addr, data frame not copied.
111       Error, valid dest addr, data frame copied.

Passwords and any other types of sensitive data, such as customer's SSN, banking info, e-mail, etc., should always be encrypted on a LAN since the data can be intercepted by any malicious users who know how to interpret and modify data on a Token Ring or a Token Bus. It was noted in class that these issues are common to all LANs.