2 Internet Protocol Centrex Interactive Communications over Internet Protocol The Internet Protocol (IP) was devised in the 1960s and has been subse- quently revised as a universal networking language for the network of net- works that we now know as the Internet.Some of the relative objectives of IP are that it should enable: ◆ Data to be transmitted over different media and data links; ◆ Data packets to be transmitted reliably; ◆ New networks to be joined to the Internet without disruption; ◆ Equipment of different types and from various vendors to be inter- connected smoothly. Layer 3 and 4 Protocols IP exists at layer 3 (the network layer) of the seven-layered Open System Interconnection (OSI) protocol stack, above the data link layer, where technologies such as ATM, frame relay, and Ethernet apply, as illustrated in Figure 2.1. 9 10 Centrex or PBX: The Impact of IP Layer OSI IP Layer 7 Application 6 Presentation Application: Web Browser, 4 HTTP 5 Session 4 Transport Transport: RTP, TCP, UDP 3 3 Network Internet: IP 2 2 Data link Network interface: 1 1 Physical Ethernet, ATM, FR Figure 2.1 IP and OSI Protocol stacks. While IP, at layer 3, defines the addressing and routing rules for data packets, at layer 4 there are three other relevant protocols: user datagram protocol (UDP), Real-Time Transport Protocol (RTP), and transmission control protocol (TCP).UDP is a connectionless protocol, with an emphasis on the minimizing of packet transit time, which is ideal for the interactions that are essential to voice and video traffic.TCP is the connection-oriented protocol that provides confirmation of a packets receipt at its destination, if necessary after retransmission and without con- cern about delay.With a Voice or Video-Conferencing over Internet Pro - tocol (VoIP) session it is usual to employ TCP/IP for the call setup, any in-progress signaling, and teardown, while UDP/IP is better suited to the real-time transmission of the voice or video conversations.TCP provides the same functionality on networks for VoIP as signaling system number 7 (SS7) does over circuit-switched, TDM networks for conventional, long- distance telephony. RTP is an Internet Engineering Task Force (IETF) standard that defines the streaming of interactive voice and video packets over switched networks.This is a so-called thin protocol that does not provide any quality of service (QoS) features. IP Telephony For several years the term Voice over Internet Protocol (VoIP) has been used to identify the transmission of voice signals over any IP-based Internet Protocol Centrex 11 network.In the business context, VoIP now refers to the transmission of voice and/or video communications over a managed WAN or LAN. Some consultants and one or two telecom manufacturers have used the term telephony over Internet protocol (ToIP) to describe the switching of real-time, conversational traffic in systems that are attached to IP data networks. The words IP telephony have emerged as the umbrella term that cov- ers both VoIP and ToIP, which can be delivered either by an IP-PBX (housed on the customers premises) or by IP-Centrex (for which the call processor is owned and accommodated by the carrier). Some organizations have been using VoIP to reduce the cost of long- distance service, particularly international service, for several years.In countries that have highly competitive interexchange carriers it is generally no longer worthwhile to use VoIP, after allowing for the cost of gate- way hardware and software.Several international carriers, such as the T-Systems division of Deutsche Telekom, have specifically promoted the use of IP networks for international traffic. IP telephony has been validated since the year 2000 by the availability of some IP-PBXs (see Chapter 4).An IP-PBX system or IP-Centrex service may deliver only VoIP (through appropriate trunk and line interfaces), only ToIP (by retaining interfaces for the existing analog and digital devices), or both in the form of IP telephony.Now that IP has become, by far, the most popular protocol for data transmission and is widely deployed in networks of all sizes, from one room to worldwide, organizations have access to the appropriate transport technology to gain the advantages of convergence.Full convergence between multimedia and data applications requires the availability of both VoIP and ToIP, which are both inherent to the concept of IP-Centrex. Interface Standards The key to success with IP-Centrex will be a high level of interoperability between devices and systems or applications.This will be a major move away from the proprietary interfaces that have kept acquisition costs up and made application implementations complex with PBX systems.The acceptance of a limited number of open standards will also facilitate more competition between a broader range of manufacturers and service providers. The earliest call control standard for mapping users names or tele- phone numbers into and IP source