 done in the cable vault; an example is shown in Figure 3-24. The large cable on the left side of the splice case is broken down into the collection of smaller cables, exiting on the right. Into the Central Office We could say a great deal more about the access segment of the network, and will later. For now, though, let's begin our tour of the central office. By now our cable has traveled over aerial and underground traverses and has arrived at the central office. It enters the building via a buried conduit that feeds into the cable vault, the lowest subbasement in the 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. Telephony Chapter 3 office. (Remember, this is where the New York City fire began.) Because the cable vault is exposed to the underground conduit system, there is a always a danger that noxious and potentially explosive gases (methane, mostly) could flow into the cable vault and be set afire by a piece of sparking electrical equipment. These rooms are, therefore, closely moni- tored by the electronic equivalent of the canary in a coal mine, and will cause alarms to sound if gas is detected at dangerous levels. The cables that enter the cable vault are large and encompass hun- dreds if not thousands of wire pairs, as shown in Figure 3-25. Their secu- rity is obviously critical, because the loss of a single cable could mean loss of service for hundreds or thousands of customers. To help maintain the integrity of the outside plant, large cables are pressurized with very dry air, fed from a pressurization pump and manifold system in the cable vault (see Figure 3-26). The pressure serves two purposes: It keeps mois- ture from leaking into a minor breach in the cable, and serves as an alarm system in the event of a major cable failure. Cable plant techni- cians can analyze the data being fed to them from pressure transducers along the