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A repeater forwards every frame; it has no filtering capability
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It is tempting to compare a repeater to an amplifier, but the comparison is inaccurate An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it A repeater does not amplify the signal; it regenerates the signal When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength
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A repeater is a regenerator, not an amplifier
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The location of a repeater on a link is vital A repeater must be placed so that a signal reaches it before any noise changes the meaning of any of its bits A little noise can alter the precision of a bit's voltage without destroying its identity (see Figure 153) If the corrupted bit travels much farther, however, accumulated noise can change its meaning completely At that point, the original voltage is not recoverable, and the error needs to be corrected A repeater placed on the line before the legibility of the signal becomes lost can still read the signal well enough to determine the intended voltages and replicate them in their original form Figure 153
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Function ofa repeater
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Active Hubs
An active hub is actually a multipart repeater It is normally used to create connections between stations in a physical star topology We have seen examples of hubs in some Ethernet implementations (lOBase-T, for example) However, hubs can also be used to create multiple levels of hierarchy, as shown in Figure 154 The hierarchical use of hubs removes the length limitation of 10Base-T (100 m)
A bridge operates in both the physical and the data link layer As a physical layer device, it regenerates the signal it receives As a data link layer device, the bridge can check the physical (MAC) addresses (source and destination) contained in the frame
Figure 154 A hierarchy of hubs
One may ask, What is the difference in functionality between a bridge and a repeater A bridge has filtering capability It can check the destination address of a frame and decide if the frame should be forwarded or dropped If the frame is to be forwarded, the decision must specify the port A bridge has a table that maps addresses to ports
A bridge has a table nsed in filtering decisions
Let us give an example In Figure 155, two LANs are connected by a bridge If a frame destined for station 712B13456142 arrives at port 1, the bridge consults its table to find the departing port According to its table, frames for 7l2B 13456142 leave through port 1; therefore, there is no need for forwarding, and the frame is dropped On the other hand, if a frame for 712B13456141 arrives at port 2, the departing port is port 1 Figure 155
4 3 2 1
A bridge connecting two LANs
5 4 3 2
1 f-+,---l1-l1
Address 71:2B: 13:45:61:41 71:2B: 13:45:61:42 64:2B: 13:45:61:12 64:28:13:45:61:13
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1 1 2 2
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64:2B: 13:45:61: 12 64:2B: 13:45:61: 13
and the frame is forwarded In the first case, LAN 2 remains free of traffic; in the second case, both LANs have traffic In our example, we show a two-port bridge; in reality a bridge usually has more ports Note also that a bridge does not change the physical addresses contained in the frame
A bridge does not change the physical (MAC) addresses in a frame
Transparent Bridges
A transparent bridge is a bridge in which the stations are completely unaware of the bridge's existence If a bridge is added or deleted from the system, reconfiguration of the stations is unnecessary According to the IEEE 8021 d specification, a system equipped with transparent bridges must meet three criteria:
I Frames must be forwarded from one station to another
2 The forwarding table is automatically made by learning frame movements in the network 3 Loops in the system must be prevented
A transparent bridge must correctly forward the frames, as discussed in the previous section
Learning The earliest bridges had forwarding tables that were static The systems administrator would manually enter each table entry during bridge setup Although the process was simple, it was not practical If a station was added or deleted, the table had to be modified manually The same was true if a station's MAC address changed, which is not a rare event For example, putting in a new network card means a new MAC address A better solution to the static table is a dynamic table that maps addresses to ports automatically To make a table dynamic, we need a bridge that gradually learns from the frame movements To do this, the bridge inspects both the destination and the source addresses The destination address is used for the forwarding decision (table lookup); the source address is used for adding entries to the table and for updating purposes Let us elaborate on this process by using Figure 156 1 When station A sends a frame to station D, the bridge does not have an entry for either D or A The frame goes out from all three ports; the frame floods the network However, by looking at the source address, the bridge learns that station A must be located on the LAN connected to port 1 This means that frames destined for A, in the future, must be sent out through port 1 The bridge adds this entry to its table The table has its first entry now 2 When station E sends a frame to station A, the bridge has an entry for A, so it forwards the frame only to port 1 There is no flooding In addition, it uses the source address of the frame, E, to add a second entry to the table 3 When station B sends a frame to C, the bridge has no entry for C, so once again it floods the network and adds one more entry to the table 4 The process of learning continues as the bridge forwards frames
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