or destination addresses was H.323, 12 Centrex or PBX: The Impact of IP which was adopted by the International Telecommunications Union (ITU).H.323,was originally intended to define how multimedia commu - nications were to be transmitted over a data network between teleconfer- ence units and, as used for VoIP, defines only a restricted feature set, with different enhancements being made by various manufacturers.H.323does not define transmitted voice quality, is considered to be too processing intensive, and depends on the use of intelligent workstations. A working group of the IETF created the session initiation protocol (SIP) to lessen call setup times and take better advantage of the Internet infrastructure than H.323. SIP is most likely to become the interface stan- dard of choice between telephone sets and computers with ToIP systems. A third standard, officially known as H.248 but more generally as MGCP or Megaco, is being jointly developed by the IETF and ITU, with support from some, but not all, major manufacturers.The H.248/MGCP protocol addresses the needs of multimedia conferencing and is intended for use with media gateway controllers (MGC). Every communicating device on an Internet protocol-conforming net- work must have an IP address, so that a desktop with a telephone, a PC, and a softphone (within the PC) or a video terminal needs to be allocated three addresses.There must be a process with the network for mapping tele- phone numbers to corresponding IP addresses. Also, any endpoints address must be known in order to be accessible; in some circumstances, this requirement becomes a security concern.This situation can be problematic in that multiprotocol label switching (MLPS), which is widely used in managed wide area networks (WANs), does not allow for any spoofing (i.e., the alteration and retransmission of any part of a signal in order to hide the address contents and therefore discourage hacking).For details regarding the use of MPLS, see Managed Networks in Chapter 3. IP Centrex Configurations The development of IP-Centrex is quickly following the same path trod by the manufacturers of IP-PBXs over the past few years.There are two main stages in this development, where IP-Centrex is first delivered from an existing, circuit-switched type of serving CO and then, launched from a new controller that handles only packet-switched traffic. Internet Protocol Centrex 13 The first stage of IP-Centrex could be described as a hybrid service, supporting existing terminals and new IP phones.We may think of the sec - ond stage as pure IP-Centrex, with a server in the carriers network pro- viding control signals to various gateways and links to application processors (APs), for applications such as unified messaging (UM), auto- matic call distribution (ACD), and CRM. Hybrid IP-Centrex Service In the examples that are shown in Figures 2.2 and 2.3, a mixture of conven- tional (analog and digital) and IP phones (both separate sets and soft- phones on PCs) is used in the single or multilocation situations, served by one legacy Centrex system. At an organizations branch location, as in Figure 2.3, an on-site IP-Centrex gateway may be placed on the customers premises to support Customers premises Serving central office ISDN/BRI Toll Analog Trunk Digital Fax machine Class 5 switch Laptop (Softphones) IP Centrex Desktop PC gateway Broadband access link Edge Simple router Managed IP network Executive IP phones IP phones Figure 2.2 Single-location hybrid IP-Centrex. 14 Centrex or PBX: The Impact of IP Head office Serving central office Toll ISDN/BRI Analog Trunk Digital Fax machine Class 5 switch (Legacy CTX) Branch offices IP Centrex Desktop PC gateway (Softphones) Edge router Broadband links Managed Simple Executive IP network IP phones IP phones Figure 2.3 Multilocation hybrid IP-Centrex. non-IP phones and, possibly, facsimile machines, through analog and digi- tal ports. Pure IP-Centrex is based on a new type of CO switch that is now appearing on the market and is frequently known as a softswitch. Part of a typical softswitch configuration is shown in Figure 2.4. With this devel- opment the call control functions are physically separated from the various outboard gateways, in a way that has not been obvious with legacy, circuit- switched CO systems. Trends to Full IP-Centrex The softswitch-based IP-Centrex solution must continue to provide for a significant proportion of legacy phones, both analog and digital, while accommodating a growing population of IP-compatible devices.We do not encounter many large greenfield situations, where hundreds of users need new phones or computers at one time, in any one year.Most organi - zations wish to retain their present inventory of phone sets and a few fax machines for several years.A complete switch to local area network (LAN)-based voice and workstations is much more likely in home offices, for telecommuters, and in branch locations, as at the bottom of Figure 2.4.