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.InteroperabilityThis is necessary to protect the capital that has already been invested in theexisting networking and telephony service delivery infrastructures.These infra-structures include both legacy PSTN facilities and the existing (if any) VoIPservice delivery systems from di¤erent equipment manufacturers.The PSTNfacilities are the POTS phones, the PSTN switches, access and transmissionTDM networks, and the associated billing, operations, and call feature hosts.In the same way, there may also exist a variety of first-generation IP tele-phony related equipment such as H.323 and MGCP phones and the relatedGWs, GKs, and billing systems in both enterprise and public telephone ser-vice providers networks.Therefore, the network elements for implementing orenhancing the existing VoIP service should be selected to support both legacysystems and earlier generations (and versions) of the line (and/or trunk) inter-faces and VoIP protocols.These help the network designers to choose theproper software and hardware configurations of the network elements for rapiddeployment of VoIP service via incremental evolution of the network.Authentication and SecurityBecause of the openness of IP and the ubiquity of the Internet, IP-based tele-phony endpoints and VoIP network elements are susceptible to both mali-cious attacks and inadvertent damage (during configuration change, softwareupgrade, etc.).To minimize the chances of these events, multiple levels of user(or client) authentication can be utilized before allowing access to or approvinga VoIP call over the network.Also, a variety of security enforcement devicessuch as firewalls, proxy servers, and so on can be utilized to safeguard usersaccess to the critical networking and service hosting facilities.To achieve securecommunication over the shared IP network, large (e.g., 1024 bit) key-basedencryption can be utilized.However, this may add further overhead such asmaintenance of key distribution centers and clients need to communicate withthese centers for each session, and may cause degradation of the quality of voicesignal transmission.Other solutions include utilization of VoIP session setup(e.g., the latest version of SIP; RFC 3261, RFC 3262) and IP communication(e.g., IPv6, RFC 2460) protocols that have built-in security-related services.Legal and Public Safety Related ServicesTo comply with regulatory requirements, the public telecom service providerswho o¤er VoIP or IP telephony-based basic telephony services to residential EPILOGUE 135customers must implement services like CALEA, routing of 911 calls to thePSAPs with callers identification and location information, and so on.How-ever, the enterprises may need to implement these services as well once theybecome fully dependant on IP-based telecommunication services.Since the IPnetworks operate using shared resources in a distributed fashion, it is some-times di0 cult to detect the identities and locations of the communicatingparties in a VoIP session.However, many distributed server, wiring diagram,and media access control (MAC address) level filtering mechanisms are cur-rently being explored to resolve these issues.Cost-E¤ective ImplementationDelivery of voice and data services over multiservice IP networks will not bewidespread until and unless the implementation of highly available and reli-able VoIP service becomes economically feasible.The network elements thatare required to implement the basic IP telephony and VoIP-related servicesare significantly less expensive (possibly one-tenth) than the traditional PSTNswitches.The costs for transmission of a packetized voice signal over shared IPlinks are also much smaller (e.g., could range from one-tenth to one-fifth) thanthose of voice transmission over traditional TDM (circuit-switch) networks.This is equally true for both intero0 ce telephone calls within an enterpriseand domestic LD and international telephone calls in public telecom networks.However, the prices of IP phones and the related CPE/IAD, as well as theexpenses for implementing PSTN-grade VoIP service in the access networks(e.g., 99.999% of availability of service or 5.256 min of downtime of service peryear), may be higher.These higher costs can be attributed, to some extent, tothe lack of embedded reliability, security, and QoS features in the software andhardware components and to the protocols that are utilized for implementingthe VoIP service.Many work groups are currently developing mechanisms toovercome these limitations.EPILOGUEImplementation of VoIP promises low-cost realization of telephony and manyother feature-rich real-time and non-real-time communications services.How-ever, various issues are posing realistic challenges to network element devel-opers, network designers, and service providers alike.These concerns includemanaging the PSTN-grade availability, reliability, security, service-completionstandards, and customer satisfaction of the telephony service in open and flexi-ble (i.e., IP) networking environments.Many industry consortiums, task forces, and standardization forums areworking with equipment manufacturers and service providers to develop pro-tocols, architecture frameworks, and interoperability specifications to resolvethese issues.These include various study groups of ANSI (www.ansi.org) and 136 CONCLUSIONS AND CHALLENGESITU-T, work groups of IETF, MSF, and the International Softswitch Con-sortium (ISC), IEEE (standards.ieee.org), Cable Labs (www.cablelabs.com/projects/), and the DSL and ATM forums.We expect to see publication ofpractical implementation and interoperability recommendations from theseindustrywide joint e¤orts in the foreseeable future.REFERENCES1.B.Khasnabish, Optical Networking Issues and Opportunities: Service ProvidersPerspectives, Optical Networks Magazine, Vol.3, No.1, pp.53 58, January/February 2002.2.G.Jakobson and B.Khasnabish, Guest Editors, Enterprise Network and ServiceManagement, Special Issue of IEEE Network Magazine, Vol.16, No.1, pp.6 7,January/February 2002.3.B.Khasnabish, Next-Generation Corporate Networks, IEEE IT Pro Magazine,Vol.2, No.1, pp.56 60, January/February 2000. Implementing Voice over IP.Bhumip KhasnabishCopyright © 2003 John Wiley & Sons, Inc.ISBN: 0-471-21666-6APPENDIX ACALL PROGRESS TIMEMEASUREMENT IN IP TELEPHONY1In IP telephony, a voice call is usually established through multiple stages.Inthe first stage, a phone number is dialed to reach a near-end or call-originatingor ingress IP-telephony (or IP-PSTN) GW.The next stages involve user iden-tification by delivering an m-digit user ID to the authentication and/or billingserver, followed by user authentication using an n-digit personal identificationnumber (PIN)
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