sponding input. BLOCKING Output blocking occurs in a switch when two packets are simultaneously destined for the same egress port. Obviously, in this case, one of the conflicting packets must be buffered (or blocked) in order to 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 SWITCH FABRICS 269 allow the other to exit the switch. No switch can avoid the output-blocking problem. A technique that can be used on TDM switches based on crosspoint architectures in order to avoid this undesirable eventuality is to use flavors of time-space (TS) switching, which essentially means using a TSI on each input and/or output line before and/or after the crosspoint module. One talks then about time- space-time switching (TST). We say that internal blocking occurs when two or more packets from different inputs that are des- tined for different outputs run into an internal bottleneck and consequently one of them must be tem- porarily buffered. For example, the simple crossbar is internally nonblocking in the sense that no sample is blocked in the switch while waiting for an output line to become available. A nonblocking switch is one, which is immune to situations of internal blocking. In general, a switch is rearrangeably nonblocking (RNB) if any new connection from a free input link to a free output link can be physically made, but it may require the switch to be reconfigured. A switch is strictly nonblocking (SNB) if a new connection from a free input link to a free output link can be made at any time without rearranging the internal configuration of the switch. A fully inter- connected switch has N inputs going to N different switches, which each corresponds to one of the N outputs. A fully interconnected switch is necessarily a nonblocking one. We talk about head of line (HOL) blocking when in some designs at one of the input stages of a 