the process. Each nameserver record has a TTL that specifies the time they may be cached. When TTL expires, the record is discarded and a new query is performed. Servers build a hierarchy. At the top of the hierarchy are root name- servers. They have information about all top-level domain nameservers like .net or .fi nameservers. These nameservers, in turn, know about all name- servers immediately under their domain. One nameserver can serve several domains. Several nameservers may also serve one domain. In fact, at least two nameservers for each domain are strongly recommended. This ensures service for the domain in case one of the nameservers is temporarily out of order. One of the nameservers serv- ing a domain contains the master or primary copy of the zone information. All changes are made to this copy. Other nameservers are slave or sec- ondary nameservers for this domain. In GRPS, the internetwork operations between two or more PLMNs are extremely important to handle the demands of a roaming user. Therefore, DNS is a crucial service and has a significant impact on the operation of the network. 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. Function of GPRS Elements Chapter 4 DNS and Inter-PLMN Network Each PLMN operator should have at least two DNS servers. This makes it possible to upgrade one of the servers without service interruption. The servers should keep a cache of recently queried DNS records. Caching reduces query response time and decreases network traffic. A necessary DNS hierarchy can be arranged through two possibilities. The first is to configure the nameserver of each domain at the inter-PLMN network indi- vidually at each PLMN operator. Each time a new domain is added to the inter-PLMN backbone network name service or any authoritative name- server address is changed every operator must update the DNS servers. Over t