The maximum distance between stations is governed by the relationship between the propagation time of the
signals between the stations and the transmission time of the minimum-length frame, so that in the worst case
a station can always detect a collision before the transmission is finished. Not being able to act on the assigned
propagation speed and on the transmission rate, which characterizes the network hardware, the simplest thing
is to change the minimum frame length in order to satisfy the required constraint. So Equation 8.1 can be rewritten
Determine the maximum throughput achievable with the Fast Ethernet structure with two-level
topology shown in Figure 8.6 in which the three network devices are all switches instead of hubs. Compare
the result obtained with what would be obtained if all the terminals were interfaced to a single switch.
In this case the maximum throughput is given by the sum of the traffic that can be generated/received in the
terminal equipment and therefore . The two-level configuration of the switches has no effect on the maximum carried traffic and therefore a single switch interfacing the 6 terminal devices would give the same maximum carried traffic performance. However, it should be noted that in the two-level case there is a constraint on the maximum flows of traffic transported, since no more than 100 Mbit/s can transit between the two sets of terminals (A,B,C) and (D,E,F).
Calculate the maximum efficiency that characterizes the IEEE 802.11b access protocol taking
into account only the MAC layer and physical layer (PL) overhead, knowing that the latter defines 144 or 72
preamble bits, depending on the modulation type, and 6 bytes of headers, neglecting the times required to
send control frames and the propagation times.
Given that the minimum overhead of a MAC layer frame consists of 32 bytes and that the information field
of a MAC frame comprises a maximum of 2312 bytes, it follows that the maximum efficiency varies from to .
A network configuration is given which includes 5 bridges (, , , , ) and 5 LANs
(A, B, C, D, E) with the following connections: bridge connected to networks A (port 1), B (port 2), C
(port 3), bridge connected to networks C (port 1), E (port 2), D (port 3), bridge B3 connected to networks D (port 1), E (port 2), bridge B4 connected to networks C (port 1), E (port 2), bridge connected to networks B (port 1), E (port 2). Apply the spanning tree protocol procedure to obtain the status of all the ports of the bridges whose BI is equal to the respective numerical index, assuming unit cost of crossing each of the networks
The assigned network configuration is shown in Figure E8.1. The application of the spanning tree algorithm
is shown in Table E8.1. The column labeled x,y shows the current status of port y of bridge x and the BPDUs
that pass through it in transmission (Tx-BPDU) and in reception (Rx-BPDU) after updating the RPC field
(Rx-BPDU cost-update), or without updating the same field (Rx-BPDU). The lack of updating of the RPC
field in the last Rx-BPDU row serves to correctly compare the cost of the different routes that lead from each
network to the tree root, so as to determine the ports to be blocked for each of the networks (there is only one port D in each network). In the initial state (1) all ports are labeled D. Bridge x sends on port k a BPDU of the type (x,0,x,k) on the
interfaced LAN which is then received by the other bridges on the same LAN. In state (2) bridge remains
root-bridge as it receives BPDUs with larger RI. Bridge recognizes bridge as root-bridge and elects port
number 1 as its port R, since on that it has received the BPDU with the highest priority (1,1,1,3), also taking
into account the cost of the last crossed network, and the other ports as port D, emitting on the latter BPDUs
(1,1,2,k), which indicate bridge as root-bridge with cost RPC = 1 to reach it. Bridges and behave in
a similar way, electing port number 1 as port R and the other as port D. Bridge instead believes that bridge
is the root-bridge, elects port 2 as port R, as it receives the BPDU with higher priority determined this time
by the PI field, and therefore sends the relative BPDU (2,1,3,1) on port 1, labeled as D. In state (3), the blocking
state of ports is determined and the cost of the last network crossed is not taken into account in determining
the highest priority. Therefore ports number 2 and 3 of bridge remain labeled as ports D, since they
both receive BPDUs with lower priority than the sent BPDU. The ports number 2 of the bridges , and
and the port number 1 of the bridges become ports B, and therefore disabled for transmission, as BPDUs
with higher priority are received there than the BPDUs sent on the respective ports (sent RI larger for bridge and sent BI larger for bridges and ). The final configuration of the network with labeled bridge ports is shown in Figure E8.2, which highlights the tree connecting the bridges.
Figure E8.1 Configuration of interconnected networks according to Exercise 8.12.
Table E8.1 Spanning tree construction according to Exercise 8.12.
Figure E8.2 Network configuration according to Exercise 8.12 after spanning tree construction.
A network configuration is given which includes 5 bridges and 5 LANs
(A, B, C, D, E) with the following connections: bridge connected to networks A (port 1), C (port 2), bridge connected to networks B (port 1), D (port 2), bridge connected to networks A (port 1), B (port 2), E (port 3), bridge connected to networks C (port 1), E (port 2), bridge connected to networks D (port 1), E (port 2). Apply the procedure of the spanning tree protocol to obtain the status of all the ports of the bridges whose BI is equal to the respective numerical index, assuming unit cost of crossing each of the networks.
The assigned network configuration is shown in Figure E8.3. The application of the spanning tree algorithm, shown in Table E8.2 determines the network configuration in which is obviously the root bridge, having
the highest priority, and the loops are eliminated by blocking port 2 of the bridge and port 1 of bridge . It is interesting to observe how in this case after the first BPDU exchange, there are two bridges, and , which believe to be root bridges, while with the second step, only bridge remains as root bridge. Furthermore, a change of root port in bridge is observed in the passage from the first exchange of BPDU to the second exchange. The resulting spanning tree is shown in Figure E8.4.
Figure E8.3 Configuration of interconnected networks according to Exercise 8.15.
Table E8.2 Spanning tree construction according to Exercise 8.15.
Figure E8.4 Network configuration according to Exercise 8.15 after spanning tree construction.
Consider the network configuration with spanning tree shown in Figure 8.31 where 10 hosts
are interfaced. It is assumed that all forwarding tables are initially empty and that only 9 frames have been
successfully transmitted with the following source-destination pairs (SA-DA): Q-S, R-X, S-R, T-V, V-Y, WU,
X-W, Y-R, Z-X, as already described in Exercise 8.17. Now consider the displacement of the stations R,
T, U which are connected to the networks D, A and B, respectively. Determine the new content of the forwarding
tables of bridges , and after the successful transfer of the 3 frames with the following source-destination addresses: R-Z, T-V, S-U.
Figure 8.31 Ethernet networks interconnected by bridges according to Example 8.7 (Exercise 8.19).
Table E8.3, ignoring the entries in italics that appear after an arrow, shows the status of the forwarding tables of the bridges , , after the initial transfer of the 9 frames specified in the initial configuration of networks and stations. After moving the stations R, T, U, the network configuration and related stations shown in Figure E8.5 is obtained. Sending the R-Z frame does not imply any change in the forwarding tables; in fact, station R is reachable from the same ports of bridges , , (the relative entry in bridge would change, but this is not examined in the exercise). After the transmission of the T-V frame, only the entry relating to station T in bridge changes, since both the stations involved interface the same network and the location of station V is known to bridge which therefore does not forward the frame in flooding. The transmission of the S-U frame implies that a new entry relating to station S is recorded in bridges , , (this entry was present exclusively in bridge ), being a flooding transmission since the destination station U is unknown to all network bridges. All these changes are indicated in Table E8.3 in italics with an arrow.
Table E8.3 Forwarding table of bridges , , according to Exercise 8.19.
Figure E8.5 Interconnected networks after moving stations according to Exercise 8.19.
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