while a space represented a zero. A point of historical inter- est is that the terminology "mark" and "space" is common parlance in modern networks: In T-Carrier, the encoding scheme is called Alternate Mark Inversion (AMI), because every other one alternates in polarity from the ones that surround it. Similarly, Alternate Space Inversion is used in signaling schemes such as on the ISDN D-Channel. At any rate, the entire message would be delivered in this fashion, from office to office to office, ultimately arriving at its final destination, a technique called message switching. Over time, of course, the process became fully mechanized and the telegraph operators disappeared. There was one major problem with this technique. What happened if the message, upon arrival, was found to be corrupt, or if it simply did not arrive for some odd reason? In that case, the entire message would have to be resent at the request of the receiver. This added overall delay in the system and was lawfully inefficient because, in most cases, only a few characters were corrupted. Nevertheless, the entire message was retransmitted. Once the system was fully mechanized, it meant that the switches had to have hard drives on which to store the incoming mes- sages, which added yet more delay, since hard drives are mechanical devices and by their very nature relatively slow. Improvements didn't come along until the advent of packet switching. 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. Protocols Chapter 2 Packet Switching With the arrival of low-cost, high-speed solid-state microelectronics in the 1970s, it became possible to take significant steps forward in switch- ing technology. One of the first innovative steps was packet switching. In packet switching, the message that was transmitted in its entirety over the earlier message-switched store-and-forward networks