Token Bus was a proprietary Ethernet-based network. It worked like Token Ring in

Question

Token Bus was a proprietary Ethernet-based network. It worked like Token Ring in that a small token packet was sent from one station to the next in agreed-upon order, and a station could transmit only when it had just received the token.

(a). If the data rate is 10 Mbps and the token is 64 bytes long (the 10-Mbps Ethernet minimum packet size), what is the average wait to receive the token on an idle network with 40 stations? (The average number of stations the token must pass through is 40/2 = 20.) Ignore the propagation delay and the gap Ethernet requires between packets.

(b). Sketch a protocol by which stations can sort themselves out to decide the order of token transmission; that is, an order of the stations S0 ... Sn-1 where station Si sends the token to station S(i+1) mod n.

Explanation / Answer

Solution to part a:

Transmission delay (for the token) = 64 bytes / 10 Mbps = 5.12 x 10-5 seconds

Given, average number of stations that the token must pass = 20.

Hence, average wait to receive token on an idle network with 40 stations = 20 x 5.12 x 10-5 = 1.024 x 10-3 seconds

= 1.024 miliseconds (Answer)

Solution to part b :

Stations can compare themselves to one another according to their MAC Addresses(numeric value of MAC addresses). The station with the smallest MAC address becomes S0, then the station with the next smallest address becomes S1 and so on. The above protocol can help the stations in deciding the order of token transmission. S0 --> S1 --> S2 .....--> Sn-1 in the increasing order of MAC address values.

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