switch a specific packet is denied transmission across the switch fabric core and it therefore blocks all other packets following it at the same input. This causes a backup in the queue that is entering the switch through that same port and stage. BASIC SWITCHING ELEMENTS We discussed how the two fundamental functions of a switch can be physically separated from each other. We will now show how this functionality can be implemented for a 2X2 switching element, which is often referred to in the industry as a B-element (pronounced "beta element"). In the case of larger switches (which involve more inputs and outputs than shown in this example), synthesizing a network that is composed of similar B-elements easily creates this functionality. Figure 14.4 shows the structure of a B-element. Delay/Buffer Delay/Buffer Decision circuits Crossconnect switch Latching/Buffering FIGURE 14.4 The structure of a basic switching element. (Source: RAD Networks) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SWITCH FABRICS 270 NETWORK PROCESSORS The two input signals arrive at the element shown here from the top. Each must undergo a certain delay stage in order to synchronize the packet/cell contents with the decision that will be made based on the decision logic block. The decision logic block is operating on the packet header based on some application-specific sets of procedures. The shown latch is required to temporarily hold the result of that decision for the duration of the packet or, in the case of ATM, the duration of the cell, which is defined as 53 bytes long. The first 5 bytes contain header information such as source and destination addresses, whereas the remaining 48 bytes are the payload. The crossconnect block is a dual multiplexer, which can be set in either a bar or a cross state. This means that each input can be routed to either one of the 